Varanosaurus, Ophiacodon and the Origin of the Therapsida

Figure 1. Varanosaurus, Ophiacodon, Cutleria and Ictidorhinus. These are taxa at the base of the Therapsida. Ophiacodon did not cross into the Therapsida, but developed a larger size with a primitive morphology. This new reconstruction of Ophiacodon is based on the Field Museum (Chicago) specimen. Click to enlarge.

Figure 1. Varanosaurus, Ophiacodon, Cutleria and Ictidorhinus. These are taxa at the base of the Therapsida. Ophiacodon did not cross into the Therapsida, but developed a larger size with a primitive morphology. This new reconstruction of Ophiacodon is based on the Field Museum (Chicago) specimen. Click to enlarge.

Earlier we talked about the role of Ophiacodon at the base of the Therapsida. Several years later that hypothesis is still in direct contrast to traditional thinking (Laurin and Reisz 1996 and other refs below) that holds sphenacodonts and Tetraceratops were basal to Therapsida.

Here (Figs. 1, 2)  we’ll add the tall-skull holotype of Varanosaurus, which nests as an outgroup to Ophiacodon, a genus known from several species of increasing size. Cutlerlia nests at the base of the Therapsida, basal to Biarmosuchus and tiny Ictidorhinus (wonder if it is a juvenile lacking fangs?). Cutleria is also close to the other branch of the Therapsida, the Anomodontia (dromasaurs, dicynodonts and their kin.)

Figure 2. Closeup of figure 1 focusing on the smaller skulls at the base of the Therapsida. Click to enlarge.

Figure 2. Closeup of figure 1 focusing on the smaller skulls at the base of the Therapsida. Click to enlarge.

References
Amson E and Laurin M 2011. On the affinities of Tetraceratops insignis, an Early Permian synapsid. Acta Palaeontologica Polonica 56(2):301-312. online pdf
Conrad J and Sidor CA 2001. Re−evaluation of Tetraceratops insignis (Synapsida: Sphenacodontia). Journal of Vertebrate Paleontology 21: 42A.
Matthew WD 1908. A four-horned pelycosaurian from the Permian of Texas.Bulletin of the American Museum of Natural History 24:183-185.
Laurin M and Reisz RR. 1996. The osteology and relationships of Tetraceratops insignis, the oldest known therapsid. Journal of Vertebrate Paleontology 16:95-102. doi:10.1080/02724634.1996.10011287.
Sidor CA and Hopson JA 1998. “Ghost lineages and “mammalness”: Assessing the temporal pattern of character acquisition in the Synapsida”. Paleobiology 24: 254–273.

 

New nesting for Echinerpeton with Secodontosaurus

Figure 1. Just move the mandible forward so the last tooth is anterior to the orbit and Echinerpeton becomes a long snouted pro to-secodontosaur.

Figure 1. Just move the mandible and maxilla forward so the last tooth is anterior to the orbit (as in other pelycosaurs) and Echinerpeton becomes a long snouted proto-secodontosaur. 

Raising my hand to proclaim a nesting error
Earlier (now trashed) I recovered Echinerpeton at the base of the Synapsida and Diapsida, but those elongate dorsal spines seemed odd at that node. Then I noticed that all other pelycosaurs had teeth only in front of the orbit. The skull is largely missing, so there’s no harm in shifting the jaws forward a bit. And suddenly Echinerpeton made more sense.

Echinerpeton intermedium (Reisz 1972), Late Carboniferous, 308 mya. Reisz (1972) tentatively classified Echinerpeton as an ophiacodontid in his initial description, and in 1986 he considered it an indeterminate “pelycosaur“. Benson (2012) could not nest Echinerpeton with certainty, perhaps because he used the wrong outgroups and mistakenly included caseasaurs because he followed tradition without the benefit of a large gamut reptile tree like we have here (Fig. 2).

Figure 1. Secodontosaurus and its ancestors going back to Varanosaurus. Secodontosaurus is the only sphenacodont with a varanopid-like skull.

Figure 1. Secodontosaurus and its ancestors going back to Varanosaurus. Secodontosaurus was the only sphenacodont with a varanopid-like skull. No Echinerpeton has one too.

