Grade Synapsids - The early evolution of Synapsida (Vertebrata, Amniota) and the quality of the

Hypotheses of the Relationships of Pelycosaurian-grade Synapsids

Romer and Price (1940) provided the first review of relationships of pelycosaurian-grade synapsids incorporating information from all specimens known at the time. This review proposed that basal synapsids were divided into three suborders: Ophiacodontia, Edaphosauria and Sphenacodontia (Figure 6). Ophiacodontia were considered the basalmost group from which the other clades evolved, and contained Ophiacodontidae and Eothyrididae.

Edaphosauria was a grouping of the two herbivorous clades, Edaphosauridae and Caseidae.

The Sphenacodontia contained Varanopidae (supposedly basal within this clade), Sphenacodontidae and Therapsida. Since this review by Romer and Price (1940), the sister-group relationship between sphenacodontids and therapsids has been largely accepted, whereas the relationships of all other basal synapsid clades have undergone many revisions.

Figure 6: The relationships of pelycosaurian-grade synapsids suggested by Romer and Price (1940)

Reisz (1980) was the first to employ phylogenetic treatment of characters to decipher the relationships of pelycosaurian-grade synapsids. Unlike Romer and Price (1940), who focused mostly on the postcranium, 20 out of the 24 characters used by Reisz (1980) were cranial characters. This analysis supported a basal split between the Caseasauria (a clade containing caseids and eothyridids) and Eupelycosauria (all other synapsids). Romer and

Price’s Sphenacodontia was still supported, with Varanopidae, Edaphosauridae, Ophiacodontidae, and Caseasauria as successive outgroups (Figure 7A). Brinkman and Eberth (1983) also used a phylogenetic approach, but employed a different character list compiled through study of six well-known species representing the major clades. For the first time a sister-group relationship between Edaphosauridae and the clade containing sphenacodontids and therapids was suggested, a grouping named Sphenacomorpha (Ivakhnenko, 2003;

Spindler et al., 2015), that is also considered valid in the most recent phylogenetic analyses of basal synapsids. Ophiacodontidae was suggested to represent the sister taxon to Sphenacomorpha, whereas Varanopidae were found to be the sister to Caseidae (Figure 7B).

Figure 7: Four hypotheses of the phylogenetic relationships of pelycosaurian-grade synapsids. A) Reisz (1980);

B) Brinkman and Eberth (1983); C) Reisz (1986); D) Benson (2012)

Reisz (1986), in the volume of the Handbook of Paleoherpetology dedicated to Pelycosauria, used a set of 26 characters to infer the relationships that have since become the widely accepted consensus (Figure 7C). The basal split between Caseasauria and Eupelycosauria was supported and Edaphosauridae, Ophiacodontidae and Varanopidae were found to be successive outgroups to Sphenacodontia (the clade containing sphenacodontids and therapsids). These relationships have been sustained by the introduction of computer

algorithms for phylogenetic analysis (Gauthier et al., 1988; Modesto, 1994; Berman et al., 1995; DeBraga and Rieppel, 1997; Reisz and Fröbisch, 2014).

Most recent phylogenetic analyses of pelycosaurian-grade synapsids have examined the relationships within clades. Varanopidae has been a particular focus, not only with the description of new taxa and specimens (Reisz and Dilkes, 2003; Anderson and Reisz, 2004;

Maddin et al., 2006; Campione and Reisz, 2010; Modesto et al., 2011; Berman et al., 2014) but also the reassignment to this clade of several species previously thought to be diapsids or therapsids (Reisz et al., 1998; Reisz and Laurin, 2004; Reisz and Modesto, 2007; Reisz et al., 2010). Sphenacodontidae has also come under scrutiny (Reisz et al., 1992; Laurin, 1993;

Kissel and Reisz, 2004; Fröbisch et al., 2011; Spindler et al., 2015), leading to the realisation that one of the subfamilies traditionally included within this clade, the Haptodontinae (Romer and Price, 1940), are in fact a paraphyletic grade outside Sphenacodontidae. Other analyses have examined the relationships within Ophicaodontidae (Berman et al., 1995), Caseidae (Maddin et al., 2008; Reisz and Fröbisch, 2014), and Edaphosauridae (Modesto, 1994; 1995;

Mazierski and Reisz, 2010).

