A temnospondyl trackway from the early Mesozoic of western Gondwana and its implications for basal tetrapod locomotion

Abstract

Background: Temnospondyls are one of the earliest radiations of limbed vertebrates. Skeletal remains of more than 190 genera have been identified from late Paleozoic and early Mesozoic rocks. Paleozoic temnospondyls comprise mainly small to medium sized forms of diverse habits ranging from fully aquatic to fully terrestrial. Accordingly, their ichnological record includes tracks described from many Laurasian localities. Mesozoic temnospondyls, in contrast, include mostly medium to large aquatic or semi-aquatic forms. Exceedingly few fossil tracks or trackways have been attributed to Mesozoic temnospondyls, and as a consequence very little is known of their locomotor capabilities on land.

Methodology/principal findings: We report a ca. 200 Ma trackway, Episcopopus ventrosus, from Lesotho, southern Africa that was made by a 3.5 m-long animal. This relatively long trackway records the trackmaker dragging its body along a wet substrate using only the tips of its digits, which in the manus left characteristic drag marks. Based on detailed mapping, casting, and laser scanning of the best-preserved part of the trackway, we identified synapomorphies (e.g., tetradactyl manus, pentadactyl pes) and symplesiomorphies (e.g., absence of claws) in the Episcopopus trackway that indicate a temnospondyl trackmaker.

Conclusions/significance: Our analysis shows that the Episcopopus trackmaker progressed with a sprawling posture, using a lateral-sequence walk. Its forelimbs were the major propulsive elements and there was little lateral bending of the trunk. We suggest this locomotor style, which differs dramatically from the hindlimb-driven locomotion of salamanders and other extant terrestrial tetrapods can be explained by the forwardly shifted center of mass resulting from the relatively large heads and heavily pectoral girdles of temnospondyls.

Figure 1. Location of Moyeni tracksite, Lesotho.

Top, paleogeographic reconstruction of southern Pangea during the Late Triassic, 200 Ma (map produced at the University of Texas Institute for Geophysics, 2012; http://www.ig.utexas.edu/research/projects/plates/). Locality is indicated by a star. Bottom, Google Earth image of southern Lesotho showing location of Moyeni tracksite (building indicated by arrow) just outside the town of Quthing. A2 and A4 are major roadways leading to Quthing. Abbreviations: AF, Africa; AN, Antarctica; EU-AS, Eurasia; IN-MA, Indo-Madagascar; NA, North America; SA, South America.

Figure 2. Moyeni tracksite.

Track surface exposed during 1960s and 1970s, with original interpretations of tracks and surface morphology by Ellenberger (; foldout e). Current position of footbridge and Visitor Center are shown schematically by translucent gray polygons. The trackway of Episcopopus ventrosus is highlighted in blue, and the first and last recorded steps are indicated.

Figure 3. Moyeni tracksite, overview of scanned surface.

This map, which includes Episcopopus ventrosus manus-pes pairs 49–70 (Ellenberger's numbering), was constructed from a series of 50×50 cm scans.. For higher resolution images, see Figures 4–7.

Figure 4. Episcopopus ventrosus.

Scanned image (top) and interpretive line drawing (bottom) of manus-pes pairs 65–70. Odd numbers represent left manus-pes pairs; even numbers indicate right manus-pes pairs. Direction of travel and left (L) and right (R) are indicated by the arrow at right. Abbreviations: I–V, digits; An ., Anamoepus; Gr ., Grallator; m, manus; p, pes. White arrowheads indicate straight drag marks probably made by the base of the tail; black and white arrowheads indicate manual digit drag marks. Scale equals 1 m. m.

Figure 5. Episcopopus ventrosus.

Right manus-pes pair 56 demonstrating the superposition of the pes print over the manus drag. Note that this is photograph of a cast (positive), with light directed from the upper left. The pentadactly pes (I–V) is positioned forward of the tetradactyl manus (I–IV) and oversteps truncates the digital drag marks (dd) of the manus. The microbial-matted surface (mm) has been smoothed in the immediate vicinty of the tracks. Scale equals 5 cm. cm.

Figure 6. Episcopopus ventrosus.

Sinusoidal manus drag between successive footfalls. Image at left shows right manus-pes pair 68 and and right manus 70. The rightmost edge of the belly drag, which smoothed the microbially-matted surface, can be seen at left. The interpretive drawing at right shows schematic outlines (filled in black) of the right manus and pes along with a dotted line indicating the arc of curvature of the drag. Abbreviations: I–V, digit numbers; dd, digit drag; m, manus; p, pes.

Figure 7. Episcopopus ventrosus.

Belly drag between manus-pes pairs 65–70. In this photograph of a plaster cast of the trackway, made on site by the authors, the belly drag was recognized by the smoothing of the microbial-matted surface between the successive footfalls. Our interpretation, which is marked in the translucent white tone bordered by a dotted line, differs from that portrayed by Ellenberger across nearly the same interval (: fig. 70, pl. I), which was continuous, broader, and slightly more regularly undulating. That is, our interpretation is that the belly drag was narrower, more discontinuous, and irregular. The scale bar is in centimeters (lower central portion of image).

Figure 8. Secondary overstep in a ‘long-coupled’ tetrapod trackmaker.

The illustration shows the underside of a stereospondyl as it progresses along a hypothetical trackway surface. The lower portion of the illustration shows the sequence of impressions made by the right and left manus (RM, LM) and pes (RP, LP) leading up to the moment just before the left pes contacts the substrate. Note that in this ‘long-coupled’ form the pes oversteps the manus of the previous stride. We infer that the Episcopopus trackmaker made tracks in this fashion.

Figure 9. Salamander manus and pes surface morphology.

Photographs show the impressions left by the undersides of the right manus and pes of the salamander Dicamptodon ensatus (UMMZ 135631). Roman numerals indicate digit number. Scale equals 1 cm. cm.

Figure 10. Comparison of the body shape in a Siderops-like temnospondyl (left), which is inferred to be the Episcopopus trackmaker, and a salamander (right).

Skeletal silhouettes are shown in ventral view. We calculated the glenoacetabular distance for the temnospondyl to be ca. 130 cm, and its body form is based on comparisons with complete skeletons of Mesozoic stereospondyls, such as the Australian chigutisaurid Siderops and the European capitosauroid Mastodonsaurus . The salamander body is based on Dicamptodon ensatus (UMMZ 135631) and skeletal information from http://digimorph.org/specimens/Ambystoma_tigrinum/whole/. Important differences between these two include relative size and ossification of the skull and pectoral girdle, robustness of dorsal ribs, and degree of ossification of carpal, tarsal, and metapodial elements. Silhouette reconstructions are not to scale; see schematics at the bottom right size proportions.

Figure 11. Putative Mesozoic temnospondyl tracks.

A, aubold (1971); B, Pohlig (1893); C, Peabody (1948); D, Fuglewicz et al. (1990). Scale bar equals 10 cm for A and C; 5 cm for D. The original image in B was published without a scale.