Here Echinerpeton nests with Secodontosaurus. The snout was long because the last maxillary tooth was in front of the orbit. The maxilla was straight while the dentary was concave dorsally. Both were filled with long teeth.

The dorsal spines were long, but not as long as those of Secodontosaurus.The scapula was small and both the humerus and femur were short and slender. The ankle bones were round elements. Together these point to an aquatic, rather than a terrestrial niche. So Echinerpeton was a crocodile-like sphenacodont pelycosaur.

References
Benson RBJ 2012. Interrelationships of basal synapsids: Cranial and postcranial morphological partitions suggest different topologies. Journal of Systematic Palaeontology: 601-624.
Reisz R 1972. Pelycosaurian reptiles from the Middle Pennsylvanian of North America. Bulletin of the Museum of Comparative Zoology 144 (2): 27–62. online here

 

Something about Clepsydrops (Dimetrodon natalis)

Figure 1. Clepsydrops (Dimetrodon) natalis. Here we find the original reconstruction with a too large pelvis. Putting the loose elements on the old reconstruction gets us closer to reality. There's a scaled version over the scale bar compared to a full-size Dimetrodon.

Figure 1. Clepsydrops (Dimetrodon) natalis. Here we find the original reconstruction with a too large pelvis. Putting the loose elements on the old reconstruction gets us closer to reality. There’s a scaled version over the scale bar compared to  Dimetrodon grandis. Ghosted area represents a restoration.

We first met Dimetrodon natalis a few posts ago when Romer and Price presented it along with a selection of other Dimetrodon skulls. I don’t know much about Clepsydrops (Cope 1875), a name which seems to have gone out of favor, or to the back of the museum drawer. Romer and Price in their Review of the Pelycosauria (1940), considered it an ophiacodont, close to Varanosaurus, perhaps based on size, but this was before the skull became known. The femur is robust. The data is a little hard to read. In the old days they used to say “image at 1/4 size” rather than applying scale bars.

Shelton et al. 2103 reported, “The Briar Creek Bonebed (Artinskian, Nocona Formation) in Archer County is oneof the richest sources of Dimetrodon bones in the Lower Permian of Texas, USA. Based on size, a small (Dnatal is ), an intermediate (D. booneorum), and a large species (D. limbatus) have been described from this locality. It has been proposed that these traditionally recognised species represent an ontogenetic series of only one species.

… The external fundamental systems observed in the largest humerus and the two largest femora confirm that D. natal is is not the juvenile of a larger species. The presence of the EFS in the cortex of their long bones unquestionably indicates that  these animals had attained skeletal maturity. 

…The results thus partially refute Bakker’s (1982) hypothesis,that the bones of D. natalis, D. booneorum and D. limbatus only represent an ontogenetic series of a single species, which may in turn disprove the juvenile/adult habitat shift hypothe-sis. Juveniles and adults of D. natalis are found in the samebonebed. However, the findings are insufficient for fully test-ing the habitat shift hypothesis. The results support Brinkman(1988), as well as the morphological classification used by Romer & Price (1940).”

References
Bakker RT 1982. Juvenile–adult habitat shift in Permian fossil reptiles and  amphibians. Science 217: 53–55.
Brinkman D 1988. Size-independent criteria for estimating relative age in Ophiacodon and Dimetrodon (Reptilia, Pelycosauria) from the Admiral and Lower Belle Plains formations of West-central Texas. Journal of Vertebrate Paleontology 8, 172–80.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of American Special Papers 28, 538 pp.
Shelton CD, Sander PM, Stein K, Winkelhorst H 2013. Long bone histology indicates sympatric species of Dimetrodon (Lower Permian, Sphenacodontidae) Earth and Environmental Science Transactions of the Royal Society of Edinburgh 09/2012; 103(3-4). DOI:10.1017/S175569101300025X

The Strange Case of Secodontosaurus

At first, you don’t believe what you’re seeing.
Secodontosaurus (Fig. 1, Cope 1880) looks like a varanopid skull grafted onto a sphenacodont body, complete with dorsal sail. And there’s a genetic reason for that. It’s not modular evolution, although it seems like it at first. It’s more like a reversal because there’s a long line of other taxa between Secodontosaurus and Varanosaurus. And they left their genetic markers too.

