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.

Wooster celebrates National Fossil Day

October 13th, 2010

Crinoid holdfasts and bryozoans on a cobble from the Ordovician of northern Kentucky.

WOOSTER, OHIO–Today we are celebrating the first annual National Fossil Day (or at least I am!). Be sure to check out that link from the National Park Service — it contains the official National Fossil Day song! My recognition of this special day is to post some photographs of nice fossil specimens from the Wooster collections. You can find larger versions of these photos — and hundreds more — on my Wikimedia page. Here’s to fossils: beautiful messengers from the distant past.

Shark teeth (Scapanorhynchus) from the Upper Cretaceous of southern Israel. These were collected by Andrew Retzler ('11).

Rudist bivalves from the Upper Cretaceous of the Omani Mountains.

Tentaculitids from the Devonian of Maryland.

Thecideide brachiopods, cyclostome bryozoans and serpulids encrusting a bivalve shell from Zalas Quarry (Jurassic: Callovian-Oxfordian) in southern Poland.

Fossil leaf (Viburnum lesquereuxii) with insect damage; Dakota Sandstone (Cretaceous) of Ellsworth County, Kansas.

Wooster Paleontologists in Indiana!

September 12th, 2010

RICHMOND and LIBERTY, INDIANA–The College of Wooster Invertebrate Paleontology class had its field trip today to sunny eastern Indiana. We collected bags and bags of fossils from Upper Ordovician strata for research projects throughout the rest of the course. Each student will be reconstructing a paleocommunity from the fossils, and along the way will learn several paleontological techniques and principles. Our specimens include many strophomenid and orthid brachiopods, trepostome and cyclostome bryozoans, rugose and heliolitid corals, crinoids, nautiloids, a few trilobites, and some mystery fossils I find perplexing. (Always scientific opportunities there!) We hope to show some of our discoveries in later blog posts.

The challenge of this trip was the size of the group: 21 people in five vehicles. It all worked out well for a spectacular field day.

The Invertebrate Paleontology class spreads out along an Upper Ordovician outcrop. Note the great weather.

Travis Louvain and Nick Fedorochuk enjoy a nice exposure.

Stony bryozoans get their day

August 1st, 2010

Trepostome ("stony") bryozoan on a carbonate hardground from the Kanosh Formation (Ordovician, Whiterockian) of west-central Utah.

KIEL, GERMANY–The first day of the International Bryozoology Association meeting is traditionally devoted to workshops where participants can listen to experts on a particular group, technique or idea and then ask questions, work out exercises, or study specimens. I went to the workshop on a group of extinct bryozoans called trepostomes. The Order Trepostomata usually produced thick skeletons of the mineral calcite so they are commonly known as “stony bryozoans”. They lived from the Ordovician into the Triassic, and then disappeared forever. They are a difficult group to work with because their diagnostic features are internal and microscopic (thus requiring thin-sections or acetate peels to identify) and the number of important defining characters is still debated. I went to this workshop because Ohio can be considered the Trepostome Capital of the World with its abundant and diverse varieties found in the Ordovician of the Cincinnati area. Any Wooster geology student who has taken the Invertebrate Paleontology course will remember the buckets of these fossils we’ve collected over the years on field trips.

Wooster played a small role in this workshop, to my delight. One of the interesting and somewhat odd trepostome bryozoan types is found in the Ordovician (Whiterockian) at a place called Fossil Mountain in the western desert of Utah. A generation of Wooster Independent Study students worked here with me studying carbonate hardgrounds and the fossils associated with them. We collected many examples of a strange bryozoan we called “Trepostome Species A” because we could not identify it. Later Andrej Ernst, Paul Taylor and I described it as a new genus: Kanoshopora. It is still odd with its variable walls and colony forms. This meeting may have stirred some interest in pursuing its functional morphology (essentially how it lived) and evolutionary placement. A nice contribution from those days in the late 20th Century when we walked up and down the sunny slopes of Fossil Mountain trying to sort it all out.

Longitudinal thin-section view of Kanoshopora droserae showing its complex zooecial walls.

Fossil Mountain, west-central Utah -- the scene of much Wooster geology Independent Study fieldwork in the 1980s and 1990s, and the home of many of the oldest and strangest trepostome bryozoans.

The utility of trace fossils

July 16th, 2010

LOGAN, UTAH–Today we hiked north of Tony Grove Lake in Logan Canyon to explore an Ordovician sequence of rocks. The most interesting unit (to my surprise) was the Swan Peak Quartzite, an orange-brown unit at the base of a white dolomite and gray limestone. It weathers in sharp edges and large blocks tumbled down the slopes. Quartzite began as a quartzose sandstone which was metamorphosed into a rock where the silica cement is as hard as the quartz grains. Most fossils are destroyed in this metamorphosis, along with almost all sedimentary features, so I didn’t expect to see much in the Swan Peak.

Outcrop of the Swan Peak Quartzite (Ordovician) north of Tony Grove Lake, Utah. This photograph was taken facing north at N41° 54.198', W 111° 38.72'.

Google Earth image of the Swan Peak Quartzite exposure. The green arrow is the location from where the outcrop photograph above was taken. The stepped nature of the rocks is easily visible in the middle of the image.

The Swan Peak Quartzite is exposed here as a series of large steps looking something like a side of an eroded Egyptian pyramid. Why does it erode into this kind of stairway instead of one large block? A close examination of the rock surfaces revealed trace fossils were preserved — the kind of fossils which could be clues to the character of this formation.

Horizontal burrow systems preserved in remnant bedding planes of the Swan Peak Quartzite. These traces give the rock its local name of "Fucoidal Quartzite". Fucoidal is an old word for trace fossils.

Vertical burrow systems (Skolithos) exposed on vertical faces of the Swan Peak Quartzite.

It appears that the step surfaces have horizontal burrow systems, and the “risers” have vertical burrows (Skolithos). There is an alternation of beds with horizontal burrows and beds with vertical burrows through the Swan Peak Quartzite. The horizontally-burrowed units are less resistant than the vertically-burrowed units, so this alternation produces the stepped erosion pattern.

My hypothesis is that the horizontal burrows represent slightly deeper water than the vertical burrows, based on the distribution to such burrow systems in other formations and in today’s oceans. The alternation between the two trace fossil types may thus show fluctuating sea levels as the Swan Peak sediments were deposited way back in the Ordovician.

So, this unpromising rock may indeed be providing clues to its ancient depositional environment through the trace fossils it contains.

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