Wooster’s Fossil of the Week: A receptaculitid (Middle Ordovician of Missouri)

September 18th, 2011

This week’s fossil is a long-standing paleontological mystery. Above is a receptaculitid from the Kimmswick Limestone (Middle Ordovician) near Ozora, Missouri. I think I found it on a field trip with Frank Koucky in the distant mists of my student days at Wooster, but so many outcrops, so many fossils …

Below is a nineteenth century illustration of a typical receptaculitid fossil. They are sometimes called “sunflower corals” because they look a bit like the swirl of seeds in the center of a sunflower. They were certainly not corals, though, or probably any other kind of animal. Receptaculitids appeared in the Ordovician and went extinct in the Permian, so they were confined to the Paleozoic Era. Receptaculitids were bag-like in form with the outside made of mineralized pillars (meroms) with square or diamond-shaped heads. The fossils are usually flattened disks because they were compressed by burial. You may notice now that the fossil at the top of this post is a mold of the original with the dissolved pillars represented by open holes. (Paleontologists can argue if this is an external or internal mold.)So what were the receptaculitids? When I was a student we called them a kind of sponge, something like a successor of the Cambrian archaeocyathids. In the 1980s a convincing case was made that they were instead a kind of alga of the Dasycladales. Now the most popular answer is that they belong to that fascinating group “Problematica”, meaning we have no idea what they were! (Nitecki et al., 1999). It’s those odd meroms that are the problem — they appear in no other known group, fossil or recent.

I find it deeply comforting that we still have plenty of fossils in the Problematica. We will always have mysteries to puzzle over.
Another Wooster receptaculitid specimen, this time seen from the underside showing side-views of the meroms.
Diagram of a receptaculitid in roughly life position showing its inflated nature and pillar-like meroms. From Dawson (1880, fig. 25): a, Aperture (probably imaginary here). b, Inner wall. c, Outer wall. n, Nucleus, or primary chamber. v, Internal cavity.

Finally, this is what a typical receptaculitid looks like in the field (Ordovician of Estonia). Note that nice sunflower spiral of the merom ends.

References:

Dawson, J.W. 1880. The chain of life in geological time: A sketch of the origin and succession of animals and plants. The Religious Tract Society, 272 pages.

Nitecki, M.H., Mutvei, H. and Nitecki, D.V. 1999. Receptaculitids: A Phylogenetic Debate on a Problematic Fossil Taxon. Kluwer Academic/Plenum, 241 pages.

Fossils in the Wild: Invertebrate Paleontology Field Trip

September 11th, 2011

CAESAR CREEK LAKE, OHIO–The 2011 Invertebrate Paleontology class had a productive field trip on a beautiful Ohio day. Thunderstorms roamed the state, but we saw them only when we were comfortably on the bus.

We worked in the emergency spillway at Caesar Creek Lake in southwestern Ohio, roughly halfway between Cincinnati and Dayton. This site is maintained by the US Army Corps of Engineers as a fossil-collecting preserve. You obtain a free permit at the visitor center, agree to follow the rules, and extraordinary fossils await your picking. (Last time I was here it was very cold.)

The fossils are in the Arnheim, Waynesville, Liberty and Whitewater Formations of the Richmondian Stage in the Cincinnatian Series of the Ordovician System. These are shaly units with shell-rich limestones formed during storms. Brachiopods, bryozoans, crinoids, trilobites, clams, snails, nautiloids, corals — the whole Ordovician menagerie. Perfect for student collections and our later exercises.

Brachiopod-rich storm layer in the Liberty Formation. Note the circular bryozoan attachment.

Bryozoan colony and brachiopod shell interior from the Waynesville Formation.

Our fancy bus. The design insures that the back seats are rather bouncy.

Last of the summer flower field photos! It was such a beautiful day.

