Wooster’s Fossil of the Week: Upper Ordovician bivalve bioimmured by a bryozoan

November 7th, 2014

DSC_4503This week’s fossil is a simple and common form in the Cincinnatian Series (Upper Ordovician) of the Ohio, Indiana and Kentucky tri-state area. We are looking above at the base of a trepostome bryozoan that encrusted the outside of an aragonite bivalve shell. The bivalve shell (probably a species of Ambonychia) dissolved away, leaving its impression in the base of the calcitic bryozoan. This fossil is from the Upper Whitewater Formation (Richmondian) in eastern Indiana near Richmond itself.
DSC_4516In this closer view you can see the plications (“ribs”) of the bivalve preserved in negative relief on the attachment surface of the bryozoan. Close examination shows the individual zooecia of the bryozoan exquisitely molding the bivalve topography.

This is a kind of substrate bioimmuration, a preservational mode in which a skeletal organism (the bryozoan here) overgrows another organism (with a soft body or hard skeleton), making an impression of it in its base. The overgrown organisms is rots or dissolves away, leaving the exposed mold. You can also think of it as a kind of external mold produced by a living organism (the encruster). Such “vital immuration” was first described by Vialov (1961), and it is thoroughly covered by Paul Taylor in his 1990 paper cited below.

Again, these fossils are common in the Cincinnatian, and this one is far from being the fanciest. It is the Fossil of the Week because of its very ordinary nature, yet it provides extraordinary information. The aragonitic shell the bryozoan encrusted would have been lost forever after it dissolved if this bryozoan hadn’t occupied it and built a calcitic memorial. I’ve collected now hundreds of these substrate bioimmurations, and they have been critical in many studies, from the preservation of soft-bodied sclerobionts (see Wilson et al., 1994) to the revelation of boring interiors (and thus the behavior of the borers) and skeletal sclerobiont paleoecology. I’m also convinced there are many aragonitic mollusk taxa in the Cincinnatian that are known only through this bioimmuration process. These are fascinating fossils my students and I will continue to collect and study.


Taylor, P.D. 1990. Preservation of soft-bodied and other organisms by bioimmuration—a review. Palaeontology 33: 1-17.

Vialov, O.S. 1961. Phenomena of vital immuration in nature. Dopovidi Akademi Nauk Ukrayin’ skoi RSR 11: 1510-1512.

Wilson, M.A., Palmer, T.J. and Taylor, P.D. 1994. Earliest preservation of soft-bodied fossils by epibiont bioimmuration: Upper Ordovician of Kentucky. Lethaia 27: 269-270.

Wooster’s Fossils of the Week: Upper Carboniferous seed casts from northeastern Ohio

October 31st, 2014

Trigonocarpus trilocularis Hildreth 1838We haven’t had a paleobotanical fossil of the week for awhile, so here are a couple of nice seed casts from the Upper Carboniferous Massillon Sandstone exposed near Youngstown, Ohio. They fall within the “form genus” Trigonocarpus Brongniart 1828. A form taxon is one that may not have any systematic or evolutionary validity, but it is a convenient resting place for taxa that share a particular morphological pattern but can’t be easily classified elsewhere. Trigonocarpus consists of seed casts that are “radially symmetrical, decorticated, and have their surface marked by three prominent ridges” (Gastaldo and Matten, 1978, p. 884). These particular seeds appear to be Trigonocarpus trilocularis (Hildreth, 1837). The taxa here are problematic, of course, because these seeds belong to larger plants that have their own names.
Trigonocarpus trilocularis Hildreth 1838_585These seeds appear to be from medullosalean trees, which were small relatives of today’s cycads. They were common in wetlands throughout North America and Europe during the Carboniferous, especially the Late Carboniferous. The seeds we have were likely attached to small stalks. You can see what appears to be a circular attachment scar above.
Samuel Prescott Hildreth (1783–1863)
Dr. Samuel Prescott Hildreth (1783-1863) was a physician and historian with a keen eye for natural history, especially including fossils and rocks. He was born in Massachusetts of strong Patriot stock and moved to the dangerous territory of Ohio in 1806, settling in Marietta in 1808. Dr. Hildreth is often cited as one of the first scientists in the country west of the Alleghany Mountains. His prolific writing is fast-moving, diverse and interesting, so he must have been a great traveling companion. Dr. Hildreth served in the Ohio Legislature and was on the first Ohio Geological Survey.
HildrethNutThe above is a figure from Hildreth (1837, p. 29) showing the fossil seed he named Carpolithus trilocularis. He wrote that “[t]his nut is probably the fruit of some antediluvian palm”, which is not far from what we think now (apart from the Flood reference!).


