Laosuchus naga enters the LRT

Updated January 30, 2019
with a new nesting of Laosuchus between Eryops and the Cochleosaurus clade, not as a chroniosuchid, as originally nested.

The question today is:
what are chroniosuchians? Are they reptiles or not? Arbez, Sidor and Steyer 2018 say: ‘not’ (Fig. 1). Here that mistake is due to tradition and taxon exclusion, based on their cherry-picked outgroups. Heretically. chroniosuchians are amphibian-like reptiles.

Figure 1. Cladogram from xx 2018 with Laosuchus nesting with chroniosuchians in the absence of Solenodonsaurus.

Figure 1. Cladogram from Arbez, Sidor and Steyer 2018 with Laosuchus nesting with chroniosuchians in the absence of Solenodonsaurus.

Arbez, Sidor and Steyer report from their abstract:
“Chroniosuchians were a clade of non-amniotic tetrapods known from the Guadalupian (middle Permian) to Late Triassic, mainly from Russia and China.” Asaphestera is the proximal outgroup followed by Limnoscelis, Seymouria, Gephyrostegus and other taxa.

By contrast and using more outgroup taxa
the large reptile tree (LRT 1391 taxa, ) nests chroniosuchians within the base of the archosauromorph branch of reptiles. When more taxa are included in the LRT, Limnoscelis and Gephyrostegus nest as reptiles (= amniotes) while Asaphestra and Seymouria nest as unrelated traditional microsaur lepospondyls and seymouriamorphs respectively.

Arbez, Sidor and Steyer 2018 introduce a new taxon,
Laosuchus naga (Fig 3), as a long-snouted chroniosuchian, but here nests with long-snouted eryopid temnospondyls. 

Figure 1. Laosuchus in dorsal and lateral views. Colors added with some difficulty here as all the bones are fused and their surfaces are ornamented.

Figure 1. Laosuchus in dorsal and lateral views. Colors added with some difficulty here as all the bones are fused and their surfaces are ornamented.

Laosuchus naga traits include:

  1. an extremely reduced pineal foramen
  2. absence of palatal dentition
  3. well-developed transverse flange of the pterygoid that contacts the maxilla
  4. internal crest on and above the dorsal side the palate
  5. otic notch closed by the tabular horn and the posterior part of the squamosal, forming a continuous wall
  6. thin and high ventromedial ridge on parasphenoid.
Figure 4. Solenodonsaurus skull in situ and reconstructed. That brown bone on top of the frontal/parietal suture is a displaced lacrimal that nicely fills the gap in the reconstruction.

Figure 4. Solenodonsaurus skull in situ and reconstructed. That brown bone on top of the frontal/parietal suture is a displaced lacrimal that nicely fills the gap in the reconstruction.

Something I learned while reexamining Solenodonsaurus
The displaced bone atop the skull is actually part of the broken lacrimal. The quadratojugal is displaced on the posterior mandible. The prefrontal is broken but not very displaced. The posterior jugal is broken into several pieces. Using DGS allows one to cut and paste and fit these puzzle pieces back into the missing parts of the skeleton where they belong. If they don’t fit, they don’t belong, but they never fit perfectly. It’s like putting Humpty Dumpty together again. There are always a few pieces left over.

References
Arbez T, Sidor CA and Steyer J-S 2018. Laosuchus naga gen. et sp. nov., a new chroniosuchian from South-East Asia (Laos) with internal structures revealed by micro-CT scan and discussion of its palaeobiology. Journal of Systematic Palaeontology DOI: 10.1080/14772019.2018.1504827

http://zoobank.org/urn:lsid:zoobank.org:pub:11D07FA3-0F4C-4EF9-A416-E8E6BE76C970

 

Solenodonsaurus revealed by DGS

Solenodonsaurus is a crushed fossil basal reptile (Fig. 1). The skull has been difficult to interpret. Previous workers, including Carroll 1970 and Danto et al. 2012 both came up with the same outline, but the details were difficult to ascertain.

Figure 1. Solenodonsaurus interpreted using DGS. That's a 13 cm skull

Figure 1. Solenodonsaurus interpreted using DGS. That’s a 13 cm skull. The pineal opening has been difficult to find because a bony rod (hyoid? parasphenoid process?) sticks up through it. The naris is just beginning to bud off an antorbital fenestra. On the right the layers are segregated, and see how much clarity that brings! And then you can fit the parts together in a reconstruction, then compare that to sister taxa. It’s a longer process than just tracing.

Solenodonsaurus nests with chroniosuchids near the base of the Reptilia. If I made any mistakes, I’ll correct them with valid input.

References
Broili F von 1924. Ein Cotylosaurier aus der oberkarbonischen Gaskohle von Nürschan in Böhmen. Sitzungsberichte der Mathematisch-Naturwissenschaftlichen Abteilung der Bayerischen Akademie der Wissenschaften zu München 1924: 3-11.
Brough MC and Brough J 1967. Studies on early tetrapods. III. The genus Gephyrostegus. Philosophical Transactions of the Royal Society B252: 147-165.
Carroll RL 1970. The ancestry of reptiles. Philosophical Transactions of the Royal Society B257: 267-308.
Danto M, Witzmann F and Müller J 2012. Redescription and phylogenetic relationships
of Solenodonsaurus janenschi Broili, 1924, from the Late Carboniferous of Nyrany, Czech Republic
Laurin M and Reisz 1999. A new study of Solenodonsaurus janenschi, and a reconsideration of amniote origins and stegocephalian evolution. Canadian Journal of Earth Sciences 36:1239-1255.
Pearson HS 1924. Solenodonsaurus (Broili), a seymouriamorph reptile. Annals and Magazine of Natural History 14:338-343.

wiki/Solenodonsaurus

Chroniosaurus: suture or crack?

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).

Figure 1. Click to enlarge. Chroniosaurus as is and colorized (DGS) for visual presentation. How did I do? Did I miss anything? How is it missing an entire squamosal, unless it was loose and was removed, which I suspect. PIN 3585 ⁄ 124

Figure 1. Click to enlarge. Chroniosaurus as is and colorized (DGS) for visual presentation. How did I do? Did I miss anything? How is it missing an entire squamosal, unless it was loose and was removed, which I suspect. PIN 3585 ⁄ 124. Or it disappeared during taphonomy. 

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.”

Figure 2. Chroniosuchus and Chronioaurus to scale with PIN.

Figure 2. Chroniosuchus and Chroniosaurus to scale with PIN 3585 ⁄ 124. The lower palate is from PIN 3585/99, which is considered a juvenile but is generally the same size as the other specimens shown here. The spratemporals are purple here for clarity, changed from the yellow in the fossil tracing. 

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).

Figure 3. Skull of Chroniosaurs by Ruta from Klembara and Clack 2009. Note the lacrimal does not contact the orbit, different than the tracing in Fig. 1.

Figure 3. Skull of Chroniosaurs by Ruta from Klembara and Clack 2009. Note the lacrimal does not contact the orbit, different than the tracing in Fig. 1. Their parietal is also much narrower than in Fig. 1.

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.

References
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.

hwiki/Chroniosaurus