Linnaeus' Infernal Fury

The starting point of modern zoological nomenclature (Clerck notwithstanding) has been established as the tenth edition of Linnaeus' Systema Naturae, published in 1758. Linnaeus divided the animal kingdom between six classes, with vertebrates making up four (Mammalia, Aves, Amphibia and Pisces) and invertebrates assigned to just two. One of these, Insecta, essentially corresponded to modern arthropods, and all other invertebrates were included in the class Vermes, 'worms'. Linnaeus' concept and arrangement of Vermes bears little resemblance to anything that exists in modern zoological classifications; with the study of invertebrate anatomy still in its absolute infancy, he was largely classifying animals based on their overall external appearance alone. One of Linnaeus' orders of Vermes, the 'Intestina', defined as 'simple, shell-less and limb-less', included animals now classified as annelids, nematodes, molluscs and even a chordate (the hagfish Myxine glutinosa). It also included a species whose identity would be debated for the next several decades: the 'infernal fury', Furia infernalis.

A reconstruction of Furia infernalis, from Piter Kehoma Boll.

Furia infernalis was described by Linnaeus as "Corpus filiforme, continuum, aequale, utrinque ciliatum: aculeis reflexis corpori appressis" ('body thread-like, continuous, uniform, ciliated on both sides with reflexed spinules appressed to the body'). It was found in marshes of southern Sweden and Finland. Linnaeus went on to record that F. infernalis was, "Pessima omnium, ex aethere decidua in corpora animalium, ea momento citius penetrat, intra horae quadrantem dolore atrocissimo occidit": the 'worst of all, falling from the sky onto the bodies of animals, into which it rapidly penetrates within a moment, striking [the victim] down with the most atrocious pain within quarter of an hour'. Linnaeus had good reason to highlight this animal's unpleasantness: he had been attacked by one himself when collecting botanical specimens in 1728, and barely escaped the resulting ailment with his life. A more detailed description of "der Höllenwurm" was compiled by Jördens (1802): it was a very slender worm, about the length of a nail, of a pale yellow or fleshy colour (other authors described it as greyish), with one end black. It climbed up standing vegetation, from whence it was carried by the breeze onto the exposed skin of humans and animals into which it rapidly burrowed. For victims, the first sign of its presence was usually a sudden pain in the afflicted spot, like the stab of a needle, and a small black spot marking the worm's entry point. A violent itching followed that developed into severe and extensive inflammation, often accompanied by fever; in the majority of cases, the affliction was so violent that the victim was dead within a matter of days if immediate action was not taken. If applied quickly enough, the worm could sometimes be drawn out with a poultice of fresh cheese curds. Otherwise, treatment required the careful dissection of the worms from between the muscle tissue into which they had entered, a process that (considering the surgical facilities available at the time) must have nearly as hazardous as the original infection.

As can be imagined, the attacks of this animal were greatly feared. In 1823–1824, an epidemic of Furia attacks spread through herds of livestock in Swedish and Finnish Lapland; thousands of head of reindeer perished, as well as countless cattle and sheep. Scavengers such as wolves feeding on the carcasses themselves sickened and died. One account from the time involves a young woman who was shearing wool from a recently deceased sheep (on a waste not, want not principle, I suppose) when she felt the tell-tale sting on a knuckle. Her life was saved by her master who was working nearby, when he quickly chopped off the affected finger with an axe. So great was the devastation that Norway, which had hitherto been free of the worm, passed an edict banning the import of animal furs from affected areas (Brooke 1827).

There were some, however, who greeted the description of Furia infernalis with skepticism. The idea of a tiny worm that somehow flew through the air and caused almost instantaneous mortality seemed fantastic. Even more problematic was the dearth of specimens. Many had seen the wounds caused by the worm and observed its effects; very few had seen the worm itself. Linnaeus himself had only seen a single, very poorly preserved specimen submitted to him by a church pastor. Most of the details about the worm's supposed appearance came from a single source, an article written by Solander, a student of Linnaeus'. The Academy of Sciences at Stockholm, naturally keen to discover all they could about such a scourge afflicting their country, offered generous rewards to anyone who could procure them a genuine specimen; no such specimen was forthcoming. Eventually, a consensus was reached: the worm Furia infernalis was an entirely fabulous animal, with no place in the annals of physical zoology. By 1827, notwithstanding the epidemic of only a few years previously, Brooke was able to comment that one could quite easily accept that something had affected the supposed victims of Furia without presuming that that something had to be the Furia itself. Even Linnaeus eventually came to accept that his inclusion of Furia in the Systema Naturae had been an error.

That Furia infernalis never existed outside the realms of fantasy remains the accepted wisdom to this day. But in that case, what did afflict Linnaeus and other unfortunates wandering the marshes of Sweden in the early 1700s? One thing that struck me was how much I was reminded of the more recent phenomenon here in Australia of 'white-tailed spider bites'. In recent decades, many people (including many medical professionals) have attributed serious ulcerative skin lesions, sometimes so serious that treatments such as skin grafts are required, to the bite of white-tailed spiders Lampona spp., common ground-running spiders often encountered near human dwellings. The actual evidence linking white-tailed spiders to such injuries is minimal; indeed, a clinical survey of 130 confirmed white-tail bites by Isbister & Gray (2003) found not a single incidence of one leading to ulceration. In both the 'Furia attacks' and the 'white-tailed spider bites', it seems likely that the primary culprit is bacterial infections resulting from opportunistic pathogens such as Streptococcus and Staphylococcus species. The initial wound may indeed have been caused by something like an animal bite or sting, or for that matter a splinter or pin-prick. Germ theory would not become widely accepted until the mid- to late 1800s; when Linnaeus compiled the Systema Naturae, flying worms probably seemed as good an explanation as any. The first 'attack' recorded by Furia victims may have simply been the first moment they noticed the infection's symptoms. And the 'worms' dissected out of advanced victims? Personally, I'm inclined to suspect that they may have been small pieces of tissue from the unfortunate sufferers themselves.

The exact causes of the 1823 epidemic are probably lost to history. Brooke (1827) stated that faculty at the Stockholm academy "had been led to consider the disorder by which [the reindeer] were attacked as a particular variety of hydrophobia". He also mentioned another possibility: reindeer were known to be vulnerable to inflammation of the brain, and dissections of the brains of deer killed by this condition sometimes revealed the presence of "a small vesicular worm". We can now recognise these vesicles as the cysts of hydatid tapeworms, which can hatch to cause tapeworm infections in any predator that eats the flesh of their host. So perhaps the 1823 epidemic was caused by a worm after all—just not the worm that was blamed.

REFERENCES

Brooke, A. de C. 1827. A Winter in Lapland and Sweden, with various observations relating to Finmark and its inhabitants; made during a residence at Hammerfest, near the North Cape. John Murray: London.

Isbister, G. K., & M. R. Gray. 2003. White-tail spider bite: a prospective study of 130 definite bites by Lampona species. Medical Journal of Australia 179: 199–202.

Jördens, J. H. 1802. Entomologie und Helminthologie des Menschlichen Körpers, oder Beschreibung und Abbildung der Bewohner und Feinde desselben unter den Insekten und Würmern vol. 2. Gottfried Adolph Grau: Hof.

Linnaeus, C. 1758. Systema Naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis 10^th ed., revised, vol 1. Laurentius Salvius: Copehagen.

A Place for Worms

When we think of endangered species, we tend to focus on the charismatic vertebrates, such as pandas, parrots, tigers or turtles. But endangered species may come from all walks, crawls or wriggles of life. Have you ever considered, for instance, the plight of endangered earthworms?

An unidentified species of Glossodrilus, copyright Thibaud Decaens.

Glossodrilus is a genus of earthworms found in tropical and subtropical regions of Central and South America. They are mostly fairly small as earthworms go, averaging only a few centimetres long and one or two millimetres in diameter. The largest, G. oliveirai from Brazil's Roraima State and Guyana, is about 25 centimetres long; the smallest, G. tico from Roraima and Venezuela, is less than two centimetres in length. Most species lack pigmentation, meaning that they appear greyish from the colour of their gut contents. A single species, G. freitasi from Amapá State in Brazil, is a bright violet in colour. Other diagnostic features of the genus include: eight setae per segment, arranged in regular series; a pair of (or sometimes one) calciferous glands sitting above the oesophagus in segments XI to XII; two or three pairs of lateral hearts in segments VII to IX, and two pairs of intestinal hearts in X and XI; and a pair of testes in segment XI. Glossodrilus is distinguished from a closely related earthworm genus, Glossoscolex, by the absent of a pair of muscular copulatory chambers associated with the male ducts in the latter genus (Righi 1996).