Gastaldo, R.A. and Matten, L.C. 1978. Trigonocarpus leeanus, a new species from the Middle Pennsylvanian of southern Illinois. American Journal of Botany 65: 882-890.

Hildreth, S.P. 1837. Miscellaneous observations made during a tour in May, 1835, to the Falls of the Cuyahoga, near Lake Erie: extracted from the diary of a naturalist. American Journal of Science and Arts 31:1-84

Zodrow, E.L. 2004. Note on different kinds of attachments in trigonocarpalean (Medullosales) ovules from the Pennsylvanian Sydney Coalfield, Canada. Atlantic Geology 40: 197-206.

Wooster’s Fossils of the Week: A pair of molded nautiloids from the Upper Ordovician of northern Kentucky

October 24th, 2014

1 Nautiloid pair 091314Two nautiloids are preserved in the above image of a slab from the Upper Ordovician of northern Kentucky. (I wish I knew which specific locality. This is why paleontologists are such fanatics about labeling specimens.) The top internal mold (meaning it is sediment that infilled a shell now dissolved away) has been covered in a previous blog entry. This week I want to concentrate on the nautiloid at the bottom.

These nautiloids belong to the Family Orthoceratidae McCoy, 1844, which existed from the Early Ordovician (490 million years ago) through the Triassic (230 million years ago). They had conical, aragonitic shells with walls inside separating chambers (camerae) and a central tube (the siphuncle) connecting them. They were swimming (nektic) predators that could control their buoyancy through a mix of gases and liquids in the camerae mediated by the siphuncle.

What is most interesting here is the preservation of these nautiloids. The aragonitic shells were dissolved away at about the same time the internal sediment was cemented, forming the internal molds. These molds were exposed on the seafloor, attracting encrusting organisms. This means the dissolution and cementation took place quickly and in the marine environment, not after burial. This rapid dissolving of aragonite and cementation by calcite is typical of Calcite Sea geochemistry, something we don’t see in today’s Aragonite Seas.
2 Nautiloid siphuncle 091314Above is a close view of the cemented siphuncle of the lower nautiloid, heavily encrusted by a trepostome bryozoan.
3 Bryozoan undersideEven more cool, the outside of the lower nautiloid was encrusted by several trepostome bryozoan colonies. When the shell dissolved it left the undersides of these bryozoans exposed, as seen above. These undersides often contain the remains of shelly organisms the bryozoans encrusted (see the Independent Study project by Kit Price ’13) and even soft-bodied animals (epibiont bioimmuration; see Wilson et al., 1994).

A neat package here resulting from biological, sedimentological and geochemical factors.


Palmer, T.J., Hudson, J.D. and Wilson, M.A. 1988. Palaeoecological evidence for early aragonite dissolution in ancient calcite seas. Nature 335 (6193): 809–810.

Sweet, W.C. 1964. Nautiloidea — Orthocerida, in Treatise on Invertebrate Paleontology. Part K. Mollusca 3, Geological Society of America, and University of Kansas Press, New York, New York and Lawrence, Kansas.

Wilson, M.A., Palmer, T.J. and Taylor, P.D. 1994. Earliest preservation of soft-bodied fossils by epibiont bioimmuration: Upper Ordovician of Kentucky. Lethaia 27: 269-270.