Figure 1. Secodontosaurus and its ancestors going back to Varanosaurus. Secodontosaurus is the only sphenacodont with a varanopid-like skull.

Figure 1. Secodontosaurus and its ancestors going back to Varanosaurus. Secodontosaurus is the only sphenacodont with a varanopid-like skull.

Varanosaurus and Secodontosaurus shared traits
Long low rostrum. Naris close to jaw rim. Strongly concave dorsally dentary.

Varanosaurus and Secodontosaurus differences
Quadratojugal greatly reduced in Secodontosaurus. Lacrimal does not contact the naris. Reflected lamina on angular. Loss of mandibular fenestra. Posteriorly shorter postorbital, not in contact with supratemporal.

And that’s just for starters on the skull alone. If anyone has good refs for the post-crania, I’ll complete a reconstruction.

There have been some additions and revisions to reptileevolution.com around the synapsids as I’ve added new taxa recently.

Thanks for all your support.

References
Cope ED 1880. Second contribution to the history of the Vertebrata of the Permian formation of Texas. Proceedings of the American Philosophical Society 19:38-58.

 

Anningia – still an enigma?

Updated February 23, 2015 with an updated reconstruction of Mycterosaurus.

Anningia megalops (Broom 1927, Middle Permian, Transvaal Museum N. 4024) is only known from a poorly preserved back half of a skull (Fig. 1). My introduction was through Romer and Price (1940, Fig. 1) as they associated it with sphenacodonts, like Dimetrodon. Later Reisz and Dilkes (1992) considered “available information insufficient to determine taxonomic position or phylogenetic relationships beyond the likelihood that it may be the remains of a pelycosaur.” 

 

Figure 1. Anniningia megalops as illustrated by Romer and Price 1940 (below) and in two views from Reisz and Dilkes 1992 with elements colorized and hypothetical rostrum and jaws added.

Figure 1. Anningia megalops as illustrated by Romer and Price 1940 (below) and in two views from Reisz and Dilkes 1992 with skull element shapes colorized and hypothetical rostrum and jaws added. Remember, the bones are largely missing here. These are impressions of the inner side of the bones.

Comparing Anningia to other candidate taxa brings a pretty close match to Archaeovenator (among tested taxa, Fig. 3), a much smaller and much earlier genus. Broom, Romer and Price and Reisz and Dilkes noted superficial similarities to Mycterosaurus (Fig. 2), which nests close toArchaeovenator, but closer to Heleosaurus.

Figure 2. Mycterosaurus based on Berman and Reisz 1982.

Figure 2. Mycterosaurus based on Berman and Reisz 1982.

The Middle Permian of Anningia (Tapinocephalus Zone) is later than typical pelycosaur strata in the Early Permian. Archaeovenator of the Late Carboniferous, preceded the Permian. Elliotsmithia, another basal pelycosaur, was also a relic taxon in the Middle Permian. So some of these taxa had a long run.

Broom identified a pre-parietal bone, a bone found otherwise only in gorgonopsians and anomodonts and so thought Anningia was a transitional taxon leading to these.

It’s a mess.
There has been extensive fracturing  and erosion of the skull surface of Anningia, which complicates interpretation. Only impressions of the skull roofing bones are preserved. Reisz and Dilkes found “no sutures to suggest the presence of a pre parietal bone.”

Reisz and Dilkes note: “All the osteological features noted above can be interpreted as primitive features for synapsids, and are consistent with placement of A. megalops within varanopseids. Unfortunately, these features are not diagnostic for varanopseids, but are also present in primitive ophiacodontids.”

Reisz and Dilkes report that most of the critical portions of the skull have been lost because most of the type material (left side of the skull) was destroyed when transverse sections were made of the braincase.

Figure 2. Archaeovenator, a smaller relative of Anningia, itself a synapsid outgroup to the Diapsida.

Figure 3. Archaeovenator, a smaller relative of Anningia, itself a synapsid outgroup to the Diapsida. If you think this is starting to look kind of like Petrolacosaurus, you’re right.

