It is usually quite difficult to attribute fossil trackways to the particular organism that produced the tracks. In a few rare cases the critter may literally “die in in tracks” and we can be certain, but usually the critter has moved on, and the track preserves the impression of soft tissues that may not be obvious in the body fossils of the track maker, to say nothing of the variation of tracks based on local conditions like the type of substrate. As a result, paleoichnology (the study of tracks) has its own nomenclature. Tracks are given their own scientific names, using a process that mirrors the system of nomenclature of animals. So we have ichnofamilies that contain inchnogenera, that contain ichnospecies. We can hypothesize that a particular ichnospecies correlates to a particular biological species, but there’s often a fair amount of uncertainty, and the retention of the ichnospecies name reflects this.
The Triassic deposits at the Solite Quarry have produced a fairly large number of ichnofossils, including a variety of vertebrate tracks. One of the most distinctive tracks from the site are tiny five-toed footprints called Gwyneddichnium (above).
Gwyneddichnium was originally described by Bock (1952) based on a number of specimens collected at Gwynedd, Pennsylvania from the Upper Triassic Lockatong Formation. The tracks are distinctive because of their small size and the five slender digits on both the front and back feet. Bock actually described three different species of Gwyneddichnium, but they differ in only minor respects and all were found in the same beds in the Lockatong.
Gwyneddichnium is one of those relatively rare instances in which we can be reasonably certain what made the tracks; the small protorosaur Tanytrachelos (below):
Admittedly, no Tanytrachelos skeleton has ever been found with its feet sitting in Gwyneddichnium tracks. But in all the places where Gwyneddichnium has been reported, Tanytrachelos has also been found. Moreover, in those deposits Tanytrachelos is the only known animal that is the correct size and morphology to be the Gwyneddosaurus-maker. In particular, Olsen and Flynn (1989) convincingly argue that the hind foot track in Gwyneddosaurus has particular features in the proportions and orientations of the toes that are uniquely shared with Tanytrachelos. The front feet, while not as unique, are still a good match. Compare the tracks at top to the front foot of Tanytrachelos (below):
The apparent correlation between Gwyneddichnium and Tanytrachelos also raises an interesting nomenclatural problem. Although Bock (1952) didn’t provide an etymology of Gwyneddichnium, I believe he named it for Gwyneddosaurus, a small reptile that Bock described in 1945 based on some disarticulated bones. Gwyneddosaurus and Gwyneddichnium both come from the same beds in the Lockatong, and Bock (1952) suggested that Gwyneddosaurus was the Gwyneddichnium trackmaker.
So what about Tanytrachelos? Tanytrachelos was described much later (Olsen, 1979) based on a skeleton from Solite. If Tanytrachelos and Gwyneddosaurus are the same taxon, then Gwyneddosaurus is the older name and would have priority.
Olsen and Flynn (1989) also addressed this issue. They pointed out that Gwyneddosaurs
appears to be composed of bones from multiple animals, probably including both Tanytrachelos and coelacanth remains. The whole clump of bones seems to be gastric ejecta (puke) from some larger animal. With Gwyneddosaurus being a chimera composed of multiple taxa, they suggested that the name should be considered a nomen dubium and suppressed in favor of Tanytrachelos, an opinion with which I agree. But Gwyneddichnium will live on, a relict of the old Gwyneddosaurus.