Fossil of the month: Conostichus
This month’s fossil is a conical to cup-shaped feature that is not an actual part of the body of an organism, but is the only fossil evidence of the soft-bodied sea anemones that made it; meet Conostichus.
Description. Conostichus is a trace fossil, also called an ichnofossil. Trace fossils are any evidence of past life that are not part of the actual organism. Conostichus is a broadly cup- or cone-shaped trace fossil. It is the fossilized dwelling burrow (technically called a Domichnia) of a burrowing sea anemone. The upper surface is the widest end of the burrow. It is commonly depressed into a shallow bowl shape, but may also be flat and infilled, or covered.
The base of the Conostichus burrow is a small, circular bulb- or disc-shaped structure. In some cases, the basal bulb or the cone just above the basal bulb is divided into a series of sections or lobes, arranged radially around the axis of the base, somewhat like a flower. As many as 12 lobes (septation) may be preserved, although they are often very faint, and the basal bulb or disc may be smooth. The sides of Conostichus traces may also be sculpted with longitudinal (vertical, transverse to bedding) markings, lobes, or ridges extending up from the basal disc region, although these may be faint or absent. Some specimens may also exhibit transverse (horizontal) wrinkles or ridges.
In cut or broken sections through Conostichus, the structure often consists of stacked cone shapes. In some cases, a vertical structureless tube extends up from the basal bulb through the center of the stacked cones (Chamberlain, 1971; Pemberton and others, 1988).
In Kentucky, Conostichus generally ranges in height and width (diameter) from 1 to 10 cm, although most specimens are less than 5 cm in height and width. Specimens can be much wider than tall, slightly wider than tall, or nearly equant. Post-depositional compaction, and reworking or superposition of burrows and traces of other organisms can complicate and alter the shape and texture of individual Conostichus specimens.
Conostichus typically is found in interbedded sandstones and shales but also can be found in gray shales or siderite beds within shales. Where one Conostichus is found in a bed, more will generally be found. Commonly, Conostichus is found in layers with other trace fossils, including Arenicolites, Asterosoma, Rosselia, and Teichichnus. Beds containing these trace fossils may lack other hard-part fossils (sea shells, etc.) but will exhibit different degrees and shapes of bioturbation (churning of sediment by organisms).
Other fossils that could be confused with Conostichus. At least three types of plug- or cone-shaped trace fossils are reported from Kentucky. Conostichus is one, and Conichnus (similar spelling) and Bergauria are the others. Bergauria sometimes has a basal disc similar to Conostichus, or may have a smooth, rounded base. Bergauria, however, is generally much shorter than Conostichus and is smooth sided. Conichnus has a narrower cone shape than Conostichus, and lacks a basal disc or bulb (Chamberlain, 1971; Pemberton and others, 1988). Because these are trace fossil genera which encompass a range of shapes, there can be some overlap between the three forms. For example, Conostichus can be wider than tall (like Bergauria) or taller than wide (like Conichnus). Small Conostichus can look like Bergauria. Likewise, narrow Conostichus with a broken or poorly developed basal bulb can look like Conichnus. There are also other tall, narrow, sub-conical to cylindrical traces that are similar to Conichnus, but they are narrower and more cylindrical than Conostichus.
Cone-shaped Conostichus with bowl-shaped depressions in their upper surface might also be confused with horn corals because of their shape. Horn corals do not have layers of sediment crossing through their cone shapes, and have distinct septa (radiating lines) in their cups and sometimes along the outside of their cones (see Grewkingkia for example), so can easily be distinguished from Conostichus.
Species. Several species of Conostichus have been defined, although specific species of these forms are generally not reported in non-paleontological research. Pemberton and others (1988) reviewed many of the known species and validated five species based on (1) width to height ratios, (2) basal disc lobation, and (3) sculpturing (ornamentation). Because these are trace fossils, rather than body fossils, it is important to recognize that species names for this genus only define different shapes of Conostichus burrows, and not different species of prehistoric anemones. The same species of prehistoric burrowing anemone might produce different burrow shapes depending on conditions.
