Finally, a single basal therapsid: Cutleria

Updated May 10, 2016 with a new cladogram. 

Earlier we considered Cutleria (Fig. 1, Lewis and Vaughn 1965, USNM 22099, early Permian, 299-290mya) a basal therapsid based on its morphology, but not tested in phylogenetic analysis. Laurin (1994) redescribed Cutleria wilmarthi as a sister to the Sphenacodontidae + Therapsida at a time when therapsids were thought to descend from sphenacodontids. Wiki considers Cutleria a basal sphenacodontid.

Before today
The large reptile tree recovered a diphyletic Therapsida, with Anomodontia on one branch and the remainder of the therapsids on another.

Figure 3. Basal therapsid tree.

Figure 3. Basal therapsid tree.

Today
a phylogenetic analysis nests Cutleria as THE basal therapsid (among the 50 included taxa, Fig. 2) and it still descends from Ophiacodon. So Cutleria is still just another ophiacodontid. It had simply taken the next evolutionary step toward mammals, dicynodonts, etc. etc. And it probably was not too far from sphenacoodontids even so.  Evidently there was an early Permian radiation that included tailback pelycosaurs and basal therapsids, largely at the same time, both derived from ophiacodontids.

Figure 1. Ophiacodon, Cutleria and Stenocybus, the lineage of basal therapsids.

Figure 1. Ophiacodon, Cutleria and Stenocybus, the lineage of basal therapsids.

Earlier
we looked at the diphyletic nature of the therapsid family tree. One branch led to the Anomodontia (dromasaurs + dicynodonts and kin). The other branch (let’s call them Biarmosuchiformes) led to dinocephalians, theriodontids and by way of extension mammals and us humans.

Today we have a common ancestor for these two therapsid branches, Cutleria, in the therapsid tree at right. Cuteria does not have a single large canine, but two small canines, as in Raranimus and Stereophallodon.

Liu et al. (2009) reported, “While sphenacodontid synapsids are considered the sister-group of therapsids, the place of origin of therapsids is an enigma, largely because of a long standing morphological and temporal gap (Olson’s Gap) in their fossil record.”

Note: Tetraceratops, once thought to be a basal therapsid, is not part of this clade, as it nests with the limnoscelid, Tseajaia.

Benson (2012) recovered several basal synapsid trees, almost all with Cutleria close to the base of the Therapsida, but far from Ophiacodon. Benson erred when he included caseasaurids, which nest with millerettids in the large reptile tree. His outgroups (save Protorothyris) were largely new lepidosauromorphs like Captorhinus and Limnoscelis, which are not recovered by the large reptile tree. Hylonomus and Coelostegus would have been more appropriate and phylogenetically closer. Benson’s tree did recover Haptodus as a basal sphenacodontid, not far removed from Ophiacodon, if we get rid of caseasaurs.

References
Benson RBJ 2012. Interrelationships of basal synapsids: cranial and postcranial morphological partitions suggest different topologies. Journal of Systematic Palaeontology 10(4):601-604.
Brinkman D and Eberth DA 1986. The anatomy and relationships of Stereophallodonand Baldwinonous (Reptilia, Pelycosauria). Breviora 485: 1-34.
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]
Laurin, M 1994. Re-evaluation of Cutleria wilmarthi, an Early Permian synapsid from Colorado. Journal of Vertebrate Paleontology 14(1): 134-138.
Lewis GE and Vaughn PP 1965. Early Permian Vertebrates from the Cutler Formation of the Placerville Area Colorado. United States Geological Survey Professional Papers 503-C:1-50.
Liu J, Rubidge B and Li J-L 2009. New basal synapsid supports Laurasian origin for therapsids. Acta Palaeontologica Polonica 54(3): 393-400.

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)

wiki/Stenocybus

Reconstructing the Skull of Stenocybus

Stenocybus accidentusis (Cheng and Li 1997) is known from a skull and a set of skull parts. Earlier we looked at comparisons to anteosaurs as reported by Kammerer (2011).

The holotype specimen, IGCAGS V 361, is complete, but skewed parasagittally. Photographs of the specimen (Kammerer 2011), my only data source were taken from less than ideal angles using a fairly wide angle lens (which does not flatten perspective). Unfortunately Kammerer (2011) did not delineate the bones, so I attempted to differentiate sutures from cracks. So, based on observation and comparison with potential sister candidates, here they are (having never seen the actual fossil). If I made any mistakes, please bring them to my attention. I certainly made mistakes earlier. This is a learning process. The parietal portion was particularly troublesome as the posterior frontals have a medial hole into which the preparietal snakes. Moreover the parietal was unexpectedly narrow. Nevertheless, it all seems to fit together and no “rules” were broken.

Stenocybus in situ

Figure 1. Stenocybus in situ with bones identified. Colors match those in figure 2. A longer lens (not such a wide angle) and a more direct lateral and dorsal view would have made reconstruction much easier to do. From Kammerer 2011.

