What is Polymorphodon adorfi?

Sues et al. 2020 bring us a new Middle Triassic German diapsid,
Polymorphodon adorfi (Fig. 1; SMNS 91343, SMNS 91400) with a large toothy premaxilla and a hint of an antorbital fenestra.

Figure 1. Skull elements of Polymorphodon.

Figure 1. Skull elements of Polymorphodon. Consider the possibility that the quadrate had an anterior lean, as in figure 2,, elongating the postorbital region of the skull.

At first glance Polymorphodon looks a lot like
Archosaurus (Fig. 2; Late Permian, eastern Europe), a taxon not yet tested in the LRT due to a paucity of material.

Figure 2. Archosaurus is not in the LRT, but shares several traits with Polymorphodon.

Figure 2. Archosaurus is not in the LRT, but shares several traits with Polymorphodon.

From the Sues et al. abstract
“Skeletal remains of a small reptile with a distinctive dentition from the Lower Keuper (Erfurt Formation; Middle Triassic, Ladinian) of the Schumann quarry near Eschenau, in the municipality of Vellberg in Baden-WÃrttemberg, Germany, represent a new taxon of non-archosaurian archosauriforms, Polymorphodon adorfi.”

That’s a wee bit general. Let’s see if the large reptile tree (LRT, 1698+ taxa; subset Fig. 5) can nest it more precisely.

The Sues et al. abstract continues:
“It is diagnosed by various craniodental autapomorphies, including mesial and distal carinae of labiolingually flattened maxillary and dentary tooth crowns with large, somewhat hook-shaped denticles aligned at distinct angle to apicobasal axis of tooth crown; premaxilla with long, leaf-shaped posterodorsal process that is slightly longer than body of element; presence of prominent lateral fossa on premaxilla anteroventral to external narial fenestra; premaxilla with five gently recurved, conical teeth; medial surface of maxilla with distinct ledge above the interdental plates; and maxilla and dentary with distinctly heterodont dentition”

The Sues et al. diagnosis is focusing on the dentition, plus the premaxilla and maxilla. Again, not much to work with, even though quite distinctive.

The Sues et al. abstract continues:
“Phylogenetic analysis recovered Polymorphodon adorfi in a position crownward of Erythrosuchus africanus but in an unresolved polytomy with derived non-archosaurian archosauriforms such as Proterochampsidae and Euparkeria capensis and with Archosauria.”

The first red flag: Proterochampsidae is not related to Euparkeria in the LRT. Simply add taxa to fix this.

The Sues et al. abstract continues:
“The maxillary and dentary teeth of Polymorphodon adorfi differ from those of other non-archosaurian archosauriforms and indicate a different, possibly omnivorous diet, suggesting that these reptiles were more diverse in terms of feeding habits than previously assumed.”

This abstract plus Wikipedia information plus results from the LRT indicate taxon exclusion is the issue here.

In the LRT (subset Fig. 5, not yet updated)
Polymorphodon nests at the base of the Pararchosauriformes, basal to all the many included proterosuchids, choristoderes, phytosaurs and proterochampsids in that order (Fig. 5). In the LRT the clade Pararchosauriformes is a sister to the clade Euarchosauriformes, which begins with two specimens of Euparkeria and ends with crocs, dinos and birds. All these are derived from the UC 1528 specimen of Youngoiides (Fig. 3), the most derived of the various non-archosauriform younginids.

Figure 3. Cladogram on the Polymorphodon Wikipedia page based on Ezcurra 2016. Note the lack of taxa preceding the taxon "Proterosuchidae", which is where the LRT nests Polymorphodon.

Figure 3. Cladogram on the Polymorphodon Wikipedia page based on Ezcurra 2016. Note the lack of taxa preceding the taxon “Proterosuchidae”, which is where the LRT nests Polymorphodon.

So, yes, taxon exclusion is the problem
with the Sues et al. 2020 cladogram based on the Ezcurra 2016 cladogram, which suffered from taxon exclusion, as detailed here four years ago.

Polymorphodon is a late survivor
of a Late Permian radiation and is a key taxon with many pararchosauriform descendants. This hypothesis of relationships was overlooked by the original authors due to taxon exclusion.

Figure 3. Updated image of various proterosuchids and their kin. When you see them all together it is easier to appreciated the similarities and slight differences that are gradual accumulations of derived taxa.

Figure 4. Updated image of various proterosuchids and their kin within the LRT clade, Pararchosauriformes. When you see them all together it is easier to appreciate the similarities and slight differences that are gradual accumulations in derived taxa.

I still have not seen the Sues et al. 2020 PDF.
When it arrives (see below) we’ll see if it includes Youngoides, Archosaurus and many of the pertinent taxa in figure 4. Since they are using Ezcurra 2016 the odds are reduced. For now the nesting of Polymorphodon in the LRT is more certain and more stem-ward than originally proposed (Fig. 3).

Figure 2. Cladogram of basal archosauriforms. Note the putative basalmost archosauriform, Teyujagua (Pinheiro et al 2016) nests deep within the proterosuchids. The 6047 specimen that Ewer referred to Euparkeria nests as the basalmost euarchosauriform now.

Figure 5. Cladogram of basal archosauriforms from 2016. Polymorphodon nests basal to Proterosuchus BPI 1 4016, awaiting an update soon.

Adding taxa solves so many problems.
Not sure why academics are hesitating to do what needs to be done. Sure it’s hard work, but it only has to be done once.

Via email
Hans Sues indicated that a PDF of the paper will be ready within a week due to some publisher issues linking the supplemental information. We may explore this taxon again if that data provides information not available from present sources. For now, only a little data from a new taxon was enough to nest it with confidence, so long as taxon exclusion is minimized, as it is in the LRT. This helpful online resource is free for all to use.


References
Sues H-D, Schoch RR, Sobral G and Irmis RB 2020. A new archosauriform reptile with distinctive teeth from the Middle Triassic (Ladinian) of Germany. Journal of Vertebrate Paleontology Article: e1764968 (advance online publication)
doi: https://doi.org/10.1080/02724634.2020.1764968
https://www.tandfonline.com/doi/abs/10.1080/02724634.2020.1764968Â

wiki/Polymorphodon

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