Range and geographic occurrence. Conostichus ranges from the Ordovician to at least the Cretaceous (Pemberton and others, 1988), and possibly extending as far back as the Cambrian (Stachacz, 2016). Similar structures are recorded in modern sediments (Shinn, 1968). In Kentucky, Conostichus is most common in Lower Pennsylvanian estuarine to marine sandstones of the Grundy Formation, Breathitt Group. These outcrop at the surface along the margins of the Eastern Kentucky Coal Field. Specimens have also been recorded in sandstones and shales of other Pennsylvanian units in eastern and western Kentucky. Examples from Pennsylvanian rocks in Kentucky, Illinois, Tennessee, and West Virginia are discussed in Pfefferkorn, 1971; Miller and Knox (1985), Martino (1989), Martino and Sanderson (1993), Greb and Chesnut (1992, 1994), Archer and others (1994), and Greb and Archer (1995).
Life and paleoecology. Historically, Conostichus was interpreted as the body fossil of a sponge or algae. By the 1960s, it was generally considered a trace fossil of a soft-bodied organism (Pfefferkorn, 1971). Different types of organisms have been inferred for the structure, but ultimately they were determined to be the burrows of sea anemones (Chamberlain, 1971; Pemberton and others, 1988; Seilacher, 2007). Burrowing sea anemones are slightly different in appearance, but related to, the anemones seen in fish tanks, or in the movie Finding Nemo. All anemones are essentially soft-bodied polyps with a mouth region (oral disc) surrounded by tentacles (arms). Common sea anemones like the ones seen in aquariums are vase-shaped to tubular but can expand and contract into different shapes. They have many tentacles and a flat, disc-shaped base (pedal disc) capable of attaching to a variety of surfaces. Burrowing anemones have fewer tentacles and tend to be much narrower. Rather than a basal (pedal) disc region, burrowing anemones have a bulb-shaped feature at their base, called a physa.
Scientists have interpreted a sea anemone origin for the Conostichus structure through observation of modern burrowing anemones on the sea floor. Some modern burrowing anemones create burrows which form stacked cone structures. When feeding, only the tentacles of the anemone extend from its burrow. If the burrow is buried, the anemone can expand and contract its body so it migrates up through the sediment, creating more stacked cone structures (Shinn, 1968). The stacked cone shapes are similar to those found inside Conostichus. The wrinkles and longitudinal furrows on the margins of some Conostichus burrows were formed by movement of the anemone digging its burrow or by the contractions and expansions of its body if the anemone (or anemone-like organism) had to move up through newly deposited sediment (Chamberlain, 1971; Pemberton and others, 1988; Seilacher, 2007).
Anatomical comparisons of modern burrowing sea anemones with some of the features in Conostichus also support a sea anemone origin for these structures. The small bulb at the base of Conostichus is the type of feature that could be formed by the physa bulb at the base of a burrowing anemone. Many burrowing anemones have six pairs of structures called septa (and arms), so 12 divisions in their body (e.g., Chamberlain, 1971). Some Conostichus and Bergauria have basal bulbs divided into 6 to 12 sections or lobes (e.g., Alpert, 1973), and some Conostichus have as many as 12 ridges or furrows radially arranged around their cone.
Because burrowing sea anemones (and similar organisms) lack hard parts that can easily be fossilized, trace fossils like Conostichus and similar-appearing Bergauria are important as the only evidence (usually) of their existence in the distant past! Also, because the size of modern burrowing sea anemones is usually similar to the size of the burrows they make, even if the actual prehistoric anemones aren’t preserved, their burrows provide some evidence of their likely sizes in the geologic past.
Where one Consotichus is found in a bed, more will generally be found. This isn’t surprising as burrowing sea anemomes usually live in groups on the sea floor today. Also, Conostichus is typically found in layers with other trace fossils called a Cruziana trace-fossil association. Trace fossil associations help geologists interpret depositional origins for the surrounding rock, and are especially useful in rock beds where body fossils (brachiopod shells, gastropods shells, etc.) are lacking. The Cruziana association is common in nearshore, shallow marine and estuarine environments, which have been interpreted for some Pennsylvanian-age units with Conostichus in Kentucky.
