Heleosuchus – the enigma has nested in the Rhynchocephalia

Figure 1. Heleosuchus, a former enigma, nests in the middle of the Rhynchocelphalia, between Planocephalosaurus and Sphenodon.

Figure 1. Heleosuchus, a former enigma, nests in the middle of the Rhynchocelphalia, between Planocephalosaurus and Sphenodon. Here is Heleosuchus in situ and Planocephalosaurus restored to scale. Click to enlarge.

Heleosuchus (Fig. 1) has been an enigma since first described by Owen 1876. Several heavy-hitters in paleontology (Broom 1913, Evans 1984, Carroll 1987) have taken a whack at it without resolving its relations.

According to Wikipedia, “It was originally described as a species of Saurosternon, but was later recognized as a separate taxon by R. Broom. Heleosuchus is suggested as being either an early diapsid reptile, not closely related to other lineages, or as being an aberrant and primitive lepidosauromorph. Heleosuchus shares the hooked fifth metatarsal found in some other diapsids, such as primitive turtles (Odontochelys), lepidosauromorphs, and archosauromorphs, but it also resembles ‘younginiform’-grade diapsids in its gross morphology.  Heleosuchus may also share a thyroid fenestra with these higher diapsid reptiles as well, but the identity of this feature is disputed.”

Based on tracings by Carroll (1987) the large reptile tree (not updated yet) Heleosuchus nested between Planocephalosaurus (Fig. 1) and the clade of Sphenodon and Kallimodon in the middle of the Rhynchocephalia. What was identified as a scapula must be a portion of the interclavicle instead.

However, even Carroll was not sure of the identification of several elements. Unfortunately it appears as though the last time someone published on Heleosuchus was prior to the advent of computer-assisted phylogenetic analysis. Carroll notes, “if a thyroid fenestra is present and the fifth metatarsal is hooked, Heleosuchus would definitely represent a lineage distinct from the younginoids. These features are present in Late Triassic sphenodontids and Jurassic lizards, but they are also present in other groups. In conclusion, the characters that are preserved point to a position near the base of the lepidosauromorph assemblage, possibly close to the younginoids but perhaps representing a distinct lineage.”

What appears to be bothering Carroll is the early appearance of Heleosuchus in the Late Permian of South Africa relative to the lepidosaurs known to him at the time. That early appearance doesn’t bother the large reptile tree, which nests several other Permian contemporaries just as high if not higher in the reptile family tree.

References
Broom R 1913. A revision of the reptiles of the Karroo. Annals of the South African Museum 7: 361–366.
Carroll RL 1987. Heleosuchus: an enigmatic diapsid reptile from the Late Permian or Early Triassic of southern Africa”. Canadian Journal of Earth Sciences24: 664–667.
Evans SE 1984. The anatomy of the Permian reptile Heleosuchus griesbachi. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte, 12: 717-727.
Owen R 1876. Descriptive and illustrated catalogue of the fossil Reptilia of South Africa in the collection of the British Museum. Trustees of the British Museum (Natural History), London, UK.

wiki/Heleosuchus

Adding Homo sapiens to the large reptile tree

Not sure why I didn’t think to do this earlier.
I added Homo sapiens to the large reptile tree (still not updated) and ran a phylogenetic analysis to see where we nest. To no one’s surprise Homo (Fig. 1) nested with the cynodont Procynosuchus among the Therapsida (no other mammals are yet entered).

Figure 1. Homo sapiens alongside sister taxa Australopithecus and Ardipithecus (both in gray).

Figure 1. Homo sapiens alongside sister taxa Australopithecus and Ardipithecus (both in gray). Click to learn more.

Entering characters for Homo and Procynosuchus
was more than enlightening as so many traits were shared between the two. You should try it sometime!

Figure 2. Procynosuchus, a basal cynodont therapsid synapsid sister to humans in the large reptile tree (prior to the addition of advanced cynodonts including mammals).

Figure 2. Procynosuchus, a basal cynodont therapsid synapsid sister to humans in the large reptile tree (prior to the addition of advanced cynodonts including mammals). Click to learn more.

