Peeling a Fish from Stone

A great deal of the romance of paleontology lies in the search for traces of prehistoric life. There’s something wonderful about hiking over naked stone and through arid badlands in the hope of finding tracks, teeth, shell, leaf impressions, or bone. Indeed, the primary reason I moved west was so that I could be in places where Earth history lies exposed in tantalizing and beautiful expanses.

Yet, despite the ubiquitous imagery of fossil hunters combing parched hills and digging under the intense desert sunshine, not all fossil deposits are so remote. Back on the east coast, where I grew up, rich fossil beds lie under forests and suburban sprawl. These ancient clues only come to light when someone has reason to cut through the blankets of overlying material, exposing the rock beneath. Such is the case in central and western Pennsylvania. Roadcuts slice deep into Devonian rock – the roughly 419 to 358 million year old “Age of Fish” when vertebrate life was almost entirely aquatic, with the exception of those pioneering tetrapods that heaved themselves around at the water’s edge. And the latest fish to come out of these human-made fossil exposures was an armor-plated oddball.

Paleontologists John Long and Edward Daeschler describe the fish in the latest Proceedings of the Academy of Natural Sciences of Philadelphia. Discovered in a roadcut along Route 15’s Covington Bypass, the creature is represented by impressions from an articulated set of armor plates and two isolated, associated plates that are ornamented by rippling ridges. These plates, Long and Daeschler propose, belonged to a hitherto unknown species of armored fish called Phyllolepis thomsoni that swam among ancient deltas sometime between 372 and 358 million years ago.

Revealing those delicate details took a little reverse engineering. The plates themselves were only preserved as scraps that clung to impressions in the rock. To recreate the look of Phyllolepis, Daeschler used latex to make casts of the plates from their natural molds:

In the big picture of the fish family tree, Phyllolepis was a placoderm. This was a long-lived group of armored fish that included the famous and fearsome Dunkleosteus among its varied ranks. Phyllolepis was not so nasty as its shear-jawed cousin, though. As reconstructed by Long and Daeschler through comparison with previously-discovered specimens, Phyllolepis thomsoni had a broad, flat head that made the fish look like something of a swimming, armored vacuum.

Given the general lack of press the weird piscine creatures of the Devonian typically receive, Phyllolepis is unlikely to become a rock star. But the discovery of the genus in Pennsylvania may nonetheless help paleontologists pinpoint places where more charismatic creatures might be found. Among other Northern Hemisphere sites, Long and Daeschler point out, Phyllolepis is often found in the same deposits as early tetrapods – the four-limbed, multi-fingered vertebrates that spanned the ecological gap between the water and the land. Therefore Phyllolepis “may serve as an indicator of favorable paleoecological conditions in the search for early tetrapods”, the researchers write. Each piece of the paleontological puzzle provides a slight outline of what the broader prehistoric picture might reveal.

Reference:

Long, J., Daeschler, E. 2013. First articulated phyllolepid placoderm from North America, with comments on phyllolepid systematics. Proceedings of the Academy of Natural Sciences of Philadelphia. 162: 33-46