A Second Look at Scallops

In a post that appeared at this site over eight years ago, I described some of the distinctive features of the Pectinoidea, the group of bivalves commonly known as scallops. It's time to look in a bit more detail at some of the points mentioned in that post.

Fossil of Pernopecten, the earliest scallop genus, from ammonit.ru.

Pectinoidea, in the sense recognised by Waller (2006), first appear in the fossil record way back in the late Devonian. They were probably derived from earlier members of the Aviculopectinoidea, an extinct group of bivalves that closely resemble scallops in their overall appearance and were included in the Pectinoidea by many earlier authors (such as in the 1969 Treatise on Invertebrate Paleontology volume on bivalves). However, the shell ligament of aviculopectinoids was reinforced by aragonite fibres (a primitive feature for bivalves) rather than having the specialised rubbery core found in pectinoids. As such, aviculopectinoids would have lacked the swimming abilities of true scallops. The Palaeozoic pectinoids belong to a single genus, Pernopecten, that possesses a number of features such as details of the shell crystalline structure that indicate a position outside the pectinoid crown group. In the early Triassic, Pernopecten begat the family Entolioididae that includes the ancestors of living pectinoids.

As mentioned in the previous post, four pectinoid families survive to the present day: the Pectinidae, Propeamussiidae, Entoliidae and Spondylidae. The first three families diverged in the early Triassic. Spondylids (usually classified in a single genus, Spondylus) were not to appear until the mid-Jurassic and Waller (2006) argued for their derivation from within the Pectinidae. The Pectinidae are otherwise distinguished from other pectinoids by a structure called the ctenolium. This is a row of teeth that develops on the shell in the gap between the disc and one of the auricles (the triangular 'wings' at the top of the shell). During the earlier part of the scallop's life, when it lives attached to the ocean bottom by a byssus (what in mussels we call the 'beard'), the ctenolium functions to hold the byssus threads in place and help stop the shell from twisting. In those pectinid species that lack a byssus in the latter part of their life, the ctenolium may end up getting overgrown by the expanding shell and disappearing, but all pectinids (ignoring the aforementioned Spondylus question) have a ctenolium for at least part of their life.

The propeamussiid Cyclopecten secundus, copyright Museum of New Zealand Te Papa Tongarewa.

The Pectinidae is the largest scallop family in the present day, followed by the Propeamussiidae. The Entoliidae were diverse during the Mesozoic but declined dramatically after the end of the Cretaceous (I'm not clear whether or not their decline was a direct part of the end-Cretaceous mass extinction). Indeed, entoliids are completely unknown from the fossil record between the Palaeocene and the late Pleistocene; like the tuatara, it might be that the post-Mesozoic survival of entoliids could have gone completely unrecognised were it not for the single surviving relictual genus.

In the earlier post, I implied that propeamussiids lack the eyes and guard tentacles of other pectinids; it turns out that this was a mistake on my part. Many propeamussiids found in the deep sea do indeed lack these features but they are present in shallow-water propeamussiids. It appears that these features are ancestrally common to all crown-group pectinoids but have been lost as an adaptation to life below the photic zone. The anatomy and lifestyle of many propeamussiids remains poorly known but those species that have been investigated have simplified gills compared to pectinids. The filaments of the gills are free rather than being connected by ciliary junctions. The lips of the mantle are also simplified, lacking the complex lobes found in pectinids. These features may be related to the carnivorous diet of many propeamussiids that feed on zooplankton rather than smaller phytoplankton and organic particles.

REFERENCE

Waller, T. R. 2006. Phylogeny of families in the Pectinoidea (Mollusca: Bivalvia): importance of the fossil record. Zoological Journal of the Linnean Society 148 (3): 313–342.

The Neomiodontids: Brackish-Water Bivalves of the Mesozoic

Cast from internal mould of Myrene tetoriensis, from here.