Wooster’s Fossils of the Week: Bivalve borings, bioclaustrations and symbiosis in corals from the Upper Cretaceous (Cenomanian) of southern Israel

October 17th, 2014

Fig. 2 Aspidiscus1bw_scale 585The stark black-and-white of these images are a clue that the fossil this week has been described in a paper. Above is the scleractinian coral Aspidiscus cristatus (Lamarck, 1801) from the En Yorqe’am Formation (Cenomanian, Upper Cretaceous) of southern Israel. The holes are developed by and around tiny bivalves and given the trace fossil name Gastrochaenolites ampullatus Kelly and Bromley, 1984. This specimen was collected during my April trip to Israel, a day recorded in this blog. I crowd-sourced the identification of these corals, and they were highlighted as earlier Fossils of the Week. Now I’d like to describe them again with new information, and celebrate the publication of a paper about them.

En Yorqe'am040914aThis is the exposure of the En Yorqe’am Formation where Yoav Avni and I collected the coral specimens approximately 20 meters from its base in Nahal Neqarot, southern Israel (30.65788°, E 35.08764°). It is an amazingly fossiliferous unit here with brachiopods, stromatoporoid sponges, zillions of oysters, gastropods, ammonites and the corals.

The abstract of the Wilson et al. (2014) paper tells the story: “Specimens of the small compound coral Aspidiscus cristatus (Lamarck, 1801) containing evidence of symbiosis with bivalves have been found in the En Yorqe’am Formation (Upper Cretaceous, early Cenomanian) of southern Israel. The corals have paired holes on their upper surfaces leading to a common chamber below, forming the trace fossil Gastrochaenolites ampullatus Kelly and Bromley, 1984. Apparently gastrochaenid bivalve larvae settled on living coral surfaces and began to bore into the underlying aragonitic skeletons. The corals added new skeleton around the paired siphonal tubes of the invading bivalves, eventually producing crypts that were borings at their bases and bioclaustrations at their openings. When a boring bivalve died its crypt was closed by the growing coral, entombing the bivalve shell in place. This is early evidence of a symbiotic relationship between scleractinian corals and boring bivalves (parasitism in this case), and the earliest record of bivalve infestation of a member of the Suborder Microsolenina. It is also the earliest occurrence of G. ampullatus.”

Fig. 3 BoringPair2bw_scale 585 Paired apertures of Gastrochaenolites ampullatus in the coral Aspidiscus cristatus.

Fig. 4 EmbeddedBivalve1bw_scale_rev 585Polished cross-section through a specimen of Gastrochaenolites ampullatus in an Aspidiscus cristatus coral. In the lower left of the chamber are layered carbonates (A) representing boring linings produced by the bivalve. An articulated bivalve shell (B) is preserved in the chamber. The chamber has been roofed over by coral growth (C).

Thank you very much to Tim Palmer and Olev Vinn for their critical roles in this paper, and, of course, thanks to Yoav Avni, the best field geologist I know.


Avnimelech, M. 1947. A new species of Aspidiscus from the Middle Cretaceous of Sinai and remarks on this genus in general. Eclogae geologicae Helvetiae 40: 294-298.

Gill, G.A. and Lafuste, J.G. 1987. Structure, repartition et signification paleogeographique d’Aspidiscus, hexacoralliaire cenomanien de la Tethys. Bulletin de la Societe Geologique de France 3: 921-934.

Kleemann, K., 1994. Associations of corals and boring bivalves since the Late Cretaceous. Facies 31, 131-140.

Morton, B. 1990. Corals and their bivalve borers: the evolution of a symbiosis. In: Morton, B. (Ed.), The Bivalvia: Proceedings of a Memorial Symposium in Honour of Sir Charles Maurice Yonge (1899-1986) at the 9th International Malacological Congress, 1986, Edinburgh, Scotland, UK. Hong Kong University Press, Hong Kong, pp. 11-46

Pandey, D.K., Fürsich, F.T., Gameil, M. and Ayoub-Hannaa, W.S. 2011. Aspidiscus cristatus (Lamarck) from the Cenomanian sediments of Wadi Quseib, east Sinai, Egypt. Journal of the Paleontological Society of India 56: 29-37.