Romer and Price reported it had a very short face, even though “the snout has vanished, but the complete lower jaw is present.” Unfortunately that jaw is not shown in Reisz and Dilkes. They report, “Contrary to previous interpretations of this material, we feel that there is insufficient information to determine the length of the complete lower jaw. These earlier interpretations were made before the wide range of jaw proportions of Paleozoic synapsids was known.”

Anningia comes from the same horizon as Galesphyrus capensis (Fig. 4), which is skull-less but Romer and Price repeat Broom’s suggestion that it is of a size that could accommodate the Anningia skull.

No way.
The skull of Anningia is at least as long as the glenoid-acetabulum length. Just check out those scale bars, which are nearly identical in figures 1 and 3.

Figure 3. Galesphyrus, a basal protodiapsid from the same horizon as Anningia.

Figure 3. Galesphyrus, a basal probable diapsid (look at the long ulnare and very short pedal digit1) from the same horizon as Anningia. Unless this is a juvenile, the Anningia skull would never mate to this neck.

In the large reptile tree, protodiapsids nest with varanopids (and the entire synapsid clade), as an outgroup, so the paradigm of calling these reptiles varanopids is entirely reasonable — but they are on another branch, not the Varanops branch. This point is overlooked in smaller prior studies that ignore and exclude basal diapsids (because, after all… we’re not talking about diapsids when we talk about synapsids, are we? [sarcastic droll rise of one eyebrow].

References
Berman DS and Reisz RR 1982. Restudy of Mycterosaurus longiceps (Reptilia, Pelycosauria) from the Lower Permian of Texas. Annals of Carnegie Museum 51, 423–453.
Broom R 1927. On a new type of mammal-like reptile from the South African Karroo beds (Anningia megalops). Proceedings of the Zoological Society of London 227-232.
Reisz RR and Dilkes DW 1992.
The taxonomic position of Anningia megalops, a small amniote from the Permian of South Africa. Canadian Journal of Earth Sciences 29: 1605-1608.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.

New Elliotsmithia semi-skull

This doesn’t add a lot to what we knew of Elliotsmithia (Broom 1937), but it does provide a canine tooth, and an upturned mandible tip. The question is: is this also Elliotsmithia? Or something else?

Figure 1. The new specimen referred to Elliotsmithia. It's the same size and shares many traits. In the original Elliotsmithia (Fig. 2) the posterior teeth appear larger. The part and counterpart are overlaid here. I'm guessing that the anterior lower dentary is buried here. Otherwise there would be no room for the next erupting tooth to develop.

Figure 1. The new specimen referred to Elliotsmithia. It’s the same size and shares many traits. In the original Elliotsmithia (Fig. 2) the posterior teeth appear larger. The part and counterpart are overlaid here. I’m guessing that the anterior lower dentary is buried here. Otherwise there would be no room for the next erupting tooth to develop.

The type Elliotsmithia had giant serrated teeth and a forward leaning squamosal. Even so, this is a very close match in most respects.

Figure 2. The type of Elliotsmithia. Those are really big teeth beneath the orbit. The pineal is more centrally placed. And the jugal appears to lean in more.

Figure 2. The type of Elliotsmithia. Those are really big teeth beneath the orbit. The pineal is more centrally placed. And the jugal appears to lean in more.

Elliotsmithia nests at the base of the Synapsida in the large reptile tree. As such it represents the most primitive appearance of the lateral temporal fenestra, but not the earliest. Living among therapsids in the Middle Permian, it was a relic taxon, like the living Sphenodon.

References
Broom R 1937. A further contribution to our knowledge of the fossil reptiles of the Karroo. Proceedings of the Zoological Society, Series B 1937:299-318.
Modesto S, Sidor CA, Rubidge BS and Welman J. 2001. A second varanopseid skull from the Upper Permian of South Africa: implications for Late Permian ‘pelycosaur’ evolution. Lethaia 34: 249-259.

Sphenacodonts to scale

Yesterday we looked at ophiacodonts from Romer and Price (1940) and showed them to scale. Today we’ll do the same with sphenacodonts (Figs.1,2).

Figure 1. Sphenacodont pelycosaur skulls from Romer and Price 1940.

