Enigmatic vertebrate trackway from the Scalby Formation (Middle Jurassic) Yorkshire, United Kingdom, with discussion of archosaur and ‘mammal’ trace fossils

Abstract

We describe a new and unusual vertebrate trackway from the Middle Jurassic Scalby Formation of the Cleveland Basin, Yorkshire, United Kingdom. The Enigmatic Burniston Trackway (EBT) is the first and only example of such a trackway known from this region. The best preserved EBT print, belonging to a pentadactyl tetrapod, does not resemble any known Middle Jurassic ichnogenus, but shares features with Triassic and Cretaceous archosaur and synapsid ichnotaxa. EBT most closely resembles the Triassic ichnogenus Synaptichnium in having the longest digit III, shortest digits I and V, and digit V positioned posterior to the other digits. Synaptichnium has been assigned to various trackmakers, including crocodylomorphs, and early archosaurs (‘thecodonts’ and aetosaurs). However, the anteriorly oriented digits and reduced and posterolaterally placed digit V of EBT also resemble Sederipes from the Cretaceous, and Dicynodontipus from the Permian-Triassic (both representing large-bodied synapsid or ‘mammal’ trackmakers). Unlike most traces assigned to cynodont (including mammalian) or crocodylomorph makers, EBT has low total digit divergence. Digit imprints end in short claws and the pes is rotated by ∼20° outwards in relation to the manus. Preservation is fragmentary for most of the trackway and the specimen could not be collected, but we record it here and provide comparative context within the existing ichnofossil record. We discuss the issues regarding ‘mammal’ ichnofossil literature, including taxonomic nomenclature, inconsistent diagnostic criteria, and assumptions made previously about Mesozoic synapsid body mass.