You heard it here first: Orovenator had diapsid AND varanopid traits—for good reason!

This is a YouTube video of a
talk given by postgraduate David Ford recorded at The 65th Symposium on Vertebrate Palaeontology and Comparative Anatomy, University of Birmingham. His incredibly detailed  observations found diapsid traits AND varanopid traits, which was cause for consternation. Click to view.

Ford used µCT data
to recover in Ororvenator what the large reptile tree (LRT, 1181 taxa) was able to recover from published drawings. Ford nested Orovenator and Synapsida within Diapsida. Although heretical, that’s not the correct solution when you add more pertinent taxa.

By contrast, in the LRT
basal synapsids split at their genesis between Synapsida and Prodiapsida following Vaughnictis, another late-surviving taxon. Ford was unaware of that split at the time. In the LRT, late-surviving early Permian Orovenator was derived from basal synapsids (varanopids) AND ancestral to basal diapsids like Petrolacosaurus in the Late Carboniferous.

We looked at Orovenator relationships earlier
here in 2014 and here in 2017. Key to testing any taxonomic relationships is appropriate taxon inclusion. Let’s hope Ford has expanded his taxon inclusion set appropriately when the paper comes out. He’s got a good handle on the details, but the big picture evidently was not in his ken due to the exclusion of pertinent taxa.

Figure 2. The Prodiapsida now include the holotypes of Ascendonanus and Anningia.

Figure 2. The Prodiapsida now include the holotypes of Ascendonanus and Anningia.

The Early Permian Ascendonanus assemblage

There are five specimens from the same pit
that were assigned to the varanid taxon Ascendonanus. Spindler et al. 2018 thought they were all conspecific.

Given their distinct proportions
(Fig. 1) and the phylogenetic differences recovered in 2 of the 5 so far (earlier one nested as a basal iguanid), we’re going to need some new generic names for at least one of the referred specimens. The others have not yet been tested in the large reptile tree (LRT, 1179 taxa).

The holotype
remains Ascendonanus, but here it’s no longer a varanopid synapsid. Here it nests as a derived prodiapsid and the basalmost tested diapsid (Fig. 2), a little younger than the oldest diapsid, Petrolacosaurus.

Figure 1. The five specimens from the Ascendonanus quarry, all to the same scale. Most images from Spindler et al. 2018. Some have skulls 3x the occiput/acetabulum length. Others as much as 5x, the first hint that these taxa are no conspecific.

Figure 1. The five specimens from the Ascendonanus quarry, all to the same scale, counter plate flipped in every specimen. Most images from Spindler et al. 2018. Some have skulls 3x the occiput/acetabulum length. Others as much as 5x, the first hint that these taxa are no conspecific.

Some of these specimens
(Fig. 1) have an occiput/acetabulum length distinct from the others, ranging from 3x to 5x the skull length, the first clue to their distinct morphologies.

Figure 2. The Prodiapsida now include the holotypes of Ascendonanus and Anningia.

Figure 2. The Prodiapsida now include the holotypes of Ascendonanus and Anningia. Remember, the Diapsida does not include any Lepidosauriforms, which nest elsewhere.

Spindler et al. 2018
did not include several taxa typically included in pelycosaur studies and should not have included any caseasaurs, despite their traditional inclusion. Spindler et al. did not include any diapsids nor did they understand the role of the former varanopids now nesting as ancestors to the Diapsida (sans Lepidosauriformes).

Figure 3. Cladogram from Spindler et al. 2018. Colors refer to clades in the LRT.

Figure 3. Cladogram from Spindler et al. 2018. Colors refer to clades in the LRT.

The holotype 0924 specimen has more of a varanopid skull
than the 1045 specimen we looked at earlier. Prodiapsid sisters include varanopids ancestral to synapsids. Prodiapsids, as their name suggests, are late-surviving ancestors to diapsids like the coeval Araeoscelis (Early Permian) and the earlier Spinoaequalis (Late Carboniferous).