Figure 1. Click to enlarge. Sphenacodont pelycosaur skulls from Romer and Price 1940. Notice how similar the skull of sail-less Sphenacodon is to that of Dimetrodon. Little Anningia was shown to be undiagnosable by Reisz 1992. D. natal is is evidently the baby Dimetrodon. If so most of the skull is imaginary here.

It’s a different picture, isn’t it, when scale is applied to illustrations.

Figure 2. Click to enlarge. Sphenacodont skulls to scale. Figure 2. Click to enlarge. Sphenacodont skulls to scale.

Figure 2. Click to enlarge. Sphenacodont skulls to scale.

Paleontologist Alfred Romer erected the species Dimetrodon natalis in 1936, previously described as Clepsydrops natalisD. natalis was the smallest known species of Dimetrodon at that time, and was found alongside remains of the larger-bodied D. limbatus. Sternberg (1942) didn’t mention it in his description of an “immature Dimetrodon cf. grandis,” which is a different and smaller specimen, the baby Dimetrodon found here.

Secodontosaurus is the oddball here, but the postcrania is indistinguishable from Dimetrodon. More on that one later.

References
Brinkman D 1988. 
Size-independent criteria for estimating relative age in Ophiacodon and Dimetrodon (Reptilia, Pelycosauria) from the Admiral and lower Belle Plains formations of west-central Texas. Journal of Vertebrate Paleontology 8 (2): 172–180.
Reisz RR, Dilkes DW 1992. The taxonomic position of Anningia megalops, a small amniote from the Permian of South Africa. Canadian Journal of Earth Sciences, 1992, 29(7):1605-1608.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.
Sternberg CW 1942. The skeleton of an immature pelycosaur, Dimetrodon cf. grandis, from the Permian of Texas. Journal of Paleontology 16 (4): 485–486.

Ophiacodonts to scale

Romer and Price (1940) did a bang-up piece on pelycosaurs back in the day. From their tome I pulled two images, one of ophiacodonts known then (Fig. 1) and of sphenacodonts (we’ll do that tomorrow).

Figure 1. Ophiacodont skulls. Here Tetraceratops and Eothyris do not belong.

Figure 1. Ophiacodont skulls as Romer and Price drew them. Here Tetraceratops and Eothyris do not belong, so they are grayed out. Some skulls, like Elliotsmithia, are not so well known as pictured.

Romer and Price were kind enough to put mm lengths under each picture. I thought it might be nice to see how they look when scaled (Figs. 3,4). Ophiacodon retroversus gets pretty impressive, especially when it develops two lateral temporal fenestrae!

Figure 3. Ophiacodont skulls to scale. Gives a greater appreciation for their variety, doesn't it?

Figure 3. Ophiacodont skulls to scale. Gives a greater appreciation for their variety, doesn’t it?

According to the large reptile tree, Elliotsmithia was the most primitive. Eothyris and Tetraceratops were not ophiacodonts, but caseasaurs and limnoscelids, respectively. Varanops and Varanosaurus skulls we looked at earlier here. They don’t look quite the same in bone as they do here.

References
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.

wiki/Ophiacodon

Baby Dimetrodon – Chimaeras and Fakes – Part 1

Many have seen this cast (fig. 1) entitled, “Baby (or Juvenile) Dimetrodon.” It’s a common piece of plaster merchandise sold at fossil fairs, etc.

Is it a complete fake?
The specimen has not been illustrated in the literature (that I know of), but it has been described (Sternberg 1942) and the description is a perfect match. Parts (Fig. 1 in gray) have been added to the cast to make it more interesting and complete. Sternberg (1942) reported the specimen was originally at the Walker Museum in Chicago, but in 1953 most Walker paleo exhibits, perhaps including this one, were moved to the Field Museum.

Figure 1.  Sternberg 1942 described this specimen he attributed to a baby or juvenile Dimetrodon. Parts added by artisans are in gray.

Figure 1. Sternberg 1942 described this specimen he attributed to a baby or juvenile Dimetrodon. Parts added by artisans are in gray.

On the plus side
One of the more complete Permian fossils is this baby/juvenile Dimetrodon (Sternberg 1942, Figs. 1, 2), less than a quarter the size of an adult with a much shorter sail and much longer legs. If this is a juvenile Dimetrodon, these proportions change allometrically during growth. The mandible was slightly shorter, compared to the adult, indicating the skull was likewise not larger relative to the body.

