Grand Canyon tetrapod tracks: odd, or misinterpreted?

Short summary for those in a hurry.
There are several reasons to think only the original interpretation (Fig. 1) is odd.

Rowland, Caputo and Jensen 2020 bring us their interpretation
of an odd 313mya trackway (Figs. 1–3) from the latest Early Carboniferous (Pennsylvanian) in Grand Canyon National Park (AZ, USA).

Figure 1. Animation of the interpretation of Rowland, Caputo and Jensen 2020 of the Grand Canyon Early Carboniferous trackmaker.

Figure 1. Animation of the interpretation of Rowland, Caputo and Jensen 2020 of the Grand Canyon Early Carboniferous trackmaker.

From the abstract
“We report the discovery of two very early, basal-amniote fossil trackways on the same bedding plane in eolian sandstone of the Pennsylvanian Manakacha Formation in Grand Canyon, Arizona. 

Only trackway 1 (Figs. 1–3) is under review here. And that may be more than one trackway.

It displays a distinctive, sideways-drifting, footprint pattern not previously documented in a tetrapod trackway. We interpret this pattern to record the trackmaker employing a lateral-sequence gait while diagonally ascending a slope of about 20˚, thereby reducing the steepness of the ascent.”

Only one interpretation was provided by the authors. Here you’ll see one more.

“These trackways are the first tetrapod tracks reported from the Manakacha Formation and the oldest in the Grand Canyon region. The narrow width of both trackways indicates that both trackmakers had relatively small femoral abduction angles and correspondingly relatively erect postures.”

Is this the correct interpretation? Another (Fig. 2) is presented.

“They represent the earliest known occurrence of dunefield-dwelling amniotes―either basal reptiles or basal synapsids―thereby extending the known utilization of the desert biome by amniotes, as well as the presence of the Chelichnus ichnofacies, by at least eight million years, into the Atokan/Moscovian Age of the Pennsylvanian Epoch.”

“The depositional setting was a coastal-plain, eolian dunefield in which tidal or wadi flooding episodically interrupted eolian processes and buried the dunes in mud.”

Could these tracks be interpreted more parsimoniously?
What if the trackmaker was just an ordinary lissamphibian, like Celtedens (Figs. 2, 6) or a reptilomorph, like Amphibamus (Fig. 4)? Both have more of a matching manus and pes than any coeval amniote (details below). What if the trackmaker had a more sinuous spine, similar to that of coeval tetrapods and Celtedens or Amphibamus? What if the trackmaker had sprawling limbs, similar to other coeval tetrapods and Celtedens or Amphibamus? The trackmaker did not leave a tail drag mark, so what if the trackmaker had a short tail, like Celtedens or Amphibamus? What if there were multiple trackmakers? Is it possible that one or two trackmakers closely followed another one in a mating ritual or pursuit?

Figure 2. The Chelichnus-like tracks together with Celtedens, an amphibian trackmaker with a short tail, sinuous spine, splayed limbs and fewer digits than coeval amniotes.

Figure 2. The Chelichnus-like tracks together with Celtedens, an amphibian trackmaker with a short tail, sinuous spine, splayed limbs and fewer digits than coeval amniotes. The unused tracks would have been created by a pursuing Celtedens-like trackmaker.

If so, 
here’s an animation based on an alternate taxon walking in a more typical fashion (Fig. 2) with less freehand invention. More splayed limbs and a sinuous spine are employed here matching coeval tetrapods in morphology and gait. In this scenario the unused tracks (Fig. 2) were created by a second and third Celtedens-like trackmaker pursuing in lock step with the first trackmaker.

Figure 3. Imagery from Rowland, Caputo and Jensen 2020, with color overlays and PILs added.

Figure 3. Figure from Rowland, Caputo and Jensen 2020, with color overlays and PILs added at left.

From the first line of the Introduction
Amniotes evolved early in the Pennsylvanian or late in the Mississippian Epoch.”

The authors were so sure the tracks were made by amniotes
they plugged the word “Amniotes” into the first line of the Introduction. In the large reptile tree (LRT, 1725+ taxa) we have several amniotes (= reptiles) from the EARLY Mississippian (Viséan). These were overlooked by Rowland, Caputo and Jensen 2020. The authors and those they cited were not up to date with the most recent phylogenic hypotheses of interrelationships.

Still on the subject of amniotes, the authors note,
“Because this trackway records the presence of relatively long digits with acuminate claws, we infer that the trackmaker was an amniote.” The Early Cretaceous lissamphibian, Celtedens (Figs. 2, 5) and the Late Carboniferous reptilomorph, Amphibamus (Fig. 4), also have long, slender digits with claws that taper to a point. The longest digits are medial on each manus and pes. Amniotes had more asymmetrical extremities with digit 4 typically the longest. The authors followed their initial bias and did not consider morphologically similar, but phylogenetically dissimilar trackmakers.

The keyword “Lissamphibian”
is not found in the Rowland, Caputo and Jensen text. Celtedens is a lissamphibian known only from two Early Cretaceous specimens. However, given the presence of related Gerobatrachus, Apteon and Doleserpeton specimens in the Early Permian, the radiation of Celetedens-like taxa was likely in the Carboniferous. The Late Carboniferous basal reptilomorph, Amphibamus, is likewise not mentioned and was not considered a potential trackmaker despite its appropriate match both morphologically and temporally.

The manus and pes of the Grand Canyon trackmaker 
were nearly equal in size. The pes in Carboniferous amniotes is typically larger than then manus. The authors agree, noting, “the manus prints of the Manakacha tracks are not conspicuously smaller than the pes prints, contrary to the typical pattern in Chelichnus.” Celtedens also has a pes that is larger than the manus, but the lissamphibians Apteon, Doleserpeton and Triassurus have subequal extremities. So does the reptilomorph, Amphibamus (Fig. 4).