Over sixty species have been assigned to Glossodrilus; as is usual with earthworms, they are mostly distinguished by internal characters such as features of the reproductive systems. They are most diverse in upland regions, with many species inhabiting high rain forest. A few species in the northernmost or southernmost parts of the genus' range inhabit secondary grasslands. Glossodrilus species are conspicuous by their absence in the Brazilian central plateau, and only infrequently present in lowland Amazonia (Righi 1996).

And this is where the question of conservation comes in. You see, the greater number of Glossodrilus species are known only from a very restricted area (Lavelle & Lapied 2003). Part of this may be an artefact of sampling: in more recent decades, our understanding of South American earthworm diversity has been heavily shaped by one researcher, Gilberto Righi of the Universidade de São Paulo (I referred to him briefly in an earlier post on Amazonian earthworms), and we know little of areas where Righi did not collect specimens himself or from where he did not receive specimens supplied by ecological surveys. Nevertheless, sampling has probably been extensive enough to expect that the low number of shared species between different regions will hold firm at the broad scale at least. Most Glossodrilus species (and other native South American earthworms) are dependent on old-growth habitats; as land is cleared for farming, forestry and the like, exotic and invasive earthworm species take over. It would be all to easily for the little Glossodrilus to find themselves homeless, and slip into extinction without any to mark their passing.

REFERENCES

Lavelle, P., & E. Lapied. 2003. Endangered earthworms of Amazonia: an homage to Gilberto Righi. Pedobiologia 47: 419–427.

Righi, G. 1996. Colombian earthworms. Studies on Tropical Andean Ecosystems 4: 485–607.

Who Knows Which Way the Water Flows?

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Dorsal and lateral views of specimens of Trenella bifrons, from Parkhaev (2001).

There is no question that the molluscs are one of the most significant groups of animals in the marine environment. And thanks to the production by many species of mollusc of a hard shell, they are also one of the best-known groups in the fossil record. A rich and detailed picture of molluscan evolution is available to us as far back as the earliest Cambrian. But, of course, the further back in time we go the more questions we have about what the picture means. And it is in the earliest part of their history that the picture becomes the most opaque.

The Trenellidae are part of that early picture. This family of molluscs is known from the early Cambrian (Parkhaev 2002). They are part of the assemblage of early molluscs referred to as the helcionelloids, whose overall position in the molluscan family tree is very much open to question. Helcionelloids are simple, more or less cap-shaped or cone-shaped shells that are usually also tiny. The type species of the Trenellidae, Trenella bifrons, for instance, is only about 1 to 1.5 millimetres along the longest axis, and only one-half to one millimetre tall (Parkhaev 2001). This all adds up to a general shortage of morphological details that might help us pin down which, if any, modern molluscan groups helcionelloids are connected to. Possession of a undivided dorsal shell has lead many to compare them to gastropods. Others have pointed to the monoplacophorans like the modern Neopilina. In both cases, though, the resemblance is fairly superficial and confirming things one way or another would depend on identifying features of the soft anatomy, such as torsion, that are difficult if not impossible to infer from features of the shell alone.

Within the helcionelloids, trenellids are characterised by having the lower rim of one end of the shell's long axis drawn out into a siphonal groove. It seems likely that this groove was somehow involved in the passage of water around the gill(s), but whether its position indicates the front end or the back end of the shell, and whether it was used to draw water in or expel water out, depends again on what each author expects its original soft anatomy to have been. Unfortunately, evidence for the latter in trenellids is almost completely non-existent; while muscle scars have been identified in some helcionelloids, they remain unknown for this family.

The Trenellidae are closely related to, and probably include the ancestors of, the Yochelcionellidae in which the siphonal groove become raised and closed ventrally, turning it into a snorkel-like structure (one yochelcionellid, Yochelcionella daleki, has been featured on this site before). However, comparing trenellids to yochelcionellids raises something of a question in my mind. In general, mollusc shells grow through secretion from the mantle around the shell's rim only, meaning that once shell growth has passed a certain section the mollusc usually cannot go back and rearrange it. Assuming that helcionelloids grew in the usual molluscan manner, surely yochelcionellids would have gone through a stage in their development before the lower part of the 'snorkel' was closed off where they looked a heck of a lot like a trenellid? Is it even possible to distinguish a mature trenellid from a juvenile yochelcionellid?

REFERENCES

Parkhaev, P. Yu. 2001. Trenella bifrons: a new helcionelloid mollusk from the Lower Cambrian of South Australia. Paleontological Journal 35 (6): 585–588.

Parkhaev, P. Yu. 2002. Phylogenesis and the system of the Cambrian univalved mollusks. Paleontological Journal 36 (1): 25–36.

Depending on the Liver

Stained specimen of Asian liver fluke Clonorchis sinensis, from here.

Every year, tens of millions of people worldwide (particularly in tropical Asia) suffer the effects of clonorchiasis and opisthorchiasis, conditions caused by infections with liver flukes of the family Opisthorchiidae. Exactly which condition the victim is suffering from depends on just which species of flukes they find themselves infected with, but there is little immediate difference between the clinical symptoms of either. Issues arising from clonorchiasis include fever, jaundice, diarrhoea and malnutrition. Long-term or heavy infections may result in cirrhosis, pancreatitis or even cancer (King & Scholz 2001). But just what is responsible for these debilitating illnesses?

Flukes are a diverse group of endoparasitic flatworms that reach maturity in association with vertebrates. As with other parasite lineages, different fluke species prefer different hosts and infect different parts of the host's system. Many have complex life cycles involving multiple larval stages and the successive infection of up to three distinct hosts on the way to maturity. Opisthorchiidae have such a three-host life cycle; their adult (or 'definitive') hosts span the gamut of vertebrates from fish to birds to mammals. Opisthorchiids in the strict sense are invariably associated with the liver of these hosts, taking up residence in the bile duct and gall bladder (however, phylogenetic studies have indicated that the closely related Heterophyidae, which infect the intestine, are probably paraphyletic with regard to opisthorchiids and the two families may be merged into an expanded Opisthorchiidae—Thaenkham et al. 2012). When mature they are elongate and flattened with the mouth near the front of the body surrounded by a sucker for attachment to host tissue. A second sucker is present on the underside of the body not too far behind the first (Dawes 1956).

Like other internal parasites, liver flukes are incredibly fecund. A female of Clonorchis sinensis, one of the main opisthorchiid species of concern to humans (yes, flukes reproduce sexually; I'll allow a moment for the disgusting implications to fully sink in), may produce up to 4000 eggs in a single day. These eggs are released into the host's digestive system, passing out in the faeces. They do not hatch until after they are ingested by the first larval host, an aquatic snail (many sources will say a freshwater snail but at least one opisthorchiid genus, Delphinicola, paratises marine dolphins so presumably has a correspondingly marine gastropod host). The egg hatches into a ciliated larva called a miracidium that over the course of the next few hours will find a likely spot in the snail's gut to develop into the next larval stage, the sporocyst. The sporocyst is immobile and mouthless, and feeds by absorbing nutrients directly from the host tissue. It also contains a mass of germ tissue that develops into multiple individuals of the next larval stage, the redia, that are released from the parent sporocyst after a couple of weeks or so. The rediae are worm-like and mobile, chomping their way through suitable sections of host tissue. They also develop multiple individuals of the next stage within them just as the sporocysts did. In this way, a single egg may eventually result in an exponentially increased number of larvae.

Life cycle of Clonorchis sinensis, from here.

The next larval stage is called the cercaria. In opisthorchiids, the cercariae look a bit like tadpoles with a dorsoventrally finned tail. I haven't found exactly how opisthorchiid cercariae are released into the water column but in other flukes they may be released with the discharge from the abcess or cyst that forms as the rediae feed on their host, or escape from the host tissue after the snail dies as a result of its infection. The cercaria is a dispersive stage that seeks out the next host in the life cycle. This they do by hanging head-down in the water column and allowing themselves to slowly sink until disturbed by contact with a potential host or water-currents created by its movement. At this point the cercaria rapidly swims upwards before allowing itself to sink again, hopefully onto the new hosts skin. The cercaria will then dig its way into the host's muscle tissue and transform into the last larval stage, a cyst called a metacercaria. Opisthorchiid cercariae most commonly attach themselves to some kind of fish but they are a bit less picky about their host than the other stages in their life cycle; opisthorchiid metacercariae have also been found in crustaceans and have been shown in the laboratory to even be capable of infecting mammals (specifically guinea pigs).

The developing liver fluke reaches its definitive host when the second larval host is eaten. A young fluke hatches from the metacercaria inside the definitive host's gut and make their way to the liver which they find by detecting the traces of its chemical products and/or by detecting the physical track of the bile duct. There they will mature into fully adult flukes, all ready to begin the cycle again (by doing the nasty in some poor sod's gall bladder).