Wooster’s Fossil of the Week: A Biserial Graptolite (Middle Ordovician of Tennessee)

August 28th, 2011

This week’s fossils are graptolites (from the Greek for written rocks) I found many years ago in the Lebanon Limestone near the town of Caney Springs south of Nashville, Tennessee. They are of the genus Amplexograptus and probably belong to the species A. perexcavatus (Lapworth, 1876).

Graptolites were colonial organisms consisting of hundreds and sometimes thousands of tiny zooids (individuals) connected together in a flexible proteinaceous skeleton (the rhabdosome). They first appeared in the Late Cambrian (around 510 million years ago) and disappeared forever in the Early Carboniferous (around 350 million years ago). Amplexograptus colonies were probably attached to floats so they could drift through the ancient oceans filtering out organic particles; they would be officially “passively mobile planktonic suspension feeders”. They belong to the Phylum Hemichordata, although there have always been disputes about their actual evolutionary relationships. This matters because graptolites are important index fossils for sorting out the age relationships of Lower and Middle Paleozoic rocks.

Graptolites are usually preserved as thin carbonaceous films on dark shales, making them rather hard to see (as my paleontology students will readily agree). The great 18th Century naturalist Linnaeus even said that they were “pictures resembling fossils rather than true fossils”. Sometimes, though, they are found in lighter-colored rocks like limestones, as above. Goldman et al. (2002) found Amplexograptus in limestones preserved in three dimensions, possibly because the limestones were cemented early around them before they collapsed with decay. They even studied this same species from the Lebanon Limestone. The 3-D preservation allows for a much more detailed analysis of the tiny cups (thecae) which held the individual zooids. It is possible that I could dissolve the limestone shown above and retrieve some delicate three-dimensional graptolites — but I could also just as easily destroy them.

Amplexograptus perexcavatus was originally described in 1876 by the famous geologist Charles Lapworth (1842-1920), who referred it to the genus Diplograptus. Actually, he had two species in his D. perexcavatus group, so it took some taxonomic detective and legal work to fix the current naming system. Lapworth, who I’ve figured below with an inset of his not-very-helpful diagram of the original D. perexcavatus, is well known by paleontologists for his work with graptolites as index fossils. Scientists and historians of science know him as the man who invented the Ordovician Period in 1879 to solve a bitter dispute between Roderick Murchison and Adam Sedgwick who each claimed the same rock interval in Wales for the Silurian and Cambrian periods respectively. Lapworth’s primary biostratigraphic argument for the Ordovician as a separate period was the distribution of graptolites, including our friend Amplexograptus perexcavatus. (Murchison and Sedgwick were long gone by the time their dispute was settled.)

(Charles Lapworth. Image courtesy of The Lapworth Museum of Geology.)

References:

Goldman, D., Campbell, S.M. and Rahl, J.M. 2002. Three-dimensionally preserved specimens of Amplexograptus (Ordovician, Graptolithina) from the North American mid-continent: taxonomic and biostratigraphic significance. Journal of Paleontology 76: 921-927.

Lapworth, C. 1876. The Silurian System in the South of Scotland, p. 1–28. In: Armstrong, J. Young, J. and Robertson, D. (eds.), Catalogue of Western Scottish Fossils. Blackie and Son, Glasgow.

Wooster’s Fossil of the Week: A trilobite hypostome (Upper Ordovician of southern Ohio)

August 21st, 2011

We had a familiar trilobite last week, so this week we’ll look at a poorly-known part of a trilobite: the hypostome. Above is an incomplete forked, conterminant hypostome of the large trilobite Isotelus. (Isotelus, by the way, is the state fossil of Ohio. Do you know your state fossil?)

Hypostome means “under mouth”. On trilobites it is found underneath the cephalon (head) near what we think was the mouth. They are not common in the fossil record. It is obvious from their color and composition that they are part of a trilobite, but most people don’t know about this little plate on the otherwise soft underside (the ventral side) of the animal. The hypostome is important in some new taxonomic schemes for sorting out the trilobites (Fortey, 1990), and they are useful for interpreting a particular trilobite’s feeding habits (Fortey and Owens, 1999).
Trilobite hypostome forms from Wikipedia (via Obsidian Soul). The small green plates are the hypostomes seen against the gray cephalon above. A – Natant: Hypostome not attached to doublure; aligned with front edge of glabella (shown in red broken lines). B – Conterminant: Hypostome attached to rostral plate of doublure. Aligned with front edge of glabella. C – Impendent: Hypostome attached to rostral plate but not aligned with glabella.

