THE INFLUENCE OF HYDRODYNAMIC FORCES ON TRACK MORPHOLOGY: TURTLE TRACKS FROM THE LATE CRETACEOUS CREVASSE CANYON FORMATION, NEW MEXICO

Testudinatan skeletal materials are among the most ubiquitous fossils recovered from Mesozoic-aged strata in western North America. By contrast, there is a paucity of documented turtle tracks from the region, particularly from Cretaceous-aged formations. Here we report on multiple turtle track-bearing surfaces from fine-grained channel facies in the Coniacian/Santonian Crevasse Canyon Formation, northwestern New Mexico. These monospecific turtle track assemblages are consistent with the freshwater channel interpretations of other Chelonipus-bearing strata in the Western Interior Basin.

Numerous tracks are preserved as natural casts on a fallen block of sandstone bearing ripple marks and desiccation cracks; trough cross-bedding is visible in cross section. We attribute these tracks to Chelonipus in that they present as rostrally tapered features mainly in sets of three (occasionally two to four) that are indeterminately assignable to the manus or pes, typically wider than long (~2-4 cm x ~1.5-5 cm), and bear a more extensive digit III phalangeal drag mark. Tracks are fairly symmetrical in appearance and may display an arching connection of the sandstone cast features. The majority are bounded posteriorly by an ovate positive hyporelief structure suggesting the animal used force to kick off from the substrate. Elongate dragging furrow impressions are commonly associated with these “pushup” structures and are variable in length, up to a maximum of 25 cm. Distinguishable stride length can be measured between two pairs of features and ranges from 11-12 cm.

The turtle track Chelonipus is interpreted as a semi-buoyant behavior of the organism (a flow-assisted motion trace) as evidenced by the incomplete nature and parallel relations of trackways. Elongate “dragged” phalangeal impressions on this specimen supports this interpretation. Exposure of five sides of the fall block including the track surface and opposing current indicators allows us to test the flow-assisted motion hypothesis using photogrammetric reconstruction. Contrast of the traces against that of the paleocurrent indicator show that these Chelonipus tracks are predominantly oriented against the current. Such an interpretation also explains the presence of posterior ovate displacement structures.