Wooster’s Fossils of the Week: Trace fossils making ghostly shells (Upper Cretaceous of Mississippi)

January 26th, 2014

Entobia gastropod Prairie Bluff Chalk FormationThe unusual fossil above was collected by Megan Innis (’11) and myself in Mississippi during a May 2010 paleontological expedition with Caroline Sogot and Paul Taylor of The Natural History Museum, London. That splendid trip has contributed already to one high profile publication (Sogot et al., 2013) and no doubt more will come from the excellent collections we made. All the fossils in this post came from the Prairie Bluff Chalk Formation (Maastrichtian) exposed at the intersection between Highway 25 and Reed Road in Starkville, Mississippi (locality C/W-395).

The specimen is a marine gastropod (fancy name for a snail), which is hard to believe considering no shell is preserved. The shape of the original aragonitic shell has been taken by a series of interlocking blobs, each with a sediment-filled tube extending outwards. These are casts of chambers made by a boring clionaid sponge. The resulting trace fossil is known as Entobia, a form we have seen several times in this blog. The sequence of events: (1) The sponges excavated cavities connected by tunnels into the aragonite shell of the gastropod, maintaining connections to the seawater for filter-feeding; (2) the cavities and tubes filled with fine-grained calcareous sediment after the death of the sponges; (3) the aragonite gastropod shell dissolved away, probably at the same time the sediment filling the cavities was cemented; (4) the fossil was exhumed as a series of natural casts of the sponge cavities — the trace fossil Entobia.
Entobia bivalve 1 exterior Prairie Bluff Chalk FormationThere were many other such fossil ghosts at this locality, such as the apparent bivalve shell fragment above.
Entobia cast close Prairie Bluff Chalk FormationIn this closer view (taken with my new extension tubes on the camera) we see some of the interlocking sponge chamber casts. On the surfaces of some you can just make out a reticulate pattern that represents tiny scoop-like excavations by the sponges. In the upwards-extending tubes there are a few green grains of the marine mineral glauconite.

As a paleontologist it is always sobering to see a fossil preserved in such an odd way. Were it not for these circumstances of boring, filling and cementation, the shells would have completely disappeared from the fossil record. Every fossil we have, really, is a victory of improbable preservation.

References:

Bromley, R.G. 1970. Borings as trace fossils and Entobia cretacea Portlock, as an example. Geological Journal, Special Issue 3: 49–90.

Schönberg, C.H. and Shields, G. 2008. Micro-computed tomography for studies on Entobia: transparent substrate versus modern technology, p. 147-164. In: Current Developments in Bioerosion. Springer; Berlin, Heidelberg.

Sogot, C.E., Harper, E.M. and Taylor, P.D. 2013. Biogeographical and ecological patterns in bryozoans across the Cretaceous-Paleogene boundary: Implications for the phytoplankton collapse hypothesis. Geology 41: 631-634.

Sohl, N.F. 1960. Archeogastropoda, Mesogastropoda, and stratigraphy of the Ripley, Owl Creek, and Prairie Bluff Formations, p. A1-A151. In: Late Cretaceous gastropods in Tennessee and Mississippi: U.S. Geological Survey Professional Paper 331-A.

Taylor, P.D. and Wilson, M.A. 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1-103.

Wilson, M.A. 2007. Macroborings and the evolution of bioerosion, p. 356-367. In: Miller, W. III (ed.), Trace Fossils: Concepts, Problems, Prospects. Elsevier, Amsterdam, 611 pages.

Wooster’s Fossils of the Week: Tiny little oysters (Lower Paleocene of Mississippi)

June 26th, 2011

This week’s fossils are by no means rare — last year Megan Innis and I picked up dozens of them at a muddy outcrop near Starkville, Mississippi, on our Cretaceous-Paleogene boundary expedition (click “Alabama” and “Mississippi” in the tags to the right for entries from that trip). They are, though, significant indicators of a particular kind of ecological system that appeared in the oceans of southeastern North America after the cataclysm of the Cretaceous Extinction.

