More on Ophiacodon + therapsids

A new nesting for the two new Varanosaurus specimens according to the large reptile tree. These two nest at the base of the main group of synapsids and close to the protodiapsids (synapsid taxa leading toward the diapsid, Petrolacosaurus). Note, Ophiacodon nests three nodes away.

Figure 1. New nestings for Varanodon here, but therapsids still evolve from a sister to Ophiacodon.

Yesterday I updated my earlier Varanosaurus post because new data (taxa) had come in and certain old data reexamined with a more critical eye. Mickey Mortimer’s instructive note (see below) was important to this next step. As a result, Varanosaurus moved closer to Ophiacodon in the large reptile tree (still an outgroup, Fig. 1) and Archaeothyris nested further away, more primitively. If Archaeothyris is still considered an ophiacodontid, then Apsisaurus and Varanosaurus are too. So are all the sphenacodontids.

Moreover, as you’ll see below, several basal therapsids also share traits with Varanosaurus + Ophiacodon. This makes therapsids and mammals ophiacodontids. This is a heretical relationship previously and still overlooked that we examined and demonstrated earlier here. Now, courtesy of Mortimer’s list of Benson’s (2012) traits describing the Ophiacondontidae, the evidence is more substantial for a therapsid relationship.

M. Mortimer wrote in response to my earlier Varanosaurus post here (answers follow bullets — this starts slow, but gains momentum):

MM: Benson (2012) found the following [traits] to support Ophiacodontidae-

7. Temporal fenestra morphology: narrow dorsoventrally (<0.5x temporal height) with deep temporal bar [posterior process of the jugal] (0). (not in your matrix)

  • good trait. In evolution, fenestrae tend to start small (in this case narrow), coming from nothing (solid cheek). But note the small temporal fenestra has the same shape in therapsids only larger.

37. Maxilla, supracanine buttress on medial surface: present, may be expanded into lateral margin of internal naris [choana] (1). (not in your matrix)

  • I have few to no traits visible on inner bone surfaces. For my work I need more commonly found external traits for 340 taxa.

94. Stapes, shaft: rod-like, quadrate process small or indistinct (0); blade-like with prominent quadrate process, substantially longer than dorsal process (1). (not in your matrix)

  • I have few to no traits so tiny as this.

192. Humerus, ridge connecting deltopectoral crest to head: single, fossa absent (1). (not in your matrix)

  • I have no traits based on limb fossa.

219. Pubis, pubic tubercle anteroventral to acetabulum: present, projects dorsally (2). (not in your matrix)

  • This is character 189 on the large reptile tree. I did not notice the dorsal projection on ophiacodontids.

Further, he [Benson 2012] found Varanosaurus and Ophiacodon (but not Archaeothyris, or unknown in the latter) to share-

Here’s where it gets interesting… Yesterday I found Varanosaurus and Apsisaurus to be the outgroup to the Ophiacodontidae (including Therapsida).

2. Snout proportions, antorbital length in relation to temporal length: snout very long, antorbital length at least twice the temporal length (3). (you have a few snout length characters, but I’m not sure if any are equivalent)

14. Premaxilla, orientation of ascending (supranarial) process: slopes posterodorsally at an angle < 75° (2). (your character 13 is similar, but seemingly not equivalent)

  • Also in Biarmosuchus, Nikkasaurus many other therapsids

17. Premaxilla, marginal tooth count: five–six (1). (character 108 in yours)

  • Also in Biarmosuchus, Stenocybus, Nikkasaurus.

27. Maxilla, ventral surface: pronounced convexity (1). (character 28 in yours)

  • Also in Biarmosuchus, Stenocybus, many other therapsids

32. Maxilla, precanniniform tooth count: six or more (2). (not in your matrix)

  • Yes, perhaps by convergence or homology, not in Archaeothyris and unknown in Apsisaurus.

47. Nasal, length: longer than frontal (2). (character 25 in yours, but I’d say it’s correlated with character 2 in Benson’s, so that’s a case of his matrix being equally bad)

  • Perhaps. Also in BiarmosuchusStenocybus, many other therapsids

54. Prefrontal, lateral surface: concave, forming antorbital recess [prefrontal pocket] (1). (not in your matrix)

70. Postorbital and jugal, medial orbital process (deep, dorsoventrally tall medial flange): present (1). (not in your matrix)

  • Also in Stenocybus, unknown in Archaeothyris.

75. Jugal, posterior ramus length: long, extending to posterior end of temporal fenestra (or past temporal midlength in taxa that lack a temporal fenestra) (1). (character 63 in yours)

  • Also in Stenocybus.

76. Jugal–squamosal ventral contact, perforated by small, elongate fenestra: present, upper margin enclosed by anteroventral exension of the squamosal (1). (not in your matrix)

  • Also in Stenocybus, Nikkasaurus, many other therapsids

79. Squamosal, anteroposterior breadth of posttemporal portion on lateral surface of skull: broad, just over half the breadth of the temporal region, restricting the posterior extent of the temporal fenestra (1). (not in your matrix)

  • Also in Stenocybus, Nikkasaurus, many other therapsids

86. Tabular morphology: subrectangular sheet located dorsal to posttemporal fenestra (0). (not in your matrix)

  • Also in Stenocybus, Nikkasaurus, many other therapsids

96. Opisthotic, paraoccipital process: short and knob-like (2). (not in your matrix)

  • Also in Biarmosuchus (hard to tell, impossible on the other basal therapsids listed above)

102. Basal articulation, orientation of basipterygoid process: posteroventrally directed (2). (not in your matrix)

  • Difficult to assess in available crushed data

137. Lateral mandibular fenestra between dentary and angular: present (1). (character 124 in your matrix)

  • Also present in Stenocybus, Anomodonts.

140. Splenial, contact with posterior coronoid: present (1). (not in your matrix)

  • Don’t have many any traits visible medially.

43. Angular, cross-section shape of ventral border of angular: prominent, sheet-like keel with strongly convex posterior edge (1). (not in your matrix)

  • Don’t have any traits visible in cross-section.

155. Dorsal centra, anteroposterior length: short, subequal to height (0). (not in your matrix)

  • I have cervical ratios, but not dorsals, true.

178. Scapula, posterolateral surface of blade immediately dorsal to glenoid: deep, triangular concavity bounded anteriorly by prominent supraglenoid buttress (1). (not in your matrix)

  • I am not able to see this in that data available for Ophiacodon or Varanosaurus. Something like this appears in the later therapsid Cynognathus. Good posterior views of posterior scapulae hard to find. That’s why it’s not in my matrix.

227. Femur, mound-like eminence on dorsal surface of proximal end: small (1). (not in your matrix)

Also in Biarmosuchus. Other basal therapsids known only from skulls.

So in summary
Benson’s traits describing Ophiacodon + Varanosaurus are good, but they also apply to basal therapsids more often than not. A larger study including several missing generic taxa might have separated the Apsisaurus + Varanosaurus clade from the Ophiacodon + Therapsid clade in the Benson (2012) tree. It’s touch’n’go. They’re all very close.

I do move taxa when the results indicate to do so. Still these often incomplete taxa are all within just a few steps of each other.

Soon we’ll look at two double-canine ophiacodontids known from scraps that may help cross the therapsid transition.

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