Dusting for prints from a fossil fish to understand evolutionary change

In 370 million-year-old red sandstone deposits in a highway roadcut, scientists have discovered a new species of armored fish in north central Pennsylvania.

Illustration of the Devonian armored fish Phyllolepis thomsoni as it may have looked when alive
[Credit: Jason Poole, Academy of Natural Sciences of Drexel University]

Fossils of armored fishes like this one, a phyllolepid placoderm, are known for the distinctive ornamentation of ridges on their exterior plates. As with many such fossils, scientists often find the remains of these species as impressions in stone, not as three-dimensional versions of their skeletons. Therefore, in the process of studying and describing this fish's anatomy, scientists took advantage of a technique that may look a lot like it was stolen from crime scene investigators.

Dr. Ted Daeschler has shown the fossil and made a rubber cast by pouring latex into its natural impression in the rock. Once the latex hardened, Daeschler peeled it out and dusted its surface with a fine powder to better show the edges of the bony plates and the shapes of fine ridges on the fish's bony armor -- a lot like dusting for fingerprints to show minute ridges left on a surface. With this clearer view, Daeschler and colleagues were better able to prepare a detailed scientific description of the new species.

This is a dorsal view of the dermal armor of the newly identified fossil fish species,
Phyllolepis thomsoni [Credit: Academy of Natural Sciences of Drexel University]

This placoderm, named Phyllolepis thomsoni, is one of two new Devonian fish species described by Daeschler in the Bicentennial issue of the Proceedings of the Academy of Natural Sciences of Philadelphia, with different co-authors. The other new species is a lobe-finned fish discovered in northern Canada.

Both the Pennsylvania placoderm and the Canadian lobe-finned fish species are from the late Devonian period, at a time long before dinosaurs walked the Earth -- but, geologically speaking, not long before the very first species began to walk on land. Daeschler studies Devonian species in particular to help describe the evolutionary setting that gave rise to the first vertebrate species with limbs. He has dug for Devonian species in Pennsylvania since 1993, and in northern Canada since 1999.