SUBAQUEOUS TETRAPOD SWIM TRACKS FROM THE MIDDLE JURASSIC: BIG HORN BASIN, WYOMING U.S.A

Recent discoveries indicate that marine carbonates and carbonate-rich siliclastics of the Middle Jurassic (Bajocian) Gypsum Spring Formation contain tetrapod tracks of swimming animals. There are two distinctive vertebrate swim track types tentatively assigned to crocodilians and to possible bipedal dinosaurs. This swim track horizon is laterally extensive and can be traced throughout the Big Horn Basin where ever the Gypsum Spring Formation outcrops.

Importantly, the swim track horizon is located stratigraphically one meter above a well documented, multiple layered, tridactyl dinosaur footprint bed. The tridactyl tracks are preserved on multiple surfaces and geographic localities in the northeastern Bighorn Basin. The swim tracks are preserved as convex hyporelief “negative relief impressions” on a single exposed flat bedding plane surface. The swim traces are subparallel and parallel scrape marks or dimples” that occur either in pairs or (rarely) in threes. Lateral spacing between the sub-parallel marks is typically a few centimeters. Many traces in the Gypsum Spring Formation are characterized by two parallel 1 cm wide grooves, spaced approximately 3.5 cm apart. Each groove set is approximately 4-8 cm long. “Dimples” are subequal, or equal, non-linear, indentations sometimes preserved in two's, three's, and rarely in four's. Tracks exhibit “impact rims” and/or “pressure release structures” at the termination of the “grooves” or “dimples” suggesting a piling-up of the sediment behind the track. In most cases, the groves are perpendicular to the bedding plane. However, some arcuate forms have been found. These traces are interpreted to represent toe/claw scratch marks made by buoyed animals briefly touching bottom while swimming over a muddy carbonate substrate.

Some unusual nearly in-line Gypsum Spring traces reflect a swimming behavior of a dinosaur rather than crocodile. The in-line traces do not seem to be consistent with a sprawling swim pattern, but rather a more erect motion of bipedal (?) swimming. The more arcuate (inclined to the bedding plane) traces may, however, reflect a more crocodile-like sprawling gait swim behavior. Similar arcuate, rectilinear (parallel and sub-parallel) scratches and “dimpled” traces are common and have been found in Permian, Triassic and Jurassic rocks.