Click to enlarge. Figure 1. Baby Dimetrodon (above) compared to adult (below) to the same scale and to different scales. Note the smaller sail and longer legs and tail. Regressing the baby to egg size suggests the sail developed after hatching.

Figure 2. Click to enlarge. Figure 1. Baby Dimetrodon (above) compared to adult (below) to the same scale and to different scales. This is the first reconstruction of this specimen that I am aware of. Restored parts in light red. Note the smaller sail and longer legs and tail in the juvenile.  Regressing the baby to egg size suggests the sail developed after hatching. I’m curious about the rib length from front to back on the juvenile, different from the adult.

So, longer legs on a juvenile synapsid?
That’s not the pattern we see in mammals or Heleosaurus, a varanopid(?) protodiapsid in which adults have the longer legs. In Dimetrodon the juveniles didn’t have marginally longer legs. Juveniles had legs relatively twice as long as those on adults. Generally longer legs provide more speed to attack prey or avoid predators.

Is this really a baby Dimetrodon?
Or is it a different smaller species? Bakker(1982) suggested different habitats for Permian juveniles would help them avoid adult predation. Brinkman (1988)  cast doubt on Bakker’s idea by showing that the specimens found in floodplain and swamp sediments represented two different species, not adult and juvenile populations of the same species.

Do we need more tiny specimens and a few teenage specimens to help determine what the situation is here? Both sides make sense.

Sternberg (1942) wrote,
“The preservation of the bone is poor: It is probable that the bony elements were never well ossified.” He also wrote that three or four partial skeletons of Dimetrodon grandis were found in the same pocket, which lies in the breaks of Coffee Creek in Baylor County, Texas. If we assume those were adults, there goes Bakker’s and Brinkman’s hypotheses. Brinkman did not reference the Sternberg paper, but noted that poor ossification attended smaller Dimetrodon specimens.

Only parts are fake
Just because parts of this specimen have been added with restoration, doesn’t mean the rest of the skeleton is useless or should be labeled “a fake.” In this case, we should use what is real and avoid what is fake. The size and proportion relationships are still good data that make a good story.

References
Bakker RT 1982. Juvenile-Adult Habitat Shift in Permian Fossil Reptiles and Amphibians. Science 217 (4554): 53–55. doi:10.1126/science.217.4554.53.PMID 17739981.
Brinkman D 1988. Size-independent criteria for estimating relative age in Ophiacodon and Dimetrodon (Reptilia, Pelycosauria) from the Admiral and lower Belle Plains formations of west-central Texas. Journal of Vertebrate Paleontology 8 (2): 172–180.
Sternberg CW 1942. The skeleton of an immature pelycosaur, Dimetrodon cf. grandis, from the Permian of Texas. Journal of Paleontology 16 (4): 485–486.

A paper written on fossil fakes is online here.

Is Stenocybus an Anomodont Ancestor?

Earlier we looked at the unlikely connection between Stenocybus and the anteosaurs Sinophoneus. We also looked at a skull reconstruction. Today we’ll take a look at another candidate for a closest kin to Stenocybus at the base of the Therapsida.

The dicynodonts were plant-eating therapsids with bizarre skulls distinctly different from those that retained a carnivorous diet and eventually spun off the mammals and a number of other plant-eating clades along the way. The origin of the dicynodonts has been pegged to the basal anomodonts Patranomodon (Fig. 1) Anomocephalus and their kin.

In our search for the closest kin to Stenocybus its worthwhile to consider a comparison to Patranomodon, with which it appears to share more traits.

 

Figure 1. The skulls of Stenocybus and Patranomodon, side by side and to scale (the smaller Patranomodon illustrations are to scale with Stenocybus). If larger eyes and a shorter snout are indeed signs of neotony in this lineage then we may be seeing the evolution of one form into another here. While the skull of Patranomodon appears wider, that is about the only dimension that has remained the same in the otherwise overall reduction of this taxon from Stenocybus. Note, in particular, the similar palates as Stenocybus transits from the pelycosaurian-grade palate to that approaching the dicynodont grade. This illustration improves on an earlier attempt.