The hypostome of Isotelus is attached to the anterior edge of the skeleton (thus “conterminant”) and has two distally-directed prongs (making it “forked”). Hegna (2010) has recently suggested this hypostome with its unusual shape and terraced outer structure may have been used for grinding food rather than serrating it. Turns out our hypostome has a unique form among the common trilobites!

References:

Fortey, R.A. 1990. Ontogeny, hypostome attachment and trilobite classification. Journal of Paleontology 33: 529-576.

Fortey, R.A. and Owens, R.M. 1999. Feeding habits in trilobites. Palaeontology 42: 429–65.

Hegna, T.A. 2010. The function of forks: Isotelus-type hypostomes and trilobite feeding. Lethaia 43: 411-419.

Wooster’s Fossil of the Week: An edrioasteroid (Upper Ordovician of Kentucky)

July 24th, 2011

This week’s fossil appeared previously in this blog when we discussed hiatus concretions and their fossil fauna. It is one of my favorites for both how we found it (see the entry linked above) and the way it introduced me to hard substrate fossils (it was my first). The edrioasteroid is the circular fossil in the center. Above it is a branching cyclostome bryozoan that will be the subject of another story someday. These fossils were found in the Kope Formation (Cincinnatian Group) of the Upper Ordovician in northern Kentucky, making them about 450 million years old.

Edrioasteroids (“seated stars”) were echinoderms (spiny-skinned animals) that lived from the Cambrian through the Permian periods (Sumrall, 2009). Their living relatives today include sea stars, sea urchins, sand dollars and crinoids. Edrioasteroids have a flattened disk-like body called a theca covered with plates of calcite. They attached themselves to hard substrates like shells, hardgrounds or cobbles (as in the photo above). On the upper surface of the theca are ambulacra extending outward from a central mouth. The anus is a little circular set of plates between two of the ambulacra. The ambulacra themselves had tiny little tube feet that extended upwards into the seawater  for filter-feeding suspended organic matter.

The fossil above, also represented in the diagram below, is Cystaster stellatus (Hall, 1866). It is a small edrioasteroid, as the group goes, and is characterized by straight, wide ambulacra.

(Image from the Cincinnati Dry Dredgers’ wonderful website.)

(Image from the public domain Encyclopaedia Britannica, 11th Edition.)

Edrioasteroids are favorite fossils for collectors. I learned this when I published a paper on the fauna that included the fossils above (Wilson, 1985) and later the outcrop was pillaged — not a single edrioasteroid remains there from the hundreds originally found.

References:

Sumrall, C.D. 2009. First definite record of Permian edrioasteroids; Neoisorophusella maslennikovi n. sp. from the Kungurian of northeast Russia. Journal of Paleontology 83: 990-993.

Wilson, M.A. 1985. Disturbance and ecologic succession in an Upper Ordovician cobble-dwelling hardground fauna. Science 228: 575-577.

Wooster’s Fossil of the Week: A strange little echinoderm (Ordovician of Russia)

June 12th, 2011


This small fossil was completely new to me when I found it during my research trip to the Ordovician of Russia in the Fall of 2009.  A side view is shown on the left of this conical skeleton, and the top view is right.  I could tell it was an echinoderm because it has a characteristic structure in its calcitic skeleton known as the stereom (a network of tiny passageways inside the crystals).  Other than that, it was a mystery to me.