The economic impact of opisthorchiids around the world is estimated to amount to hundreds of millions of dollars each year. Unfortunately, as with many other illnesses more widespread in developing nations, there still remains a lot to be learned about their control. Cooking fish before consumption to kill metacercariae is one of the more obvious methods, though it should be noted that metacercariae can be devillishly difficult buggers to kill. Installation of sanitation and sewerage systems can also help by reducing the chance of egg-carrying faeces to make it into water bodies, though medically significant opisthorchiids may also infect animals other than humans such as cats, dogs or pigs. For now, it looks like liver flukes will be with us for some time.

REFERENCES

Dawes, B. 1956. The Trematoda, with special reference to British and other European forms. University Press: Cambridge.

King, S., & T. Scholz. 2001. Trematodes of the family Opisthorchiidae: a minireview. Korean Journal of Parasitology 39 (3): 209–221.

Thaenkham, U., D. Blair, Y. Nawa & J. Waikagul. 2012. Families Opisthorchiidae and Heterophyidae: are they distinct? Parasitology International 61: 90–93.

Typhloesus: The 'Alien Goldfish' of Bear Gulch

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Above: Typhloesus wellsi, external appearance, and the same with major anatomical details shown. Based on figure of specimen U.M. 6027 in Conway Morris (1990).

Recently, the interwebs became all agog at the suggestion that the hitherto-mysterious Carboniferous fossil Tullimonstrum gregarium could possibly represent a vertebrate, distantly related to modern lampreys. But there are other fossil animals whose relationships remain inexplicable and one of these is another child of the Carboniferous, the so-called 'alien goldfish' Typhloesus wellsi.

When I announced my plan to write this post, I referred to Typhloesus as coming from Mazon Creek, the fossil deposit from whence comes Tullimonstrum. This, as it turns out, was a mistake on my part: Typhloesus actually comes from a different deposit, Bear Gulch in Montana. Bear Gulch is perhaps most famous for its fossils of early fish, such as symmoriiform sharks (the ones with the weird shoebrush headgear) and heavily armoured palaeoniscoids. Indeed, compared to other Carboniferous deposits, Bear Gulch is unusual for its preponderance of swimming rather than benthic animals. Typhloesus is represented in the deposit by a number of individuals in varying states of preservation.

In some ways, Typhloesus is more famous for what it is not than for what it is. It was one of the first body fossils found in association with conodonts, minute teeth-like fossils that had been subject to much speculation as to what sort of animal they might have come from. Initially, there was much excitement that the conodont animal may have finally been found, but it did not take very long for questions to be raised about the nature of this association. By the time Typhloesus was reviewed in detail by Conway Morris (1990), it was clear that the conodont fossils had been preserved within its gut, not its mouth, and Typhloesus was a conodont-eater rather than a conodont-bearer (it has since been found that conodont animals were eel-like chordates).

Externally, Typhloesus was a fairly simple, cigar-shaped animal, with its body laterally compressed and higher than wide. It grew to a decent size, with the largest specimens being a little under ten centimetres in length. There is no sign of eyes or any other prominent sensory structure, and so far as is known the external skin or cuticle was smooth and unornamented. The most distinctive external feature is a large 'tail-fin' at the rear. This fin was supported by an arrangement of criss-crossing rods or fibres, and would have been fairly stiff in life. Another pair of folds or fins ran along most of the underside of the body with a noticeable gap towards the rear. Typhloesus probably swam in a not dissimilar manner to an active modern fish, using sweeps of the tail-fin to provide thrust; the ventral fins may have provided stability and steerage. The visible line of the foregut comes to a halt slightly before reaching the front of the body, and it seems that the mouth would have been slightly ventral and contained within a 'hood'. Though its overall conformation and known gut-contents (most commonly conodonts, but sometimes worm jaws or fish scales) suggest an active predator, I am at a loss to understand how it located its prey without eyes. Perhaps the hood contained some sort of chemical sensors in life.

When it was first found, it was thought that its overall appearance suggested a relationship of Typhloesus to the chordates. However, Conway Morris (1990) saw its internal anatomy as incompatible with this view. Fossils of this animal show a narrow foregut leading into a voluminous, sack-like midgut. Below the midgut is a pair of dark, disc-shaped organs showing a concentration of iron deposits called the ferrodiscus; though a striking element of all Typhloesus fossils, the function of this structure is completely unknown. What Conway Morris found conspicuous by its absence, however, was an anus: there appeared to be no sign of any gut structures in the rear of the animal. The gut was a blind sack, with the only way out being the same as the way in. The absence of a through-gut would be unprecedented in a chordate, or indeed in many animals except jellyfish or flatworms. Conway Morris was also unable to identify other chordate-specific structures such as muscle-blocks, gill openings or a notochord; though he confessed that the first two might be obscured by the vagaries of decay, he felt that the third at least should have left more of a sign. It was this combination of an overall fish-like appearance with a very un-fish-like anatomy that led Conway Morris to later dub Typhloesus the 'alien goldfish'.

With the exclusion of a chordate connection as a possibility, Conway Morris found himself at a loss as to just where Typhloesus fitted into animal evolutionary history. Finned swimmers are also known among molluscs, nemerteans and chaetognaths, but Typhloesus is no more like any of these than it is like a chordate. Conway Morris felt himself compelled to declare the affinities of Typhloesus completely unknown. Personally, though, I can't help wondering if the 'alien goldfish' might not be so alien after all: maybe it is a chordate. The overall similarities of Typhloesus to a chordate are remarkable; in particular, the hooded mouth is very similar to that of a lancelet. But what about that missing anus, you say? Where is that all-important butthole? To which I respond, is it really missing? Looking at the figures of Typhloesus fossils in Conway Morris (1990) (which is of course a poor competitor to Conway Morris' ability to look directly at the fossils themselves), I see that directly below the midgut is the ferrodiscus. And directly below that is a streak running between the ferrodiscus and the animal's venter. Conway Morris saw this structure (which he called the 'midventral strand') as some sort of connection between the ferrodiscus and the exterior, but could it in fact be the tail-end of the reargut? It is certainly not unknown for the anus in chordates to not be right at the very rear of the animal; in some fish (such as the scorpionfish-like Aploactinidae) it is even moved so far forward as to be almost underneath the head. And the missing notochord? Considering that despite the presence of specimens numbering in the thousands, a notochord was only announced in Tullimonstrum within the past year, maybe on that front Typhloesus could reward a second look.

REFERENCE

Conway Morris, S. 1990. Typhloesus wellsi (Melton and Scott, 1973), a bizarre metazoan from the Carboniferous of Montana, U.S.A. Philosophical Transactions of the Royal Society of London Series B 327: 595–624.

Predatory Ribbons

Some of you may have seen something like this doing the rounds:

The animal in the clip is called a nemertean. The Nemertea, commonly known as ribbon worms, are a group of more than 1200 known species of mostly predatory worm-like animals. The majority of nemerteans are marine, but there are also species found in freshwater or even terrestrial environments (the clip above shows a terrestrial species). As their vernacular name suggests, the majority of ribbon worms are flattish, slender animals with little in the way of external elaborations. Most are small and unassuming, but there are exceptions: one ribbon worm species from coasts of northern Europe, Lineus longissimus, grows to estimated lengths of over 30 m and may even be the longest animal in existence*. The most characteristic feature of ribbon worms is a long proboscis that they use in capturing prey; when not being deployed, this proboscis is retracted within a cavity called the rhynchocoel that runs much of the animal's length. Other than this, nemerteans have little in the way of internal body cavities other than the gut. They do have a simple blood-vascular system consisting of a few blood vessels but no actual heart; instead, the blood just kind of sloshes back and forth as a result to the animal's body contractions as it moves.

*Some uncertainty over the exact lengths of Lineus longissimus specimens is inevitable because, despite their remarkable length, they are still only a centimetre or so wide. When you're trying to extract something like that from among a bunch of rocks, it's gonna stretch and break. Still, thirty metres is a fairly conservative estimate of its length; Wikipedia cites a supposed maximum nearly twice that. These mega-nemerteans are definitely one of those animals that make me wonder, how does this thing even exist? I mean, what is the point of being so incredibly long and slender? How does it collect enough food at the front end to nourish itself all the way to the back end? How does it not just fall apart of its own accord, let alone when subjected to any external pressure?

Lineus longissimus, from here.

The relationships of nemerteans to other animals are rather uncertain, and they have generally been classified as their own independent phylum. Because of their simple body plan, many early authors compared them to flatworms, at least on a grade level, but this fell out of favour as it became accepted that the rhynchoel and blood-vascular system probably correspond to anatomical structures in more complex animals. More recent evidence from molecular and other sources has converged on a position within the Lophotrochozoa, the major animal clade that also includes molluscs, brachiopods and annelids, but their exact placement within this clade remains open to debate.