During the early part of the Cretaceous, an area corresponding to the modern Sea of Japan was occupied by a massive brackish-water lake that has been called Lake Tetori, after the geological Tetori Formation that it left behind it. My impression is that Lake Tetori was not an overly hospitable place: warm, shallow, and probably low in oxygen, it was home to a fairly depauperate fauna dominated by only two species of clam, known as Tetoria yokoyamai and Myrene tetoriensis (Kondo et al. 2006). Tetoria I shall leave for another time; Myrene (and its ilk) is the one I want to look at today.

Myrene tetoriensis belonged to a now-extinct family of bivalves known as the Neomiodontidae that lived during the Jurassic and Cretaceous periods. Neomiodontids were most diverse in the northern continents, though species have also been assigned to this family from India and Australia (Moore 1969). Not dissimilar in appearance to a modern pipi, though generally smaller in size, neomiodontids are primarily known from brackish-water or freshwater deposits. The late Triassic/early Jurassic saw something of a flush of bivalve lineages in low salinity environments: the separation of the ex-Pangaean continents resulted in an increase in continental margins, while high carbon dioxide concentrations in the atmosphere stymied the growth of calcium-heavy marine forms (Kondo & Sano 2009).

A rock full of fossils of Neomiodon, from here.

Most neomiodontids were found in sandy habitats, though Myrene tetoriensis lived in mud. They were shallow burrowers, living buried in the sand with the tip of the shell at surface level. Deep-burrowing bivalves possess elongate tubular siphons through which they breath and feed; the mantle boundary inside the shell of such species has a cavity called the pallial sinus into which the siphons can be retracted. In neomiodontids, the pallial sinus is undeveloped, indicating a proportional lack of development of any siphons. Neomiodontids would have mostly been suspension feeders, capturing food particles floating in the water; because of its muddier habitat, Myrene may have been a deposit feeder (Nishida et al. 2013). The narrow, relatively slim shape of neomiodontid shells suggests that they could probably burrow into their substrate rapidly if they became exposed by water action or potential predators.

When the Neomiodontidae was first established as a distinct family, Casey (1955) suggested that it could include the ancestors of the Sphaeriidae, a living group of small freshwater clams known as the pea clams. If this was the case, then neomiodontids did not truly go extinct during the Cretaceous but live on in their descendants. However, more recent authors do not seem to support this relationship. Kondo et al. (2006) note that the decline of brackish-water shallow burrowers such as neomiodontids correlated with the diversification of deeper-burrowing families and suggest a causal connection between the two, but it is worth noting that Tetoria, mentioned above as peaceful cohabitant of Myrene, was a deep-burrower. Nishida et al. (2013) see the extinction of neomiodontids somewhat differently. Citing the often transient nature of the habitats preferred by neomiodontids and other non-marine bivalves, they suggest that the neomiodontids were not a single lineage but represented numerous independent colonisations of non-marine habitats by members of the related marine family Arcticidae, with the 'neomiodontid' habitus the result of convergent evolution.In this view, the reasons for 'neomiodontid' extinction should be sought not with the neomiodontids themselves, but with the extinction of their arcticid progenitors.

REFERENCES

Casey, R. 1955. The Neomiodontidae, a new family of the Arcticacea (Pelecypoda). Proceedings of the Malacological Society 31: 208–222.

Kondo, Y., T. Kozai, N. Kikuchi & K. Sugawara. 2006. Ecologic and taxonomic diversification in the Mesozoic brackish-water bivalve faunas in Japan, with emphasis on infaunalization of heterodonts. Gondwana Research 10: 316–327.

Kondo, Y., & S. Sano. 2009. Origination of extant heteroconch families: ecological and environmental patterns in post-Paleozoic bivalve diversification. Paleontological Research 13 (1): 39–44.

Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology pt N. Mollusca 6. Bivalvia vol. 2. The Geological Society of America, Inc., and The University of Kansas.

Nishida, N., A. Shirai, K. Koarai, K. Nakada & M. Matsukawa. 2013. Paleoecology and evolution of Jurassic–Cretaceous corbiculoids from Japan. Palaeogeography, Palaeoclimatology, Palaeoecology 369: 239–252.