‘Tracking’ the Origin and Early Evolution of Reptiles

From the Marchetti et al. 2021 abstract,
“The origin of Reptilia and the biostratigraphic and palaeobiogeographic distribution
of its early representatives are still poorly understood.”

Not true. The large reptile tree (LRT, 1899+ taxa; subset Fig. 2) documents the origin of Reptilia with clarity, and takes it back to Silurian fish and Ediacaran worms. So the origin of Reptilia is well understood, just not by Marchetti et al., as they confess above. Sometimes academic authors make situations seem worse so they can seem better. Other times authors overlook and omit. Everyone in paleo hopes to shed new light on the origin of clades. This time that light was shed years ago in the LRT (subset Fig. 2) by standard phylogenetic analysis.

“An independent source of information may come from the extensive Carboniferous footprint record of reptiles, which is arguably richer and more complete than the skeletal record.”

Trying to figure out which taxa first produced amniote eggs by looking at their footprints (Fig. 1) while omitting their skeletons seems to be the wrong way to go about achieving that goal. What were they thinking? Lead author, Lorenzo Marchetti, is a PhD at the Museum für Naturkunde in Berlin. Ironically, a taxon close to the origin of reptile in the LRT, Gephyrostegus, is housed at the same museum. This taxon is mentioned only once in Marchetti et al. 2021.

Marchetti et al. 2021 report,
“In fact, a large part of the supposed early reptile tracks can be assigned to the anamniote ichnotaxon Hylopus hardingi.” See figure 1, lower right photo + graphic, with expanded toe tips and without ungual marks.

Basal reptiles have five fingers,
inherited from their seymouriamorph and reptilomorph ancestors. Five fingers are also found in a few basal tetrapods (Fig. 2) like Proterogyrinus, Parotosuchus and Cacops. Fingers are notoriously absent in many basal tetrapod fossils, so a few others may have had five fingers.

Figure 2. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Marchetti et al. 2021 report,
“Here, we revise the ichnotaxon Hylopus hardingi based on anatomy-consistent material, attribute it to anamniote reptiliomorphs, and distinguish it from Notalacerta missouriensis, the earliest ichnotaxon that can be attributed to reptiles, and the somewhat younger Varanopus microdactylus (attributed to parareptiles, such as bolosaurians) and Dromopus lacertoides (attributed to araeoscelid reptiles and non-varanodontine varanopids).”

A few misunderstandings here.
Parareptilia is a paraphyletic clade (e.g. mesosaurs are not related to pareiasaurs).
No bolosaur preserves post-crania, let alone manus and pes data. Eudibamus is often considered a bolosaur, but when tested with other taxa nests with basal diapsids, like Petrolacosaurus (Fig. 1).
Non-varanodontine varanopids are not varanopids, nor are they synapsids. The LRT shows these taxa are proto-diapsids, still retaining a synapsid skull architecture. Remember traits do not determine clade membership (that would be “Pulling a Larry Martin“). Only a last common ancestor determines clade membership.

Marchetti et al. 2021 report,
“The origin of reptiles has been consistently placed in the Carboniferous, and more specifically, a late Mississippian to Early Pennsylvanian origin of this group is inferred (e.g., Didier and Laurin, 2020; Pardo et al., 2020 and references therein).”

Note that Marchetti et al. do not list a last common ancestor for the clade Reptilia. Why? Because they don’t know. They don’t present a cladogram. The last common ancestor method is the only way to find out which taxon evolved the ability to produce amniote eggs (Fig. 2). This paper should never have been started without this key bit of knowledge. (And it was under their nose the whole time, in the vaults of their own museum).

“The skeletal record of the earliest reptiles is, however, rather incomplete, scattered and in need of revision (e.g., Mann et al., 2019).”

This is how paleontologists blow smoke to cover their own incomplete and scattered knowledge of reptile origins. The skeletal record (Fig. 3) is actually quite good!

Figure 3. Basal amniotes to scale. Gephyrostegus is at upper right. Click to enlarge.

Marchetti et al. 2021 report,
“Moreover, the phylogeny of the earliest representatives of reptile groups is often unclear or not fully resolved, because of the overall similarity among these forms and the difficult distinction from closely related anamniote reptiliomorphs and synapsids (e.g., Ford and Benson, 2020).”

More smoke. Actually (when you do the work) the phylogeny is quite clear and fully resolved. Marchetti et al. need to do the work.

“An independent source of information may come from the tetrapod footprint record”

Given the choice between skeletons and footprints, Marchetti et al. choose footprints. That was not a good decision. Fewer precise data points, more muddy blur. Nothing to do with egg-laying.

Marchetti et al. provide several manus and pes skeletal reconstructions,
that more or less match Carboniferous ichnites. These are presented side-by-side, which invites comparisons (imagine your eyeballs going back and forth looking for unlisted differences and similariites). The authors’ presentation does not break down phalangeal lengths or provide PILs as in Peters 2000. Pes and manus data were presented for the archosauromorph reptiles, Casineria, Anthracodromeus, Erpetonyx, Eudibamus, Paleothyris, Petrolacosaurus, Araeoscelis and Mesenosaurus. None were presented for lepidosauromorph reptiles. The other two skeletal illustrations were basal tetrapods, Proterogyrinus and Greererpeton. Neither nest anywhere close to the origin of reptiles in the LRT (subset Fig. 2).

See why it is so important to have a valid cladogram?
In their quest for the origin of the amniotic membrane, Marchetti et al. not only wasted their time looking at footprints from unrelated taxa, they did not look at skeletons of pertinent taxa. Marchetti et al. should not have called their paper ‘Tracking the Origin and Early Evolution of Reptiles’, then omit the taxa that actually nest at the origin and early evolution of reptiles (Figs. 2, 3) because they restricted their search to footprint data. They should have performed an analysis of skeletal data to determine the last common ancestor of all amniotes. Footprints can’t help recover when ovaries start producing special membranes.

References
Marchetti L, Voigt S, Buchwitz M, MacDougall MJ, Lucas SG, Fillmore DL, Stimson MR, King OA, Calder JH and Fröbisch J 2021. Tracking the Origin and Early Evolution of Reptiles. Front. Ecol. Evol. 9:696511. doi: 10.3389/fevo.2021.696511
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.