What happens when taxa are excluded? (How deep can we go?)
The contribution of cynodont traits to the story of human evolution was more powerful than I thought. I was surprised at one happened when I took one step further.

Deleting only Procynosuchus
results in Homo nesting results in Homo nesting between Dibamus and Tamaulipasaurus (Fig. 2) two burrowing skinkomorph squamates. My guess is the fusion/loss of so many skull bones, the brevity of the rostrum, the great depth of the coronoid process of the dentary and the complete lack of postcranial characters for the two burrowing taxa are attracting the taxon Homo with similar skull traits.

With the present character and taxon list, these skinkomorphs nest closer to humans than Biarmosuchus and more basal synapsids, like Dimetrodon. They’re just not human enough.

Biarmosuchus, the most basal therapsid.

Figure 3. Biarmosuchus, the most basal therapsid and not a cynodont. Despite nesting as a basal therapsid, its traits do not attract the taxon Homo more than others do. 

You think THAT’S ridiculous. Let’s take the next step…

Figure 2. Tamaulipasaurus nests with Homo sapiens when the basal cynodont, Procynosuchus, is excluded.

Figure 2. Tamaulipasaurus nests with Homo sapiens when the basal cynodont, Procynosuchus, is excluded. The fusion of skull bones, the short rostrum, and the large coronoid process of the dentary are traits shared with humans.

Deleting all the skinkomorph squamates
results in Homo nesting as a turtle/pareiasaur ancestor. Here the short face, anterior nares, tall pelvis and loss of manual and pedal phalanges appear to attract Homo to turtles like Proganochelys.

Proganochelys. Formerly the most primitive turtle.

Figure 2. Proganochelys. Formerly the most primitive turtle. Click to learn more. 

See what happens with taxon exclusion?
Strange bedfellows can result. So many current problems and enigmas in paleontology can be readily settled with a large enough family tree.

In the same light, I challenge paleontologists
to add thalattosaurs to Vancleavea studies… to add fenestrasaurs to pterosaur studies… to add mesosaurs to ichthyosaur studies… and to add millerettids to caseasaur studies. There’s no harm in doing so, and we all might learn something.

 

Restoring Scoloparia as a procolophonid AND as a pareiasaur

Today I have a quandary…
Is Scoloparia a procolophonid or a pareiasaur? I’ve looked at it both ways (Figs. 1, 2). It nests both ways (depending on the restoration), and at least one way is wrong.

This problem highlights more basic problems
found within the Procolophonidae, some of which nest in the large reptile tree (still not updated)  with diadectids (Procolophon and kin, Fig. 1), with pareiasaurs (Sclerosaurus) and the rest nest as pre-Lepidosauriformes (Owenetta and kin). Conventionally procolophonids are considered parareptiles. Cisneros lists Nyctiphruretus as the outgroup and owenettids as basal taxa within the Procolophonidae. The large reptile tree replicated that outgroup only for the owenettids.

Scoloparia glyphanodon (Sues and Baird 1998) is currently represented by several specimens, three of which are figured, colorized and restored here (Figs. 1, 2). All three differ in size. Comparable skulls differ in morphology. This has been attributed to ontogeny.

Figure 1. Scoloparia restored here as a procolophonid together with other procolophonids.

Figure 1. Scoloparia restored here as a procolophonid together with other procolophonids. Click to enlarge. The large YPM mandible is a definite procolophonid. The small 82.1 specimen is a definite procolophonid. The holotype is the big question mark.

Clearly the referred specimens
(the dentary and the small 82.1 specimen) are procolophonids. Only seven blunt and rotated teeth in a mandible that tips down anteriorly along with gigantic orbits mark these taxa as procolophonids. They compare well with other procolorphonids.

Figure 2. Scoloparia restored as a pareiasaur close to Elginia along with several other pareiasaurs for comparison. Sclerosaurus typically nests as a procolophonid, but even with the removal of all skull traits, it nests as a small pareiasaur.