The specimens pictured above are Pycnodonte pulaskiensis, a local species of oyster that belongs to a very prolific genus found around the world. Pycnodonte ranges from the Lower Cretaceous (about 140 million years ago) to, it appears, today. Kase and Hayami (1992) appear to have found this oyster — or a close relative — still living in submarine caves near Japan. This makes them a kind of “living fossil”, a group with a very long history of evolutionary stability.

This longevity fits into our Pycnodonte pulaskiensis story. These fossils are very common in the lowest Paleocene sediments just above the extinction horizon that marks the fiery end of the Cretaceous. After all that devastation (and Alabama was uncomfortably close to the impact site of Chicxulub), P. pulaskiensis appeared first to reoccupy the seafloor muds. They were virtually alone in this muddy habitat, and so lived there in great numbers. We call this kind of early successional species an “opportunist” (in the good sense!) taking advantage of a recently emptied niche.

Our little oysters in the Clayton Formation near Starkville, Mississippi.

Paul Taylor, Megan Innis and George Phillips at the Cretaceous-Paleogene boundary near Starkville, Mississippi in May 2010. Our oysters were found directly below Megan's feet.

These little oysters weren’t entirely alone, though. Many of them have small beveled holes in the center of their left valves, producing the ichnofossil Oichnus. These are apparently the traces of naticid gastropod predators (see Dietl, 2003) that drilled the holes to kill and eat the oyster soft parts.  (And who can blame them?) Several shells also have encrusting foraminiferans like Bullopora and Ramulina. Small hints of a recovering ecosystem setting the stage for the modern fauna we see in the northern Gulf of Mexico today.

References:

Dietl, G.P., 2003. Traces of naticid predation on the gryphaeid oyster Pycnodonte dissimilaris: Epifaunal drilling of prey in the Paleocene. Historical Biology 16: 13-19.

Kase, T. and Hayami, I., 1992. Unique submarine cave mollusc fauna: composition, origin and adaptation. Journal of Molluscan Studies 58: 446-449.

Wooster’s Fossil of the Week: A scaphitid ammonite (Late Cretaceous of Mississippi)

April 24th, 2011

The beauty above is Discoscaphites iris (Conrad, 1858) from the Owl Creek Formation of Ripley, Mississippi. Megan Innis and I collected it during our expedition to the Cretaceous-Paleogene boundary in the southern United States last summer. It is a significant index fossil in biostratigraphy: the Discoscaphites iris Zone is the latest in the Cretaceous (the late Maastrichtian Stage). This animal lived in the final days of the Mesozoic Era just before the mass extinction 65.5 million years ago.

Discoscaphites iris is an ammonite, a type of extinct cephalopod mollusk related to the modern octopus, squid and nautilus. It had a planispirally-coiled shell with chambers divided from each other by complexly-folded walls. If you look closely near the top of the fossil above, you will see where the shell has flaked away revealing an internal mold of sediment and a peek at the folded walls inside. “Ammonite”, by the way, is a very old term for these fossils. Pliny the Elder himself used a variant of the name, which comes from the Egyptian god Amun with his occasional coiled ram’s horn headgear.

Reconstruction of an ammonite by Arthur Weasley (via Wikipedia).

Ammonite shells were made of the carbonate mineral aragonite. This is the mineral that makes many modern mollusk shells have prismatic colors, which we call nacreous. You may know it best as “mother of pearl” or as pearls themselves. Aragonite has an unstable crystal structure and so is not common in rocks older than a few million years. The original aragonite in our ammonite fossil is thus a bonus.

In an oddly topical note, Discoscaphites iris was recently found in the Upper Cretaceous of Libya, giving it a disjunct range from the US Gulf and Atlantic coasts to the Mediterranean coast of northern Africa (Machalski et al., 2009).