Wilson, M.A., Vinn, O. and Palmer, T.J. 2014. Bivalve borings, bioclaustrations and symbiosis in corals from the Upper Cretaceous (Cenomanian) of southern Israel. Palaeogeography, Palaeoclimatology, Palaeoecology 414: 243-245.


Wooster’s Fossil of the Week: An early bryozoan on a Middle Ordovician hardground from Utah

October 10th, 2014

ORBIPORA UTAHENSIS (Hinds, 1970) 072014Last week I presented eocrinoid holdfasts on carbonate hardgrounds from the Kanosh Formation (Middle Ordovician) in west-central Utah. This week we have a thick and strangely featureless bryozoan from the same hardgrounds. It is very common on these surfaces, forming gray, perforate masses that look stuck on like silly putty. Above you see one on the left end of this hardground fragment. (The circular object to the right is another eocrinoid holdfast.)
Kanosh bryo eo 072014Here is a closer view of the bryozoan, again with one of those ubiquitous eocrinoids encrusting it. The holes are the zooecial apertures. Each zooecium is the skeletal component of a living bryozoan individual (zooid). Note that the walls are thick and granular between the zooecia. All the zooecia look pretty much the same, and there are no other structures like spines, pillars or maculae. This is about as simple as a bryozoan gets.

It is impossible to be certain without a thin-section or acetate peel showing the interior, but I’m pretty sure this Kanosh bryozoan is Orbipora utahensis (Hinds, 1970). It matches fairly well the description in Hinds (1970), who named it Dianulites utahensis, and it fits within the redescription by Ernst et al. (2007).

Several years ago we would have called this a trepostome bryozoan and left it at that. These are, after all, the “stony bryozoans” with thick calcite skeletons and long zooecia. However, the group to which Orbipora belongs is unusual because they have no polymorphs (small zooecia different from the primary zooecia) and have granular skeletal textures rather than laminated. We think the granular walls may be because the original skeletons were made of high-magnesium calcite that later altered to low-magnesium calcite and dolomite, losing details of the microstructure. Orbipora is thus in an as yet undescribed new order of bryozoans. [Update: See comment below from Paul Taylor.]

The Kanosh hardgrounds and their attaching faunas are important in geological and biological history because they are telling us something about the geochemical conditions of the seawater when they formed. We think this was a peak time of Calcite Seas, when low-magnesium calcite was a primary marine precipitate and carbon dioxide levels were high in the atmosphere and seawater. Hardgrounds would have formed rapidly because of early cementation, and aragonite and high-magnesium skeletons would have altered soon after death. The abundant Kanosh communities and substrates are critical evidence for these conditions that were superimposed on the Great Ordovician Biodiversification Event (GOBE). We thus have a delightful combination of seawater geochemistry (and, ultimately, the tectonics that controls it) and evolution intertwined in the history of these rocks and fossils.


Ernst, A., Taylor, P.D. and Wilson, M.A. 2007. Ordovician bryozoans from the Kanosh Formation (Whiterockian) of Utah, USA. Journal of Paleontology 81: 998-1008.

Hinds, R.W. 1970. Ordovician Bryozoa from the Pogonip Group of Millard County, western Utah. Brigham Young University Research Studies, Geology Series 17: 19–40.

Marenco, P.J., Marenco, K.N., Lubitz, R.L. and Niu, D. 2013. Contrasting long-term global and short-term local redox proxies during the Great Ordovician Biodiversification Event: A case study from Fossil Mountain, Utah, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 377: 45-51.