Figure 2. Scoloparia restored as a pareiasaur close to Elginia along with several other pareiasaurs for comparison. Sclerosaurus typically nests as a procolophonid, but even with the removal of all skull traits, it nests as a small pareiasaur. The new restoration reidentifies several bones. Note the convergence with the procolophonids in figure 1.

The problem is in the large holotype
The 83.1 specimen holotype of Scoloparia was preserved without a skull roof or palate, so the nasals, frontals and parietals are restored here.

Originally
the size and morphological differences were attributed to the juvenile status of the smaller specimen. H. Sues wrote to me, “Both specimens have the same peculiar ‘cheek’ teeth, which are unlike those of any other procolophonid.” 

I think what Dr. Sues means is shown below in figure 3. The teeth of the referred specimen attributed to Scoloparia have multiple cusps, unlike most procolophonids, but approaching the serrated morphology of pareiasaurs. The convergences are mounting!! And now you see why this is a quandary!

Figure 4. Teeth compared. Elginia, Scolaparia (referred), Leptopleuron and Diadectes.

Figure 3. Teeth compared. Elginia, Scolaparia (referred), Leptopleuron and Diadectes, a stem procolophonid. Oddly the very procolophonid Scoloparia (referred specimen) does have peculiar teeth for a procolophonid. They are serrated somewhat like those in the pareiasaur, Elginia.

I have asked to see images of the teeth for the Scoloparia holotype. No reply yet.

The mystery of the holotype 
Teeth were not illustrated by Sues and Baird for the holotype 83.1 specimen, who reported the mandible was articulated. The authors described two premaxillary, six maxillary and eight dentary teeth. That low number of teeth point toward a procolophonid ancestry. The upper anterior four teeth are described as incisiform with bluntly conical crowns that are rounded in cross section. The first premaxillary tooth is reported to be much larger than the other teeth. A large medial pmx tooth also points toward a procolophonid ancestry, as we’ve already seen with Colobomycter. In Elginia (Fig. 3)  the many small teeth are slightly constricted at the base and serrated at the crown as in other pareiasaurs.

Figure 4. Elginia colorized in four views. Note the rotation of the tabulars to the dorsal skull.

Figure 4. Elginia colorized in four views. Note the rotation of the tabulars to the dorsal skull. Click to enlarge. Note the many similarities to the pareiasaur-like restoration of Scoloparia. 

Nuchal osteoderms
Sues and Baird noted “nuchal (neck) osteoderms” preserved posterior to the skull in the 83.1 holotype of Scoloparia. Cisneros (2008) reports osteoderms have only been found in Sclerosaurus and Scoloparia. Since Sclerosaurus nests here as a pareiasaur, that means no other procolophonids have osteoderms. Hmmm.

Reversals in the skull roof of pareiasaurs 
In the large reptile tree pareiasaurus are sisters to turtles (all derived from Stephanospondhylus) and bolosaurids, all derived from Milleretta. In Stephanospondylus (Fig 5) a reversal takes place in which the postparietals (or are they tabulars?) rotate to the dorsal surface of the skull and the supratemporals develop small horns. These traits usually appear on pre-amniotes.

Figure 2. Stephanospondylus skull in two views. Note the rotation of the post parietals to the dorsal skull along with the transformation of the supratemporals into small horns.

Figure 5. Stephanospondylus skull in two views. Note the rotation of the post parietals to the dorsal skull along with the transformation of the supratemporals into small horns.

This dorsalization of the tabulars
becomes even more apparent in pareiasaurs (Fig. 2) and Elginia (Fig. 4). If the purported nuchals of Scoloparia are actually large supratemporals, tabulars, and opisthotics, then it’s a pareiasaur. If so, a foramen magnum is also present topped by a supraoccipital and two flanking exoccipitals. What a quandary!

Not quite enough to go on
I am working from a 2D line drawing here (from Sues and Baird 1998), not a photograph. So I await images of the teeth and any other data that may come down the pike. If new data ever comes in, I will let you know. For now, can’t tell if we’re dealing with autapomorphic nuchal osteoderms on a procolophonid or dorsalized tabulars and an occiput on a pareiasaur.