Molecular data have also influenced our understanding of relationships within the Nemertea. An influential classification of the group divided them between the Enopla, in which the proboscis is usually armed with a stabbing stylet or stylets, and the Anopla, in which the proboscis is unarmed (members of this latter group often have the proboscis branched as in the clip above; I'm guessing that in the absence of a stylet the proboscis probably works through adhesion). The two groups also differ in that Anopla always have the proboscis emerging from a separate pore to the mouth, whereas in many Enopla the mouth and proboscis pore share a common opening (Kvist et al. 2014). The Anopla were further subdivided into the Heteronemertea, which have a distinctive tissue layer called the dermis underneath the outer epidermis, and the Palaeonemertea which lack such a differentiation of skin layers. However, one need not be an expert in nemerteans to spot that the Anopla and Palaeonemertea were mostly defined by their lack of derived features (no stylets, no dermis) and so it should come as little surprise that molecular studies of the group have failed to offer resounding support for their monophyly. Instead, a number of studies have suggested that the Heteronemertea and Enopla together form a clade that Thollesson & Norenburg (2003) dubbed the Neonemertea. When they did so it was on the basis of molecular data only, but later authors have identified possible synapomorphies of the Neonemertea in features of the nervous and blood-vascular systems. One family of 'Palaeonemertea', the Hubrechtidae, has been suggested to also belong within the Neonemertea as sister-taxon to the Heteronemertea. This is of interest because the Hubrechtidae and Heteronemertea share a distinctive type of ciliated planktonic larva called a pilidium (other nemerteans either develop directly or have a creeping planula-type larva). Ciliated planktonic larvae are known a number of groups of animals, such as the veliger of molluscs, the trochophore of annelids, or the tornaria of acorn worms, and there has been a lot of discussion over the years as to whether similarities between these larvae represent a shared ancestry, or whether they might have evolved independently. In the case of nemerteans, at least, the current evidence seems to favour the latter. As for the other 'palaeonemerteans', there seems to be less of a consensus as to whether they form a single clade or a paraphyletic series relative to the Neonemertea.

A polystiliferan, Drepanogigas albolineatus, copyright Peter Wirtz.

As for the Enopla, it appears to form a valid clade. Previous authors divided the enoplans between the Hoplonemertea, including the majority of species, and the Bdellonemertea, including the single distinctive genus Malacobdella. The Hoplonemertea were in turn divided between the Monostilifera, in which the proboscis has a single long stylet, and the Polystilifera, in which it bears a pad of small stylets, and molecular analyses support the separation of these groups. Malacobdella (which lacks proboscis stylets but has the conjoined mouth-proboscis pore) has a sucker at the posterior end of its body, by which it lives attached to the gills of a mollusc. Malacobdella is not a parasite of the mollusc, per se: instead, it feeds on food particles drawn in by water flowing through the mollusc's gills. However, the recent analyses have indicated that Malacobdella is in fact a derived monostiliferan, and a number of recent authors have used the Hoplonemertea as an equivalent name to the old Enopla.

Live individual of the pelagic nemertean Dinonemertes shinkaii (head towards the right), from here.

Also distinctive within the Hoplonemertea are two clades, the polystiliferan Pelagica and the monostiliferan Korotkevitschiidae, that have left the ocean floor and adopted a pelagic life style. Members of both these groups are gelatinous and eyeless; the Pelagica have lost further internal organs such as nephridia. The Korotkevitschiidae (which also lack a proboscis stylet) are found towards the surface of the ocean; the Pelagica are found in much deeper waters (Chernyshev 2003). The pelagic nemerteans are among the most poorly known of all ribbon worms; they are rarely encountered (about half of the 100 or so described species of Pelagica are known only from single specimens) and their relatively simple morphology makes them difficult to compare to other nemerteans. If the individual in the photograph is any indication, however, they are beautiful animals.

REFERENCES

Chernyshev, A. V. 2003. Classification system of the higher taxa of enoplan nemerteans (Nemertea, Enopla). Russian Journal of Marine Biology 29 (Suppl. 1): S57–S65.

Kvist, S., C. E. Laumer, J. Junoy & G. Giribet. 2014. New insights into the phylogeny, systematics and DNA barcoding of Nemertea. Invertebrate Systematics 28: 287–308.

Thollesson, M., & J. L. Norenburg. 2003. Ribbon worm relationships: a phylogeny of the phylum Nemertea. Proceedings of the Royal Society of London Series B—Biological Sciences 270: 407–415.

The Erisocrinoidea: Shallow Crinoids

Articulated calyx of Erisocrinus typus, copyright Richard Paselk.

The close of the Permian period saw the largest mass extinction ever recorded. It has been estimated that about 95% of all marine species were wiped out. Many prominent Palaeozoic lineages disappeared entirely; others were reduced to a mere remnant of their former selves.

One of the casualties of the end-Permian extinction was the crinoid group known as the Erisocrinoidea (or Erisocrinacea in older texts). These were a diverse group of crinoids divided between several families, recorded from the Carboniferous and Permian periods. One species, Erisocrinus typus, is known from a large number of well-preserved, articulated specimens from the mid-Late Carboniferous of the United States and is one of the best representatives of the Palaeozoic cladid crinoids. Erisocrinoids are characterised by a low cup, dominated by the ring of radial plates. The base of cup was often recessed, meaning that the basal and infrabasal plate rings were often partially or entirely obscured in outer view. Most significantly, the array of anal plates found in other crinoids was reduced to a single plate or even lost. The insertion points of the arms bear signs of strong muscular articulation, indicating that these were animals of higher-energy environments requiring more exertion to maintain an ideal feeding position. The anal sac, where it is preserved, was only weakly plated and would have been reasonably soft in life (Moore et al. 1978).

In other respects, though, the erisocrinoids could be somewhat disparate. Many, such as the type family Erisocrinidae and the families Protencrinidae and Catacrinidae, have biserial arms in which the arm's skeleton is comprised of paired rows of plates. In other families, such as the Graphiocrinidae and Diphuicrinidae, the arms were uniserial, with only a single row of plates. Webster & Maples (2006) noted that, even though all erisocrinoids shared the character of a reduced anal plate array, the exact position in the cup of the anal plate or its remnant differed between families. They therefore suggested that the erisocrinoids might not be a monophyletic group, but members of a number of different lineages that had converged on a similar morphology and presumably lifestyle.

This was not an entirely novel suggestion. Even while recognising a single superfamily Erisocrinacea, Moore et al. (1978) had suggested connections between individual erisocrinoid families and families placed in other superfamilies. The integrity of the Erisocrinoidea had also been questioned in relation to Encrinus, a genus from the Middle Triassic that had been included with the erisocrinoids on the basis of its combination of biserial arms and lack of an anal plate. If this assignment was correct, erisocrinoids would have survived the end-Permian extinction: the only crinoid lineage to do so other than the Articulata, the clade including the living sea lilies and feather stars. Articulates retain uniserial arms, a more plesiomorphic characteristic. However, while investigating the evolutionary origins of the articulates, Simms & Sevastopulo (1993) pointed out that Encrinus shared derived features with articulates that were absent in erisocrinoids. For instance, while Encrinus and the erisocrinoids both had each of the basic five echinoderm arms branching to form a total array of ten arms, in Encrinus they branched from the second primibrachial plate as in articulates, instead of from the first as in erisocrinoids. Rather than being a late-surviving erisocrinoid, Encrinus was an early side-branch of the articulates, and as far as is known only a single crinoid lineage survived the Permian.

REFERENCES

Moore, R. C., N. G. Lane, H. L. Strimple, J. Sprinkle & R. O. Fay. 1978. Inadunata. In: Moore, R. C., & C. Teichert (eds.) Treatise on Invertebrate Paleontology pt T. Echinodermata 2. Crinoidea vol. 2 pp. T520–T759. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

Simms, M. J., & G. D. Sevastopulo. 1993. The origin of articulate crinoids. Palaeontology 36 (1): 91–109.

Webster, G. D., & C. G. Maples. 2006. Cladid crinoid (Echinodermata) anal conditions: a terminology problem and proposed solution. Palaeontology 49 (1): 187–212.

Petrosia: The Sexual Life of the Sponges

It has to be admitted that sponges are not one of the best-publicised of animal groups. Even when they are given some grudging mention, there is little reference to the variety of sponges that can be found on our planet. But don't go thinking that all sponges are the same.

Stony sponge Petrosia ficiformis, copyright Véronique Lamare.

