Wooster’s Fossils of the Week: Silicified sclerobionts (Middle Permian of southwestern Texas)

October 21st, 2012

During my work at the National Museum of Natural History last week, I had my research desk amongst the many cabinets housing the famous Permian brachiopod collection made by the eminent paleontologist Richard E. Grant (1927–1995). Most of these specimens are from the Middle Permian of southwestern Texas, and they are preserved in a fantastic way. I peaked into some of these drawers and was just amazed at the beauty and delicacy of these fossils.

Many years ago I received a block of limestone from the Road Canyon Formation (Middle Permian, Roadian, about 270 million years old) found in the Glass Mountains of southwestern Texas. This rock was from an ancient reef system and so nearly completely filled with fossils. The fossils are replaced with very fine-grained quartz (“silicified”), yet the rock matrix around them is limestone (composed of calcium carbonate). The trick, then, is to dissolve away the limestone in hydrochloric acid and watch the delicate replaced fossils emerge. I did this with the Road Canyon Formation rock and recovered hundreds of extraordinary specimens. One set is shown above. Previous Fossils of the Week have included an aberrant brachiopod and a set of reef-forming brachiopods.
While at the Smithsonian, Kathy Hollis showed me a polished block of original Road Canyon Formation limestone (above) and then next to it the results after dissolving a similar block in acid (below). The complex mass of bryozoans, corals and brachiopods is preserved in exquisite detail.
Now, back to the Wooster specimens at the very top of this entry and just above. The platform is the wavy outer layer of a bivalve shell. Attached to it are encrusting organisms (sclerobionts). The long, gorgeous tube is a rugose coral. At its base is a ribbed athyrid brachiopod. Also in this vignette are bryozoans, additional corals and some really tiny productid brachiopods. Beautiful.

References:

Cooper, G.A., and Grant, R.E., 1964, New Permian stratigraphic units in Glass Mountains, West Texas: American Association of Petroleum Geologists Bulletin 48: 1581-1588.

Cooper, G.A., and Grant, R.E. 1966. Permian rock units in the Glass Mountains, West Texas, In: Contributions to stratigraphy, 1966: U.S. Geological Survey Bulletin 1244-E: E1-E9.

Olszewski, T.D. and Erwin, D.H. 2009. Change and stability in Permian brachiopod communities from western Texas. Palaios 24: 27-40.

 

Wooster’s Fossil of the Week: A giant oyster (Eocene of Texas)

May 6th, 2012

It’s no ordinary oyster, of course, because it comes from Texas. It certainly is the largest oyster I’ve ever seen. Wooster received it as part of a large donation in 2010. (You can see students studying it in this previous blog entry.)

All we know is that it came from Texas (a notoriously big place) and the Eocene Series. It appears to be the extinct oyster Crassostrea gigantissima (Finch, 1824). Curiously, this is the first fossil species described from the Paleogene of North America (see Howe, 1937). It is worth quoting the entire description:

Fossils. This extensive formation is chiefly composed of a large species of ostrea, which I believe has not yet been described. A specimen of it may be seen in the Philadelphia museum; it is twelve inches long and two and three-quarters wide, and each valve from half to two and a quarter inches thick — Major Ware says they occur larger; on account of their great size I propose to call them Ostrea Gigantissima. The shells appear but slightly changed by their residence in the earth, and are in many parts used for burning into lime. (Finch, 1824, p. 40)

This is what it took to name a new species in 1824! Since then, of course, we have a detailed set of rules for naming animal taxa detailed in the International Code of Zoological Nomenclature. The Lawrence (1991) reference below is an example of what we often have to do in order to bring old names like “Ostrea Gigantissima” up to, well, Code.

The interior of the attaching valve of Crassostrea gigantissima.

The top surface of our giant oyster is riddled with these small holes. They are produced by the boring sponge Entobia, which is the next Fossil of the Week.

References:

Finch, J. 1824. Geological essay on the Tertiary Formations in America. The American Journal of Science and Arts 7: 31-42.

Howe, H.V. 1937. Large oysters from the Gulf Coast Tertiary. Journal of Paleontology 11: 355-366.

Lawrence, D.R. 1991. The neotype of Crassostrea gigantissima (Finch, 1824). Journal of Paleontology 65: 342-343.

Wooster’s Fossil of the Week: A new microconchid genus and species (Permian of Texas)

December 4th, 2011

Two years ago I was invited to Texas by Tom Yancey (Texas A&M) to look at some curious wiggly tubular fossils in the Lower Permian (about 280 million years old). They form small reefs a meter or so across and have traditionally been referred to as serpulid worm tubes. We suspected otherwise. After field and lab work, and collaboration with our Estonian colleague Olev Vinn, we determined that they are a new genus and species of microconchid. Our paper describing this taxon has just appeared: Wilson, Vinn and Yancey (2011).

A tangled collection of Helicoconchus elongatus Wilson, Vinn and Yancey 2011.