References
Cisneros JC 2008. Phylogenetic relationships of procolophonid parareptiles with remarks on their geological record. Journal of Systematic Palaeontology): 345–366.
Sues HD and Baird D 1998. Procolophonidae (Reptilia: Parareptilia) from the Upper Triassic Wolfville Formation of Nova Scotia, Canada. Journal of Vertebrate Paleontology 18:525-532.

The vampire pterosaur has a new sister: Daohugoupterus

Cheng et al. (2014)
present a new small, late Jurassic pterosaur, Daohugoupterus. They were not quite sure what it was, assigning it to Pterosauria incerta sedis. The specimen is represented by an articulated skeleton lacking hind limbs, the anterior skull and two proximal wing phalanges (Fig. 1). Wing tip soft tissue was preserved. I believe the ulna and radius are just beneath the surface based on the positions of the humerus and carpus/metacarpus. The rest of the wing is likely twisted beneath these elements as the distal two wing phalanges frame the soft tissue.

Figure 1. Click to enlarge. Daohugoupterus in situ, colorized (left) and as originally traced (right). You'll note that DGS pulled out more details than firsthand tracing.

Figure 1. Click to enlarge. Daohugoupterus in situ, colorized (left) and as originally traced (right). You’ll note that DGS pulled out more details than firsthand tracing.

From their abstract:
“Daohugou is an important locality of the Jurassic Yanliao Biota, where
only two pterosaurs have been described so far (Jeholopterus and
Pterorhynchus). Here we report a new genus and species, Daohugoupterus
delicatus gen. et sp. nov. (IVPP V12537), from this region, consisting
of a partial skeleton with soft tissue. The skull is laterally
compressed, differing from Jeholopterus. The antorbital fenestra is
larger than in Pterorhynchus. The upper temporal fenestra is unusually
small. The short cervical vertebrae bearing cervical ribs indicate
that it is a non-pterodactyloid flying reptile. The sternal plate is
triangular, being much wider than long. The deltopectoral crest of
humerus is positioned proximally and does not extend further down the
shaft, a typical feature of basal pterosaurs. Daohugoupterus also
differs from the wukongopterids and scaphognathids from the Tiaojishan
Formation at Linglongta, regarded to be about the same age as the
Daohugou Bed. The new specimen increases the Jurassic
non-pterodactyloid pterosaur diversity of the Yanliao Biota and is the
smallest pterosaur from Daohugou area so far.”

DGS
Digital Graphic Segregation was used to pull details out of the skeleton. While the original paper described small upper temporal fenestra (that are indeed there) the figure did not show this detail. No skull bones were identified. The vertebrae were outlined without details. Color tracing and reconstruction (fig. 2) help bring this specimen ‘back to life.’ The length of the rostrum is unknown, but after phylogenetic analysis nesting with Jeholopterus, the rostrum was reconstructed like it’s sister taxon.

Reconstruction
A reconstruction of all available elements resulted in a sister to Jeholopterus, sharing many traits including the strong reduction of anterior cervical vertebrae, robust cervical vertebrae posteriorly, wide ribs creating a pancake-like torso, and a fragile skull with very large orbit (Fig. 2). Notably, Jeholopterus was a contemporary from the same Late Jurassic formation.

Figure 2. Click to enlarge. Daohugoupterus reconstructed.

Figure 2. Click to enlarge. Daohugoupterus reconstructed.

If you take a bone-by-bone survey
of the the DGS tracing vs. the original tracing (Fig. 1), you’ll find many differences. This is a difficult fossil and the accuracy of my tracings depending to a large part on testing each part within an evolving reconstruction (Fig. 3). Attempting reconstructions of roadkill pterosaurs is something conventional paleontologists are loathe to do, and they never ask me to help. Hence this blog.

Figure 1. Jeholopterus in lateral view. Note the extreme length of the dermal fibers, unmatched by other pterosaurs.

