SVP 18 – the pelycosaur Dimetrodon via Dr. Robert Bakker

Bakker et al (2015)
show evidence that Dimetrodon (Fig. 1) fed on aquatic prey as there were too few terrestrial reptilian herbivores to sustain their numbers.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

From the abstract
“In restorations, Dimetrodon often appear feeding upon large land herbivores, e.g., Diadectes and Edaphosaurus. 􀁄􀁑􀀃􀁄􀁏􀁗􀁈􀁕􀁑􀁄􀁗􀁌􀁙􀁈􀀃􀁙􀁌􀁈􀁚􀀏􀀃􀀲􀁏􀁖􀁒􀁑􀂶􀁖􀀃􀀤􀁔􀁘􀁄􀁗􀁌􀁆􀀃􀀩􀁒􀁒􀁇􀀃 Base Theory (AFBT) recognizes non-terrestrial prey as key for dimetrodont food webs. Over 45% of the bones are severely tooth-marked; ubiquitous shed Dimetrodon teeth are mingled with tooth-marked bones in every depositional unit. The CBB lacks any structures that indicate high current energy, so the hydraulic forces probably did not wash in bones from beyond the trough, though bloated whole carcasses could have floated in. There are 39 Dimetrodon, one each of the large herbivores Edaphosaurus and Diadectes, three of the large non-herbivore, non-apex carnivore Secodontosaurus, and three of the semi-terrestrial amphibian Eryops calculated form postcrania. Did benthic amphibians and fish fill the gap in prey? The benthic amphibian Diplocaulus is abundant in every bone-rich unit. Xenacanth sharks are very common in several layers; each shark carried a large, well ossified head spine. AFBT is corroborated: dimetrodonts fed intensively on aquatic prey at the CBB.”

Combine this with what we know of Spinosaurus, and finback reptiles appear to have been largely aquatic in habitat. That’s heresy joining the mainstream.

There is also a good Dimetrodon video (52 min.)
on YouTube featuring Dr. Bakker as he describes how the vast majority of Dimetrodon tails are missing, neatly cut and probably carried away for their meat (because that’s where the most of it is!) by other Dimetrodons.

References
Bakker RT et al. 2015. Dimetrodon and the earliest apex predators: The Craddock bone bed and George Ranch Facies show that aquatic prey, not herbivores, were key food sources. Journal of Vertebrate Paleontology abstracts.

Early Permian Sail-back Synapsids

Everyone knows
about Dimetrodon and Edaphosaurus, the two Early Permian sail back synapsid reptiles (Figs. 1, 2). Ianthasaurus was a more primitive sister to Edaphosaurus. Secodontosaurus was a sister to Dimetrodon. A taxon without a sail, Haptodus, was basal to both clades.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

Dimetrodon 
was a meat-eater. Edaphosaurus was a plant-eater. Every grade-schooler knows this. Skull size and sail design readily distinguish these two iconic taxa. Other traits, from teeth to toes also distinguish them.

Figure 2. Edaphosaurus, a sailback pelycosaur synapsid reptile of the Early Permian.

Figure 2. Edaphosaurus, a sailback pelycosaur synapsid reptile of the Early Permian. Note the tall caudal neural spines, distinct from Dimetrodon (figure 1).

Several specimens
of Dimetrodon are known (Fig. 3). Several attempts at reconstructing the skull of Edaphosaurus have been made (Fig. 2). I have the impression that there is not yet a single complete skull known for this taxon.

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

Figure 3. Click to enlarge. Sphenacodont skulls to scale. See Figure 2 for Edaphosaurus skulls. Not sure why Sphenacodon is not considered a species of Dimetrodon. The skulls are nearly identical.

The two sails
are either convergent or homologous. At this point, we don’t know. They both have individual designs with Edaphosaurus having curved neural spines with short spars on each “mast”. If they are homologous, Ianthsaurus (Fig. 4) is close to that common ancestor. At present, sail-less Haptodus is the last common ancestor.

Figure 4. Ianthasaurus, a basal edaphosaur.

