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A Middle Pennsylvanian (CA. 315 Million Years) Amphibian Trackway from the Cross Mountain Formation, East Tennessee Cumberlands

By Martin S. Kohl, Tennessee Division of Geology and Jonathan R. Bryan, Department of Geology, Florida State University

On April 9, 1985, Karl T. Henn, Josip R. Galetovic, and Steven K. Wood, all formerly with the Tennessee Division of Surface Mining, discovered a footprint bearing slab of sandstone in coal mine spoil during a site inspection in the Cumberland Mountains of Campbell County. The slab had probably been transported during mining and reclamation from strata immediately overlying the Grassy Springs Coal at the base of the Cross Mountain Formation. They searched for additional material with disappointing results. Two subsequent visits in 1990 and 1991 by the authors also yielded no additional tracks.

Amphibian Trackway Slab

Figure 1. Amphibian Trackway Slab

The trackway slab (Figure 1) consists of thinly cross-laminated, very fine-grained sandstone with a silty clay shale parting over the surface containing the footprints. The entire slab is about 45 by 70 cm (2.54 cm = 1 inch) in size and 1.8 to 3.5 cm thick. The upper surface is undulating, probably reflecting ripples or sediment load structures. What are thought to be raindrop impressions are found near one edge. The surface in which the footprints are impressed is interpreted as a residual mud from a receding flooding event. Over 65 cm of trackway and at least 33 separate footprints are preserved. A tail drag impression is conspicuous.

Two distinct types of foot marks are present (Figure 2). The smaller, with the narrower track width, show at most four digits and are inferred to be manus (forefoot) prints, while the wider and somewhat larger, many of which show five digits, are inferred to be pes (hindfoot) impressions. Manus imprints average about 2.8 cm long and 3.1 cm wide, and have four digits of roughly equal size. They are oriented within the trackway so that digit I or II is parallel to the direction of travel.

Amphibian foot marks

Figure 2. Manus and Pes Measurements (Scale Bar = 1 cm). 1) left manus with average length and width, 2) right manus with average interdigital angles, 3) best left pes with average length and width, 4) best right pes with average interdigital angles.

Pes impressions are somewhat larger than the manus, with five digits of distinctly unequal size. The outermost toe is slightly divergent in angle from the others, and in some impressions there is also a degree of displacement away from them. Pes imprints average 3.8 cm long and 3.5 cm wide, and are oriented within the trackway such that the inner or middle toe is parallel to the direction of motion. Track width (see Figure 3) averages 5.49 cm for manus and 6.73 cm for the pes. Stride averages 8.45 cm.

Trackway Measurements

Figure 3. Trackway Measurements (Scale Bar = 5 cm). Representative trackway measurements of stride length and width, pace angulation for manus and pes, and glenoacetabular distances (body lengths).

The tail drag impression is continuous across the entire length of the trackway, and shows only a very slight degree of sinuousity. The minimum radius of curvature in the tail drag impression of 9 mm represents the maximum possible radius of the portion of the tail that contacted the sediment. There is no evidence for a flattened bladelike end, common on the tails of semiaquatic animals such as salamanders and crocodilians.

Reconstructed Trackmaker

Figure 4. Reconstructed Trackmaker. 1) Long Wheelbase, 2) Short Wheelbase.

Pace angulation is an indication of the efficiency of an animal’s locomotion. This measure is very high (approaching 180°) in modern animals such as dogs, horses, and humans that have long legs and walk with their feet directly under their bodies. In the case of the trackmaker, just the opposite is true. Its pace angulation averages 76.1° for the manus impressions and 63.1° for the pes (due to the greater track width), both very much lower than almost all modern animals. We must conclude it was an animal having either relatively short legs or using a wide sprawling gait. The pes prints fall roughly halfway between manus prints, so that in the trackway, manus and pes impressions are almost exactly opposite each other. This condition permits two interpretations concerning the length of the animal (Figure 4).

The “short wheelbase” animal placed the pes immediately behind the manus during walking, while the “long wheelbase” animal’s pes would make its impressions one full stride behind. Estimated body lengths are 8.2 or 16.7 cm, depending on which tracks are assumed to have been occupied simultaneously by the trackmaker. The longer animal is considered the more likely for the following reasons:

  • The impressions are large relative to the track width and stride, implying a larger, heavier animal with proportionately larger feet.
  • The placement of the impressions does not exhibit the high degree of regularity relative to each other that is expected in the “short wheelbase” animal.
  • The continuous, well impressed tail drag suggests a large heavy tail, in turn implying a larger animal.

The four-digit manus is significant, and considering the relatively large size of the trackway, we conclude that the trackmaker was most probably an amphibian of the Order Temnospondyli, perhaps an ancestor to the large and ponderous Eryops. Taxonomic clues preserved in fossil trackways such as phalangeal formulae, the presence or absence of claws, size, and various trackway measurements may prove to be more diagnostic of trackmaker identity, that is, its identification relative to skeletal remains. As a trace fossil, it has been namedMatthewichnus caudifer.

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