Vertebrate Trackways
GRI #1342
Slab GRI #1342 is about 1.5 m long and up to 75cm wide. It is 3.5 cm thick on the narrow end and 4.8 cm thick on the wide end. This slab is particularly interesting because vertebrate trackways are preserved on both of its sides.
The top side has one linear trackway with at least eighteen shallow, concave impressions, alternating at regular distance on opposite sides of the trackway midline. Some prints in the series show a low relief expulsion rim on the posterior side. It is possible to distinguish superposition of the smaller manus print over the pes print in several elements of the trackway. It is difficult to detect clear toe marks, as is often the case even with neoichnological experiments (i.e., Brand, 1996). Because of the general low relief and the lack of detail, it could be possible that these are underprints from the layer directly above.
The bottom side of the slab has two clearly visible trackways, mostly preserved in concave epirelief. Many other faint tracks are preserved on this surface, but the following analysis focuses only on the two prominent trackways. The trackways are nearly parallel and do not overlap, although they are almost contiguous at one point. They were left by two animals of different size, as inferred from the size of the prints. The trackway with larger prints consists of 35 impressions, regularly spaced, alternating on opposite side of the trackway’s midline, and mostly representing superimposed manus/pes tracks, with up to four detectable toe marks per print. The pes prints are about 2 cm across, and the manus prints are about 1.5 cm across. The impressions are deeper on the anterior side and taper toward the posterior side, to the point of being slightly concave around their posterior termination, probably indicating the presence of expulsion rims.
The second trackway starts with five superimposed manus/pes impressions, alternating left/right diagonally on either side of the midline, then continues with 12 consecutive, superimposed manus/pes impressions on the right, with no corresponding impressions on the left. There is one more impression on the right, which is very faint, but may be part of the trackway, due to its position and spacing compared to the tracks before and after. The trackway continues with a superimposed manus/pes on the left, then the alternating pattern continues on the left and right sides of the midline with six more superimposed manus/pes impressions. There are two more superimposed manus/pes impressions on the right, followed by a gap of about one step where it seems neither a right nor a left set of prints were preserved. There are two more superimposed manus/pes impressions on the left side, and a possible, faint manus/pes impression on the right. The pes prints of this trackway are about 1.5 cm across, and the manus prints are about 1 cm across. Toe marks are not easily discerned.
Both trackways appear to shift slightly to the left near the middle of the slab.
The presence of lineations on the slab surface and the fact that the trackways are orientated in a similar direction suggests that the vertebrate track makers were moving upward along the dip direction of the sand dunes, as explained in the analysis of slab GRI #1341. Researchers have found that almost all of the Coconino Sandstone trackways are following an uphill path (Brand, 1979).
This slab is quite significant in that it shows that multiple animals were walking in a similar direction at different times. How much time did it take for the sand that separates the trackways at the bottom and top of the slab to be deposited? It would seem unlikely that a very long period of time would elapse between the occurrence of such similar behavior and conditions documented by the trackways.
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.
References:
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.