Wilson, M.A., Palmer, T.J., Guensburg, T.E., Finton, C.D. and Kaufman, L.E. 1992. The development of an Early Ordovician hardground community in response to rapid sea-floor calcite precipitation. Lethaia 25: 19-34.

Wooster’s Fossils of the Week: Eocrinoid holdfasts on a Middle Ordovician hardground from Utah

October 3rd, 2014

Kanosh Hardground 072014 smBack in the late 1980s and early 1990s, several students and I did fieldwork in the Middle Ordovician Kanosh Formation in west-central Utah. One year we were joined by my friend Tim Palmer of the University of Aberystwyth. Together, Chris Finton (’91), Lewis Kaufman (’91), Tim and I put together a paper describing the carbonate hardground communities in this remarkable formation (Wilson et al., 1992). At top is an image of one of the surface of one of these hardgrounds. It is covered with holdfasts of rhipidocystid eocrinoids, a kind of primitive echinoderm.
Fossil Mountain UtahMost of the hardgrounds we studied in the Kanosh Formation were found here at Fossil Mountain near Ibex, Utah. (If you want to consider Ibex a place, at least.) It was a beautiful place to work, and it is still highly productive for geologists and paleontologists (see Marenco et al., 2013, for the latest investigation).

Kanosh eocrinoid 2The encrusters on the Kanosh hardgrounds are dominated by two groups: bryozoans (which we’ll highlight next week) and stemmed echinoderms (this week’s subject). The echinoderms are represented by thousands of these small attachment structures called holdfasts. The stem of the echinoderm was attached here to the hardground. The entire skeleton of the echinoderm, including the hardground, is made of low-magnesium calcite, so they are very well preserved. Surprisingly, the hardground communities in the Kanosh have very few sponges or borings.

Kanosh eocrinoid 3 072014The holdfasts come in a few varieties with subtle morphological differences. Here we have one with a tri-radiate center.

Kanosh eocrinoids 1Sometimes the holdfasts blended together on the hardground surface, which was probably the result of competition for attachment space. Note the tri-radiate centers.

Mandalacystis diagramFrom a few plates we found, it appears that the rhipidocystid eocrinoid holdfasts are from a creature like Mandalacystis, which is pictured above from Figure 1 of Lewis et al. (1987). We can’t tell for certain without more of the skeleton, but the holdfasts are very similar to what has been described for the genus.

These Middle Ordovician hardgrounds were formed at an interesting time in the chemistry of the oceans and the development of marine invertebrate faunas. More on that next week!


Ernst, A., Taylor, P.D. and Wilson, M.A. 2007. Ordovician bryozoans from the Kanosh Formation (Whiterockian) of Utah, USA. Journal of Paleontology 81: 998-1008.

Lewis, R.D., Sprinkle, J., Bailey, J.B., Moffit, J. and Parsley, R.L. 1987. Mandalacystis, a new rhipidocystid eocrinoid from the Whiterockian Stage (Ordovician) in Oklahoma and Nevada. Journal of Paleontology 61: 1222-1235.

Marenco, P.J., Marenco, K.N., Lubitz, R.L. and Niu, D. 2013. Contrasting long-term global and short-term local redox proxies during the Great Ordovician Biodiversification Event: A case study from Fossil Mountain, Utah, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 377: 45-51.

Wilson, M.A., Palmer, T.J., Guensburg, T.E., Finton, C.D. and Kaufman, L.E. 1992. The development of an Early Ordovician hardground community in response to rapid sea-floor calcite precipitation. Lethaia 25: 19-34.

Wooster’s Fossil of the Week: A crinoid calyx from the Upper Ordovician of southern Ohio

September 26th, 2014

Xenocrinus baeri (Meek, 1872)_585This week’s contribution from the Wooster collections will be short. If all is going well, as this is posted I’m on my way to the Fourth International Palaeontological Congress in Mendoza, Argentina. I hope to have a few posts from that exotic place!

