Varanosaurus and the Ophiacodontidae

This post was updated on August 12, 2013 prompted by the addition of several taxa. 
Dr. Roger Benson (2012) recently proposed a family tree of basal synapsids (pelycosaurs) that was covered earlier. Benson (2012) added Varanosaurus as a late surviving member of the Ophiacodontia (Fig. 1) following Berman et al. (1995). The long rostrum and short cranial region of Varanosaurus (Fig. 2) is indeed shared with Ophiacodon. They are closely related, along with Apsisaurus. the ophiacodontid, Archaeothyris is basal to them.

Varanops and Varanodon nest in a different clade, with Elliotsmithia and Aerosaurus, taxa with the orbit at mid skull.

Figure 1. This time calibrated strict reduced consensus cladogram recovered by Benson 2012. This tree assumes the Varanopidae became extinct, whereas the large reptile tree found one part (yellow portion) of the Varanopid branch gave rise to diapsids, including living birds and crocs. Casesauria do not belong in this tree as they are related to Millerettids.  Therapsids arise from ophiacodontids in the large reptile tree.

Figure 1. This time calibrated strict reduced consensus cladogram recovered by Benson 2012. This tree assumes the Varanopidae became extinct, whereas the large reptile tree found one part (yellow portion) of the Varanopid branch gave rise to diapsids, including living birds and crocs. Casesauria do not belong in this tree as they are related to Millerettids. Therapsids arise from ophiacodontids in the large reptile tree.

Romer and Price (1940) published a reconstruction of Varanosaurus. However it differs in several respects from a complete skull image found on the web (Fig. 2). It is a distinct specimen, MCZ 1366.

Varanosaurus.

Figure 2. Above – Images from Romer and Price (1940) of the MCZ specimen. Middle – Photo image  of the FMNH specimen of Varanosaurus. Below – Tracing of the photo. This appears to be a distinct specimen and species, not just the result of crushing. Note the second lateral temporal fenestra, likely the result of an increase in the size of the jugal and squamosal.

Three Different Varanosaurus Specimens
Benson (2012) reports that the earlier Romer and Price (1940) specimen, MCZ 1366, is highly reconstructed. Berman et al. (1995) reported on FMNH PR 1760 (Fig. 2) and BSPHM 1901 XV 20 (Fig. 3.

Figure 3. The BSPHM specimen of Varanosaurus

Figure 4. 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 4. A new nesting for the two new Varanosaurus specimens according to the large reptile tree, as outgroups to Ophiacodon and derived from a sister to Apsisaurus.

The large reptile tree includes several times more taxa than either the Benson (2012) tree or the Berman et al. (1995) tree. It recovered a tree in which both of the new Varanosaurus specimens were sisters to one another and nested with Apsisaurus at the base of the Ophiacondontidae. This clade includes therapsids and mammals. The Sphenacodontidae, formerly considered therapsid ancestors, is a sister clade to Ophiacodon and Apsisaurus (Fig. 4). Diapsids evolved from basal synapsids, like Heleosaurus.

The MCZ specimen was not tested.

So Varanosaurus was a late-surviving member of a basal synapsid clade. Varanosaurus had the proportions of a modern Varanus, the monitor lizard, and likely had a similar lifestyle.

Duplicating Results
Both the Berman (1995) and Benson (2012) studies could have benefitted by including more outgroup taxa (Fig. 4) and dropping Caseasauria, which are not related. Apsisaurus needs to be included.

References
Benson RBJ 2012. Interrelationships of basal synapsids: cranial and postcranial morphological partitions suggest different topologies. Journal of Systematic Palaeontology, iFirst 2012, 1-24.
Berman D S, Reisz RR, Bolt JR, and Scott D. 1995. The cranial anatomy and relationships of the synapsid Varanosaurus (Eupelycosauria: Ophiacodontidae) from the Early Permian of Texas and Oklahoma. Annals of Carnegie Museum 64: 99-133.
Romer AS and Price LW1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.

2 thoughts on “Varanosaurus and the Ophiacodontidae

  1. But Benson (2012) found the following 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)
    37. Maxilla, supracanine buttress on medial surface: present, may be expanded into lateral margin of internal naris [choana] (1). (not in your matrix)
    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)
    192. Humerus, ridge connecting deltopectoral crest to head: single, fossa absent (1). (not in your matrix)
    219. Pubis, pubic tubercle anteroventral to acetabulum: present, projects dorsally (2). (not in your matrix)
    Further, he found Varanosaurus and Ophiacodon (but not Archaeothyris, or unknown in the latter) to share-
    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)
    17. Premaxilla, marginal tooth count: five–six (1). (character 108 in yours)
    27. Maxilla, ventral surface: pronounced convexity (1). (character 28 in yours)
    32. Maxilla, precanniniform tooth count: six or more (2). (not in your matrix)
    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)
    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)
    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)
    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)
    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)
    86. Tabular morphology: subrectangular sheet located dorsal to posttemporal fenestra (0). (not in your matrix)
    96. Opisthotic, paraoccipital process: short and knob-like (2). (not in your matrix)
    102. Basal articulation, orientation of basipterygoid process: posteroventrally directed (2). (not in your matrix)
    137. Lateral mandibular fenestra between dentary and angular: present (1). (character 124 in your matrix)
    140. Splenial, contact with posterior coronoid: present (1). (not in your matrix)
    143. Angular, cross-section shape of ventral border of angular: prominent, sheet-like keel with strongly convex posterior edge (1). (not in your matrix)
    155. Dorsal centra, anteroposterior length: short, subequal to height (0). (not in your matrix)
    178. Scapula, posterolateral surface of blade immediately dorsal to glenoid: deep, triangular concavity bounded anteriorly by prominent supraglenoid buttress (1). (not in your matrix)
    227. Femur, mound-like eminence on dorsal surface of proximal end: small (1). (not in your matrix)

    So of 25 characters supporting this relationship, you include 6. Thus you can't say Varanosaurus is not an ophiacodont, because you didn't use three fourths of the evidence that say it is. While your analysis does include "several times more taxa" than Benson's, most of these are not particularly close to ophiacodonts, and it includes many that are close to them but which your analysis lacks (e.g. Stereophallodon, Pyozia). Thus which has the better taxon selection isn't clear. Note that Benson's characters 2, 7, 14, 86, 96, 102, 143 and 219 are poorly formed like yours though, more evidence even professionals get things wrong sometimes.

    • I will have to have another look at this, but the key differences here are inappropriate taxon inclusion (Caseasauria) and inadvertent taxon exclusion (several of the protosynapsids and protodiapsids, sensu the large reptile tree). .

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.