Figure 4. Ianthasaurus, a basal edaphosaur not far from the common ancestor to all tailback pelycosaurs.

Interestingly,
at the same time that sails were developing in one synapsid clade, another clade, the Therapsida, led by Cutleria and Stenocybus was developing in different ways. At present only skulls are known, but more derived therapsids had longer legs and apparently a more active lifestyle, again dividing at their origin into meat-eaters, like Biarmosuchus, and plant-eaters, like Niaftasuchus and the Dromasauria.

The Early Permian
reminds me of the Early Triassic with regard to the great amount of evolutionary novelty appearing then, likely in response to new environs, weather patterns, predators and experiments in raising the metabolism in several clades. At this time basal diapsids and basal lepidosaurs were diversifying as well.

References
Case ED 1878. Descriptions of extinct Batrachia and Reptilia from the Permian formation of Texas. Proceedings of the American Philosophical Society xvii pp. 505-530.
Cope ED 1882. Third contribution to the history of the Vertebrata of the Permian formation of Texas. Proceedings of the American Philosophical Society (20): 447–461.
Marsh OC 1878. Introduction and succession of vertebrate life in America: Popular Science Monthly, v. 12, p. 513-527, 672-697.
Modesto SP 1994. The Lower Permian Synapsid Glaucosaurus from Texas. Palaeontology 37:51-60
Reisz RR and Berman DS 1986. Ianthasaurus hardestii n. sp., a primitive edaphosaur (Reptilia, Pelycosauria) from the Upper Pennsylvanian Rock Lake Shale near Garnett, Kansas. Canadian Journal of Earth Sciences 23(1): 77–91.
Reisz R R, Berman DS and Scott D 1992. The cranial anatomy and relationships of Secodontosaurus, an unusual mammal-like reptile (Pelycosauria: Sphenacodontidae) from the early Permian of Texas. Zoological Journal of the Linnean Society 104: 127–184.
Romer, AS 1936. Studies on American Permo-Carboniferous tetrapods. Problems of Paleontology, USSR 1: 85–93.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.

wiki/Ianthasaurus
wiki/Edaphosaurus
wiki/Secodontosaurus
wiki/Dimetrodon
wiki/Sphenacodon

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

re: Eupelycosaur Skin Impressions

Body impressions of tetrapods are very rare
Fossils with skin and no bones are rarely found. Most belong to amphibians from the Carboniferous/Permian of Europe and North American. Not talking about footprints with drag marks here. A recent paper by Niedziedzki and Bojanowski (2012) describes foot, belly and tail imprints from the Lower Permian ascribed to a eupelycosaur, like Dimetrodon.

From their abstract
“We describe a new specimen of a supposed Paleozoic tetrapod body impression from the Lower Permian Slupiec Formation in the Intra-Sudetic Basin, Poland. The size, integument morphology of belly and part of tail imprints, and the morphology of a well-preserved pes track diagnose the specimen and readily distinguish it from other described specimens of body impressions of Paleozoic tetrapods. The eupelycosaur identity of this new specimen is based on the identification of the footprint Dimetropus leisnerianus (Geinitz, 1863), which is connected with the inferred body imprint. The morphology of integument impressions indicates the presence of the various-sized square or rectangular-shaped scales on the bottom part of the belly and tail of this eupelycosaurid trackmaker.”

The trackmaker evidently ‘sat down’ after taking a walk. (I have not seen the paper, yet, btw.)

Luckily the Eupelycosauria are still with us
Laurin and Reisz (1997) redefined the Eupelycosauria “to designate a clade of synapsids that includes most pelycosaurs, as well as all therapsids and mammals.” That was meant to include all pelycosaurs (sans caseaurids, which are not related anyway) and all their descendants. If that indeed included Varanopidae, then the large reptile tree indicates that the Eupelycosauria also includes all new Archosauromorpha diapsids, which includes crocs, dinos and birds. That unfortunate bit of news is valid because current members of the Eupelycosauria include the Varanopidae, which includes the Mycterosaurinae, which includes Heleosaurus and Mesenosaurus, which are currently the best diapsid ancestor candidates in the large reptile tree. Take those few taxa out of the definition and the original intention of the Laurin and Reisz (1997) definition remains.