Recently, Benson (2012) reviewed the relationships of pelycosaurian-grade synapsids with a global phylogenetic analysis of 53 taxa. This analysis included evaluation of all characters from analyses published prior to that date, as well as the addition of new characters. Recognising that the majority of phylogenetic analyses thus far had been heavily biased towards the cranium, Benson (2012) added large numbers of new postcranial characters. The relationships obtained by this analysis did not recover the basal split between Caseasauria and Eupelycosauria. Instead, Caseasauria was found to be the sister to Sphenacomorpha, while Ophiacodontidae and Varanopidae formed a monophyletic grouping that was the sister to all other synapsids (Figure 7D). It was the postcranial characters which forced this set of relationships; when these characters are removed, Caseasauria is returned to a basal position. However, Benson (2012) acknowledged that the lack of information on the early evolution of both caseids and eothyridids was a problem. At that time, no postcranial information was available on either Eothyris or Oedaleops, and caseids earlier than the latter part of the Early Permian were unknown. The discovery of the Late Carboniferous caseid Eocasea provided fresh information on the postcranial anatomy of caseids early in their evolution (Reisz and Fröbisch, 2014) and re-analysis of Benson’s matrix with Eocasea included found Caseasauria in their more “traditional” position as the sister to all other synapsid clades (Reisz and Fröbisch, 2014).

Expanding Phylogenetic Analysis of Pelycosaurian-grade Synapsids

Institutional Abbreviations

MCZ – Museum of Comparitive Zoology, Harvard.

FMNH – Field Museum of Natural History, Chicago

USNM – National Museum of Natural History, Washington DC

Many of the analyses undertaken in this thesis required an up-to-date and comprehensive phylogeny of pelycosaurian-grade synapsids. As such, the data matrix of Benson (2012) was expanded by the addition both of new material and new characters. The new material includes four previously described species as well as the postcranial material of Oedaleops campi (Sumida et al., 2014), unpublished at the time of Benson’s original analysis.

The four species newly added to the matrix are:

1: Apsisaurus witteri (MCZ 1474) includes a partial skull and lower jaw, a string of vertebrae from the posterior cervicals to the anterior caudals, several ribs and parts of both limbs. This specimen from Archer City Formation of Texas was first described as a diapsid by Laurin (1991), and so was not included in most subsequent phylogenetic analyses of pelycosaurian-grade synapsids. However, in more recent years, knowledge of basal synapsid morphology and relationships, in particular that of varanopids (Reisz and Dilkes, 2003;

Anderson and Reisz, 2004; Maddin et al., 2006), has increased. With the benefit of this knowledge, Reisz et al. (2010) re-assigned Apsisaurus to Varanopidae, an assignment supported by a phylogenetic analysis including both varanopids and eureptiles. Reisz et al.

(2010) noted that several of the previously considered synapomorphies of diapsids were in fact present in varanopids, such as the short quadratojugal. Apsisaurus did lack the recurved, laterally compressed teeth thought characteristic of varanopids, but the discovery of Archaeovenator (Reisz and Dilkes, 2003) showed that conical teeth are the primitive condition for varanopids. The presence of a tubercle on the jugal and similarities of the mandible to that of Archaeovenator confirmed the varanopid affinities of Apsisaurus.

However, this specimen was not included in the global analysis of Benson (2012). Due to its importance as a basal member of the varanopids it was added to the analysis presented herein.

2: Casea nicholsi is a large caseid represented by two specimens (FMNH UR 85 and 86) from the late Kungurian Upper Vale Formation of Texas (Olson, 1954). The specimens

include partial vertebral columns, fragments of the skull roof, partial pelvic girdles, a forelimb, a pes, and a distal femur. The specimen was re-examined by Olson in his review of the family Caseidae (Olson, 1968), but has never been included in a phylogenetic analysis.

