Tuesday, December 22, 2009

Fossil of the Week

This beautiful black spiky thing, that you might at first think is a tooth, is actually part of an extinct fish – it’s a spine from the tail of a fossil skate (something line a modern sting-ray). It is the holotype* (PRI catalog number 25065) of Machaeracanthus retusus, named in the journal American Midland Naturalist by John Wells in 1940. As far as I know, it is the only part of the fish that is known so far, which might be because the fish was probably an elasmobranch (sharks, skates, and rays), who all have cartilaginous skeletons (that don’t fossilize as well as bone). This is a pretty impressive spine, now 10.3 cm long (a little over 4 inches) but it was estimated (from the impression left by the spine in the rock) to be at least 16 cm (more than 6 inches) in length when the animal was alive. The name “retusus” means blunt, referring to the relatively blunt-tapering end of the spine (which was apparently not caused by wear and tear). A cross section of the spine in the original three-page paper describing the species (which, unusually, was dedicated to description of this one species) shows that each side of the spine was flattened into a sharp keel or blade.

The name: Machaeracanthus retusus actually refers only to the spine, not to the fish that made it. It is called an “ichthyodorulite” and is a trace fossil. Trace fossils receive their own scientific names in paleontology. They might or might not be connected to a whole animal, and even if they eventually are, they still retain their own names. So this spine is Machaeracanthus retusus, whether or not we ever know anything more about the fish that it came from! (So far, I don’t think we do.) The best-known trace fossils are footprints, and we have a very famous one here in New York: Grallator is the name of the only dinosaur remains in New York State – they are footprints of what might have been the dinosaur Coelophysis (our own “Cecil” mascot). But paleontologists acknowledge that Grallator footprints might have been made by any number of small, three-toed, bipedal theropod dinosaurs.

The scientist: The author of this fossil, John West Wells (1907-1994), was a graduate student of PRI’s founder Gilbert Harris (receiving his PhD from Cornell in 1933), past President of the PRI Board of Trustees, a long-time geology faculty member at Cornell University, and one of the world's leading authorities on fossil and living corals. According to the original description, this fossil was in his private collection. It might have come to us along with his private library, donated shortly before he died in 1994. This donation included what has been described as the largest private collection of books on corals. Memorial gifts from Wells’ colleagues and former students allowed PRI Library to create the John Wells Rare Book Room in 1996.

The locality: This is a Devonian fossil, from the Lowest Geneseo Shale at a location very familiar to all at Museum of the Earth - Taughannock Gorge, Cayuga Lake, New York. This place is famous for Taughannock Falls, a spectacular 215-foot waterfall just outside of the village of Trumansburg, New York. Although a narrow cataract, it is taller than Niagara Falls and sits in a supremely picturesque setting, now a NY State Park. Definitely worth a visit next time you’re here!

*See Fossil of the Week 8/19/09 - Cerithium gainesensis for a definition of "holotype."

Text by Paula Mikkelsen

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