Petrosia is a genus of sponges found in tropical and subtropical oceans around the world. Members of this genus come in a variety of forms: branching, cylindrical, globular, lamellate or bowl-shaped. They may reach large sizes, with some species up to a metre or two in diameter, though others may be much smaller. Most species are dark colours such as red, brown or black, though the Sulawesi species Petrosia alfiani is a bright canary yellow (de Voogd & van Soest 2002). The reasons for classifying such superficially divergent forms in a single genus lie beneath the surface. However, it has a high proportion of skeletal spicules to soft tissue, giving Petrosia species a hard, brittle texture (hence they are sometimes known as 'stony sponges'). The spicules of Petrosia are mostly long, slightly curved rods that may be rounded or pointed at the ends; they may be large or smaller, with smaller spicules tending to be more common closer to the sponge's surface. Two subgenera are recognised within Petrosia on the basis of whether the spicules are mostly in a tangential (subgenus Petrosia) or reticulate (Strongylophora) arrangement. The subgenus Petrosia is known from the Atlantic and Pacific Oceans, whereas Strongylophora species are found in the Indian Ocean and the western Pacific (Desqueyroux-Faúndez & Valentine 2002).

Magnified view of surface of Petrosia ficiformis specimen, showing arrangement of spicules, from (Desqueyroux-Faúndez & Valentine (2002).

One of the best-studied species in this genus is the Mediterranean Petrosia ficiformis, which tends towards a cylindrical growth habit in sheltered spots. Like other sponges, P. ficiformis may provide an important habitat for other organisms. Smaller invertebrates live in and around the sponge, and molecular studies have shown that different sponge species tend to host their own distinct communities of bacteria. However, the niche provided by Petrosia in the Mediterranean can be vulnerable to damage: field observations have indicated that stony sponges grow exceedingly slowly. Maldonado & Riesgo (2009) found that in twenty years of diving off the Spanish coast, they saw almost no growth in individual sponges. When they took small (one by one-half centimetre) tissue samples from the sponges, it could take up to three months for the removed patch to regrow. Such a slow rate of growth definitely makes one wonder just how old some of the large Petrosia referred to above must be.

Bowl-shaped Petrosia lignosa, from de Voogd & van Soest (2002).

Maldonado & Riesgo (2009) were taking their samples to study how the sponges reproduced. Petrosia species are free spawners, releasing eggs and sperm directly into the water column. In the case of P. ficiformis, this happens in late autumn. Eggs develop at scattered locations through the sponge, but migrate within the body to form clusters before being released. The sexes are separate, with an individual sponge only producing either eggs or sperm. After fertilisation, the eggs develop into small ciliated larvae that may shift between a spherical and a multilobate form. Whereas the larvae of other sponges may be quite mobile, those of P. ficiformis are not active swimmers, presumably relying on the motion of water currents to carry them to a suitable resting spot. Maldonado & Riesgo (2009) noted that in the two years they observed Petrosia spawning, it occured at times when surge levels had risen immediately prior to the onset of stormy weather. Despite the regular associations of Petrosia with particular microbial populations, the larvae do not carry any sort of culture propagule from their parents, indicating that each individual sponge reacquires its associates from the surrounding waters. Larvae attach themselves to the substrate after two to four weeks of growth, and proceed to grow slowly (though, as is the way of sponges, if multiple larvae settle immediately adjacent to one another they may fuse into a single aggregate individual). Larvae grown in the lab took about one and a half months to develop distinct choanocyte chambers (the ciliated chambers in which a sponge filters water for food particles). They may share their environment with sea hares, but there is no question that Petrosia are sea tortoises.

REFERENCES

Desqueyroux-Faúndez, R., & C. Valentine. 2002. Family Petrosiidae van Soest, 1980. In: Hooper, J. N. A., & R. W. M. van Soest (eds) Systema Porifera: A guide to the classification of sponges pp. 906–917. Kluwer Academic/Plenum Publishers: New York.

Maldonado, M., & A. Riesgo. 2009. Gametogenesis, embryogenesis, and larval features of the oviparous sponge Petrosia ficiformis (Haplosclerida, Demospongiae). Marine Biology 156 (10): 2181–2197.

Voogd, N. J. de, & R. W. M. van Soest. 2002. Indonesian sponges of the genus Petrosia Vosmaer (Demospongiae: Haplosclerida). Zool. Med. Leiden 76 (16): 193–209.

Serpularia: A Rightly Forgotten Problematicum

I think it may be time to rock out something that hasn't been seen on this site for a while. Horns at the ready...

(Credit, again, to Neil from Microecos). And I'm afraid that may just be the most excitement that we get in this post. While some fossils are problematic because they're so strange that they can't be easily compared to living animals, others are problematic simply because they're rubbish.

In 1840, the palaeontologist Georg Graf zu Münster ('Graf' being a German title that generally gets translated as 'Count') published his Beiträge zur Petrefakten-Kunde, in which he described a number of fossils held in his collection. This book included a section on fossils from the Ordovician Orthoceratite Limestone of the Fichtel Mountains in Bavaria. Which, close to the end, included this little tidbit:

Unter mehreren Bruchstücken einiger mir noch unbekannten Versteinerungen kommen auch einige röhrenformige Korper vor, welche ich anfänglich für den von Murchison aus der 27sten Tafel abgebildeten Myrianites hielt, allein genaue Untersuchung zeigte, dass diese Korper formliche Schalen hatten und daher vielleicht zu den Serpuliten gehört hatten, daher ich sie vorläufig Serpularia genannt habe. Aus der Taf. IX. Fig. 14 und 15 sind zwei Arten von dergleichen Bruchstücken abgebildet; Fig. II. Serpularia crenata; glatt gebogene Röhre, aus dem Rücken crenulirt. Fig. 15. Serpularia bicrenata; glatte etwas zusammengedrückte ganz grade Röhrchen, die an beiden Seiten crenulirt sind.

Translated with the help of Google Translate, I think this means: "Among several fragments of fossils unknown to me occured a tube-like body, which I initially took for Myrianites as figured by Murchison in the 27th plate, until close examination showed that this body had distinct signs of segmentation and was therefore perhaps one of the Serpulidae. Therefore, I have provisionally called it Serpularia. On Plate IX Figs 14 and 15 are shown two types of the like fragments; Fig. 14, Serpularia crenata: smooth curved tube crenulated from the back. Fig. 15, Serpularia bicrenata: smooth, slightly compressed, quite straight tubes that are crenulated on both sides".

Münster's (1840) original figures of the two Serpularia.

As perfunctory as it was, that seems to be all there was to say on the matter. The good Graf's Serpularia has pretty much never been mentioned again*, beyond being cited to cause a name change in a later homonymous gastropod genus, and a brief listing in Howell's (1962) coverage of worm fossils for the Treatise on Invertebrate Paleontology that adds nothing to the original description.

*Though if it were to be mentioned again, it would probably have to be under a different name. The name 'Serpularia' had earlier been used by Fries in 1829 for a genus of slime moulds. At the time, slime moulds were treated as fungi, and hence fell under the purview of botanical rather than zoological names, but with the recognition that they are amoebozoans an increasing number of authors would move them into the field of the Zoological Code.

Münster believed that his fossils belonged to the Serpulidae, a family of annelid worms. Annelids, being mostly soft and squishy things that do not stand up well to decay, have a pretty deplorable fossil record, but serpulids are a bit of an exception. These are sessile worms that secrete a calcareous tube in which they live their lives (modern serpulids appeared on this site in this post). Unfortunately, while these tubes are eminently fossilisable, they are also a bit nondescript, and have little to mark them as uniquely serpulid.

Because of the dominance of annelids among modern worms, there has been a definite tendency in the past to assume that any given worm-like fossil represents an annelid. Howell's (1962) aforementioned list of annelids includes the Ediacaran Spriggina (identity still under debate, but probably not an annelid) and the Cambrian Pikaia (now generally regarded as an early chordate). Similarly, any worm-like tube has been assumed a serpulid. But even among annelids, serpulids are not the only tube-bearing worms. At least two other families, the Sabellidae and the Cirratulidae, include species producing calcareous tubes. There are also other groups of non-annelid worms that, though relatively uncommon or unprepossessing today, may have been more prominent in the past. After all, we are talking here about a period of hundreds of millions of years. We know that vertebrates have gone through a great deal of evolutionary change over that period; why should we assume that worms have not?

So while fossils have been assigned to the serpulids going back as far as the Cambrian (if not beyond), there is little reason to take those assignations at face value. When so-called Palaeozoic serpulids have been examined critically in recent years, they have so far proven to lack features that would definitely confirm their identification (Vinn & Mutvei 2009). Weedon (1994) found that Palaeozoic fossils that had been assigned not only to the Serpulidae, but to the modern genus Spirorbis, had a shell microstructure that suggested a relationship to bryozoans or brachiozoans rather than to annelids. Without a similar close analysis, we could not assume a priori that Münster's Serpularia were not serpulids, but odds would currently be against it.

REFERENCES

Howell, B. F. 1962. Worms. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W144–W177. Geological Society of America and University of Kansas Press.