Helicoconchus elongatus is, as you may suspect from the name, an elongate coiled tube. The walls are impunctate (meaning they have no pores) and have diaphragms (horizontal partitions) with little dimples in their centers. They have two kinds of budding: fission (shown in the top image) and lateral budding (shown below). They grew into thick intertwined disks in shallow marine waters where they lived with snails, clams, echinoids and foraminiferans.

A small lateral bud on the side of a microconchid tube.

An acetate peel showing a longitudinal cross-section of a microconchid tube. The thin diaphragm running vertically in this image shows an inflection for the "dimple".

Microconchids (Ordovician – Jurassic) are an evolutionarily interesting group because they appear to be related to bryozoans and brachiopods (much to everyone’s surprise). This is based on their shell structure and their manner of budding (Zatoń and Vinn, 2011). Helicoconchus elongatus will tell us much about the relationships of microconchids to other groups because of the detail we can see in its budding styles and its marvelous preservation.

Helicoconchus elongatus in the field.

References:

Wilson, M.A., Vinn, O. & Yancey, T.E. 2011. A new microconchid tubeworm from the Artinskian (Lower Permian) of central Texas, USA. Acta Palaeontologica Polonica 56: 785-791.

Zatoń, M. & Vinn, O. 2011. Microconchids and the rise of modern encrusting communities. Lethaia 44:5-7.

Wooster’s Fossil of the Week: an aberrant brachiopod (Permian of Texas)

October 9th, 2011

Funny word to apply to a fossil: aberrant, meaning “deviating from the normal”. It’s an old-fashioned word rarely used these days, primarily because we have a hard time defining “normal”. It was the word used when I was introduced to the above brachiopod, though, so I employ it in honor of my old-timey professors.

On the left is the dorsal valve exterior and on the right the ventral valve interior of Leptodus americanus Girty 1908. (Both valves are broken.) This species is a member of the Family Lyttoniidae in the Order Productida, which some of my students may have just figured out. The large ventral valve relative to the reduced dorsal valve is the clue. The specimen was found in the Word Limestone (Wordian Stage, Guadalupian Series, Middle Permian System, about 265 million years old) in Hess Canyon, Texas. It is replaced by silica (“silicified”) and so was easily extracted from a block of limestone by dissolving away the calcium carbonate matrix.

These brachiopods, along with many other types, lived in extensive reefs in west Texas during the Permian. The ventral valve was cemented to other shells and extended out parallel to the substrate. The much smaller dorsal valve fit into the grooves, leaving much of the soft-part interior exposed. My professors said it was “like a leaf in a gravy boat” — and I had no idea what a “gravy boat” was then.

It is likely that Leptodus americanus had photosynthetic zooxanthellae embedded in its exposed mantle tissues. These are protists (most often dinoflagellates) that live inside the tissues of metazoans and provide them with nutrients and oxygen in return for carbon dioxide and a cozy place to live. Reef-forming corals are the best known animals to have such a mutualistic symbiotic relationship with zooxanthellae today. It would thus not be surprising to see a similar system with these reefal brachiopods.

Not so aberrant after all.

References:

Girty, G.H. 1908. The Guadalupian fauna. United States Geological Survey Professional Paper 58:1-651.

Williams, A. 1953. The morphology and classification of the oldhaminid brachiopods. Washington Academy of Sciences Journal 9: 279-287.

Wooster’s Fossil of the Week: Reef-forming brachiopods (Middle Permian of southwestern Texas)

April 10th, 2011

In my early days of teaching paleontology I had an enthusiastic trading program with colleagues around the country. I would supply fine fossils from the Upper Ordovician of southern Ohio for what I considered exotic specimens from elsewhere. In one of the trades I received a block of limestone from the Road Canyon Formation found in the Glass Mountains of southwestern Texas. It was from the Roadian Stage of the Guadalupian Series of the Permian System, so about 270 million years old.

This limestone is famous for its silicified fossils. The original calcite shells of the fossils were replaced by silica (similar to the mineral quartz), yet the matrix of the limestone remained mostly calcite. This meant that my students and I could immerse the limestone block in hydrochloric acid and watch the calcite matrix dissolve and the silicified shells remain as an insoluble residue. What emerged from the acid were beautiful fossils where even the finest spines are preserved.

Cluster of Hercosestria cribrosa brachiopods with the conical ventral valve (VV) and lid-like dorsal valve (DV) labelled.

Our particular block was part of a reef complex in which the primary framework was made by conical brachiopods attached to each other by long spines. These brachiopods are unlike any that came before or since. Each shell consists of two valves: the ventral valve is an open cone and the dorsal valve attaches to it as a hinged lid. The spines come from the ventral valve and wrap around other shells to make a wave-resistant structure — a reef. These brachiopod reefs were unique to the Permian.