Figure 3. The Ascendonanus holotype skull as originally traced and as traced here.

Figure 3. The Ascendonanus holotype skull as originally traced and as traced here. Whether an upper temporal fenestra was present (as shown in the color tracing), or not (as shown in the drawings, makes no difference as this taxon nests at the transition. 

Not sure yet
where the other three specimens assigned to Ascendonanus nest. Enough muck stirred for the moment.

References
Rößler R, Zierold T, Feng Z, Kretzschmar R, Merbitz M, Annacker V and Schneider JW 2012. A snapshot of an early Permian ecosystem preserved by explosive volcanism:
New results from the Chemnitz Petrified Forest, Germany. PALAIOS, 2012, v. 27, p. 814–834.
Spindler F, Werneburg R, Schneider JW, Luthardt L, Annacker V and Räler R 2018. First arboreal ‘pelycosaurs’ (Synapsida: Varanopidae) from the early Permian Chemnitz Fossil Lagerstätte, SE Germany, with a review of varanopid phylogeny. DOI: https://doi.org/10.1007/s12542-018-0405-9

Ford and Benson reexamine Orovenator

I have to wonder if workers
are gleaning this blog for taxa in need of study, taxa that were originally poorly nested. Probably just a coincidence that Ford and Benson 2017 took another look at Orovenator (early Permian), which was originally (Reisz et al. 2011) described as the earliest of the ‘Neodiapsida’ (archosaurs, lepidosaurs and turtles).

By contrast
in the large reptile tree (LRT, 1054 taxa) that clade is called ‘Reptilia’ and its earliest known member is Tulerpeton from the Latest Devonian. So ‘Neodiapsida’ is a junior synonym for Reptilia and needs to be ditched. Orovenator is something else.

Figure 1. Orovenator (holotype on right) along with the larger referred specimen (on left, and scaled down to the size of the holotype above right). Arrows point to mismatches.

Figure 1. Orovenator (holotype on right) along with the larger referred specimen (on left, and scaled down to the size of the holotype above right). Arrows point to mismatches.

The authors used two skulls
to recreate a more complete one, pretty much like I avoided three years ago (Fig. 1) because the referred specimen was not a good match. The LRT nests Orovenator with several of the taxa listed below as synapsid-grade pro-diapsids. In other words, they are derived from basal synapsids, like Vaughnictis, but are in the lineage of new archosaurmorph (non-lepidosaur) diapsids, like Eudibamus. This Prodiapsid clade/grade has not been recognized yet by other workers.

From their abstract
“Substantial similarity between early diapsids and varanopid synapsids has previously been noted: four taxa currently assigned to the Varanopidae were originally described as diapsids (Archaeovenator, HeleosaurusApsisaurus) or archosauromorphs (Mesenosaurus). However, our study suggests that these similarities are greater than previously appreciated, raising questions about deep phylogenetic divergences within amniotes.”

And here’s the kicker, again from their abstract
“Application of the new data on Orovenator into an existing phylogenetic matrix intended to discriminate between varanopids and diapsids resulted in an unresolved consensus. This has prompted work on a novel phylogeny, with greater taxonomic scope, to provide a working hypothesis for the interrelationships of Permo-Carboniferous amniotes.”

Yay!
We love adding taxa, increasing the taxonomic scope (gamut) and the novel phylogenies that result. If Ford and Benson recover Prodiapsida from the Synapsida, remember, you heard it here first.

References
Ford DP and Benson RBJ 2017. A re-examination of Orovenator mayorum using µCT data, and its consequences for early amniote phylogeny. SVPCA-SPPC Birmingham abstracts. September 12-15, 2017.
Reisz RR, Modesto SP and Scot DMT 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society, London B

Go back far enough in dinosaur ancestry and you come to: Heleosaurus

With our never-ending fascination with dinosaurs
it’s interesting to list some of the taxa in their deep, deep!, deep!! ancestry. One such ancestor is Heleosaurus (Fig. 1; Broom 1907; Middle Permian ~270 mya, ~30 cm snout to vent length), the first known basal prodiapsid, the clade the includes diapsids (sans lepidosaurs, which are unrelated but share the same skull topology by convergence).

