SVP 20 – a Euryodus (microsaur) -like captorhinid, Opisthodontosaurus

We looked at this taxon, Opisthodontosaurus, earlier here.
Reisz et al. 2015 describe a captorhinid basal reptile similar to a microsaur.

Figure 1. Opisthodontosaurus (above) with missing bones in color. Black lines represent the referred specimen, OMNH 77470 scaled to fit the holotype, OMNH
77469, here in ghosted lines. Colors represent missing bones.

From the abstract
“The Lower Permian fossiliferous infills of the Dolese Brothers Limestone Quarry, near Richards Spur, Oklahoma, have preserved the most diverse assemblage of terrestrial vertebrates, including small-bodied reptiles, lepospondyl microsaurs, and dissorophoid temnospondyls. One taxon that was previously only known from isolated jaw elements at the locality was the microsaur Euryodus primus. Although it is known from more complete material elsewhere, other remains of E. primus have remained elusive at the Dolese Brothers Quarry.

Figure 1. Euryodus primus, a microsaur nesting between Scincosaurus and Micraroter. Note the odd posterior canine teeth, much more exaggerated than in Opisthodontosaurus.

The recent discovery of partial articulated skulls and skeletons of a small reptile at Dolese permits the recognition that the dentigerous elements that were previously assigned to Euryodus primus from this locality belong instead to a new captorhinid eureptile. The new captorhinid represents a major departure from other members of this clade in the unique anatomy of its jaws and dentition, which are characterized by their bulbous maxillary and dentary teeth. Three enlarged teeth are present on the maxilla, one in the anterior and two in the posterior region, whereas the premaxillary dentition is homodont and small. In addition, the largest dentary tooth is present along the posterior half of the bone. The dentary is characterized by the presence of a large well-developed coronoid process and deep lateral excavation in the posterior one-quarter of the bone. A phylogenetic analysis of captorhinid eureptiles yields two most parsimonious trees, with one in which the new captorhinid is recovered as the sister taxon to Concordia, this clade in turn being the sister to all other captorhinids, and a second in which the new captorhinid is the sister to all other derived captorhinids, to the exclusion of Concordia and Thuringothyris“

The sisters to captorhinids
also include Saurorictus (actually a basal captorhinid), Romeria primus, Reiszorhinus and Cephalerpeton in the large reptile tree, none of which have enlarged posterior teeth. Cephalerpeton had a complete set of enlarged maxillary teeth with an oddly raised posterior dentary, below the orbit. All of these taxa have a much taller squamosal and a much smaller suptratemporal. The postorbital and postfrontal are triangular. None of these taxa have a dentary with a deep lateral excavation, but otherwise are all quite similar to microsaurs.

Unique among microsaurs
Euryodus is rather unique among microsaurs with its enlarged posterior teeth. So the headline of Reisz, Leblanc and Scott is a little misleading. The large reptile tree nests Euryodus in a separate clade (Microsauria) from Opisthodontosaurus (with Cephalerpeton).

References
Reisz R, Leblanc A and Scott D 2015. A new early Permian captorhinid reptile (Amniota: Eureptilia) from Richards Spur, Oklahoma, shows remarkable dental and mandibular convergence with microsaurs.

Opisthodontosaurus – not quite a captorhinid and definitely not a microsaur

Figure 1. Opisthodontosaurus (above) with missing bones in color. Black lines represent the referred specimen, OMNH 77470 scaled to fit the holotype, OMNH
77469, here in ghosted lines. Colors represent missing bones. Note the concave maxilla ventral margin and the lower postorbital region compared to Cephalerpeton, its sister in the large reptile tree. These two and other taxa are sisters to captorhinids, but have narrower skulls.

A recent paper by Reisz et al. 2015 brings us a new basal reptile, Opisthodontosaurus carrolli (Fig. 1, Reisz et al. 2015; Artinskian, Early Permian ~289 mya), with teeth so robust it brought to mind a similar microsaur with thick posterior canines, Euryodus (Fig. 2).

Figure 2. Euryodus primus, a microsaur nesting between Scincosaurus and Micraroter. Note the odd posterior canine teeth.

Reisz et al. nested Opisthodontosaurus with Concordia (which it closely matches) and not far from Reiszhorhinus and Romeria primus. The large reptile tree duplicated these nestings, but recovered an excluded big-tooth taxon, Cephalerpeton (Fig. 1), closest to Opisthodontosaurus. I do not have data on another listed sister, Rhiodenticulatus, but will add it as soon as I am able to.

Reisz et al mentioned a depressed lateral dentary posterior to the tooth row. Since no large surangular was preserved and sister taxa have such a bone, it appears likely that  that depression received the missing surangular.

Like its sisters, Opisthodontosaurus is a basal lepidosaurmorph that nests with others that have a relatively narrower skull than outgroup taxa  including captorhinids and Thuringothyris. Tooth size varied a great deal in this clade.

Narrow-skulled sisters
to the captorhinids + cephalerpetontids, the larger Orobates and the smaller Milleretta, ultimately gave rise to the rest of the lepidosauromorphs, including limnoscelids, caseasaurs, diadectomorphs, pareiasaurs, turtles, lanthanosuchids, owenettids, kuehneosaurs and lepidosauriforms including pterosaurs.

Distinct from the microsaur Euryodus, Opisthodontosaurus had a taller squamosal, a greatly reduced supratemporal, a triangular postfrontal and postorbital along with a smaller basipterygoid with a more gracile cultriform process.

Thanks to Dr. Reisz for sending his paper this morning. This is a good discovery, well written and just missing one pertinent taxon.

References
Reisz RR et al. 2015. A new captorhinid reptile from the Lower Permian of Oklahoma showing remarkable dental and mandibular convergence with microsaurian tetrapods. The Science of Nature, October 2015, 102:50.

