Kopidosaurus: no longer an enigmatic iguanian

Scarpetta 2020 bring us a tiny new Eocene lizard,
Kopidosaurus perplexus (YPM VP 8287) known from most of a disarticulated skull still in the matrix, but carefully presented in several views as µCT scans (Fig. 1).

Scarpetta warns his readers,
“Fossil identifications made in a phylogenetic framework are beholden to specific tree hypotheses. Without phylogenetic consensus, the systematic provenance of any given fossil can be volatile. Pleurodonta (Squamata: Iguania) is an ancient and frequently-studied lizard clade for which phylogenetic resolution is notoriously elusive.

Scarpetta reports,
“I address the effects of three molecular scaffolds on the systematic diagnosis of that fossil. I use two phylogenetic matrices, and both parsimony and Bayesian methods to validate my results, and perform Bayesian hypothesis testing to evaluate support for two alternative hypotheses of the phylogenetic relationships of the new taxon.”

Scarpetta did not provide a reconstruction of the skull. That is remedied here (Fig. 1).

Unfortunately Scarpetta’s three molecular scaffolds are based on genes, so they are completely useless for deep time studies, as documented several times in vertebrates. None of the three genomic studies in Scarpetta 2020 agree with each other. None agree with the LRT (Fig. 2).

Unfortunately Scarpetta’s phylogenetic analyses result in lists of suprageneric taxa, not genera, as in the LRT. Scarpetta reports, “The uncertainty of the relationships of Kopidosaurus is due in part to the mosaic morphology of the fossil and the problematic nature of pleurodontan phylogeny.”

There is no such thing as mosaic evolution. So stop using that excuse.

It doesn’t have to be this complicated. Use the LRT. It’s simple. Just Plug ‘n’ Play.

Figure 1. Kopidosaurus perplexus in situ and µCT scans from Scarpetta 2020. Reconstruction added here.

Figure 1. Kopidosaurus perplexus in situ and µCT scans from Scarpetta 2020. Reconstructions in lateral and palatal views added here.

Scarpetta reports,
“YPM VP 8287 preserves no morphological feature or combination of features that would allow clear referral to any member of Pleurodonta.” And that’s why he shouldn’t be “Pulling a Larry Martin” (relying on key traits that might converge). Instead: drop the new taxon into a comprehensive cladogram, like the LRT (Fig. 2), and let the software nest the enigma.

Definition according to Wikipedia:
Pleurodonta (from Greek lateral teeth, in reference to the position of the teeth on the jaw) is one of the two subdivisions of Iguania, the other being Acrodonta (teeth on the top [of the jaw]). Pleurodonta includes all families previously split from Iguanidae sensu lato (CorytophanidaeCrotaphytidaeHoplocercidaeOpluridaePolychrotidae, etc.), whereas Acrodonta includes Agamidae and Chamaeleonidae.”

The frontal and parietal are incomplete
and the skull is small at <2cm. Am I the first to wonder if this was a juvenile skull? Scarpetta does not bring up the subject. The large orbit relative to skull length supports that hypothesis. Otherwise this could be an adult in the process of phylogenetic miniaturization, common at the genesis of many clades (Fig. 2).

Scarpetta concluded,
“Given the phylogenetic volatility of Kopidosaurus, I refrain from favoring any biogeographic or divergence hypothesis based on the identification of the fossil and advise similar caution for other systematically enigmatic fossils, lizard or otherwise.”

Don’t give up! Use the LRT.

in the large reptile tree (LRT, 1740+ taxa) Eocene Kopidosaurus nests at the base of a clade of living iguanians (Fig. 2). It is a plesiomorphic taxon, but that doesn’t matter to the LRT. Only a suite of characters is able to nest Kopidosaurus with this level of confidence by minimizing taxon exclusion.

Figure 2. Subset of the LRT focusing on basal Squamata. Here Kopidosaurus nests at the base of a clade of living iguanians including Pristidactylus and Anolis.

Figure 2. Subset of the LRT focusing on basal Squamata. Here Kopidosaurus nests at the base of a clade of living iguanians including Pristidactylus, Basisliscus and Anolis.

As a reminder,
the LRT is still using just 238 traits, most of which were not used here due to the lack of a premaxilla, vomer and post-crania. Paleontologists still don’t want to accept the fact that the LRT continues to lump and separate with so few multi-state characters. Even those taxa previously tested without resolution, as described by Scarpetta 2020.

Scarpetta SG 2020. Effects of phylogenetic uncertainty on fossil identification illustrated
by a new and enigmatic Eocene iguanian. Nature.com/scientifcreports 10:15734.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.