Such an analysis is required to confirm the monophyly of the genus Casea. Four species were included in this genus in the review of Reisz (1986), but the analyses of Maddin et al. (2006) and Benson (2012) confirmed that “Casea” rutena did not form a monophyletic clade with the type, Casea broilii; this species has since been assigned to a new genus, Euromycter (Reisz et al., 2011). Further examination of this genus is required in order to produce reliable estimates of diversity. For this purpose, Casea nicholsi was included in the analysis presented here.

3: Eocasea martini is currently the earliest known caseid, represented by a fairly complete skeleton found in the Upper Pennsylvanian Hamilton Quarry (Reisz and Fröbisch, 2014). This is a crucial species for understanding the earliest evolution of caseids. Not only is it the earliest, and also the most basal member of the clade (Reisz and Fröbisch, 2014), but it is also not a medium-large sized herbivore, as are the other members of Caseidae. Instead, it is small, and was thought to be an insectivore (although unfortunately the sharp conical teeth were lost in preparation) (Reisz and Fröbisch, 2014). Moreover, a phylogenetic analysis presented in the supplementary materials of the description suggested that this specimen lacked many of the characters which Benson (2012) had used to unite Caseasauria with the clade containing Sphenacodontidae and Edaphosauridae, returning Caseasauria to their basal position within synapsids. It is necessary to examine this material alongside the eothyridid postcranial material now available in order to confirm or reject this hypothesis.

4: Mycterosaurus smithae was described very briefly in a catalogue of the vertebrate fauna found at the Placerville Localities of southwestern Colorado (Lewis and Vaughn, 1965).

Two specimens were assigned to this species. The holotype MCZ 2985 (Figure 8) consisted of a partial skull, five vertebrae and ribs and a proximal femur and tibia, while the referred specimen USNM 22098 was a partial femur and a string of seven vertebrae. The type species of Mycterosaurus, M. longiceps (Williston, 1915), has been included in numerous cladistic analyses which have supported its assignment to the varanopid subfamily Mycterosaurinae (Maddin et al., 2006; Botha-Brink and Modesto, 2009; Campione and Reisz, 2010; Benson, 2012). However Mycterosaurus smithae has received comparatively little attention since its original description. In order to incorporate it into the phylogeny presented herein, the holotype underwent further preparation to reveal more details of its morphology. The new

material exposed, as well as the improved knowledge of basal synapsid anatomy since its original description, allowed the re-assignment of this species to Eothyrididae. This makes

“Mycterosaurus” smithae an extremely important taxon. It is only the third known eothyridid species, and the second with postcranial material. It is also one of the earliest members of this family (see geological setting below), and as such can potentially provide a great deal of information on the earliest evolution of Caseasauria. The new information obtained from this specimen is presented here in a re-description of the type specimen, MCZ 2985.

Geological Setting

The Cutler Group spans the late Pennsylvanian and most of the Early Permian (Lucas, 2006), outcropping across New Mexico, Utah and Colorado. The Placerville Area, from which MCZ 2985 originates, is a locality where the sediments of the Cutler Group are exposed in the San Miguel Canyon (Lewis and Vaughn, 1964). Unfortunately, the biostratigraphy of the Cutler Group in Colorado is not so well established as in other areas.

Lewis and Vaughn (1964) considered the localities to represent the upper portion of the Cutler Group, equivalent to the late Sakmarian-Artinskian aged Moran, Putnam and Admiral Formations (Lucas, 2006). However, they also drew comparisons with the Dunkard Group of Ohio. Most of the taxa from Placerville which are shared with the Dunkard Group are found in the lower layers of the latter: the lower Washington Formation (Lucas, 2013), implying an earlier age, possibly Asselian-Sakmarian. Baars (1962; 1974) also supported an earlier age of the Cutler Group in southwest Colorado, suggesting equivalence with the Halgaito Tongue and lower Supai Formation of Utah and lower Abo Formation of New Mexico. These formations are considered earliest Early Permian (Asselian-Sakmarian) or possibly latest Carboniferous in the case of the Halgaito Tongue (Lucas, 2006). Since MCZ 2985 was found in the uppermost 200ms of the section, an Asselian-Sakmarian age seems best supported.

