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.

Wooster’s Fossil of the Week: A three-branched graptolite (Lower Ordovician of southeastern Australia)

January 9th, 2011


This week I’m correcting a mistake I’ve been making in my paleontology courses for nearly thirty years. Our subject is a graptolite from the teaching collections — a specimen that has been at least cursorily examined by all of my paleontology students. It is not a particularly pretty fossil, but an important one for biostratigraphy and evolution.

Graptolites were colonial marine organisms which thrived from the Late Cambrian (roughly 510 million years ago) into the Early Carboniferous (about 350 million years ago). Their colonies, termed rhabdosomes, were not mineralized, so they are most commonly preserved as thin carbon films like our featured specimen. The rhabdosome usually began with a single individual, known as a sicula, that budded branches called stipes. Each stipe is lined with cup-like thecae that held what we presume were filter-feeding individuals termed zooids (much like bryozoans). The number of stipes per rhabdosome varies considerably, from one to dozens. Some graptolites were sessile benthic epifaunal (attached to the seafloor on hard surfaces) while others were planktic (suspended or floating).

Our specimen above is the planktic graptolite Pendeograptus fruticosus from the Lower Ordovician (about 477-474 million years old) near Bendigo, Australia. There are actually two specimens overlapping here. I’ve labeled one sicula as “S1″ and the other as “S2″.

This is where my mistake began. I identified these graptolites during my first professorial year as Tetragraptus fruticosus, believing that there were two specimens with, as you might guess from the name, four stipes each. When I counted the actual stipes present, though, I found only three each. I believed, though, that graptolites had either one, two, four or many stipes, so specimens with only three had to have one stipe missing or folded over as to be invisible. So for the curious student who counted three where there should have been four, I confidently explained the preservational oddity. That both specimens lacked the fourth stipe did not apparently shake my resolve.

It was only after photographing this specimen that I looked closely enough to see that, indeed, three stipes are present and there is no evidence of a fourth for either rhabdosome. (I added numbers and letters to the above image to delineate the stipes.) Maybe there really are three-stiped graptolites? This was easily enough confirmed by simply Googling “three-stiped graptolite”. Who knew? This version of “Tetragraptus” is actually the genus Pendeograptus. Oops.

Turns out that our Pendeograptus fruticosus is part of an evolutionary series proceeding from four to three to two stipes. The three-stiped version we have is an indicator for the Australian Bendigonian Stage and a global index fossil for this narrow time interval 477 to 474 million years ago.

You can now see this specimen (without the red labels) on the Wikipedia page for graptolites. I’m correcting my mistake and sharing a useful little fossil with the world.

The paleontology of hiatus concretions: fossils without sediment

December 15th, 2010

Bryozoans (the thin branching structures) and an edrioasteroid (with the "star") encrusting a hiatus concretion from the Kope Formation (Upper Ordovician) of northern Kentucky.

Way back in 1984, when I was just a green Assistant Professor of Geology, my wife Gloria and I explored a series of Upper Ordovician (about 445 million years old) outcrops in northern Kentucky to plan a paleontology course field trip. It was a rainy day were, as is too often the case, slippery with mud. On our last roadcut exposure of the day I stepped out of the car and found at my feet the cobble pictured above. It had edrioasteroid echinoderms and bryozoans encrusting it on all sides — and we knew we had found something special. We collected dozens of the cobbles in a few minutes. It changed my research trajectory by introducing me to the splendors of hard substrate communities and hiatus concretions.

This post is a celebration of another chapter of that work published next month in the journal Facies (volume 57, pp. 275-300). This time I’m a member of a large team led by my young friend and colleague Michal Zaton of the University of Silesia in Sosnowiec. We thoroughly examined a set of bored and encrusted cobbles from the Middle Jurassic (about 170 million years old) of south-central Poland. It was a pleasure to use some of the same research techniques I employed 26 years ago to help reconstruct an ancient ecosystem and environment.

Hiatus concretions from the Middle Jurassic of Poland.

These cobbles are known as “hiatus concretions” because they collect in an environment when sediment has stopped (gone on “hiatus”, I suppose) and a lag of hard debris accumulates when fine sediment is washed away by currents. Organisms which require a hard substrate (“sclerobionts”) encrust the cobble surfaces (bryozoans, echinoderms, oysters and serpulid worms are most common) or bore into the matrix (sponges, bivalves, barnacles and worms commonly do this). A fossil record thus is formed in the absence of sedimentation, which is a bit different from the usual paradigm.

Various encrusters and borings on hiatus concretions from the Middle Jurassic of Poland.

Encrusting bryozoans on hiatus concretions from the Middle Jurassic of Poland.

I enjoy studying marine hard substrate organisms through time because they show a type of community evolution over hundreds of millions of years. These diverse fossils have also provided countless research opportunities for my Wooster students, and tracking them down has taken us all over the world and throughout the geological column. (The Cretaceous of Israel is another recent example of this work.) It is very satisfying to see a young geologist like Michal Zaton finding pleasure and research success in the same pursuit.

Bryozoans and crinoid holdfasts encrusting a cobble from the Upper Ordovician Kope Formation of northern Kentucky.

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