But, back to those scales
Similar to its Early Permian ancestors, a modern eupelycosaur (Didelphis marsupialis) still retains a scaly tail (Fig. 1), giving us a nice peek into those ancient times 295 million years ago.

Opossum tail showing rectangular eupelycosaurian scales

Figure 1. Opossum tail showing rectangular eupelycosaurian scales. From between such scales hairs emerge and therein lies the transition from reptile to mammal (among many other concurrent traits, of course).

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:
Laurin M and Reisz RR 1997. Autapomorphies of the main clades of synapsids – Tree of Life Web Project.
Niedziedzki G and Bojanowski M 2012. A Supposed Eupelycosaur Body Impression from the Early Permian of the Intra-Sudetic Basin, Poland. Ichnos 19(3):150-155.  DOI:10.1080/10420940.2012.702549 link

Oppossum tail link

Convergence at the Pelycosaur/Therapsid Transition

A good look at the base of the Therapsida (Fig. 1) reveals several interesting convergences (taxa that look alike superficially). Prior studies used shared traits between Dimetrodon in the Sphenacodontia and Eotitanosuchus in the Therapsida to support a link between sphenacodonts and basal therapsids, but that is not supported in the large reptile tree. Previously unnoticed, the sphenacodont, Haptodus, bears a superficial resemblance to a basal therapsid, Stenocybus, which may be a juvenile of a longer snouted form.

 

Figure 1. Click to enlarge. Basal Therapsida. Note superficial similarities between Haptodus and Stenocybus. Note superficial similarities between Dimetrodon and Eotitanosuchus.

Figure 1. Click to enlarge. Basal Therapsida. Note superficial similarities between Haptodus and Stenocybus. Note superficial similarities between Dimetrodon and Eotitanosuchus.

Dimetrodon and Eotitanosuchus
The  dorsally convex skull in Dimetrodon and Eotitanosuchus (Fig. 1) appears to be a shared trait, but it is convergent in the large reptile tree. Both share a deeply convex maxilla and similar proportions in the orbit vs. rostrum. Both share a posteriorly sloping cranium, broken by elevated lateral temporal fenestrae in Eotitanosuchus. The jawline was shallowest beneath the orbit in both taxa.

Haptodus and Stenocybus
The short rostrum/large orbit skull shapes of Haptodus and Stenocybus (Fig. 1) were arrived at via convergence, as are the relatively short canines and convex rostrum. A larger suite of traits and several intervening taxa separate these two. Now, Stenocybus was never allied with Haptodus in the literature. Stenocybus was previously allied with anteosaurid dinocephalians (stepped incisors a key trait), but here it nests more parsimoniously with hipposaurids.

The strength of the large reptile tree (lots of taxa, lots of traits) separates these convergent taxa. In a smaller study they would have nested closer together.

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.

Ophiacodon and the Origin of the Therapsida

Nobody cares about Ophiacodon, but we should.
Ophiacodon is an overlooked key taxon in the evolution of synapsids, therapsids and by all accounts, mammals and humans.

Ophiacodon

Figure 1. Ophiacodon, large, squat and amphibious - not the perfect therapsid precursor... or is it?

Overlooked for Good Reason
Ophiacodon was large, low-slung, pretty darn ugly and apparently nothing like the lithe little mammals it would give rise to. (As an aside, let’s not forget that — way back — pterosaurs also arose from bulky diadectids and birds had their origins with equally bulky and amphibious erythrosuchids.) Various Ophiacodon species grew larger and more specialized throughout the Early Permian, so therapsids and sphenacodonts would have arisen from less specialized, smaller, earlier members.

 Biarmosuchus, the most basal therapsid.

Figure 2. Biarmosuchus, the most basal therapsid.

The Basal Therapsid
Most studies (other than those including Tetraceratops) place Biarmosuchus at the base of the Therapsida. Now all we have to do is find the pelycosaur that most parsimoniously matches Biarmosuchus.

