THE FOOTPRINT APATOPUS LINEATUS FROM LATE TRIASSIC STRATA OF MILFORD, NJ: TO BE A PHYTOSAUR OR NOT TO BE A PHYTOSAUR, THAT IS THE QUESTION…

A historic paleontological site in the lacustrine Lockatong Formation (Late Triassic) at Milford, NJ – active since the 1880s - has yielded a fairly diverse assemblage of tetrapod ichnofossils, including the type specimens of Apatopus lineatus, Atreipus milfordensis, Atreipus sulcatus, Rhynchosauroides hyperbates, and Brachychirotherium parvum. When the ichnogenus Apatopus was named (Baird 1957, named as a new ichnogenus for an ichnospecies described by Bock 1952), the trackmaker was tentatively identified as a phytosaur, an evaluation which was later questioned but has not been studied further. This study seeks to determine the identity of the Apatopus lineatus trackmaker using a variety of methods (including cladistics as well as osteometric ratios). Identification of the A. lineatus trackmaker is important because the ichnotaxon is long-lived (223-200 Ma) and became extinct in the Triassic-Jurassic boundary extinction event.

The forefoot is symmetrical with digit II and III subequal in length and digitigrade. Digit IV is longest in the semiplantigrade hindfoot. We have taken these features and the phalangeal formulae and compared them with the distribution of derived characters within Amniota. Our results indicate that the forefoot is derived from the primitive reptilian condition and consistent with Archosauromorpha; the hindfoot is consistent with basal Archosauromorpha (e.g. rhynchosaurs, trilophosaurs), but in all Archosauria (a group within Archosauromorpha) except phytosaurs, digit III is longest; thus phytosaurs may also be a candidate trackmaker.

References:

Baird, D. 1957. Triassic reptile footprint faunules from Milford, New Jersey. Mus. Comp. Zool. Bull. 117:449-520.

Bock, W. 1952. Triassic reptilian tracks and trends of locomotive evolution. J. Pal. 26:395-433.