Münster, G. 1840. Beiträge zur Petrefacten-Kunde von Herm. v. Meyer und Georg Graf zu Münster vol. 3. In Commission der Buchner'schen Buchhandlung: Bayreuth.

Vinn, O., & H. Mutvei. 2009. Calcareous tubeworms of the Phanerozoic. Estonian Journal of Earth Sciences 58 (4): 286–296.

Weedon, M. J. 1994. Tube microstructure of Recent and Jurassic serpulid polychaets and the question of the Palaeozoic 'spirorbids'. Acta Palaeontologica Polonica 39 (1): 1–15.

The Acrotretids: Micro-brachiopods from the Dawn of... Brachiopods

Ventral valve of Acrotreta sp., copyright Ivo Paalits / TÜ geoloogiamuuseum.

When brachiopods have been featured on this site before, they have generally been representatives of the group known as the articulates. Today's subjects, the Acrotretidae, are instead members of the inarticulate brachiopods. Whereas the shells of articulate brachiopods have a hinge connecting the two valves, the shells of inarticulates do not. Instead, the valves of inarticulates are held together purely by the muscle and tissue around them. Fewer of the living brachiopods are inarticulates than articulates, and the inarticulates have been less diverse over most of brachiopod history.

The Acrotretidae are one of the earliest known families of brachiopods in the fossil record, first appearing in the early Cambrian. They were most diverse in the later Cambrian and early Ordovician, becoming less so in the later Ordovician. Only a single genus is known to have survived into the Silurian (Holmer & Popov 2000). This may be something of a pseudo-extinction: the 'Acrotretidae' as currently defined is probably ancestral to other families of the order Acrotretida that post-dated it. Nevertheless, the acrotretid lineage as a whole became extinct during the Devonian. At one time it was thought that some living brachiopod families (the craniids and discinids) might be descendants of the acrotretids; they are now believed to not be closely related.

Reconstruction of the anatomy of the acrotretid Linnarssonia constans (with a boring parasite at lower left) from Bassett et al. (2004).

The first feature that springs to attention about the acrotretids is that they were tiny. In general, their shells were only one or two millimetres across. The two valves of the shell were generally quite distinct for each other. The dorsal valve was generally low and convex, whereas the ventral valve was more or less a deep lop-sided cone. A rounded or oval opening was present in the ventral valve, usually just behind the point of the cone. In life, this would have been the opening through which extended the pedicel, the fleshy stalk that would have attached the stalk to its substrate. In brachiopods as small as acrotretids, the lophophore would have been fairly simple. Living forms with such simple lophophores open the shell wide when feeding and hold the lophophore filaments in a bell-shape; water containing food particles is drawn into the centre of the 'bell' and pushed out laterally through the filaments (Rudwick 1965).

An alternate model of the acrotretid anatomy was proposed by Chuang in the early 1970s. He compared acrotretids to the living inarticulate brachiopod Lingula, in which the pedicel does not pass through an opening in the ventral valve but instead is positioned in the centre rear of the animal, passing between the two valves. Chuang suggested that the acrotretid pedicel did likewise, and that the opening in the conical valve (which he interpreted as dorsal rather than ventral) was used to expel water after it was drawn over the lophophore. In support of this model, he conducted an experiment in which he drilled holes in a comparable position in the dorsal valve of living craniid brachiopods (demonstrating once again the concept that one can get away with anything so long as one is experimenting on 'lower lifeforms'), through which the brachiopods did indeed expel water. However, Chuang's model was dismissed by Rowell (1977) who identified a number of features confirmed that the perforate valve of acrotretids was indeed ventral. Lingula, despite being the best-known inarticulate in the modern brachiopod fauna, is a poor model for acrotretids due to its adaptations to an infaunal lifestyle buried in mud, including the modification of the pedicel into a supersized structure for digging and anchoring itself. As for Chuang's experimental observations, Rowell argued that the only thing they demonstrated was that "a system under pressure leaks when perforated", noting that "This relationship... applies equally to bicycle tires and brachiopods".

So how did acrotretids make their living? The impression I've gotten while researching this post is that they are common in deposits that would have been part of the outer continental shelf. In particular, they are often found in black shales, a rock type that was originally formed from anoxic mud. Obviously, few animals are actually able to make a living in an environment lacking oxygen. Some do, such as the "rat-tailed maggot" larvae of hoverflies that possess a long breathing tube with which to obtain air, but it is difficult to imagine acrotretids functioning in this way. The other animals found fossilised in black shales alongside acrotretids are planktonic and nektonic forms, such as graptolites or cephalopods. It is possible that many acrotretids were pseudoplankton, living attached to other organisms or objects floating in the water, such as floating seaweeds (not floating wood, though, because wood didn't exist yet). When the acrotretid died, or its host substrate disintegrated, then it would begin the long descent towards eventual fossilisation in the black muds deep below.

REFERENCES

Bassett, M. G., L. E. Popov & L. E. Holmer. 2004. The oldest-known metazoan parasite? Journal of Paleontology 78 (6): 1214–1216.

Holmer, L., & L. Popov. 2000. Lingulata. In: Kaesler, R. L. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda, Revised vol. 2. Linguliformea, Craniiformea and Rhynchonelliformea (part) pp. 30–146. Geological Society of America: Boulder, and University of Kansas: Lawrence.

Rowell, A. J. 1977. Valve orientation and functional morphology of the foramen of some siphonotretacean and acrotretacean brachiopods. Lethaia 10: 43-50.

Rudwick, M. J. S. 1965. Ecology and paleoecology. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda vol. 1 pp. H199–H214. The Geological Society of America, Inc., and The University of Kansas Press.

The Cancellothyridids: A Modern Success Story

Northern lamp shels Terebratulina septentrionalis, from Oceana.

As has been noted on this site more than once before, brachiopods are a group of animals probably more familiar to the student of palaeontology than of zoology. From the brief gloss that tends to be their only coverage in textbooks, one might be forgiven for thinking them all but inconsequential in the modern fauna. But where conditions suit them (usually sheltered locations where low levels of light and water flow favour their slow metabolisms over the higher energy requirements of bivalves), brachiopods can still be abundant, and even dominant.

One of the most diverse families of brachiopods in the modern fauna is the Cancellothyrididae. Cancellothyridids first make their appearance in the Jurassic, becoming widespread in the Cretaceous (Cooper 1973). Members of this family have shells with a large foramen (the opening at the rear of the shell through which passes the pedicel or stalk by which the brachiopod is attached to its substrate), usually with the deltidial plates surrounding the foramen greatly reduced. The main defining feature of the Cancellothyrididae is the structure of the brachidium, the skeletal structure that provides the support for the base of the lophophore, the tentacle-like feeding structures. In cancellothyridids, the two sides of the brachidium coalesce in the middle to form a tube.

Dorsal valve of the Cretaceous cancellothyridid Cricosia filosa in (A) lateral, (B) ventral and (C) posterior views, from Cooper (1973), showing the tubular brachidium.

The brachidium does not extend into the arms of the lophophore, which are instead strengthened by unattached spicules. The tubular shape of the brachidium distinguishes the Cancellothyrididae from the closely related family Chlidonophoridae, whose members share the large posterior foramen but have the two sides of the brachidium open in back. Cooper (1973) recognised two subfamilies of cancellothyridids, the living Cancellothyridinae and the Cretaceous Cricosiinae; the cricosiines have the tubular section of the brachidium longer and narrower than the cancellothyridines.

Modern cancellothyridids are found in the Indo-Pacific and the North Atlantic, but seem to be absent from the South Atlantic. The majority of living species are included in the widespread genus Terebratulina, with the other living genera all having restricted distributions in the Indo-Pacific. However, a molecular phylogenetic analysis of species of Terebratulina and the Australian genus Cancellothyris by Lüter & Cohen (2002) indicated that both Atlantic Terebratulina and Cancellothyris were nested within Pacific Terebratulina. Paraphyly of the widespread genus would also correlate with its palaeontological distribution: while the other genera are known only from the Recent fauna, Terebratulina has a fossil record dating right back to the origins of the cancellothyridids in the Jurassic (Muir-Wood 1965). Lüter & Cohen (2002) tentatively suggested the possibility of a North Pacific origin for Terebratulina (and, by implication, for Cancellothyrididae as a whole), with dispersal to the North Atlantic occurring through the gap between North and South America before formation of Central America. Their preference for this option rather than the alternative of dispersal through the Tethys (the seaway that once separated Africa from Eurasia) was based on their estimate via molecular clock of a separation of about 100 million years between the Atlantic and Pacific species, supposedly too early for the Tethys option. However, it must be stressed that their sampling of even modern cancellothyridid diversity was not comprehensive. A trans-Tethys dispersal of cancellothyridids may also be indicated by the presence of the fossil genus Rhynchonellopsis in the lower Oligocene of northern Europe (Muir-Wood 1965). Of course, there is no inherent reason why cancellothyridids could not have travelled in both directions!