Figure 3. Jeholopterus in lateral view. Note the wide ribs.

In a side-by-side comparison (Fig. 4)
Jeholopterus and Daohugoupterus do share many traits and are roughly the same size. Daohugoupterus does not have the robust limbs and surgically curved claws that Jeholopterus has, but Daohugoupterus does have enormous eyes, probably for night vison. They share a wider than deep torso which enables them to cram their bellies, but still keep an aerodynamic disc-like shape (also see Sharovipteryx for something similar). They also share a very robust neck that gets very gracile close to the skull. I presume this gives both pterosaurs a wider range of motion at the skull/neck juncture. But why does most of the neck have to be stronger than the dorsal vertebrae?

Figure 3. Jeholopterus and Daohugoupterus in side-by-side comparison to scale. The wings were relatively short in Daohugoupterus and the pelvis was small. The skull was relatively narrower, but the torso was just as broad.

Figure 3. Jeholopterus and Daohugoupterus in side-by-side comparison to scale. The wings were relatively short in Daohugoupterus and the pelvis was small. The skull was relatively narrower, but the torso was just as broad.

On a side note
Experiment.com has accepted by submission and my first crowd-source funding project has started today. See details at:
https://experiment.com/projects/the-reptile-evolution-project

References
Cheng X, Wang X, Jiang S and Kellner AWA 2014. Short note on a non-pterodactyloid pterosaur from Upper Jurassic deposits of Inner Mongolia, China. Historical Biology (advance online publication) DOI:10.1080/08912963.2014.974038

 

Mea culpa. Azendohsaurus is a sister to the protorosaur Pamelaria.

Adding taxa to the large reptile tree (now up to 425 taxa and still not updated because updates are under review at academic publications) exposes ‘strange bedfellows’ (oddballs that don’t really fit within a topology). And yes, like everyone else, I make mistakes. But I also fix mistakes whenever I find them.

Earlier I nested Azendohsaurus with Trilophosaurus. That was an error. Re-examination re-nested Azendohsaurus with the derived protorosaur Pamelaria (Fig. 1).

Figure 1. Pamelaria compared to the skull of Azendohsaurus. The description of the postcrania of Azendohsaurus by Nesbitt et al. matches that of Pamelaria.

Figure 1. Pamelaria compared to the skull of Azendohsaurus. The description of the postcrania of Azendohsaurus by Nesbitt et al. matches that of Pamelaria.

Azendohsaurus has often been considered ‘bizarre’. It has been hard to nest. Originally (Detuit 1972) it was considered a dinosaur. Only the skull has been published, but post-crania is known and was described in an abstract (see below).

Pamelaria is a little off the map for most paleontologists, so that may be why it was not previously considered as a sister. The benefit of having 425 taxa to nest with, Azendohsaurus is going to find a >most< parsimonious node.

Figure 1. The skull and palate of Azendohsaurus, a sister to Pamelaria.

Figure 1. The skull and palate of Azendohsaurus, a sister to Pamelaria.

I discovered this new nesting by removing all the previous sister taxa among the rhynchocephalians, then running PAUP to see where Azendohsaurus would nest if it didn’t nest with Trilophosaurus and kin. It’s a good experiment you can do with any taxon. It even works with turtles and pterosaurs.

I also found a probable error in an online skull reconstruction of Azendohsaurus. Notes have been passed to Flynn, Nesbitt and Parrish. We’ll see what happens.

Figure 3. It looks like the prefrontal/lacrimal in the color photo was inverted. By rotating these elements  180 degrees (in grey tones), the odd lacrimal becomes the ascending process of the maxilla, exactly matching the more complete one.

Figure 3. It looks like the prefrontal/lacrimal in the color photo was inverted. By rotating these elements 180 degrees (in grey tones), the odd lacrimal becomes the ascending process of the maxilla, exactly matching the more complete one. Like Aznedohsaurus, Pamelaria also has a major process at the dorsal quadrate. The naris is not visible in lateral view. As in Pamelaria, the naris was likely dorsal. 