My Russian colleague Andrey Dronov showed me that it is of the genus Bolboporites, a strange relative of the crinoid found only in the Ordovician of the Baltic Region and North America.  As you can see in the reconstruction on the right below, it probably lived in the sediment as an upwardly-flaring cone with a single feeding arm (the brachiole) collecting suspended organic matter from passing water for food.  In the fossil view above and right, you can see the hole where the missing brachiole fit; inside of that you can just make out an opening that is likely the mouth.

Bolboporites likely originated on the paleocontinent of Baltica and then migrated to North America.  As far as I can tell it is vanishingly rare over here — I’ve never seen Bolboporites before in the field or in collections.  Now Wooster has one of the very few of these little treasures.

References –

Rozhnov, S.V. 2009. Eocrinoids and paracrinoids of the Baltic Ordovician basin: a biogeographical report. IGCP Meeting, Ordovician palaeogeography and palaeoclimate, Copenhagen, p. 16.

Rozhnov, S.V. and Kushlina, V.B. 1994. Interpretation of new data on Bolboporites Pander, 1830 (Echinodermata; Ordovician), p. 179-180, in David, B., Guille, A., Féral, J.-P. & Roux, M. (eds.), Echinoderms through time (Balkema, Rotterdam).

Wooster’s Fossil of the Week: Encrusting craniid brachiopods (Upper Ordovician of southeastern Indiana)

May 22nd, 2011

The two irregular patches above are brachiopods known as Petrocrania scabiosa encrusting the ventral valve of yet another brachiopod (Rafinesquina). That species name “scabiosa” is evocative if not a little unpleasant — it is also the root of the English “scab”.

Petrocrania scabiosa is in a group of brachiopods we used to call “inarticulates” because their two valves are not articulated by a hinge as they are in most brachiopods. Instead they are held together by a complex set of muscles. Now we place these brachiopods in the Class Craniforma, an ancient group which originated in the Cambrian and is still alive today.

Petrocrania scabiosa was a filter-feeder like all other brachiopods, extracting nutrients from the seawater with a fleshy lophophore. The Wooster specimens are part of our large set of encrusting fossils (a type of sclerobiont) in our hard substrate collection. They have irregular shells that are circular in outline when they grew alone, and angular when they grew against each other.

Some craniid brachiopods were so thin that their shells repeated the features of the substrate underneath them, a phenomenon known as xenomorphism (“foreign-form”).

Petrocrania scabiosa brachiopods (circular) on a Rafinesquina brachiopod, along with a trepostome bryozoan that encrusted some brachiopods and grew around others. The P. scabiosa on the far left shows xenomorphic features. Specimen borrowed from the University of Cincinnati paleontology collections.

A 2007 College of Wooster paleontology field trip to the Upper Ordovician locality near Richmond, Indiana, where these specimens were found. Students are in the traditional paleontological poses.

Wooster’s Fossils of the Week: Hyoliths (Middle Ordovician of Estonia)

May 1st, 2011

The fossils above are about as simple as fossils can be. They are internal molds (sediment-fills) of conical shells that were made of the carbonate mineral aragonite.  The aragonite shells dissolved away after death and burial, leaving the cemented sediment behind.  While not complex, these fossils have historic value in paleontology.  They represent an extinct group called hyoliths, and they were found where the very first hyoliths were described by Eichwald in 1840: the Middle Ordovician of Estonia.  I collected them on my first field trip to the Baltic States in 2006.  (My original interest in picking them up, by the way, was in the faint squiggles on the outside of the molds — a trace fossil known as Arachnostega.)

Hyoliths are rather common in some rock sequences.  They are among the earliest shelly fossils known, found in the lowest Cambrian rocks (about 540 million years old).  They peaked in abundance in the Cambrian and lived throughout the Paleozoic Era, finally going extinct at the end of the Permian Period (around 250 million years ago).

Reconstruction of a living hyolith (by "Smokeybjb" via Wikipedia).