REFERENCES

Cooper, G. A. 1973. Fossil and recent Cancellothyridacea (Brachiopoda). Tohoku Univ., Sci. Rep., 2nd Ser. (Geol.), Special Volume 6: 371–390.

Lüter, C., & B. L. Cohen. 2002. DNA sequence evidence for speciation, paraphyly and a Mesozoic dispersal of cancellothyridid articulate brachiopods. Marine Biology 141: 65–74.

Muir-Wood, H. M. 1965. Mesozoic and Cenozoic Terebratulidina. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda vol. 2 pp. H762–H816.

The Rhipidothyrididae: Brachiopods of the Devonian

Specimen of Rhenorensselaeria, copyright Miguasha National Park.

In the modern world, the brachiopods are an unfamiliar group to most people. To most, they would probably not be readily distinguished from the much more abundant bivalves that they superficially resemble (a resemblance that is literally only skin deep: brachiopods and bivalves are in no way close relatives, and their internal anatomy is fundamentally different). However, this was not always the case. If one was to travel back to some point in the Palaeozoic era, one would find the situation reversed. At this time, it was the brachiopods that dominated the world's seas, while the bivalves were relegated to a minor supporting role. Their respective fortunes changed around the beginning of the Mesozoic, though whether that was because changing conditions favoured the bivalves, or whether the bivalves simply got a head start in recovering from the Rocks Fall, Everyone Dies clusterf*** that was the end-Permian extinction event, I couldn't tell you.

The fossil shown at the top of this post is one of these Palaeozoic brachiopods, a member of the family Rhipidothyrididae. Rhipidothyridids were among the earliest families of the order Terebratulida, which includes the majority of surviving brachiopods but in the Palaeozoic was just one group among many. Half a dozen genera from the Devonian period have been assigned the Rhipidothyrididae (Lee 2006). They often occur in mass assemblages, with a low diversity of other fossils (Boucot & Wilson 2004). That these assemblages represent their habits in life is indicated by the fact that the individual brachiopods in them are usually articulated; because the shells lacked a toothed hinge, the valves would soon become disassociated if transported after death.

The relationships of the rhipidothyridids are somewhat uncertain. A significant feature used in terebratulid classification is the morphology of the loop, a calcified ring at the base of the shell that provides part of the support for the lophophore in life. In some terebratulids, the loop is long and provides most of the lophophore support; in others, the loop is much shorter and lophophore support is partially taken over by free spicules embedded in the lophophore itself. However, because the loop is a quite delicate structure, its study in fossil taxa requires careful sectioning of specimens, with due consideration of the possibility of post-mortem damage. To date, this has not yet been done for the rhipidothyridids, so their loop morphology remains unknown.

REFERENCES

Boucot, A. J. & R. A. Wilson. 1994. Origin and early radiation of terebratuloid brachiopods: thoughts provoked by Prorensselaeria and Nanothyris. Journal of Paleontology 68 (5): 1002–1025.

Lee, D. E. 2006. Stringocephaloidea. In: Kaesler, R. L. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda (Revised) vol. 5. Rhynchonelliformea (part) pp. 1994–2018.

Antipatharia: The Black Corals

The black coral Antipathes, copyright Jez Tryner.

One piece of trivia I've learnt while looking stuff up for this post: the genus name Antipathes, from which the whole group of the Antipatharia derives its name, was coined to refer to the supposed ability of black coral to cure illnesses and protect against evil. It almost goes without saying that I found no indications that this evaluation was warranted.

The black corals of the Antipatharia are a group of colonial, sessile cnidarians that are found in marine waters around the world. They are predominantly deep-water animals, found mostly below the level of light penetrance. Those individuals that are found in shallower waters still keep to secluded habitats out of the light. Some of the shallowest communities are found in New Zealand at depths of only 4 m in the fiords of the South Island, where a rich concentration of tannins in the top layer of the water prevents light from reaching even that far down (Wagner et al. 2012). Black corals have been harvested in many parts of the world for jewellery (and also for their supposed curative properties referred to above), but they are very slow-growing animals. At least one colony subjected to radiocarbon dating was estimated to be over 4000 years old (Roark et al. 2009).

Wire coral Cirrhipathes, copyright Frédéric Ducarme.

Colonies of antipatharians may be highly branched, or they may form an unbranched whip (the latter forms are sometimes referred to as wire corals or whip corals). They may be only a few centimetres tall, or they may reach a length of several metres in the case of some wire corals (Wagner et al. 2012). The core of the colony is a stalk composed of chitin that varies in colour from jet black in the main stem to golden yellow at branch tips. The stalk is lined with spines that may be simple cones, or may be covered with denticles, or may even be branched and antler-like. In life, the stalk is encased in living tissue, so black corals are not actually black. Unlike other skeletonised cnidarians in which the polyps are recessed within the skeleton, those of antipatharians are entirely external to it. As a result, black corals are rarely found in locations where there is a lot of moving sediment in the water, as they lack the ability to entirely retract the polyps to protect them from abrasion. The individual polyps are usually only a few milimetres wide and up to a few centimetres tall when extended. All antipatharian polyps have six tentacles and six primary mesenteries; depending on the species, there may also be four or six secondary mesenteries, though members of the family Cladopathidae lack secondary mesenteries altogether.

The most recent classification of the Antipatharia divides it between seven families, some of which have been recognised only very recently. Because their deep-water habitat makes the study of live colonies difficult, and many features of the minute polyps become obscured in preserved material, earlier classifications focused heavily on features such as the branching arrangement of the colony, or the morphology of the spines on the skeletal axis. However, these features may be influenced by environmental factors, and their significance may have been overestimated. For instance, a molecular phylogenetic analysis by Brugler et al. (2013) found that the unbranched wire coral genus Cirrhipathes was polyphyletic and not separated from the branched genus Antipathes. Nevertheless, Brugler et al. did find that the higher-level relationships within the Antipatharia were mostly concordant with morphology, including the distinction of the seven families. These relationships included a divergent position for Leiopathes, the only genus with six secondary mesenteries; a clade including the bathyal families Schizopathidae and Cladopathidae, in which the polyps are transversely elongated; a close relationship between the families Myriopathidae and Stylopathidae, with polyps that are not elongated and have relatively short, subequal tentacles; and an association of the families Antipathidae and Aphanipathidae, in which the sagittal tentacles tend to be quite elongate relative to the lateral tentacles. There was still, of course, room for investigation: one notable anomaly is that the type species of Antipathes, A. dichotoma, was identified as a member of 'Aphanipathidae' rather than 'Antipathidae'. If correct, this would mean that aphanipathids should be called antipathids, while antipathids would be... something else.

REFERENCES

Brugler, M. R., D. M. Opresko & S. C. France. 2013. The evolutionary history of the order Antipatharia (Cnidaria: Anthozoa: Hexacorallia) as inferred from mitochondrial and nuclear DNA: implications for black coral taxonomy and systematics. Zoological Journal of the Linnean Society 169: 312-361.

Roark, E. B., T. P. Guilderson, R. B. Dunbar, S. J. Fallon & D. A. Mucciarone. 2009. Extreme longevity in proteinaceous deep-sea corals. Proceedings of the National Academy of Sciences of the USA 106 (13): 5204-5208.

Wagner, D., D. J. Luck & R. J. Toonen. 2012. The biology and ecology of black corals (Cnidaria: Anthozoa: Hexacorallia: Antipatharia). Advances in Marine Biology 63: 67-132.

The Araeolaimida: We Barely Know Ye

Axonolaimus sera, from here.

Overall, the nematodes cannot be considered one of the best-known groups of animals. This is not because they are at all uncommon: there is the oft-cited factoid that nematodes are so abundant in every corner of the world that, if everything other than them was somehow instantaneously removed, the ghostly shadow of the planet Earth would supposedly still be visible as a cloud of microscopic worms. Nematodes are even found in places other animals are not: they have been found further beneath the Earth's surface than any other multicellular organism. There are some nematode species that attract attention, such as those that cause diseases, or are notable crop or animal pests. The nematode Caenorhabditis elegans has been a workhorse of developmental biology for many a year. But these well-studied taxa represent only a small proportion of the full nematode diversity out there.

