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.

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.

A record of ancient earthquakes

December 15th, 2009

MAYSVILLE, KENTUCKY–Strong earthquakes produce seismic waves which can do much damage on land, as we well know.  They can also disturb unconsolidated sediments on shallow oceanic shelves and platforms, producing characteristically swirled structures called seismites.  The Upper Ordovician outcrop we visited today has three horizons of well-preserved seismites associated with those strange blocks described previously.

The "ball-and-pillow" structures in this view of the Kentucky Route 11 outcrop of the Fairview Formation are seismites produced by Late Ordovician earthquakes.  There are two seismite horizons, each with a flattened top produced by later erosion and redistribution of the sediments by oceanic currents.

The "ball-and-pillow" structures in this view of the Kentucky Route 11 outcrop of the Fairview Formation are seismites produced by Late Ordovician earthquakes. There are two seismite horizons visible here, each with a flattened top produced by later erosion and redistribution of the sediments by oceanic currents.

Closer view of a seismite in the Kentucky Route 11 outcrop.  I couldn't reach this high to place my hammer for scale.  The structure is about a meter thick.

Closer view of a seismite in the Kentucky Route 11 outcrop. ( I couldn't reach this high to place my hammer for scale; the structure is about a meter thick.)

The earthquakes which caused these seismites were probably associated with orogenic (mountain-building) activity to the east where the present (and much later) Appalachian Mountains sit.  Careful measurement and mapping of seismites can tell us much about the specific locations and magnitudes of these earthquakes, as well as the consistency of the sediments they disturbed long ago on those ancient seafloors.

Mysterious out-of-place rocks in the Ordovician of Kentucky

December 15th, 2009

MAYSVILLE, KENTUCKY–Our short geological expedition to northern Kentucky today was to look at some odd blocks of limestone that sit suspended in the sediments as if they were dropped in while the sequence was accumulating.

An eroded, bored and encrusted limestone block in the Fairview Formation (Upper Ordovician) of northern Kentucky at the Route 11 outcrop (N38.61243°, W83.75575°).

An eroded, bored and encrusted limestone block in the Fairview Formation (Upper Ordovician) of northern Kentucky at the Route 11 outcrop (N38.61243°, W83.75575°).

These rocks are bored by worms and encrusted by bryozoans on their top and sides, and they often sit at high angles to the surrounding strata.

Bryozoans encrusting a side of the block above. The beautiful pinkish bryozoan on the left is the holdfast of a ptilodictyoid which in life held an erect bifoliate portion of the colony. The field of view here is about 10 cm wide.

Bryozoans encrusting a side of the block above. The beautiful pinkish bryozoan on the left is the holdfast of a ptilodictyoid which in life held an erect bifoliate portion of the colony. The field of view here is about 10 cm wide.

It is difficult to imagine a mechanism which deposited large, lithified limestone blocks in the middle of a shallow carbonate ramp. They are almost certainly related to “seismite” structures in the outcrop (see next post), but how these earthquakes would have transported such rocks is a mystery.  We also do not know how quickly the limestone had been lithified before emplacement.  We do know that the sides of these blocks were exposed on the seafloor long enough to accumulate encrusters and borers.

Plenty yet to discover in these well-studied rocks.  It is a continuing lesson for scientists: the more you see the more questions you have.

Wooster Geologist in Kentucky

December 15th, 2009

MAYSVILLE, KENTUCKY–Today I visited the University of Cincinnati for a meeting of Aaron House’s thesis committee, on which I serve.  (Aaron is a 2004 geology graduate from The College of Wooster.)  It all went very well and soon after Aaron took me and two other geologists on a short field trip to an Upper Ordovician outcrop near the Ohio River town of Maysville.

Outcrop of the upper Fairview Formation (Upper Ordovician) on Kentucky Route 11 near Maysville, Kentucky (N38.61243°, W83.75575°).

Outcrop of the upper Fairview Formation (Upper Ordovician) on Kentucky Route 11 near Maysville, Kentucky (N38.61243°, W83.75575°). A distant Aaron House for scale.

Many Wooster students and alumni will immediately recognize all the elements of a typical roadside outcrop of the Cincinnatian Group in winter: gray rocks matching the gray sky, the muddy ditch at the base, and the thin verge of grass extending to the road.  Alternating limestones, siltstones and shales give the outcrop its jagged appearance.

Some of the best Ordovician fossils in the world are found in these sedimentary sequences, and the stratigraphy holds many mysteries despite over a century and a half of intensive study by geologists.  Wooster students have completed dozens of Independent Study theses with these rocks, and there are many more to come.  Aaron House is now pursuing a masters degree by assessing and interpreting the preservation of mollusk fossils in the Cincinnatian.

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