The fossil above is the crown of a monobathrid crinoid called Xenocrinus baeri (Meek, 1872). It was found by Bianca Hand (Wooster ’14) in the Bull Fork Formation (Upper Ordovician, Richmondian) on an Invertebrate Paleontology field trip to the emergency spillway at Caesar Creek State Park in southern Ohio (seen below). Thank you to my friend Bill Ausich of The Ohio State University for identifying this fossil. It is an unprepared specimen of a common species, and it is not nearly so flashy as in other collections. Still, it is one of the best finds from our class field trips, and it is cool. The calyx is on the right and mostly buried in matrix. Four filter-feeding arms extend to the left. Where the matrix is broken away on the far right you can see tiny ossicles from the pinnules on the arms. Someone using a needle very carefully under a microscope could expose more details of this crinoid, but I like leaving something to the imagination!

Schumacher, G.A. and Ausich, W.I. 198). New Upper Ordovician echinoderm site: Bull Fork Formation, Caesar Creek Reservoir (Warren County, Ohio). The Ohio Journal of Science 83: 60-64.

Wooster’s Fossils of the Week: A nest of cornulitid tubeworms and friends from the Upper Ordovician of northern Kentucky

September 19th, 2014

Cornulitids and bryozoan Bellevue 585This fascinating and complicated little cluster of cornulitid wormtubes was found by my current Independent Study student William Harrison while we were doing fieldwork near Petersburg, Kentucky. (Just down the road from the infamous Creation Museum, ironically.) It was collected from a roadcut in the Bellevue Member of the Grant Lake Formation (Upper Ordovician, locality C/W-152). We’ve seen all the elements before (cornulitids, bryozoans and stromatoporoids), but not in such a tight set of relationships. I find this aspect of paleontology to be one of the most delightful: who lived with whom and how?
reconstr1The tubes are of the common Paleozoic genus Cornulites Schlotheim 1820, and the species is Cornulites flexuosus (Hall 1847). These long-extinct little marine animals had calcitic shells and likely bore a filter-feeding lophophore, as shown in the reconstruction above by my friend Olev Vinn. They appear to be related to brachiopods, bryozoans, phoronids, and some other tubeworms that shared this feeding device and certain features of the shell. Their life goal was to keep their lophophore or equivalent apparatus free of obstructions so they could collect nutrients from the surrounding seawater.
cornulitid whole specimen 091214The bryozoan, which makes up the primary substrate of the specimen (seen above) is a trepostome. Its skeleton contains hundreds of tiny tubes (zooecia) that held individuals (zooids) in the colony (zoarium — these terms are for my paleo students this week!). Each zooid in this type of bryozoan had a lophophore for filter-feeding.
cornulitid, dermatostroma, bryozoanAbove we see a thin, light-colored, bumpy sheet in the center of the image covering three of the cornulitid tubes and some of the bryozoan. This is the stromatoproid Dermatostroma papillatum (James, 1878). Stromatoporoids were a kind of sponge with a skeletal base, so this organism was also a filter-feeder. (It was originally known as Stromatopora papillata James, 1878.) Here we see the interesting symbioses (living together) aspects of this tiny assemblage. In the top right you see a cornulitid tube growing over the bryozoan, but the bryozoan in turn is overgrowing its proximal parts. The bryozoan and the cornulitid were thus alive at the same time. The stromatoporoid is growing over the bryozoan and the three cornulitids, but it is overgrown by cornulitids on the left. In addition, the stromatoporoid did not obstruct the cornulitid apertures, an indication that they were occupying living tubeworms. My hypothesis, then, is that all three of these characters were alive at the same time growing in response to each other.

It could be that this represents a tiny hard substrate tiered assemblage, meaning that the organisms were selecting food resources at slightly different heights and particle sizes (see Ausich and Bottjer, 1982, for a start on the tiering literature). The cornulitids may have taken the largest bits, the bryozoans the next size, and then the stromatoporoids, as minuscule sponges, got the finest particles. This is another paleontological hypothesis that can be tested with further specimens.

