Not even an elevated Dimetrodon made these Dimetropus tracks

Matching tracks to trackmakers
can only ever be a semi-rewarding experience. Estimates and exclusions can be advanced. Exact matches are harder to come by. This is due to both the vagaries and varieties of sequential footprints in mud or sand, and to the rarity of having skeletal data that matches.

Which brings us to Dimetropus
Traditionally Early Permian Dimetropus tracks (Fig. 2–8; Romer and Price 1940) have been matched to the coeval pelycosaur, Dimetrodon (Fig. 1)—but only by narrowing the gauge of the Dimetrodon feet and elevating the belly off the surface, as Hunt and Lucas 1998 showed.

Today we’ll take a look at some other solutions
not involving Dimetrodon doing high-rise pushups. Several distinctly different tracks have fallen into the Dimetropus wastebasket. Let’s look at three ichnospecimens.

Traditionally, and according to Wikipedia,
citing Hunt and Lucas 1998: “Trackways called Dimetropus (“Dimetrodon foot”) that match the foot configuration of large sphenacodontids show animals walking with their limbs brought under the body for a narrow, semi-erect gait without tail or belly drag marks. Such clear evidence for a more efficient upright posture suggests that important details about the anatomy and locomotion of Sphenacodon and Dimetrodon may not be fully understood.” Hunt and Lucas blamed traditional reconstructions of Dimetrodon for the mismatch. Instead they should have looked at other candidate trackmakers from the Early Permian. Note the asymmetric manus and pes of Dimetrodon (Fig. 1). Those don’t match the tracks no matter how high the belly is above the substrate. Dimetrodon is just fine the way it is.

A better match
can be made to the Middle Triassic pre-softshell turtle pareiasaur, Sclerosaurus (Fig. 2). Note the symmetric manus and pes like those of living turtles (Fig. 3) and the Dimetropus specimen in figure 2.

Living turtle tracks
like those of the snapping turtle, Macrochelys (Fig. 3) are also symmetrical and surprisingly narrow gauge. Let’s not forget, Dimetropus tracks occur in Early Permian sediments, predating the earliest fossil turtles, like Proganochelys, first appearing in the Late Triassic. Let’s also not forget, in the large reptile tree (LRT, subset Fig. 7) Proganochelys is not the most basal turtle and valid predecessors (not eunotosaurs) had similar hands and feet.

A second set of Dimetropus tracks
(Fig. 4, right), have distinctive heels behind symmetric + asymmetric imprints. A large Dimetrodon could not have made these tracks because they are too narrow. A small Dimetrodon had extremities that were too small, as the animated GIF shows.

Fortunately,
we also have Middle Permian basal therapsid, Hipposaurus (Figs. 4, 5), a close relative of the last common ancestor of all pelycosaurs (see Haptodus and Pantelosaurus; Fig. 6). No doubt Hipposaurus elevated its torso on a narrow gauge track, with manus tracks slightly wider than pedal traces, as in Dimetropus. Both the carpus and tarsus are elongate, matching Dimetropus tracks.

Unfortunately,
we don’t have all the phalanges for the Hipposaurus manus and pes (Fig. 4). Drag marks can lengthen a digit trace. Flexing a claw into the substrate can shorten a digit trace. It is also important to note that during the last moment of the manus propulsion phase, the medial and lateral metacarpals can rotate axially, creating the impression of an ‘opposable thumb’ in the substrate. Note that no two ichnites are identical, despite being made one after another by the same animal.

At present
a more primitive sister to Hipposaurus is the best match for the Hunt et al. 1995 Dimetropus tracks and the Early Permian timing is right.

In the popular press
NewScientist.com reported, “We’ve drawn iconic sail-wearing Dimetrodon wrong for 100 years. Some palaeontologists did offer an explanation – that Dimetrodon thrashed its spine from side to side so much as it walked that it could leave narrow sets of footprints despite having sprawled legs.” That hypothesis, based on omitting pertinent taxa, is no longer necessary or valid.

Abbott, Sues and Lockwood 2017 reported the limbs of Dimetrodon were morphologically closest to those of the extant Caiman, which sits on its belly, but also rises when it walks.

It is unfortunate that no prior workers considered Hipposaurus, a nearly coeval taxon with Dimetropus having matching slender digits, long legs, an erect carriage, and the just about the right digit proportions.

A third ichnotaxon,
Dimetropus osageorum (Sacchi et al. 2014), was considered a possible caseid, rather than a sphenacodontid, but caseids have more asymmetric digits (= a shorter digit 2). Unfortunately, taxon exclusion also hampered the Sacchi et al. study. They did not consider Early Permian stephanospondylids, Late Permian pareiasaurs in the turtle lineage and Triassic turtles. No skeletal taxon is a perfect match for this ichnotaxon, but the Late Cretaceous turtle, Mongolochelys, is close  (Fig. 8). It took some 200 million years after the trackmaker of Dimetropus for the lateral pedal digits to shrink, but everything else is a pretty good match.

Also compare the hands and feet
of Early Permian Dimetropus osageorum (Fig. 8) to the Middle Triassic Sclerosaurus (Fig. 2). Dimetropus is solid evidence that turtle-ancestor pareiasaurs were present in the Early Permian (see Stephanospondylus, an Early Permian turtle and pareiasaur ancestor).

Saachi et al. conclude, “At the same time, the process of attributing ichnotaxa, on the basis of well preserved tracks and by comparison with known skeletal remains, is validated.”  True. Unfortunately all prior workers overlooked a wider gamut of skeletal taxa to compare with their ichnotaxon in their search for a ‘best match.’ Perhaps they felt restricted by time (Early Permian). As the above notes demonstrate, that is not a good excuse.

References
Abbott CP, Sues H-D and Lockwood R 2017. The Dimetrodon dilemma: reassessing posture in sphenacodonts. GSA annual meeting in Seattle, WA USA 2017. DOI: 10.1130/abs/2017AM-307190
Hunt AP and Lucas SG 1998. Vertebrate tracks and the myth of the belly-dragging, tail-dragging tetrapods of the Late Paleozoic. Bulletin New Mexico Museum of Natural History and Science. 271: 67–69.
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605
Romano M, Citton P and Nicosia U 2015. Corroborating trackmaker identification through footprint functional analysis: the case study of Ichniotherium and Dimetropus. Lethaia https://doi.org/10.1111/let.12136
Romer AS and Price LI 1940. Review of the Pelycosauria: Geological Society of America, Special Paper 28:538pp
Sacchi E, Cifelli R, Citton P, Nicosia U and Romano M 2014. Dimetropus osageorum n. isp. from the Early Permian of Oklahoma (USA): A trace and its trackmaker. Ichnos 21(3):175–192. https://doi.org/10.1080/10420940.2014.933070