Looks like a great fossil,
but the squamosal in the chroniosuchid PIN 3585 ⁄ 124 (Figs. 1, 2) is missing and it’s hard to tell the sutures from the cracks. Clack and Klembara (2009) called this specimen Chroniosaurus. But it nests with Chroniosuchus (Fig. 2) in the large reptile tree (not updated yet). This is the juvenile described by Clack and Klembara (2009) and Klembara et al. (2010), about the size of the holotype (Tverdokhlebova 1972).
Clack and Klembara (2009 ) nested chroniosuchids with Silvanerpeton, Eoherpeton and Gephyrosaurus, but also nested amniotes with microsaurs. So there’s a red flag due to taxon exclusion. Golubev (1998) nested chroniosuchids with the anamniotes. In the large reptile tree (not updated yet)chroniosuchids nest with two other amniotes, Solenodonsaurus and Brouffia, two taxa not included in Clack and Klembara (2009). This happens too often. Once again, the inclusion set was too small. Clack and Klembara (2009) concluded, “If chroniosuchians are not derived embolomeres, they remain an enigmatic group of stem amniotes whose biogeographic and phylogenetic origins are unresolved.”
The skull roof is a problem. Which bones are present? Clack and Klembara described the bones and illustrated them, but did not label the illustration. Here it is labeled (Fig. 3).
The lacrimal doesn’t contact the orbit in the Klembara and Clack reconstruction, but the prefrontal only overlaps the lacrimal in the fossil (Fig. 1). This process is completed in the textbook Chronisaurus and Chroniosuchus (Fig. 1). The nasal is broader at mid length in the fossil, but not in the Klembara and Clack reconstruction. The parietal is also broader in the fossil. Fewer and not so long and pointed teeth appear in the fossil. Finally the postfrontal has a different shape in the fossil, ever so slightly convex anteriorly.
Free-handing the reconstruction may be partly to blame here. DGS removes a certain amount of handiwork from reconstructions.
It’s a shame that the best data for the older Chroniosuchus and Chroniosaurus are line drawings. If anyone has photos of these specimens pass them on. Comparisons sometimes help figuring out the sutures from the cracks.
If you can’t tell a chroniosaurid from a chroniosuchid, or any of the other closely related types, Golubev (1998) used “(1) scute width; (2) scute sculpturing type; (3) skull surface sculpturing type; (4) presence and traits of the sculptural crests on the skull roof; (5) relative size of inter orbital space. The general chroniosuchid evolutionary direction was displayed by adult size increase, change of the dermal skull and scute armor ornament from pustular to pitted type, reduction of interorbital space, and beginning of the dorsal armor reduction in the late phylogenetic stages. Great difficulties arise in the definition of the specific position of intermediate forms.“
Buchwitz M and Voigt S 2010. Peculiar carapace structure of a Triassic chroniosuchian implies evolutionary shift in trunk flexibiliy. Journal of Vertebrate Paleontology30(6):1697-1708.
Clack JA and Klembara J 2009. An articulated specimen of Chroniosaurus dongusensis and the morphology and relationships of the chroniosuchids. Special Papers in Palaeontology, 81: 15–42.
Golubev VK 1998. Revision of the Late Permian Chroniosuchians (Amphibia, Anthracosauromorpha) from Eastern Europe. Paleontological Journal 32(4):390-401.
Klembara J, Clack J, and Cernansky A 2010. The anatomy of the palate of Chroniosaurus dongusensis (Chroniosuchia, Chroniosuchidae) from the Upper Permian of Russia. Palaeontology 53: 1147-1153.
Schoch RR, Voig S and Buchwitz M 2010. A chroniosuchid from the Triassic of Kyrgyzstan and analysis of chroniosuchian relationships. Zoological Journal of the Linnean Society 160: 515–530. doi:10.1111/j.1096-3642.2009.00613.x
Tverdochlebova GI 1972. A new Batrachosaur Genus from the Upper Permian of the South Urals, Paleontol. Zh., 1972: 95–103.
How the squamosal went missing is easy to explain: the whole area is thin and just eroded. (Happened with the lacrimal in Varanodon, giving it a wholly spurious antorbital fenestra and convincing poor Peter Vaughn, in 1965, that it was related to Archosauriformes.) You’ll notice that it’s not precisely the squamosal that’s missing; parts of the postorbital and supratemporal are gone, too, while a few fringes of the squamosal are still there (pinkish in your tracing).
I’ve read the paper those reconstructions are from (in Russian, from 1980). They’re not specimen drawings. They’re extrapolated from badly preserved fragments. The text doesn’t help either, describing e.g. “the chroniosuchian lower jaw” without mentioning which parts are actually known in which species. Don’t trust them, don’t use them for phylogenetics; I now have a paragraph in my 2016 preprint explaining why I didn’t use them.
Speaking of reconstructions, the overlap of the prefrontal on the lacrimal you illustrate is gigantic. I strongly suspect you’ve misinterpreted the photo.
Thanks for the updates. That prefrontal interpretation stands until better data comes along. I will look for it!