Reference:

Machalski, M., Jagt, J.W.M., Landman, N.H. and Uberna, J., 2009. First record of the North American scaphitid ammonite Discoscaphites iris from the upper Maastrichtian of Libya. N. Jb. Geol. Paläont. Abh. 254: 373-378.

Bioerosion on oysters across the Cretaceous-Paleogene Boundary in Alabama and Mississippi (USA) (Senior Independent Study Thesis by Megan Innis)

April 8th, 2011

This is my research team at a road-cut locality in Mississippi. (Photo courtesy of George Phillips.)

Editor’s note: Senior Independent Study (I.S.) is a year-long program at The College of Wooster in which each student completes a research project and thesis with a faculty mentor.  We particularly enjoy I.S. in the Geology Department because there are so many cool things to do for both the faculty advisor and the student.  We are now posting abstracts of each study as they become available.  The following was written by Megan Innis, a senior geology major from Whitmore Lake, Michigan. Here is a link to Megan’s final PowerPoint presentation as a movie file (which can be paused at any point). You can see earlier blog posts from Megan’s field work by clicking the Alabama and Mississippi tags to the right.

During the summer of 2010, I traveled to Alabama and Mississippi with my research team including Dr. Mark Wilson, Dr. Paul Taylor, and Caroline Sogot.  Our trip was about ten days and included fieldwork and research. The purpose of our research was to collect fossils from below and above the Cretaceous-Paleogene (K/Pg) boundary to try and understand the Cretaceous mass extinction from a microfaunal level.

I chose to focus my thesis on oysters and the sclerobionts associated with these calcareous hard substrates.  Although my study was focused on oysters, I also collected a wide variety of other specimens including nautiloids, ammonites, belemnites, corals, sharks teeth, and bryozoans.

The oyster species present in each system.

When I got back to school in August, I identified all of my oyster species (three total) and began to identify and collect data for the sclerobionts. The oysters from the Cretaceous included Exogyra costata and Pycnodonte convexa and the oysters from the Paleogene included Exogyra costata, Pycnodonte convexa, and Pycnodonte pulaskiensis.

Sample specimens that I collected in Alabama and Mississippi. The oysters in yellow boxes and circles are the oyster species that were used in my study.

I identified nine sclerobionts including Entobia borings; Gastrochaenolites borings; Oichnus borings; Talpina borings; serpulids; encrusting oysters; encrusting foraminiferans; Stomatopora bryozoans; and “Berenicia” bryozoans.  My research showed:

1) Bioerosion of oyster hard substrates was common in the Late Cretaceous and Paleogene and sclerobionts were abundant before and after the extinction.

2) Entobia sponge borings appear to increase in abundance across the K/Pg boundary and become more common in the Paleogene.

3) Gastrochaenolites borings, made by bivalves, and serpulids were more prevalent in the Late Cretaceous, suggesting boring bivalves and serpulids were significantly reduced after the extinction.

4) Encrusting oysters and foraminiferans were more common in the Late Cretaceous, but also relatively abundant on Pycnodonte pulaskiensis in the Paleogene.

5) Encrusting bryozoans were more common in the Late Cretaceous and absent in the Paleogene, suggesting bryozoans were severely affected by the extinction.

6) Talpina borings were only found on Pycnodonte pulaskiensis in the Paleogene, but no significant data was collected elsewhere.

To my knowledge, this is the first study of bioerosion on oysters across the K/Pg boundary.

The Battle of Vicksburg and Geology

May 29th, 2010

Union cannon in original positions for the Siege of Vicksburg, Mississippi (1863).