Being very small and soft-bodied, nematodes do not usually present taxonomists with a great variety of clearly defined morphological features. As a result, dividing nematodes into well-supported groupings has not been an easy task (there are some notable exceptions: try looking up the Desmoscolecida one of these days). Take, as an example, the group known as the Araeolaimida. This name spent many years as a bit of a wastebasket for various non-parasitic nematode families. Eventually, it was restricted by Ley & Blaxter (2002) to just four families: the Axonolaimidae, Comesomatidae, Diplopeltidae and Coninckiidae, with many taxa previously treated as Araeolaimida included in a separate order Plectida. Fonseca & Bezerra (2012) include a fifth family, the Bodonematidae, that was not mentioned by Ley & Blaxter. Even in this restricted sense, the Araeolaimida may not represent a coherent group. There is no single feature shared by all araeolaimidans that is not found in other nematodes, and a molecular phylogenetic study of nematodes by van Megen et al. (2009) did not recover a monophyletic araeolaimidan clade. Nevertheless, Araeolaimida normally have the ovaries outstretched within the bodies of females (in many other nematode taxa, they are folded back on themselves), and the amphids (sensory grooves on the sides of the head) are usually spiral or looped in shape. The majority of araeolaimidans are marine, with freshwater and terrestrial environments being home to two genera of Diplopeltidae, and a few species of Axonolaimidae (Fonseca & Bezerra 2012). We don't know much about their diet, but they are probably grazers on micro-algae or bacteria. About 400 species of Araeolaimida have been described, but it would be very surprising if there weren't more out there.

Head end of the freshwater diplopeltid Cylindrolaimus, photographed by Peter Mullin. Note the dark circle near the end: this is the amphid.

The separate families are a bit easier to define (Fonseca & Bezerra 2012). The single known species of Bodonematidae, Bodonema vossi, stands out by having a pharynx with the mid-part differentiated into a series of muscular bulbs, as opposed to the fairly simple pharynxes of other Araeolaimida. Coninckia, the only genus of Coninckiidae, has the amphids sitting on differentiated plaques that are not present in other taxa. The Comesomatidae have spiral amphids, while the Axonolaimidae and Diplopeltidae have simpler looped amphids. The last two families are distinguished by the shape of the buccal cavity, which is larger and more strongly sclerotised in the Axonolaimidae.

One detail which caught my eye when researching this post is that males of some axonolaimids produce two different forms of spermatozoa (Riemann 1986). The sperm cells produced in the anterior testis of Nicascolaimus punctatus are more than three times the size of those produced in the posterior testis. In another axonolaimid species, Axonolaimus helgolandicus, it is the posterior testis that produces the larger cells. Both types of sperm were shown in N. punctatus to be transferred to females, but the reason for the two different sperm types is unknown. Pomponema, a genus belonging to a separate group of nematodes from the Araeolaimida, produces dimorphic sperm in which the larger cells seem to break down before they are transferred to the female, and it is possible that only one sperm type functions to fertilise the female in axonolaimids as well. Perhaps the other sperm type represent some sort of nuptial gift? Or could they somehow interfere with fertilisation by other males? We await the nematode enthusiast who will find out.

REFERENCES

Fonseca, G., & T. N. Bezerra. 2012. Order Araeolaimida De Coninck, 1965. Zoology Online. Berlin, Boston: De Gruyter. Retrieved 3 Jun. 2014, from http://www.degruyter.com/view/Zoology/HBZ-2011-000076.

Ley, P. de, & M. Blaxter. 2002. Systematic position and phylogeny. In: Lee, D. L. (ed.) The Biology of Nematodes, pp. 1-30. Taylor & Francis: Florence (Kentucky).

Megen, H. van, S. van den Elsen, M. Holterman, G. Karssen, P. Mooyman, T. Bongers, O. Holovachov, J. Bakker & J. Helder. 2009. A phylogenetic tree of nematodes based on about 1200 full-length small subunit ribosomal DNA sequences. Nematology 11 (6): 927-950.

Riemann, F. 1986. Nicascolaimus punctatus gen. et sp.n. (Nematoda, Axonolaimoidea), with notes on sperm dimorphism in free-living marine nematodes. Zoologica Scripta 15 (2): 119-124.

Arthropods in the Precambrian?

The Ediacaran animal Spriggina floundersi, from here.

The Ediacaran biota has been touted as one of the great mysteries of palaeontology. Comprising the latest part of the Precambrian era, the Ediacaran is generally believed to have given us the earliest known animal fossils. However, palaeontologists have disagreed on just how the Ediacaran fossils relate to modern animals (see McCall 2006 for an exhaustively detailed review). Some see the Ediacarans as including the ancestors of groups that remain with us today: jellyfish, corals, comb jellies, sponges. Others see Ediacarans as outside the modern lineages: ancient animal groups that were swept aside by more modern animals at the beginning of the Cambrian. And some have even questioned whether the Ediacarans were even animals at all, suggesting links instead to fungi or Foraminifera, or even that they were an entirely independent lineage unrelated to any modern multicellular organisms.

In 1996, Benjamin Waggoner proposed the name 'Cephalata' for a clade uniting the arthropods with two groups of Ediacaran organisms: the Sprigginidae and the Vendiamorpha. These are among the most undeniably animal-like of the Ediacarans. The sprigginids (including Spriggina shown at the top of the post) have an undivided 'head' followed by a long segmented body. The vendiamorphs are shield-like organisms that also show evidence for segment-like divisions behind the 'head', such as branching internal structures that may represent side-branches of an internal gut.

The vendiamorph Vendia sokolovi, from Ivantsov (2004).

It is difficult to see these taxa as anything other than mobile animals. One supporter of non-animalian affinities for the Ediacarans, Adolf Seilacher, did suggest that Spriggina was a sessile organism, maintaining that the 'head' was in fact a holdfast while the 'body' extended upwards like the frond of a sea pen (I have seen a memorable reconstruction, though unfortunately I can't recall where, showing an individual of mobile Spriggina crawling past a cluster of sessile Spriggina). However, the numerous Spriggina specimens that have been found in Australia and Russia are invariably preserved lying flat, while sessile organisms from the same locations are preserved with the holdfast below the level of the body. Vendiamorphs, on the other hand, are simply not shaped in a way that allows them to be seen as anything other than lying flat. An immobile sprigginid or vendiamorph lying flat below the water would have been vulnerable to being buried by sediment, without any way of digging itself back out.

But if sprigginids and vendiamorphs were definitely animals, what kind of animals were they? It is at this point that things get a bit more vague. Their segmented appearance immediately suggests arthropods (and onychophorans) or annelids, but there is not a great deal to suggest one or the other. The differentiated head of sprigginids suggests the head of an arthropod, while vendiamorphs have been compared to the larvae of arthropods such as trilobites. However, it is unclear whether the Ediacaran taxa possessed anything like the limbs of arthropods and related taxa. The segments of sprigginids may be separated at the edges, and some have argued that folds in vendiamorph fossils are suggestive of limbs underneath a dorsal shield, but there is nothing that one would call unequivocal. Lateral outgrowths of sprigginids may correlate to annelid parapodia instead of arthropod limbs, and folds in the bodies of vendiamorphs may be nothing more than that. We recognise relationships between fossil and extant animals on the basis of whether they have features in common, but our assessment of what features they have may be coloured by what features we expect to see.

Another possible vendiamorph, Parvancorina minchami, from here. Note the fine parallel lines on the body, which some have interpreted as the outlines of limbs.

Some authors have drawn attention to a feature of both vendiamorphs and sprigginids that is visible in the image of Vendia above: their so-called 'glide reflectional symmetry'. Though their bodies appear segmented, the segments do not go straight across the body as one might expect. Instead, the left and right sides of the body are slightly offset from each other. For this reason, some authors have claimed that these animals do not show true bilateral symmetry and hence argued for placing them outside the Bilateria crown group, along its stem. However, others have suggested that the offset between sides may be an artefact of preservation. Even if it was indeed a feature of the living animal, glide reflectional symmetry may not necessarily force the sprigginids outside the Bilateria: a number of living bilaterians also show a certain degree of symmetry offset either as adults or during development, including basal chordates (Waggoner 1996).

During the period of the Cambrian, directly after the Ediacaran, we have access to beautifully preserved fossil deposits that have allowed us to characterise many animals from that period in exquisite detail. No such fossils exist for the Ediacaran; instead, Ediacaran animals are mostly preserved in coarse sediments that preserve only relatively broad features of the fauna. This can turn the Ediacarans into tantalising shadows, and what we see in them can say more about our assumptions than the animals themselves.

REFERENCES

Ivantsov, A. Yu. 2004. New Proarticulata from the Vendian of the Arkhangel’sk region. Paleontologicheskii Zhurnal 2004 (3): 21–26 (transl. Paleontological Journal 38 (3): 247–253.

McCall, G. J. H. 2006. The Vendian (Ediacaran) in the geological record: enigmas in geology's prelude to the Cambrian explosion. Earth-Science Reviews 77: 1-229.

Waggoner, B. M. 1996. Phylogenetic hypotheses of the relationships of arthropods to Precambrian and Cambrian problematic fossil taxa. Systematic Biology 45 (2): 190-222.