Figure 1. The skulls of Stenocybus and Patranomodon, side by side and to scale (the smaller Patranomodon illustrations are to scale with Stenocybus). If larger eyes and a shorter snout are indeed signs of neotony in this lineage then we may be seeing the evolution of one form into another here. While the skull of Patranomodon appears wider, that is about the only dimension that has remained the same in the otherwise overall reduction of this taxon from Stenocybus. Note, in particular, the similar palates as Stenocybus transits from the pelycosaurian-grade palate to that approaching the dicynodont grade. This illustration improves on an earlier attempt.

Neotony at Work
Currently the therapsid branch of the large reptile tree nests Patranomodon close to Stenocybus at the base of the Therapsida. There’s good reason for this as they share more traits than any other current candidates. Patranomodon is quite a bit smaller (judging by the only data: skulls) than Stenocybus. Patranomodon has a relatively shorter rostrum and larger eyes, both of which make it a candidate for neotony in this relationship. Patranomodon also had smaller teeth, but the breadth of its occiput changed the least.

Premaxilla Ascending Process
Shorter in Patranomodon, along with the shorter rostrum. Also it is nearly vertical, as in dicynodonts. The premaxilla likely included rake-like teeth, considering the ascent of the anterior premaxilla, though partly missing.

Disappearance and Verticalization of the Quadratojugal
In Stenocybus we see the quadratojugal rotate and reduce from chiefly horizontal, as in ophiacodonts, to chiefly vertical and associated only with the quadrate, as in more derived therapsids. We also see the squamosal overtake the quadratojugal.

Supratemporals
Pelycosaurs have them. Therapsids fuse them or lose them. In Stenocybus there are two loose supratemporal-like bones scattered randomly on the parietal, disarticulated from their original positions. So therapsids probably lost them.

Lacrimal
In primitive synapsids the lacrimal contacts the naris. In derived synapsids and all therapsids, the maxilla expands dorsally covering the lacrimal. Earlier I wondered if the lacrimal was the external septomaxilla. The septomaxilla is traditionally a bone inside the naris. In Stenocybus the anterior tip of the lacrimal and the septomaxilla are both present. Part of the broken maxilla exposes the underlying lacrimal. In Patranomodon there is no lacrimal connection, but strangely, neither is the septomaxilla delineated (Fig.1). Probably just an oversight in the original illustration (Fig. 1).

Growth of the Preparietal
Stenocybus has a small new medial bone anterior to the parietal foramen, the preparietal. It is much larger in Patranomodon.

Expansion of the Temporal Fenestrae and Adductor Chamber
In Patranomodon the adductor chamber (area devoted to jaw muscles) is relatively larger despite the smaller teeth. This probably marks the transition to herbivory or insectivory or both, considering the overall size reduction.

Shifting of the Teeth vs. Palate
In Ophiacodon, Haptodus and most therapsids, the palate bones lie between the teeth. In Stenocybus most of the palate is posterior to most of the teeth. In Patranomodon all of the palate, other than the vomers, is posterior to all of the teeth. The palatal elements in both taxa have similar shapes and proportions.

Postcrania
Little to nothing is known of the post-crania in these two taxa. However, we can make certain assumptions based on phylogenetic bracketing more primitive and more derived taxa, Haptodus and Suminia. Torso: long and low. Tail: long and meaty. Legs: splayed with a tendency toward longer toes. The tail shortens quite a bit when these basal forms evolve into dicynodonts. (Or does the body just get bigger and the tail remains the same length?)

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again. Crushing and cracking make both skulls (Fig. 1) difficult to restore the sutures. If I have made any mistakes, please bring them to my attention.

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

References
Cheng Z and Li J 1997. A new genus of primitive dinocephalian – the third report on Late Permian Dashankou lower tetrapod fauna. Vertebrata PalAsiatica 35 (1): 35-43. [in Chinese with English summary]
Kammerer CF 2011. Systematics of the Anteosauria (Therapsida: Dinocephalia), Journal of Systematic Palaeontology, 9: 2, 261 — 304, First published on: 13 December 2010 (iFirst)

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