List of Abbreviations in Figures

ac – anterior coronoid; an – angular; d – dentary; dv – dorsal vertebra; f – frontal; fe – femur;

j – jugal; l – lacrimal; m – maxilla; p – parietal; pa – prearticular; pc – posterior coronoid; pf – postfrontal; ph – phalanx; pm – premaxilla; po – postorbital; prf – prefrontal; pt – pterygoid;

qj – quadratojugal; sa – surangular; sp – splenial; sq – squamosal; st – supratemporal; ti – tibia; u – ulna.

Figure 8: MCZ 2985, after preparation.

50 Systematic Palaeontology

Synapsida (Osborn, 1903) Caseasauria (Wiliston, 1912)

Eothyrididae (Romer and Price, 1940)

“Mycterosaurus” smithae (Lewis and Vaughn, 1965)

Diagnosis: Distinguished from other members of Eothyrididae by the unusually small temporal fenestra and the large posttemporal region. Distinguished from other pelycosaurian-grade synapsids by the extension of the posterior ramus of the maxilla beyond the posterior margin of the temporal fenestra.

Holotype: MCZ 2985 (Museum of Comparative Zoology, University of Harvard), a partial skull; a string of six dorsal vertebrae; several ribs; a left femur and tibia; other fragments.

Locality and Horizon: Placerville Localities 11-13, San Miguel County, Colorado (38.0° N, 108.0° W). Cutler Group, Asselian-Sakmarian.

Description

The specimen MCZ 2985 consists of a previously articulated block bearing a skull and several postcranial fragments, including five vertebrae, ribs and a proximal femur and tibia (Figure 8). During the course of preparation, the skull has been separated from the block bearing postcranial material, and the postcranial block has been separated into multiple blocks in order to better expose the postcranial material, although these fragments still articulate.

Skull

The skull (Figures 9-11) is laterally compressed and slightly distorted, but preserved in three dimensions. The preservation quality of the skull roof makes defining sutures problematic. The sutures on the lateral sides of the skull are considerably clearer, particularly on the right (Figure 9). Most of the occiput and palate is not exposed. The orbit is relatively large, but the temporal fenestra is extremely small compared to other pelycosaurian-grade taxa, less than a quarter of the length of the orbit. Its dorsoventral height is greater than the anteroposterior length. The fenestra is oblong in shape, rather than being narrower ventrally

as in ophiacodontids or dorsally as in most other pelycosaurian-grade synapsids, including Oedaleops. Its shape is instead more similar to that of Eothyris and some mycterosaurine varanopids.

Figure 9: Skull and lower jaw of MCZ 2985 in right lateral view

The antorbital region is missing except for a separate fragment, representing a counterpart and preserving a part of the left maxilla and premaxilla with teeth as well an internal view of the tip of the right mandible (Figure 10).

Figure 10: Skull and lower jaw of MCZ 2985 in left lateral view. The premaxilla, maxilla and tip of the dentary is a separate fragment articulating with the skull, and represents an internal view of the right upper and lower jaw fragments preserved on a counterpart.

Only a small part of the premaxilla is preserved in medial view on the small separate fragment, which fits as a counterpart and extends the anteriormost preserved part of the skull.

Nothing can be said about the anatomy of the premaxilla, except for details about its teeth (see Dentition below). The septomaxillae and the nasals are not present.

The frontal is the anteriormost preserved element of the skull roof. Its anterior margin is not preserved, so it is impossible to ascertain its length. The bones around the dorsal margin of the orbits are damaged, so it is unclear whether there is a lateral lappet of the frontal

contacting the orbit. If there is one it would have to be extremely narrow, as seen in caseasaurs. The posterior process of the frontal is only visible on the right hand side. It is a short triangle of bone intruding between the parietal and the postfrontal but still leaving a substantial contact between the two, unlike in varanopids and ophiacodontids where the contact is limited.