It is also an example of the value of getting sharp-eyed students on the outcrops as often as possible. Good work, William!


Ausich, W.I. and Bottjer, D.J. 1982. Tiering in suspension feeding communities on soft substrata throughout the Phanerozoic. Science 216: 173-174.

Galloway, J.J. and St. Jean, J., Jr. 1961. Ordovician Stromatoporoidea of North America. Bulletins of American Paleontology 43: 1-102.

Morris, W. R. and H. B. Rollins. 1971. The distribution and paleoecological interpretation of Cornulites in the Waynesville Formation (Upper Ordovician) of southern Ohio. The Ohio Journal of Science 71: 159-170.

Parks, W.A. 1910. Ordovician stromatoporoids of America. University of Toronto Studies, Geology Series 7, 52 pp.

Schlotheim, E.F. von. 1820. Die Petrefakten-Kunde auf ihrem jetzigen Standpunkte durch die Beshreibung seiner Sammlung versteinerter und fossiler Ueberreste des their-und Planzenreichs der Vorwelt erlaeutert. Gotha, 437 p.

Taylor, P.D., Vinn, O. and Wilson, M.A. 2010. Evolution of biomineralization in ‘lophophorates’. Special Papers in Palaeontology 84: 317-333.

Vinn, O. and Mutvei, H. 2005. Observations on the morphology and affinities of cornulitids from the Ordovician of Anticosti Island and the Silurian of Gotland. Journal of Paleontology 79: 726-737.

Wooster’s Fossils of the Week: The mysterious Paleozoic encrusters Ascodictyon and Allonema

September 12th, 2014


1 Slide01The above pair of fossils are small sclerobionts commonly found on hard substrates in shallow marine sediments through much of the Paleozoic, especially the Silurian and Devonian. Paul Taylor and I have been studying them for a few years now and our first paper on them was published this summer (Wilson and Taylor, 2014). Ascodictyon (Silurian-Carboniferous) is on the left and Allonema (Silurian-Permian) is on the right. Both are calcitic encrusters and look, at least in this view, very different from each other. We present evidence in our paper, though, that strongly suggests Ascodictyon and Allonema are actually manifestations of the same organism. What that organism is, exactly, still eludes us. We are persuaded at the very least that they are not bryozoans as originally described by Nicholson, Ulrich and Bassler. Since they are so common their identity is important for studies of fossil diversity and paleoecology.
2 Slide07The above view through a light microscope of Ascodictyon and Allonema shows the perspective paleontologists have had of these encrusters until recently. The clear calcite skeletons sitting on a calcitic brachiopod shell (this is from the Devonian of Michigan) makes for little contrast and poor resolution, and the microscope-camera combination has a very limited depth of field. The rest of the images in this post were made with a Scanning Electron Microscope (SEM) expertly operated by Paul. The difference in morphological detail is not just astonishing, it is a revolution in the study of tiny fossils like this.
3 Slide16 siluriense UKThis is a typical view of Ascodictyon. It consists of stellate clusters of inflated vesicles (like little calcite balloons) connected by thin calcitic tubes called stolons. (Ascodictyon siluriense from the Silurian of the England.)

4 Slide24 waldronense S GotlandThis is a typical Allonema. The primary form is a series of porous vesicles attached in chains like sausages. (Allonema waldronense from the Silurian of Gotland, Sweden.)

5 Slide29 Silica MIHere is where these obscure little encrusters get interesting. This is a specimen from the Silica Shale (Middle Devonian) exposed in Michigan. It was collected in a beautiful suite of fossils by that intrepid citizen scientist, Brian Bade. It consists of Allonema sausages connected to Ascodictyon stolons which are themselves connected to Ascodictyon stellate vesicle clusters. Clear evidence that Allonema and Ascodictyon are end members of a morphological continuum produced by the same organism.

