Mississippian Amphibian

When people think of fossil vertebrates, they usually think of fossilized bones or footprints, the most common of remains. On rare occasions, paleontologists may come across other fossils that are truly exceptional, such as an entire body outline or impression. A recent rediscovery of a highly unusual specimen hidden away in the vaults of the Reading Public Museum in Reading, Berks County, has attracted international attention.

The first amphibians have a fossil record that begins in the Late Devonian, about 370 million years ago. This record is extremely limited, consisting of fossil bones from only a few localities, one of which is in Pennsylvania. There are only a few places in the world where amphibian footprints are known from younger rocks of Mississippian-Permian age (250–360 million years ago), but body impressions are extremely rare, with only two published records from the Carboniferous Period (Mississippian and Pennsylvanian periods in North America) and the Permian Period of Europe. The discovery of body impressions of three Mississippian Period amphibians in eastern Pennsylvania is significant because they provide evidence of body shape and possible social behavior in some of the earliest amphibians.

The body impressions are preserved on a small sandstone slab capped by a reddish-brown clay layer. David L. Fillmore found this specimen in 2004 in the collection of the Reading Public Museum, where it had gone unnoticed by paleontologists for more than a half-century. The slab is from the middle part of the Mauch Chunk Formation near Pottsville, Schuylkill County, rocks of Late Mississippian age, about 330 million years old (see “Following in the Foosteps of Isaac Lea’s Historic Footprints” by Spencer G. Lucas, David L. Fillmore, Edward L. Simpson, and Robert M. Sullivan, Summer 2007). The slab preserves two nearly complete amphibian body impressions (A and B in the diagram at right) and a possible third (C) that is considerably fainter.

The two more complete body impressions resemble each other, although one (A) is only about two-thirds the size of the other (B). The posterior impression is more complete. The head, broad and shovel-shaped, overlaps the tail of the body impression anterior to it. The abdomen is wide and has a distinct fold (crease) just to the side the center that extends along much of the tail. This suggests a flap of skin underneath the body, as in some living salamanders. The tail is long and tapered. The robust limbs project at essentially right angles from the trunk. The manus (hand) and pes (foot) imprints are indistinct except on the left side, where it is clear there are four toes in the manus and five in the pes.

The number of toes, their shape, full sole imprints, and foot size identify a type of extinct amphibian called a temnospondyl as the producer of these body impressions. Body proportions, particularly the relatively short trunk, are also similar to the proportions of some primitive temnospondyl amphibians. However, the proportions of the Mauch Chunk body impressions indicate a relatively terrestrial temnospondyl — with short trunk and stout limb — not matched by any species known from Mississippian period bones. The Mauch Chunk amphibians show no evidence of bony armor or of external gills in early temnospondyls; the skin appears to have been smooth. The body impressions confirm previous suggestions that temnospondyls had a sprawling stance in which the limbs projected at nearly right angles to the trunk. Most intriguing is the head-and-tail overlap of the two more complete body impressions. Given their proportions, these cannot be the consecutive body impressions of a single individual. They are the impressions of two different individuals in which the head impression of the posterior individual was impressed beneath (on top of) the tail impression of the anterior individual.

What social interactions might the Mauch Chunk amphibians suggest? Researchers look to modern amphibians for possible answers. Some living salamanders undergo a courtship ritual in which the female places her nose near the male’s vent to find the spermatophore he has left in what has been referred to as either a “tail-nudging” or “tail-straddling” walk. This ritual takes place on land and occurs in salamanders with internal fertilization. Although speculative, the fossil body impressions could be a record of such behavior in a Mississippian amphibian. In this scenario, the posterior individual would be the female, and the act of nudging for the spermatophore would explain why her head is overlapped by the tail of the male. Significantly, the female is larger than the male in such an interpretation, as in many extant salamanders. This speculation suggests that internal fertilization and associated courting behavior evolved independently in one group of amphibians more than three hundred million years ago.

Spencer G. Lucas is curator of geology and paleontology and interim director of the New Mexico Museum of Natural History and Science in Albuquerque.

David L. Fillmore rediscovered the specimen at the Reading Public Museum in 2004.

Edward L. Simpson is professor of geology and chair of the physical sciences department at Kutztown University of Pennsylvania.

Robert M. Sullivan is senior curator of paleontology and geology at The State Museum of Pennsylvania in Harrisburg.