Figure 11: Skull of MCZ 2985 in dorsal view

The parietals have been displaced so that the posterior end of the right one overlies that of the left. Despite this, one can see that the pineal foramen is large and positioned at about midlength of the parietal, as in eothyridids.

The maxilla is only fragmentarily preserved, but better on the right than the left side.

The posterior process of the maxilla is a narrow splint extending beyond the level of the temporal fenestra, and contributes to the lower orbital margin.

A fragment of the lacrimal is preserved at the anterior edge of the right orbit. A ventral process of the prefrontal incises the lacrimal and limits its contribution to the orbital margin in lateral view, a feature of eothyridids, sphenacodontids and some varanopids. A lacrimal foramen cannot be identified.

The lateral surface of the prefrontal is flat, lacking the concavity observed in sphenacodontids and ophiacodontids. In dorsal view the prefrontal has a long, narrow posterior process forming about a third of the upper margin of the orbit.

The postfrontal, best preserved on the right side, is a transversely narrow triangular element with a flat surface. It contacts the parietal posteriorly but is separated from it anteriorly by the posterior process of the frontal. The posterior margin of the postfrontal is incised by an anterior protrusion of the postorbital, a feature shared with Eothyris and sphenacodontids.

The postorbital is a robust element with a broad posterior process. It has a posterior contact with a squamosal, but this contact does not extend far back over the temporal region as in some sphenacodontids and mycterosaurine varanopids. The ventral process of the postorbital and the dorsal process of the jugal form a thick postorbital bar, similar to those of Eothyris and Eocasea.

The jugals are also robust elements: both the anterior and posterior rami are dorsoventrally thick. The anterior ramus is short, not reaching beyond the orbital midline, but the posterior ramus extends well beyond the posterior margin of the temporal fenestra reaching at least halfway along the posttemporal region (the erosion of the lateral surface of this region makes identifying the full extent impossible).

The supratemporal is preserved on the right, but eroded away on the left. It is a large element set in the parietal, more similar in proportions to that of caseasaurs than to the splint of bone seen in varanopids and sphenacodontids. It is oblong in shape.

The squamosal is broad, flat and has a lateral exposure similar to that seen in ophiacodontids: the length of the postemporal region is considerably greater than the breadth of the temporal fenestra. The temporal fenestra itself is bordered anteriorly by the jugal and dorsally by the postorbital and squamosal; there is no anterior process of the squamosal contacting the jugal dorsally.

On both sides of the skull a narrow splint of bone excludes the jugal from the ventral margin of the skull, formed from both the posterior process of the maxilla and the anterior process of the quadratojugal. The contribution of the quadratojugal to the exclusion of the jugal to the ventral margin of the skull, visible on the left side (Figure 11) is reduced relative to other caseasaurs, wherein the anterior ramus reached anteriorly beyond the temporal fenestra. “Mycterosaurus” smithae shows the condition found in mycterosaurine and varanodontine varanopids with the posterior ramus of the maxilla having the greatest contribution. In “Mycterosaurus” smithae, in fact, the posterior ramus of the maxilla extends further posteriorly than in any other pelycosaurian-grade synapsid, reaching beyond the posterior margin of the temporal fenestra.

The occipital and ventral sides of the skull of MCZ 2985 are almost entirely covered my matrix. However, a small portion of the pterygoid is exposed in left lateral view between the left maxilla and mandible. Not much can be said about the morphology of the pterygoid, but it bears a few teeth. Unfortunately not enough is exposed to say anything about their arrangement and distribution, although they are obviously large.

Mandible

Both left and right mandibles are preserved, the right as a counterpart showing the lingual sutures. The left mandible is preserved throughout most of its length, although the tip is missing. The counterpart of the right mandible is preserved throughout its entire length,

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