[N] 2006 Swimming behaviour in Early Jurassic Theropod dinosaurs
Milner, A., Lockley, M., Harris, J. and Kirkland, J. (2006) Swimming behaviour in Early Jurassic Theropod dinosaurs based on spectacular swim tracks from Soutwestern Utah. JVP 26(3) Abstracts pp.100
Numerous vertebrate track-bearing layers in the Whitmore Point Member of the Moenave Formation, within a 1 km2 area of the St. George Dinosaur Discovery Site at Johnson Farm (SGDS) preserve unique, marginal lacustrine deposits of Early Jurassic (Hettangian) “Lake Dixie.” The best-preserved track-bearing horizon, called the “Main Track Layer,” demonstrates dramatic lateral changes in facies and trace fossils from onshore to offshore.
This bedding surface records a transition from exposed mudflats, with tracks and sedimentary structures formed onshore (e.g., deep mudcracks, salt casts, and rain drop impressions), to mud, silt, and sand scoured away by longshore currents that display definitive subaqueous sedimentary structures, invertebrate traces, fish swim trails, and hundreds of theropod swim tracks. Onshore firm substrate consistencies along the “Main Track Layer” surface resulted in a preservational bias of tracks by larger theropods rather than by smaller vertebrates.
On the offshore “Main Track Layer” surface, the majority of swim (or floundering) tracks are referable to Grallator-type theropods. Exceptional details are preserved, including skin impressions, claw marks, and scale scratch lines in association with current-oriented sedimentary structures such as flute casts, scratch semi-circles, tool marks, groove casts, scours, and current ripples.
The simultaneous infilling of tracks and sedimentary structures by finesand, along with high clay content in the underlying mud, accounts for this exceptional and detailed preservation. The majority of swim tracks are oriented both parallel to and opposite the current flow direction indicated by the scour marks, suggesting animals possibly reacting to being swept off-balance by N-S currents that flowed parallel to the paleo-shoreline.
The SGDS swim tracks are by far the best-preserved and largest collection of such fossils known in the world.