The Captorhinidae

Updated October 08, 2915 with a revision to the inclusion set of the Captorhinidae.

Figure 1. Labidosaurus, a derived member of a basal reptile clade, the Captorhinidae

It is widely known that captorhinids were basal reptiles that appeared during the Early Permian and disappeared during the Late Permian, evolving from single tooth-row forms to those with multiple tooth-rows. All may be considered herbivores perhaps derived from insectivores. The problem is, which taxa are indeed captorhinids, and which are not?

The Reisz et al. 2011 List
Reisz et al. (2011) published the most recent phylogeny of the captorhinids. They found Thuringothyris nested at the base followed by Concordia, Rhiodenticulatus, Romeria, Protocaptorhinus, Saurorictus, Captorhinus, Captorhinikos, Labidosaurus, Labidosaurikos, Moradisaurus, Rothaniscus and Gansurhinus. This closely matched the phylogeny of Sumida et al. (2010).

Greater Numbers of Taxa Provide Greater Resolution
Here, in the large reptile study, the phylogeny closely matches that of Reisz et al (2011) and Sumida et al. (2010) with the following exceptions. Here, Cephalerpeton, Reiszorhinus, Concordia and Romeria primus nested just outside of the Captorhinidae at the base of all other lepidosauromorphs. Saurorictus and Romeria texana nested at the base of the Captorhinidae.

Independent Phylogenies
When independent phylogenies match, that’s a good test of validity. The exceptions merely alert others to expand the taxon list to test the exceptions on their own time using their own character lists.

Late Surviving Taxa
The presence of Concordia in the Latest Pennsylvanian indicates an earlier branching of basal captorhinids. The discovered fossils probably represent some of the broadest numerical extent of each taxon.

 

References
Muller J and Reisz RR 2006. The phylogeny of early eureptiles: Comparing parsimony and Bayesian approaches in the investigation of a basal fossil clade.” Systematic Biology, 55(3):503-511. doi:10.1080/10635150600755396
Reisz RR, Liu J, Li JL and Müller J 2011. A new captorhinid reptile, Gansurhinus qingtoushanensis, gen. et sp. nov., from the Permian of China. Naturwissenschaften 98 (5): 435–441. doi:10.1007/s00114-011-0793-0. PMID 21484260.
Sumida SS, Dodick J, Metcalf A and Albright G. 2010. Reiszorhinus olsoni, a new single-tooth-rowed captorhinid reptile of the Lower Permian of Texas. Journal of Vertebrate Paleontology 30 (3): 704–714. doi:10.1080/02724631003758078
wiki/Captorhinidae

What is Saurorictus?

Figure 1. Saurorictus australis reconstructed. The parietal, outlined in gray, is largely unknown. Click for more info.

Saurorictus australis
Captorhinids were basal lepidosauromorph reptiles that appeared in the Early Permian and evolved multiple tooth rows by the Late Permian.  Saurorictus (Modesto and Smith 2001) SAM PK-8666 was originally considered a late-surviving single-tooth row captorhinid that had “very slender marginal teeth” and reportedly lacked a supratemporal.

Figure 2. Saurorictus, Macroleter and Lanthanosuchus demonstrating the evolution of one to another and another of these three sister taxa. The derived sister taxon is Nyctiphruretus. An ancestor includes a sister to Orobates. The size increase is important.

A Larger Tree Nests Saurorictus Elsewhere
Here the large reptile tree nested Saurorictus with Lanthanosuchus and Macroleter, far from the captorhinids.  Like another sister, Nyctiphruretus, Saurorictus lacked an indented squamosal and lacked a lateral temporal fenestra. To move Saurorictus to the captorhinids requires an additional 17 steps. Saurorictus also nests between Stephanospondylus (which leads to turtles) and Nyctiphruretus (which leads to owenettids and lepidosauriformes). So this is a key taxon. And a tiny one!

Figure 3. The supratemporal in Saurorictus (ST, in pink) was originally considered a part of the parietal which is reasonable given their paradigm that Saurorictus was a captorhinid.

Missing a Supratemporal? Maybe Not.
The worst preservation in SAM PK-8666 occurs on the skull roof. The parietal is barely present and the pineal opening is nowhere to be found. Just dorsal to the squamosal is a plate-like bone that Modesto and Smith (2001) considered a parietal lacking a supratemporal between it and the squamosal. The skull of Saurorictus does indeed resemble that of captorhinids in general. The supratemporal in captorhinids is a tiny splint of bone and such a bone is indeed missing. I added the Saurorictus data (lacking a supratemporal) to the large reptile tree and was surprised to see it nested with Lanthosuchus and Macroleter, taxa with a large, plate-like supratemporal. Now the lack of a supratemporal seemed to be a very odd autapomorphy. Reexamining the published image (Modesto and Smith 2001) of Saurorictus I realized that the corner of bone originally labeled as a parietal was a large and mislabeled plate-like supratemporal, matching sister taxa.

Figure 4. Saurorictus nests with Macroleter and Lanthanosuchus.

Different and Similar
At first it would appear odd that round-skulled Saurorictus should nest with the cantilevered skulls of Macroleter and Lanthanosuchus, but round-skulled Nyctiphruretus also nests nearby. Diadectes and Procolophon also nest nearby, but Orobates is a more basal sister that shares certain plesiomorphic traits with Saurorictus. Here, apparently, we’re seeing a small, simple, pleisomorphic taxon that gives rise to the various odder, more derived sisters.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Modesto SP and Smith RMH 2001. A new Late Permian captorhinid reptile: a first record from the South African Karoo. Journal of Vertebrate Paleontology 21(3): 405–409.

wiki/Saurorictus