New Mexico Geological Society Annual Spring Meeting — Abstracts

Studying the trackways of extinct animals is one of the few ways to study social behaviors, especially herding. Estimates of trackmaker speed as well as direction of motion can be used to establish if a set of trackways was made by a herd rather than by independent, individuals. Alexander, 1989 (Dynamics of Dinosaurs & Other Extinct Giants, Columbia University Press, New York) has described a method of estimating speeds of trackmakers; this technique has been refined by Thulborn, 1990 (Dinosaur Tracks, Chapman and Hall, Melbourne). Lockley and Hunt, 1995 (Dinosaur Tracks, Columbia University Press, New York) have discussed a tracksite at Mosquero Creek. New Mexico which contains a large number of tracks of ornithopod ichnogenus Caririchnium (a presumed iguanodontid) apparently organized into groups of parallel trackways which they interpreted as indicating herds. Cotton, Cotton and Hunt 1996, (New Mexico Geology, vol. 18, p. 57) have shown the one of these groups of trackways is clustered in speed as well as direction. In addition, the estimated speeds are anti-correlated with footprint size in the sense that smaller individuals were exerting more effort, further supporting the interpretation of the tracks as having been made by a herd.

The present results include analysis of a control sample of late Triassic (Rhaetian) trackways from the Redonda Formation of Apache Canyon in eastern New Mexico. These trackways represent a mixed ichnotaxa and are not expected to show clustering in direction or speed. Eight trackways of the ichnogenera Grallator and Pseudotetrasauropus are located on a single large slab from a lacusterine shoreline deposit. Seven of the trackways are assigned to Pseudotetrasauropus, a herbivorous dinosaur, and one is assigned to Grallator, a therapod dinosaur. Analysis shows no clustering in direction or speed, suggesting independent trackmakers. The trackmakers at Apache Canyon, despite being considerably smaller than those at Mosquero Creek, were moving at higher speeds, as might be expected of exposed individuals. This null result at Apache Canyon helps validate the positive fmding at the Mosquero Creek tracksite; further strengthening the evidence for herding in the Caririchnium trackways.