For as many hyolith fossils we have, they remain an enigmatic group.  They had conical shells, usually a bit flattened, with a hinged lid (operculum) over the open end.  Extending from the space between the operculum and cone were two calcareous rods called helens (a name deliberately chosen so as not to evoke a particular function).  Some rare hyolith fossils show evidence of internal features, including muscle scars and a twisted intestinal tract.  We still can’t definitely place them in a particular animal group, though, and even their life habits are obscure.  They probably were deposit-feeders (digesting organic material from seafloor mud), but the support for this is speculative.

The hyoliths of Estonia tell us one more thing: they are different enough from other hyoliths around the world to show us that the paleocontinent of Baltica likely had its own biogeographic province.  In other words, Baltica was isolated as an island continent during the Middle Ordovician (around 460 million years ago), much like Australia today.

Baltica is the small green continent shown on this global reconstruction of the Cambrian (public domain from Wikipedia).

Wooster’s Fossil of the Week: A honeycomb coral (Upper Ordovician of southern Indiana)

February 20th, 2011

Polygons are common in nature, whether in two dimensions as desiccation cracks or in three dimensions as with columnar basalt. They result from “closely-packed” disks or tubes. The honeycomb coral (Favosites Lamarck 1816) is one of the best fossil examples of hexagonal packing.

Favosites appeared in the Late Ordovician (about 460 million years ago) and went extinct in the Permian (roughly 273 million years ago). It consists of a series of calcitic tubes (corallites) packed together as closely as possible, thus the resemblance to a honeycomb. The corallites share common walls with each other. They were occupied by individuals known as polyps that were much like today’s modern coral polyps. They had tentacles that extended into the surrounding seawater to collect tiny prey such as larvae and micro-arthropods. (I’m confident here because we actually have fossils showing the soft polyps themselves.)

A, Portion of the corallum of Favosites favosa. B, Portion of four corallites of Favosites gothlandica, enlarged, showing the tabulae and mural pores. (From H.A. Nicholson (1877): "The Ancient Life History of the Earth A Comprehensive Outline of the Principles and Leading Facts of Palæontological Science.")

As you can see in the drawings above, the corallites are distinguished by internal horizontal partitions called tabulae and holes in the walls termed mural pores. These pores most likely allowed internal soft tissue connections between the polyps so that they could share digested nutrients.

Thin-section of Favosites from the Upper Ordovician of southern Indiana. Note the gaps in some corallite walls. These are mural pores.

Favosites as a genus has a very long history. It was named by the famous French natural historian and war hero Jean-Baptiste Lamarck. It is a favorite in paleontology courses because it is so easily recognized.

Wooster’s Fossil of the Week: A cystoid (Middle Ordovician of northeastern Estonia)

January 16th, 2011

Fossils don’t get much more spherical than Echinosphaerites aurantium, an extinct creature common in the Early and Middle Ordovician of North America and Europe. These are cystoids, a somewhat informal category of filter-feeding, stalked echinoderms that are relatives of the better known crinoids. My students and I found bucketloads of them in the oil shales of the Baltic country Estonia three years ago. They are like stony golf balls.

A typical cystoid has a sac-like theca forming the bulk of the body. This theca is made of dozens to hundreds of plates of the mineral calcite fitted together like tiles. On one end of the theca is a small stem to attach it to the substrate; the other end has short brachioles, which are filter-feeding arms surrounding a tiny mouth at their base. An anus is present on the side, distinguished by a circlet of special plates.

If you look carefully at the specimen on the left in the above illustration, you’ll see at least two sclerobionts (hard-substrate dwellers) attached to the theca.  The black branching form is a graptolite (like our last Fossil of the Week) called Thallograptus sphaericola (the species name means “sphere dweller”) and the raised disk is a bryozoan.

Every once in awhile the cystoids in Estonia were buried quickly and did not fill with sediment. The hollow space within became a kind of geode with crystals of calcite growing from the thecal plates inward. Each plate is a single crystal of calcite, so the crystals grew syntaxially (maintaining crystallographic continuity). These specimens are spectacular if broken open carefully so they don’t shatter into a thousand sparkles.

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