References Cited
· Alpert, S.P., 1973, Bergaueria Prantl (Cambrian and Ordovician), a probable actinian trace fossil: Journal of Paleontology, v. 47, no. 5, p. 919–924.
· Archer, A.W., Feldman, H.R., Kvale, E.P. and Lanier, W.P., 1994, Comparison of drier-to wetter-interval estuarine roof facies in the Eastern and Western Interior coal basins, USA: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 106, no. 1-4, p. 171–185.
· Branson, C.C., 1960, Conostichus: Oklahoma Geology Notes, v. 20, p. 195–207.
· Branson, C.C., 1961, New records on the Scyphomedusan Conostichus: Oklahoma Geology Notes, v. 21, p. 130–138.
· Chamberlain, C.K., 1971, Morphology and ethology of trace fossils from the Ouachita Mountains, southeast Oklahoma: Journal of Paleontology, v. 45, no. 2, p. 212–246.
· Frey, R.W., 1970, The Lebensspuren of some common marine invertebrates near Beaufort, North Carolina. II. Anemone burrows: Journal of Paleontology, v. 44, p. 308–311.
· Greb, S.F. and Archer, A.W., 1995, Rhythmic sedimentation in a mixed tide and wave deposit, Hazel Patch sandstone (Pennsylvanian), eastern Kentucky coal field: Journal of Sedimentary Research, v. 65, no. 1b, p. 96–106.
· Greb, S.F. and Chesnut Jr, D.R., 1992, Transgressive channel filling in the Breathitt Formation (Upper Carboniferous), eastern Kentucky coal field, USA: Sedimentary Geology, v. 75, no. 3-4, p. 209–221.
· Greb, S.F., and Chesnut, D.R., Jr., 1994, Paleoecology of an estuarine sequence in the Breathitt Formation (Pennsylvanian), central Appalachian Basin: Palaios, v. 9, p. 388–402.
· Martino, R.L., 1989, Trace fossils from marginal marine facies of the Kanawha Formation (Middle Pennsylvanian), West Virginia: Journal of Paleontology, v. 63, p. 389–403.
· Martino, R.L. and Sanderson, D.D., 1993, Fourier and autocorrelation analysis of estuarine tidal rhythmites, lower Breathitt Formation (Pennsylvanian), eastern Kentucky, USA: Journal of Sedimentary Research, v. 63, no. 1, p. 105–119.
· Miller, M.F., and Knox, L.W., 1985, Biogenic structures and depositional environments of a Lower Pennsylvanian coal-bearing sequence, northern Cumberland Plateau, Tennessee, U.S.A., in Curran, H.A., ed., Biogenic structures: Their use in interpreting depositional environments: Society of Economic Paleontologists and Mineralogists, Special Publication 35, p. 67–98.
· Pemberton, S.G., Frey, R.W., and Bromley, R.G., 1988, The ichnotaxonomy of Conostichus and other plug-shaped ichnofossils: Canadian Journal of Earth Science, v. 25, p. 866–892.
· Pfefferkorn, H.W., 1971, Note on Conostichus broadheadi Lesquereux (trace fossil: Pennsylvanian): Journal of Paleontology v. 45, no. 5, p. 888–892.
· Seilacher, A., 2007, Trace fossil analyses: Springer, New York, 226 p.
· Shinn, E.A., 1968, Burrowing in recent lime sediments of Florida and the Bahamas: Journal of Paleontology, v. 42, p. 879–894.
· Stachacz, M., 2016, Ichnology of the Cambrian Ociesęki Sandstone Formation (Holy Cross Mountains, Poland): Annales Societatis Geologorum Poloniae, v. 86, no. 3, p. 291–328.
Text and illustrations by Stephen Greb, Kentucky Geological Survey