NEW MEASUREMENTS OF TRACKWAYS IN THE PASSAIC FORMATION, MILFORD, NJ: CONSIDERATION OF TRACKMAKER SOCIALITY

Fossil tracks and trackways record a connection between their maker and the paleoenvironment. Through their shape, depth, orientation, and spacing, these trace fossils can indicate how a trackmaker interacted with other life at the time, and with the substrate in which they are preserved.

The Perkasie Member of the Passaic Formation has long been known to contain fossil tracks. Here we report detailed measurements and stratigraphic context for an understudied set of tracks on an exposed bedding plane south of Milford, NJ. The host strata are interpreted as lake margin facies with red, laminated mudstone and massive siltstone layers, with NE/SW oriented (006) ripples, vertical burrows, and mud cracks on the trackway bed. The study strata were deposited during the Upper Triassic (Norian), when the Newark Basin was an active rift zone.

At our study site we documented 12 individual tracks organized in two sets. The first set of structures suggests metatarsal impressions, each measuring 6-14 cm across with a SE trackway orientation and depths that increase towards the phalanges. The second set consists of possible pedal drags measuring 6-31 cm across, with a SE orientation as well and depths that increase towards the forefoot of the impressions.

Mobile LiDAR software was used to create 3D models of the individual prints and the sets as a whole. This allows us to preserve these prints in a digital format where we are able to measure and manipulate the 3D record without damage to the actual surface.

We consider our field data at the study site in the context of social behavior in two ways. First we investigate whether a consistent track depth is more closely correlated with the size and weight of the maker or the conditions of the environment. To do this, we use quadrupeds of varying sizes and compare their created track depths in variably saturated sediments. Second, using computer modeling and aerial videos of modern social locomotory behavior, we investigate the degree to which this leads to parallel track orientations. Tracking software is used to create pathways of individuals and analyze parallelism between them.

This study’s primary fieldwork adds to our understanding of the Late Triassic paleoenvironmental conditions in the basin, with our experimental results having implications within and beyond the study strata.