 

Figure 2. Stenocybus reconstructed based on tracings in figure 1. Note the lacrimal AND septomaxilla in close proximity to each other.

Figure 2. Stenocybus reconstructed based on tracings in figure 1. Note the lacrimal AND septomaxilla in close proximity to each other.

Overall
The skull has the appearance of a small Haptodus retaining traces of its ophiacodontid ancestry. There is a median ridge on the skull.

Premaxilla
The ascending process remains long, as in ophiacodontids and most therapsids, but unlike sphenacodontids.

Lacrimal and Septomaxilla
Both appear side by side here, the latter external and the former internal, their plesiomorphic positions. One traditional trait separating pelycosaurs from therapsids is the septomaxilla becomes a surface (dermal) bone in therapsids. This can be seen to be happening in Stenocybus with the lacrimal retreating.

Preparietal
This new bone makes an early appearance in Stenocybus. Not sure why, but it appears to occupy a circular foramen in the posterior frontal, just anterior to the parietal foramen, which is relatively small here, as is the parietal itself.

Maxilla
This upper tooth bearing bone contacts the nasal and prefrontal. A tiny sliver of no consequence extends beneath the orbit.

Quadratojugal and Squamosal
Ophiacodonts have a horizontal quadratojugal. Sphenacodonts hide their’s beneath a squamosal. Therapsids have a vertical quadratojugal, sometimes visible, often not. Stenocybus shows the transitional stage, somewhat of a diagonal orientation, somewhat hidden by the squamosal. The squamosal is also overtaking the posterior jugal.

Supratemporal
Pelycosaurs have them. Therapsids don’t. Here they are found as flaked off loose elements near the parietal foramen. So they were of little consequence and probably disappeared rather than fusing to the squamosal.

Teeth
The anterior teeth of Stenocybus are rake-like. Does this indicate the start of an herbivorous diet? If so, perhaps the earliest stages of one.

Angular
In therapsids the posterior angular separates itself from the body of the mandible and develops a retroarticular process. We don’t see much separation here in Stenocybus.

Palate
The choanae are elongated here, shifting the pterygoid and palatine bones largely behind the tooth row, which presages the condition in anomodonts. However the ventral view of the palate remains quite narrow, as in ophiacodonts. The pterygoid transverse processes are vestiges. Palatal teeth were relatively larger. The epipterygoid was reduced to a slim buffer between the jawline and palate.

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
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)

wiki/Stenocybus

Is Stenocybus a Juvenile Sinophoneus?

Christian Kammerer (2011) reports that Stenocybus acidentatus (IGCAGS V 361, Middle Permian, Cheng and Li 1997) is a juvenile Sinophoneus. I don’t see it. Just too many differences, even with allometric growth. You be the judge. Here they are side by side (Fig.1).

Figure 1. Stenocybus (left) is considered a juvenile Sinophoneus (right) by Kammerer (2011). I think the changes are too great. You be the judge. Also note the differences in the new Stenocybus reconstruction, including the palate. Crushing and cracking make both skulls difficult to restore the sutures. If I have made any mistakes, please bring them to my attention.

Figure 1. Stenocybus (left) is considered a juvenile Sinophoneus (right) by Kammerer (2011). I think the changes are too great. You be the judge. Also note the differences in the new Stenocybus reconstruction, including the palate. Crushing and cracking make both skulls difficult to restore the sutures. If I have made any mistakes, please bring them to my attention.

According to Kammerer (2011), Cheng & Li (1997) “distinguished Stenocybidae from Anteosauridae on the basis of the lightly structured skull, large orbits, and small temporal fenestrae in the former. Additionally, they argued that the triangular maxilla, restriction of the septomaxilla within the naris, and anteriorly stretched lacrimal of Stenocybus are unique in therapsids and more closely resemble the condition in ‘pelycosaur’-grade synapsids.”

Kammerer continued, “Examination of the type skull reveals that some of the features used to diagnose Stenocybus are artefactual results of crushing. Examination of the type skull reveals that some of the features used to diagnose Stenocybus are artefactual results of crushing.” In contrast, I found the palate could be restored by skewing the ventral photograph in Photoshop (Fig. 1). The disarticulated palatal elements could be restored to their original positions.

Kammerer (2011) reported that a “small and light-structured skull” and “large-sized orbit and small triangular-shaped temporal fenestra are commonly observed in juvenile therapsids.” He also mentioned that juvenile anteosaurs lose their palatal teeth during maturation and that both share a “dorsal median ridge on the snout that is diagnostic for Sinophoneus,” among other traits.

I have not seen the other juvenile therapsids Kammerer is referring to. It would have been great to see these illustrated in his paper. In personal communication, Kammerer said he would try to find some images to send. When I get those we can reexamine this question with that data.

We’ll look at another pairing of Stenocybus tomorrow, another taxon that shares more traits in common. We’ll also look at the process of reconstructing the skull of Stenocybus the day after.

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)