7 Slide33 Silica MIA critical feature we see in this Ascodictyon/Allonema complex is the occurrence of “sockets” at the bases of vesicles like the above from the Silica Shale. These are almost certainly places where some erect portion of the organism extended above the substrate. Maybe these were feeding devices? Reproductive parts? We’ve found no trace of them.

8 Slide39 S GotlandOur hypothesis is that Allonema (left) and Ascodictyon (right, both from the Silurian of Gotland, Sweden) are the basal parts of some as yet unknown erect organism. They may have stored nutrients for the creature. We are convinced they were not bryozoans, foraminiferans, corals or sponges. Unfortunately we can only classify them as incertae sedis or Microproblematica. At some point we’ll have to figure out how to name this complex with two genera and over a dozen species.

It was fun work, and the project continues. For more detail, see Wilson and Taylor (2014).


Nicholson H.A. and Etheridge R. 1877. On Ascodictyon, a new provisional and anomalous genus of Palæozoic fossils. J. Nat. Hist., Series 4, 19: 463-468.

Ulrich E.O. and Bassler R.S. 1904. A revision of the Paleozoic Bryozoa. Smith. Misc. Coll. (Quart.) 45: 256-294.

Wilson M.A. and Taylor P.D. 2001. “Pseudobryozoans” and the problem of encruster diversity in the Paleozoic. PaleoBios 21 (Supplement to No. 2): 134-135.

Wilson, M.A. and Taylor, P.D. 2014. The morphology and affinities of Allonema and Ascodictyon, two abundant Palaeozoic encrusters commonly misattributed to the ctenostome bryozoans. In: Rosso, A., Wyse Jackson, P.N. and Porter, J. (eds.), Bryozoan Studies 2013. Studi trentini di scienze naturali 94: 259-266.

Wooster’s Fossils of the Week: A hardground with rugose corals from the Upper Ordovician of southern Ohio

September 5th, 2014

Hdgd small 090114The above slab is a carbonate hardground from the Liberty Formation (Upper Ordovician) of southern Ohio. Carbonate hardgrounds are cemented seafloors, so we’re actually looking at the hard rocky bottom of an Ordovician sea. I’ve long found the idea of a hardground fascinating — it is like a bit of ancient time frozen before us. This hardground is especially interesting because of the fossils associated with it. The knobby nature of the surface is probably due to a burrow system that was preferentially cemented and then exhumed by currents that washed away the loose sediment. The intersecting tunnels, now ridges, provided numerous crannies for encrusting, boring and nestling organisms to inhabit. The high points hosted encrusting bryozoans that needed currents for their filter-feeding.

brach coral 090114There are several shelly fossils found in the low points of this hardground surface. The brachiopod in the upper left is the orthid Plaesiomys subquadrata (Hall, 1847), and the conical rugose coral in the lower right is Grewingkia canadensis (Billings, 1862)

two corals 090114Here is another detailed view of the hardground showing a second rugose coral on the left. I suspect that the corals and maybe even the brachiopod are actually in place (or “in situ” to use the fancy words). I’ve seen such occurrences before and passed them off as just examples of loose fossils rolling into holes. Here, though, we can see that both corals have the calyx (the cup in which the coral polyp was located) facing upwards. These G. canadensis corals did not attach to hard substrates like some of their cousins, but lay recumbent and curved upwards on the seafloor. What better place to do so than in the cozy hollows of a hardground?

This slab is certainly a nice vignette of a marine community nearly 450 million years old.


Billings, E. 1862. New species of fossils from different parts of the Lower, Middle, and Upper Silurian rocks of Canada. Paleozoic Fossils, Volume 1, Canadian Geological Survey, p. 96-168.

Hall, J. 1847. Paleontology of New York, v. 1: Albany, State of New York, 338 p.

Palmer, T.J. 1982. Cambrian to Cretaceous changes in hardground communities. Lethaia 15: 309–323.

Wilson, M.A. and Palmer, T.J. 1992. Hardgrounds and hardground faunas. University of Wales, Aberystwyth, Institute of Earth Studies Publications 9: 1–131.

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