Figure 1. Heleosaurus is closer to the main lineage of dinosaurs. It retained canine fangs.

Figure 1. Heleosaurus is close to the main lineage of dinosaurs. It retained canine fangs. Note the squamosal distinct from the quadratojugal, as in Nikkasaurus. Also note the continuing lacrimal contact with the naris, as in Protorothyris.

But first
I want to discuss a derived Heleosaurus cousin, Nikkasaurus (Ivahnenko 2000; Fig. 2), also one of the most basal prodiapsids.

It is only by coincidence
that Ivahnenko labeled Nikkasaurus one of his ‘Dinomorpha,’ a clade name ignored by other authors. Wikipedia considers Nikkasaurus one of the Therapsida and possibly a relic of a more ancient stage of therapsid development. Like Heleosaurus, Nikkasaurus had a single synapsid-like lateral temporal fenestra. Only their nesting outside of that clade and basal to the clade Diapsida in the LRT tell us what they really are. Most of the time, as you know, we can tell what a taxon is simply by looking at it. In this case, as in only a few others, we cannot do so readily.

Figure 1. Nikkasaurus, one of the most primitive prodiapsids, direct but ancient ancestors of dinosaurs.

Figure 2. Nikkasaurus, one of the most primitive prodiapsids, direct but ancient ancestors of dinosaurs.

Nikkasaurus tatarinovi (Ivahnenko 2000) Middle Permian was a tiny basal prodiapsid with a large orbit. It retained a large quadratojugal. The fossil is missing the squamosal. Others mistakenly considered the quadratojugal the squamosal, as in therapsids. That’s an easy mistake to make. Compare this bone to the QJ in Heleosaurus (Fig. 1), another prodiapsid. Nikkasaurus has small sharp teeth and no canine fang. Nikkasaurus is a sister to Mycterosaurus. They both share a large orbit and fairly long snout. What appears to be a retroarticular process may be something else awaiting inspection in the actual fossil. Based on all other data points, I don’t trust that post-dentary data. It doesn’t match the in situ figure.

Distinct from other prodiapsids,
the Nikkasaurus, Mycerosaurus and Mesenosaurus maxilla extended dorsally, overlapping the lacrimal and contacting the nasal, as it does in Dimetrodon and basal therapsids like Hipposaurus and Stenocybus. This trait tends to be homoplastic / convergent in all derived taxa, but the timing differs in separate clades.

Figure 1. Nikkasaurus and what little is known of its postcrania. Above, in situ. Below, tentative reconstruction. If anyone has a picture of the fossil itself, please send it.

Figure 2. Nikkasaurus and what little is known of its postcrania. Above, in situ. Below, tentative reconstruction. If anyone has a picture of the fossil itself, please send it. Note the posterior mandible mismatch in the purported retroarticular process. I suspect the process is not there.

And finally we come back to Heleosaurus.
Slightly closer to the lineage of dinosaurs is the slightly more basal prodiapsid, Heleosaurus (Fig. 2), which retained canine fangs, had a more typical posterior mandible and retained a lacrimal / naris contact. This naris trait was retained by Petrolacosaurus, Eudibamus, Spinoaequalis and other basal diapsids (archosauromorpha with both upper and lateral temporal fenestra ). The maxilla did not rise again to cut off lacrimal contact with the naris in the ancestry of dinosaurs until the small Youngina specimens huddled together, SAM K 7710 and every more derived taxon thereafter, up to and including dinos.

References
Broom R 1907. On some new fossil reptiles from the Karroo beds of Victoria West, South Africa. Transactions of the South African Philosophical Society 18:31–42.
Ivahnenko MF 2000. 
Cranial morphology and evolution of Permian Dinomorpha (Eotherapsida) of eastern Europe. Paleontological Journal 42(9):859-995. DOI: 10.1134/S0031030108090013