The species we have, Hercosestria cribrosa Cooper & Grant 1969, belongs to the Superfamily Richthofenioidea in the Order Productida, so they are often called richthofenids and productids. Hercosestria had its moment of paleontological fame in the mid-1970s. Two prominent paleontologists, Richard Cowen and Richard Grant, debated the role of models in assessing the functional morphology of extinct species.  Richthofenid brachiopods were used as an example: did they flap their dorsal valves to create a current (Cowen’s suggestion), or did they crack the valves open and pump the water in and out with their fleshy lophophores? Grant showed a specimen of Hercosestria cribrosa with another brachiopod living on its dorsal valve, convincingly demonstrating that the valves did not likely flap.

On the left is a figure from Grant (1975) showing Hercosestria cribrosa with a small brachiopod living on its dorsal valve; on the right is a side view of two H. cribrosa ventral valves with attaching spines.

To make it even more interesting, by the 1980s there was considerable support for the idea that richthofenid brachiopods like Hercosestria had algal symbionts in their tissues and thus were effectively photosynthetic!

Reconstruction of a Permian reef from the University of Michigan Exhibit Museum of Natural History.

To see the other Wooster’s Fossil of the Week posts, please click on this link or the appropriate tag to the right.

References –

Cooper, G.A. and Grant, R.E. 1969. New Permian brachiopods from west Texas. Smithsonian Contributions to Paleobiology 1: 1-20.

Cowen, R. 1975. ‘Flapping valves’ in brachiopods. Lethaia 8: 23-29.

Cowen, R. 1983, Algal symbiosis and its recognition in the fossil record: in Tevesz, M.J.S. and McCall, P.L., eds., Biotic Interactions in Recent and Fossil Benthic Communities: Plenum Press, New York, p. 431-478.

Grant, R.E. 1975. Methods and conclusions in functional analysis: a reply. Lethaia 8: 31–33.

Scene from the lab

July 8th, 2010

WOOSTER, OHIO–I spent a good part of the day in the paleontology lab of Lisa Park, one of our accomplished Wooster Geology alumni who teaches at the University of Akron.  We took scanning electron microscope images of microconchid specimens I collected last November in Texas with Tom Yancey (Texas A&M).  For every day of fieldwork we probably spend another ten days in the lab studying the specimens.  Thought you might like to see one of these beautiful fossils very close-up:

Microconchid from the Bead Mountain Formation (Lower Permian) of central Texas. Note the budding and the remarkable internal diaphragms visible in the broken portion (upper right).

A very bored Permian brachiopod

November 15th, 2009

boredbrach111409

COLLEGE STATION, TEXAS–I never get tired of that too-obvious joke. I found the above productid brachiopod on the last outcrop of our little Texas expedition. It has been drilled by barnacles, which leave a distinctive slit-shaped hole with a tiny little comma shape at one end. It may not look special here photographed on my backpack in the sunlight, but it is. Hard substrate communities in the Permian are still poorly known. This specimen tells us that a future trip may reveal many more such specimens.

Paleontologists (and anyone else) should be able to tell me whether these borings were produced during the life of the brachiopod or after its death. Your determination can be posted in the comments below!

Two West Texas outcrops: which looks more inviting?

November 15th, 2009

texasoutcrop111409albanyoutcrop111409COLLEGE STATION, TEXAS–The upper one is the base of the Valera Formation on US Highway 84 (N31.88196°, W99.47115°) and the lower one is the lower Bead Mountain Formation on Route 6 near Albany; both are Permian and both have delicious microconchid fossils along with much else.  You can imagine which is the more pleasant to work on.

I have been very impressed with the Permian geology of this part of Texas.  The fossils and sedimentary rocks are very accessible and sufficiently mysterious to generate at least two paleontology and sedimentology projects, including future Independent Study work by Wooster students.  Sure there are fire ants, rattlesnakes, and very fast country road driving, but it wouldn’t be Texas without them!  (And the barbecue … all beef, dry-rubbed barbecue …)

The puzzle of gypsum

November 15th, 2009

Our Permian sections on this Texas trip have had thick beds of gypsum only a meter or three beneath our fossiliferous limestones and shales.

An outcrop of sedimentary gypsum below the Valera Formation (Permian).

An outcrop of sedimentary gypsum below the Valera Formation (Permian).

Gypsum (calcium sulfate) is an evaporite mineral, indicating when the Permian shallow sea in this case was much saltier than normal (hypersaline).  Our fossils show a restricted nature (lower diversity than normal, and generally smaller shells), but they were still living in at least close to normal salinities.  This is especially the case with our numerous echinoids.  We even have evidence of some evaporites within our fossiliferous limestones.  It is a curious juxtaposition of depositional environments.

You’re never alone on an outcrop

November 13th, 2009

Molted skin layer from a rattlesnake apparently in the cavity underneath the rock.

Molted skin layer from a rattlesnake apparently in the cavity underneath the nice piece of Permian limestone. I didn't poke around in there to wake him up, and I let that limestone stay where it was.

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