The post-crania has not been published, but an abstract appeared in 2013.
Nesbitt et al. (2013) report, Azendohsaurus madagaskarensis possessed an elongated neck, short tail, and stocky limbs. The manus and pes have unexpectedly short digits, terminating in large, recurved ungual phalanges. Together with the skull, knowledge of the postcranial skeleton elevates A. madagaskarensis to another highly apomorphic and bizarre Triassic archosauromorph.”

That sounds a lot like Pamelaria (Fig. 1), doesn’t it? We’ll see when it gets published.

References
Dutuit J-M 1972. Découverte d’un Dinosaure ornithischien dans le Trias supérieur de l’Atlas occidental marocain. Comptes Rendus de l’Académie des Sciences à Paris, Série D 275:2841-2844.
Flynn JJ, Nesbitt, SJ, Parrish JM, Ranivoharimanana L and Wyss AR 2010. A new species of Azendohsaurus (Diapsida: Archosauromorpha) from the Triassic Isalo Group of southwestern Madagascar: cranium and mandible”. Palaeontology 53 (3): 669–688. doi:10.1111/j.1475-4983.2010.00954.x .
Nesbitt, S, Flynn J, Ranivohrimanina L, Pritchard A and Wyss A 2013. Relationships among the bizarre: the anatomy of Azendohsaurus madagaskarensis and its implications for resolving early archosauromroph phylogeny. Journal of Vertebrate Paleontology abstracts 2013.

wiki/Azendohsaurus

Phylogenetic bracketing and pterosaurs – part 2

Two posts ago we looked at part 1 of this topic.

Since pterosaurs (and other tritosaurs) nest between rhynchocephalians and squamates, there are a few traits they likely shared based on phylogenetic bracketing (unless specifically excepted based on fossil evidence). Putting the rhynchocephalians aside for the moment, according to Evans (2003) squamate traits include:

(1)  a specialized quadrate articulation with a dorsal joint typically supplied by the deeply placed supratemporal, reduced squamosal, and distally expanded paroccipital process of the braincase; reduction/loss of pterygoid/quadrate overlap; loss of quadratojugal – all present in basal tritosaurs, but quadrate becomes immobile in Macrocnemus and later taxa.

(2) loss of attachment between the quadrate and epipterygoid, with the development of a specialized ventral synovial joint between the epipterygoid and pterygoid — also present up to Huehuecuetzpalli, but absent in Macrocnemus and later taxa.

(3) subdivision of the primitive metotic fissure of the braincase to give separate openings for the vagus nerve (dorsally) and the perilymphatic duct and glossopharyngeal nerve (via the lateral opening of the recesses scalae tympani ventrally). This leads to the development of a secondary tympanic window for compensatory movements and is associated with expansion of the perilymphatic system and closure of the medial wall of the otic capsule — in fossil tritosaurs these details may not be known and certainly not by me… yet.

(4) loss of ventral belly ribs (gastralia) — Basal tritosaurs, up to Homoeosaurus have gastralia. Then they don’t until Macrocnemus and all later taxa.

(5) emargination of the anterior border of the scapulocoracoid — Basal tritosaurs share this trait. Macrocnemus and tanystropheids refill the emargination. Fenestrasaurs, including pterosaurs expand the emargination resulting in a strap-like scapula and stem-like coracoid, both representing the posterior rims of these bones.

(6) hooked fifth metatarsal with double angulation — shared with tritosaurs and more complex mesotarsal joint — in tritosaurs the mesotarsal joint is simple.

(7a) a suite of soft tissue characters including greater elaboration of the vomeronasal apparatus;

(7b) a single rather than paired meniscus at the knee;

(7c) the presence of femoral and preanal organs;

(7d) fully evertible hemipenes;

(7e) and pallets on the ventral surface of the tongue tip — none of these have been noted in soft tissue fossils.

References
Evans SE 2003. At the feet of the dinosaurs: the origin, evolution and early diversification of squamate reptiles (Lepidosauria: Diapsida). Biological Reviews, Cambridge 78: 513–551.

 

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