Figure 4. Late Carboniferous Amphibamus is a potential trackmaker for the Grand Canyon latest Early Carboniferous tracks. with medial digits the longest, like the trackmaker. 

Figure 4. Late Carboniferous Amphibamus is a potential trackmaker for the Grand Canyon latest Early Carboniferous tracks. with medial digits the longest, like the trackmaker.

The digits of the trackmaker
were relatively symmetrical with digits 2, 3 and 4 making impressions. The digits in Carboniferous amniotes are typically asymmetrical with 4 the longest and largest. The authors note, “Chelichnus tracks typically consist of only three or four digits of a pentadactyl trackmaker.” 

Taxonomic affinity of the trackmaker
The authors report, “Impressions of three digits are present in each track (Figs 4 and 6), however no plausible Pennsylvanian candidate trackmaker taxon was tridactyl. Thus, we interpret the prints to be shallow undertracks made by a pentadactyl animal whose lateral digits were not impressed deeply enough into the sediment to translate into the preserved bedding plane. Without impressions of all five digits on each foot we are unable to measure foot slenderness and other characters that are useful for distinguishing among the tracks of various basal amniote taxa.”

Figure 4. Microbrachis slightly revised with a new indented supratemporal here rotated to the lateral side of the skull above the squamosal and quadratojugal. Otherwise this image is from Carroll, who did not indent the supratemporal.

Figure 5. Microbrachis slightly revised with a new indented supratemporal here rotated to the lateral side of the skull above the squamosal and quadratojugal. Otherwise this image is from Carroll, who did not indent the supratemporal.

Yes, three digits is a little unsettling for a tetrapod trackmaker. 
The Middle Pennsylvanian microsaur, Microbrachis (Fig. 5), had a three digit manus and a five digit pes with #1 and #5 smaller than the medial digits, but these were mere vestiges, unable to support the animal on a terrestrial substrate.

Figure 3. Celetendens is the closest relative to Karaurus in the LRT.

Figure 6. Celetendens is the closest relative to Karaurus in the LRT.

Celtedens and Amphibamus had four fingers and five toes. 
So, they are not a perfect match for the Grand Canyon trackmaker, but they are close, at least one finger closer than any coeval amniote. Early Cretaceous Celtedens (Fig. 5) is too small to be the trackmaker. However, two hundred million years separate the two. On the other hand, Amphibamus (Fig. 4) is a better size match to the trackmaker and much closer temporally/stratigraphically.

The authors note, 
“a lateral-sequence gait is the most parsimonious footfall-sequence interpretation that is compatible with the pattern of tracks in this trackway. Tetrapods, in fact, routinely use a lateral-sequence gait when walking slowly; while one foot is off the ground, this gait provides a larger stable triangle than other footfall sequences. Moreover, a lateral-sequence gait facilitates undulations of the spine, which lengthen the step.” 

Actually, the diagram provided by Rowland, Caputo and Jensen minimizes undulations of the spine and the steps are not lengthened, but shortened. What they inadvertently describe is the more parsimonious and typical movement of the lissamphibian Celtedens presented here (Fig. 2).

“As indicated by expulsion rims adjacent to many of the tracks (Figs 4,5B,5C and 6), interpreted to occur on the downhill side, the trackmaker’s body was oriented straight up the slope.”

In figure 2 the Celtedens-like tetrapod also ascends the hill, but diagonally, taking big steps, not tiny lateral steps.

“Fossil trackways that record diagonal movement on the slope of a sand dune are common in the ichnology literature.”

“Francischini et al. documented an occurrence within the Permian eolian Coconino Sandstone of Arizona in which the angle of progression of an Ichniotherium trackway―inferred to have been made by the diadectid reptiliomorph Orobates ―differs markedly from the angle that the feet were pointing, similar to the case documented here in the Manakacha Formation. However, none of such previously documented cases of a tetrapod moving diagonally across the face of a sand dune record such a regular pattern of impressions of all four feet, as does Trackway 1 described here, and none have been interpreted to record a lateral-sequence gait.”

The possibility of two or three trackmakers in quick succession creating trackway 1 did not occur to the authors of this paper.

The possibility of an Amphibamus-like or Celtedens-like trackmaker did not occur to the authors of this paper. Instead they went straight for an imagined, headline-generating anachronistic atypical trackmaker, a taxon not present in coeval strata walking unlike any known taxon past or present.

Best to go with Occam’s razor and maximum parsimony.

Add taxa, especially when matching tracks to trackmakers, to make sure you don’t overlook more obvious matches.

Add pursuing trackmakers if there are too many tracks for one ordinary trackmaker.


References
Rowland SM, Caputo MV and Jensen ZA 2020. Early adaptation to eolian sand dunes by basal amniotes is documented in two Pennsylvanian Grand Canyon trackways. PLoSONE 15(8): e0237636. https://doi.org/10.1371/journal.pone.0237636

The first four citations found in Rowland, Caputo and Jensen 2020:
Ahlberg PE and Milner AR 1994. The origin and early diversification of tetrapods. Nature 1994; 368: 507–514.
Clack JA 2002. Gaining Ground: the origin and evolution of tetrapods. Bloomington: Indiana University Press.
Benton MJ. 2005. Vertebrate Palaeontology. 3rd ed. Blackwell Science.
Ford DP and Benson RBJ 2020. The phylogeny of early amniotes and the affinities of Parareptilia and Varanopidae. Nature Ecology & Evolution 2020; 4: 57–65.

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