VICKSBURG, MISSISSIPPI — The Wooster Geologists southern USA team spent the better part of the day at the site of the Civil War Battle of Vicksburg (May 18-July 4, 1863).  As is the case with virtually every battle, the local geology played a prominent role here.  Union forces wanted complete control of the Mississippi River to maintain communications with the northwest, to split the Confederacy into two large parcels, and to deny the South the use of the river for transport.  Vicksburg held the key, as President Lincoln said, to the Mississippi and maybe the success of the Union war strategy.  General U.S. Grant had an innovative (and expensive) plan to attack the fortress city from the land side to the east.  To do that he faced a series of fortified bluffs which protected the city’s flanks.  Several direct Union assaults on these bluffs failed, so a long siege of Vicksburg began until it surrendered for want of supplies and low morale.

Part of the battlefield in the bluffs just east of Vicksburg. Looking from the Union lines to the Confederate positions.

The immediate geological issues are derived from the Mississippi River and its ancestor.  At the end of the Pleistocene the glacial meltwaters flowing south through this area were tremendous, producing a vast braided stream complex.  Sediment from these channels was picked up by the wind and deposited in parts of the Mississippi Valley as thick layers of loess.  Loess is an unusual sediment because it is highly uniform in composition (silt-size subangular particles and clays) and it has a very high angle of repose (meaning it erodes into very steep slopes — cliffs, really.)  As the later Mississippi River meandered through its valley, it cut a series of bluffs at its easternmost extent at Vicksburg.  The city thus has a port on the river surrounded by high bluffs well suited for artillery to protect all approaches.

A loess cliff exposed on the side of a bluff in the city of Vicksburg.

Since loess sticks together so well, it is useful for digging entrenchments and caves for protection from artillery and rifle fire.  Many people in Vicksburg lived in loess caves during the siege to protect themselves from Union cannon fire.

Cannon on the Union side aimed towards a Confederate position in the Vicksburg bluffs.

Cannon on the Union line aimed towards a Confederate position in the Vicksburg bluffs.

We can’t say that geology controlled the Battle of Vicksburg — there are numerous and decisive factors of human courage, persistence and innovation — but we can conclude that both sides had to adapt to the geological circumstances in both military and civil ways.

Vicksburg National Cemetery. We never want to forget the cost of war. That the ages of most of the soldiers etched on the tombstones is that of present college students is especially poignant to us.

Since this is our last post from the Alabama and Mississippi summer 2010 geological team, we would like to thank our excellent guides Jon Bryan (Northwest Florida State College), Peter Harries (University of South Florida), and most especially George Phillips of the Mississippi Museum of Natural Science.  George spent days with us, giving us access to sites and people we could never have dreamed of meeting on our own.  Even more important, he is an excellent paleontologist with encyclopedic knowledge of Mississippi fossils, both invertebrate and vertebrate.  Through the generosity of George and many others, we have material for many future Cretaceous-Tertiary paleontological projects.  This trip has been an excellent example of the collaborative nature of geology.

A paleontological meeting at the Owl Creek Formation

May 28th, 2010

RIPLEY, MISSISSIPPI — On our last full field day we met a team from the American Museum of Natural History (led by paleontologist Neil Landman) and converged on the famous Late Cretaceous Owl Creek Formation exposures near Ripley in northern Mississippi.  This site has been studied since 1810 and has produced extraordinary fossils, especially ammonites with pearly layers of aragonite still preserved in their shells.  As fun as the geology was, it was even more entertaining to see the mix of southern and New York accents and mannerisms on the outcrop!

The gray unit in the bottom half of the cliff is the Owl Creek Formation (Late Cretaceous); the brown and orange sands above are the Clayton Formation (Lower Tertiary). Yet another example of the K/T boundary on this trip.

A mix of geologists from England, Ohio, Michigan, Mississippi, Kansas and New York at the Owl Creek Formation section near Ripley, Mississippi.

A more recent history

May 28th, 2010

BALDWYN, MISSISSIPPI — When possible on these geological field trips we explore the local culture and history of the region in which we are temporary guests.  This morning we visited the small Civil War battlefield of Brice’s Crossroads (June 10, 1864) in Lee County, Mississippi.  It lies between our field sites at Blue Springs in the south and Owl Creek to the north.  The center of the battlefield is marked by two cannon and a stone monument which memorializes both the Union and Confederate dead.  A Confederate cemetery is nearby.

