The Terrible, Prehistoric Frog That Wasn’t

Over 300 million years ago, long before the time of the dinosaurs, giant amphibians hopped along the sandy shores of Pennsylvania. At least, that was what Pittsburgh Post-Gazette reporter James Ross explained to readers of the newspaper’s November 28th, 1948 issue.

The inspiration for the report was a set of strange tracks found in the Elk County woods not far from Pittsburgh. Bizarre scratches in stone had been discovered a week and a half before by Mike Kosinski, brother to preparator James Kosinski at the Carnegie Museum of Natural History, and it was by that route that J. LeRoy Kay – a paleontology curator at the institution – found out about the weird traces.

Exactly what sort of creature created the tracks was not immediately clear. In his report, Ross described the markings as “heavy, definite imprints in the sandstone, as though made by a giant chicken as it hopped across the surface.” But the geological context of the site indicated that the prints had been laid down millions and millions of years before the first birds had evolved. The sandstone was a slice of Carboniferous time – when huge insects crawled through the undergrowth, tiny lizard-like creatures scrambled through forests comprised of tree-like plants more closely related to quillworts, and an impressive array of archaic amphibians trundled along the water’s edge.

None of the known Carboniferous creatures seemed capable of leaving such a trail behind. “The tracks were apparently made by a good-sized animal,” Kay said, “But who he was, has me stumped.” Nevertheless, the fact that the traces were apparently created by a three-toed, hopping creature narrowed the realm of possibility down to some sort of vertebrate – maybe some kind of enormous frog with legs about two and a half feet apart. At the end of the article, Ross speculated that Kay might figure out the identity of the animal in time for the trackway’s scheduled unveiling at the Pittsburgh museum, where the fossil has been on display ever since.

The dinofrog interpretation didn’t stick. When paleontologists Derek Briggs and W.D. Ian Rolfe reanalyzed the slab in a 1983 paper, they noted that the sign accompanying the fossil display stated that the trail had been made by a eurypterid – one of the great “sea scorpions” which patrolled seas and freshwater lakes between about 460 and 250 million years ago. This sounded reasonable, but no one knew for sure. Despite the fossil’s early fame, no one had actually gotten around to describing it until Briggs and Rolfe came along.

Only a portion of the original fossil made it to the Carnegie. There were originally 20 sets of paired tracks, but only six are contained in the museum slab. (The remaining tracks may have been destroyed when the Carnegie team collected the specimen.) And, frustratingly, the geology of the slab did not provide a clear indication of the environment in which the traces were made. About all that Briggs and Rolfe were able to hypothesize was that the tracks were made in or near a shallow body of water.

Whatever the ecological circumstances of the trackway’s creation, though, Briggs and Rolfe concluded that a vertebrate could not have been responsible. The anatomy of the imprints, a continuous groove down the middle of the trackway, and, perhaps most importantly, the fact that no one had ever found a monstrous Carboniferous amphibian capable of hopping in such a manner led the paleontologists to comfortably discount the vertebrate possibility. A huge sea scorpion seemed like a better possibility.

The giant arthropod probably pulled itself along with three pairs of legs, although the trackway was unusual in that the invertebrate was using legs on both sides of the body simultaneously rather than using an alternating, side-to-side gait. This peculiar way of movement might have been a necessity given the animal’s large size, further evidenced by some of the subtle track features. “The very deep imprints and shallow median groove, which suggest that the posterior part of the trunk was dragged along,” Briggs and Rolfe wrote, “indicate that the arthropod which produced the trail was not walking fast.” This, in turn, seemed to hint that the creature was at least partially, if not entirely, out of the water as it crawled along.

Briggs and Rolfe could not point to a particular genus of arthropod that might have made the tracks. Most eurypterids were too small to leave such imprints, and ones of appropriate size lived in other places. Nevertheless, we know that at least some sea scorpions grew large enough to leave such impressive trackways. Jaekelopterus rhenaniae, a 390 million year old sea scorpion found in the strata of Germany, is estimated to have reached over eight feet in length and may have been the largest arthropod of all time. Sea scorpions, as a group, had a size range and anatomy which corresponded to the tracks. Although Briggs and Rolfe mentioned a few other possible candidates, sea scorpions seemed like the best bet, especially since previous research suggested that at least some eurypterids were amphibious.

Without some wonderful body fossils from the Carboniferous of Pennsylvania, we are left with only a fuzzy vision of how the Carnegie trackway was created. But there is a significant lesson in this historical episode. Tracks and traces left by prehistoric invertebrates have regularly been mistaken for the footprints of vertebrates. There is no shortage of examples. In 1938 paleontologist Kenneth Caster recognized that tracks attributed to early amphibians and given the name Paramphibius had actually been created by invertebrates similar to horseshoe crabs, for example, and in 1994 researchers Zbynĕk Roček and Jean-Claude Rage determined that a tetrapod footprint type called Notopus petri was probably a starfish imprint.

I wonder if another collection of supposed vertebrate tracks might eventually be reinterpreted. Two years ago, paleontologist Grzegorz Niedźwiedzki and colleagues described footprints and trackways they referred to early tetrapods. These were the first vertebrates with limbs and the capability to walk on land.

What was particularly remarkable about the tracks was that they were almost 20 million years older than Tiktaalik – a roughly 375 million year old “fishapod” with transitional features between lobe-finned fish and the earliest tetrapods. And the tracks were found in a marine setting distinct from the swampy, fresh or brackish environment that has been envisioned for the world’s first amphibious vertebrates. If true tetrapods made the 395 million year old tracks, then there are millions of years of early tetrapod evolution that we know nothing about.

But what if the seeming discrepancy between the trace fossil and body fossil record is a matter of misidentification? Could it be that the 395 million year old “tetrapod” tracks were really created by marine invertebrates or some other creature? We need more fossil evidence to figure this out, and in the meantime should take great care when envisioning prodigious amphibians from enigmatic footprints.

For another case of mistaken identity, see my post on Megarachnesee my post on Megarachne – a creature once touted as the largest spider of all time but which turned out to be something different.

References:

Briggs, D., Rolfe, W. 1983. A giant arthropod trackway from the lower Mississippian of Pennsylvania. Journal of Paleontology. 57 (2), 377-390

Caster, K. 1938. A restudy of the tracks of Paramphibius. Journal of Paleontology. 12 (1), 3-60

Niedźwiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M., & Ahlberg, P. (2010). Tetrapod trackways from the early Middle Devonian period of Poland Nature, 463 (7277), 43-48 DOI: 10.1038/nature08623

Rocek, Z., Rage, J. 1994. The presumed amphibian footprint Notopus petri from the Devonian: a probable starfish trace fossil. Lethaia. 27, 241-244

Ross, J. 1948. Museum party on trail of prehistoric animal. Pittsburgh Post-Gazette. November 28th.