Vertebrate Trackways
GRI #1341
At just over a meter long, this slab from the Coconino Sandstone contains at least four distinct quadrupedal trackways (two sets of trackways on the left side and two sets on the right side). Each track is preserved as a concavity (or depression) on the slab’s surface, a sandstone rock made of cemented grains of sand. The orientation of the trackways may indicate that several animals walked parallel to each other in the same direction.
Each set is made of prints of the hindfoot (pes print) and prints of the forefoot (manus print). In one of the trackways, the prints are filled in by sediment, but their arrangement is clearly distinguishable. Pes and manus prints are slightly different in shape and size. The pes prints are larger and round overall, and the manus prints are more triangular. The manus prints are oriented prominently inward. The number and distinctiveness of claw impressions and toe marks varies among the tracks. This is commonly observed in neoichnological experimental work with vertebrates walking on different substrates (i.e., Brand, 1996).
Note the rim of displaced sediment behind each track. The tracks were made when the sediment was not cemented, but soft and wet. As the animal walked on the substrate, the four feet made depressions in the sediment, displacing part of the substrate backward. Consistent accumulation of displaced sediment on the posterior side of the tracks suggests that the animals were walking uphill along inclined sand dune surfaces. This can often be confirmed in the field, where trackways are observed in situ on cross-stratal surfaces of the Coconino Sandstone (Brand, 1979). Although the original angle of inclination of the trackways’ surface preserved on the slab cannot be estimated, it is very likely that the animals moved parallel to the dip direction of the inclined surface. This is inferred from the preservation of lineations on the slab’s surface. These fine, low-relief features are produced by the movement of grains under a unidirectional flow, and therefore indicate the main direction of movement of a migrating dune. The lineations on the slab are parallel to the trackways, suggesting that as the dune was migrating forward, the animals were climbing it along the dip direction.
Track Measurements
Above is an example, taken from the right side of the slab, of how tracks are measured to calculate an organism’s speed, direction, and even size.
This specimen was purchased under permit by Dr. Leonard Brand from a private seller at the Gem, Mineral, and Fossil Show in Tuscon, Arizona, and donated to GRI. Based on the lithological characteristics, it is most likely that this slab came from a quarry by a hill called the Matterhorn, about 10 miles southeast of Ash Fork, Arizona.
Reference:
Brand, L., 1979. Field and laboratory studies on the Coconino Sandstone (Permian) vertebrate footprints and their paleoecological implications. Palaeogeography, Palaeoclimatology, Palaeoecology, 28, pp.25-38. https://doi.org/10.1016/0031-0182(79)90111-1
Brand, L.R., 1996. Variations in salamander trackways resulting from substrate differences. Journal of Paleontology, 70(6), pp.1004-1010. https://doi.org/10.1017/S0022336000038701