At the time of the battle, Union commander General William Tecumseh Sherman was conducting his famous March to the Sea through Georgia and other southern states.  (One of his soldiers was Corporal Julian Adolphus Wilson of the 57th Illinois Infantry — my grandfather’s grandfather.)  Confederate General Nathan Bedford Forrest and his cavalry threatened Sherman’s supply lines, so Union General Samuel Sturgis was sent into northern Mississippi to stop him.  With superior tactics, Forrest decisively defeated Sturgis at Brice’s Crossroads, forcing a long retreat.  It was a rare Confederate victory in that time and place, but Forrest was ultimately distracted from his goal of cutting Sherman’s communications.

Graves of some of the Confederate dead from the Brice's Crossroads battle.

Bryozoan Paradise at the K/T Boundary

May 27th, 2010

NEW ALBANY, MISSISSIPPI — One of the main advantages of being a geologist in a liberal arts program is the diversity of experiences our students and faculty have.  While some Wooster geologists are enjoying a “soft rock” adventure in the Cretaceous-Tertiary sediments in the Deep South, others are exploring “hard rock” quarries in the North.  Later this summer we may have simultaneous posts from Alaska, Iceland, Utah and Israel.

Today the southern expedition was very successful in its task to find bryozoans just below and just above the Cretaceous-Tertiary boundary.  Paul Taylor is a happy man.

Numerous bryozoans (the twig-like fossils) in the uppermost Cretaceous Prairie Bluff Formation east of New Albany, Union County, Mississippi.

The Cretaceous-Tertiary boundary east of New Albany, Union County, Mississippi. The uppermost Cretaceous is the brown clay, and the lowermost Tertiary is the orange sand at Megan's painted fingertip.

Which came first?

May 27th, 2010

NEW ALBANY, MISSISSIPPI — The Cretaceous oyster above was collected from the Coon Creek Beds of the Ripley Formation (Upper Cretaceous) near Blue Springs, Mississippi.  The holes are borings called Entobia which were produced by clionaid sponges which built a network of connected chambers inside the shell so that they could carry out their filter-feeding with some safety from grazing predators.  The branching white fossil is a cyclostome bryozoan, probably Voigtopora thurni.  Which was present first on the shell, the borings or the bryozoan?  Is there evidence that they were living at the same time?  The largest holes are about two millimeters in diameter.

Fixing your search images

May 26th, 2010

NEW ALBANY, MISSISSIPPI — The kind of science our paleontological field team is doing ultimately depends on unpredictable discoveries.  We came to this part of the world based on the recorded experiences of generations of geologists who assembled maps of rock types, calculated stratigraphic ages, and made long lists of fossils they found.  From this body of knowledge we could estimate our chances of finding certain kinds of fossils in certain places.  Nevertheless, as with those pioneering scientists, we ultimately have to find things on our own.

Scouring the ground for fossils in the Nixon Sand Facies of the Prairie Bluff Formation (Upper Cretaceous) in Pontotoc County, Mississippi.

The old adage that “you find what you’re looking for” has some truth in exploratory paleontology.  You have to know what your target fossils look like before you can collect them.  This means recognizing them despite their orientations in the sediment or their preservation.  We develop a “search image” over time for each particular types of fossil.  Paul Taylor, for example, can pull bryozoans off the ground right under my nose because he has trained a set of search images for decades.  On this trip we have all learned what to expect when we crawl across the Prairie Bluff or Clayton formations.  It is an honor to spend a day plucking little treasures from the ground and adding them to the store of human knowledge.

A Cretaceous oyster encrusted in the top left of the shell with a bryozoan and drilled by a predatory snail in the center, with a coin showing The Great Emancipator for scale (Troy Beds, Ripley Formation, Pontotoc County).

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