Biarmosuchus vs. the Sphenacodonts
Traditional studies have always placed sphenacodonts like Haptodus, Sphenacodon and Dimetrodon (Figure 3) as predecessors to Biarmosuchus largely due to the presence of the reflected lamina as a shared trait. A reflected lamina is that thin, circular bony leaf peeling off the back of the mandible. In reptiles that mandible bone is called the angular. In mammals the angular and reflected lamina shrinks to frame the eardrum.

The reflected lamina is important, but overall Ophiacodon looks more like Biarmosuchus (Figure 3). However, it’s not good practice to rely on just one character, but a whole suite to make a most parsimonious nesting.

No doubt therapsids were derived from pelycosaurs, but the key sister taxon has not been found yet.

 

Ophiacodon and the Origin of the Therapsida

Figure 3. Ophiacodon and its phylogenetic successors, the pelycosaurs and the therapsids.

The Problem(s) with Sphenacodonts as Therapsid Ancestors
Traditionally the sphenacodonts, Haptodus and Dimetrodon have been considered the closest sisters to the Therapsida, but sphenacodonts have a relatively shorter, taller skull, a short premaxillary ascending process, a kink at the premaxilla/maxilla jawline, a shorter, taller rostrum and a deeply concave posterior jawline. Biarmosuchus has none of these traits. But Eotitanosuchus does.

Eotitanosuchus
Eotitanosuchus
 (Figure 3) has often been compared to Dimetrodon. Both share a convex rostral margin and both lose or greatly reduce the pre-canine maxillary teeth. However, taken as a whole we find that Eotitanosuchus nests between Biarmosuchus and various higher therapsids, especially gorgonopsids in the lineage of mammals. So the characters Eotitanosuchus seemed to share with Dimetrodon were convergent.

The Reptile Family Tree
Here Biarmosuchus nests closer to Ophiacodon. Haptodus and Dimetrodon  branch off as sisters to this node. However, if we consider all the clues together, the base of the Therapsida actually lies somewhere between Ophiacodon and Haptodus, with a lean toward Ophiacodon.

Biarmosuchus vs. Ophiacodon
Several Biarmosuchus traits shared with Ophiacodon are not found in HaptodusSphenacodonand Dimetrodon: 1) Premaxilla longer than naris; 2) Rostrum twice as long as tall; 3) Quadratojugal not reduced to anearly invisible nub; 4) Premaxilla rises anteriorly; 5) Transition from premaxilla and maxilla without a kink.

Biarmosuchus vs. Haptodus
Fewer Biarmosuchus traits shared with Haptodus are not found in Ophiacodon: 1) Reflected lamina. 2) Anterior dentary deep and ventral margin sharply angled. These traits would be expected to appear in the last common ancestor of the Therapsida originating between Ophiacodon and Haptodus.

In therapsids the nasal is relatively narrow, but in sphenacodonts it is broader. The purported septomaxilla in therapsids appears to be the anterior lacrimal beneath the ascending process of the maxilla, perhaps laminated over it. Check all these out on Figure 3. Finally, let’s take a look at the right hand of our candidates. Biarmosuchus had a robust manus, not as robust as Ophiacodon, but not nearly as gracile as Haptodus.

Comparing the right manus of Haptodus, Biarmosuchus and Ophiacodon.

Figure 4. Comparing the right manus of Haptodus, Biarmosuchus and Ophiacodon. Biarmosuchus is right in the middle, literally and morphologically. The reduction of those three disc-like phalanges in Biarmosuchus signals a more erect stride.

We’ll Keep Looking
Someday we’ll find a small, early ophiacodont with longer legs, a pretty big canine, a shorter postorbital region and a reflected lamina. Essentially I’ve just described Biarmosuchus, haven’t I?

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
Marsh OC 1878. Notice of new fossil reptiles: American Journal of Science, 3rd series, v. 15, p. 409-411.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.
Tchudinov PK 1960. Diagnosen der Therapsida des oberen Perm von Ezhovo: Paleontologischeskii Zhural, 1960, n. 4, p. 81-94.

wiki/Biarmosuchus
wiki/Ophiacodon