Melanosaurus maximus, MAXIMUS GILMORE, 1928

Conrad, Jack L. & Norell, Mark A., 2008, The braincases of two glyptosaurines (Anguidae, Squamata) and anguid phylogeny, American Museum Novitates 3613 (1), pp. 1-24 : 2-5

MELANOSAURUS MAXIMUS GILMORE, 1928

LOCALITY AND AGE: AMNH FR 5168 and AMNH FR 5175 come from the Wasatch

Formation in Big Horn County, Wyoming, and are of Early Eocene age .

PRESERVATION: The braincase of the holotype ( AMNH FR 5168 ) is articulated to the rest of the skull. Consequently, only the ventral surfaces of the sphenoid and basioccipital, part of the damaged otooccipitals, and the ventral parts of the prootics are visible. The paratype ( AMNH FR 5175 ) (fig. 1) includes a disarticulated skull in which the braincase is isolated from the surrounding skull bones. Both braincases are damaged. The left paroccipital process is largely complete in AMNH FR 5168 . The sphenooccipital tubercle is best preserved on the right side in AMNH FR 5168 , but is damaged. The prootics are damaged in AMNH FR 5175 such that the anterior housing of the anterior semicircular canals and the crista alaris prootica are either not preserved or not visible in either specimen. Because the anterodorsal part of the sphenoid is damaged or hidden in the available material, the morphology of structures such as the dorsum sella and hypophyseal fossa cannot be determined. Both specimens retain the basipterygoid processes on the sphenoid, with minor damage. However, the anterior part of the sphenoid is damaged in AMNH FR 5175 and the basipterygoid processes have been separated from the rest of the braincase. Most of the remainder of the braincase is well preserved .

SPHENOID: The compound structure composed of the fused parasphenoid and basisphenoid is termed the sphenoid ( Bever et al., 2005a). The basipterygoid processes are proximodistally very short, but their pterygoid articular surfaces are extremely broad. Each pterygoid facet is a medially concave elongate oval. It narrowly clasped the pterygoid in that it overlapped the pterygoid somewhat dorsally and ventrally. Unlike the condition observed in most extant anguids, the base of the basipterygoid process is unconstricted and there is very little ‘‘neck’’ to the basipterygoid process. The posterior part of the pterygoid facet of the basipterygoid process is only slightly expanded, so that it is nearly contig- uous with the base of the process. The basipterygoid process extends well anterior to the anterior margin of the main body of the sphenoid. The interpterygoid vacuity is very narrow and the basipterygoid processes lie adjacent to each another, the space between them is a narrow ‘‘V’’ shape in ventral view (fig. 1). Thus, although the basipterygoid processes are short, they are well developed and extend far forward. This condition is in contrast to the abbreviated basipterygoid processes of mosasauroids and amphisbaenians (taxa also characterized as having short basipterygoid processes). Importantly, both Melanosaurus maximus and Helodermoides tuberculatus show a derived condition of the anterior part of the sphenoid, in which the parasphenoid rostrum lies dorsal to the medially placed anterior openings of the Vidian canal. This will be described in more detail below.

The paratype ( AMNH FR 5175 ) is broken and the anteroventral part of the sphenoid is separate from the rest of the element (fig. 1). In dorsal view, this broken anterior portion shows the anterior projection of the Vidian canal. The left side shows the course of the anterior component of the Vidian canal and the right preserves the point of separation where the common Vidian canal splits into a cranial carotid canal and the anterior Vidian canal. This occurs posterior to the level of the posterior margin of the pterygoid facet, near the posterior end of the common Vidian canal .

Both the holotype and paratype show remnants of the sphenoid-basioccipital suture. It is an anteriorly arching contact on the ventral surface of the skull. Posterolateral flanges of the sphenoid narrowly overlay the basioccipital ventrolaterally and extend onto the bases of the spheno-occipital tubercles. The exact position of the posterior opening of the Vidian canal is uncertain, but it appears to have exited at the sphenoid-prootic contact.

BASIOCCIPITAL: The basioccipital is robust laterally, but relatively thin anteromedially. It possesses robust spheno-occipital tubercles. Co-ossification between the basioccipital and otooccipitals has obscured the suture between these two bones on the occipital condyle, but it appears that the basioccipital makes up most of the occipital condyle.

In both specimens the ventral floor of the braincase at the anterior margin of the basioccipital is collapsed. This is most apparent in AMNH FR 5175, but is present in both specimens. The anterior and middle braincase floor of many squamates is thin relative to surrounding areas of the braincase. For example, the posterior part of the sphenoid and anterior part of the basioccipital are thin in Shinisaurus crocodilurus ( Conrad, 2004; Bever et al., 2005a, b) and Heloderma suspectum ( Bonine, 2005) . Some anguids possess a similar thinning of the braincase floor in tandem with a dorsally projecting fossa on the ventral surface of the sphenoid and/or basioccipital (see, for example, coronal slices 550–577 in Maisano, 2006) ( Criley, 1968; Rieppel, 1980; Good, 1987; Maisano, 2006; personal obs.). Such a thin point may have been present in the basioccipital of Melanosaurus maximus , creating a weak area that collapsed during fossilization.

In both specimens the spheno-occipital tubercles are damaged. However, it is clear that they were robust, short, and posteriorly placed. The right spheno-occipital tubercle is mostly complete on AMNH FR 5168. Its distal tip seems to show an articular surface for an unfused epiphysis.

As noted by Gilmore, the occipital condyle is somewhat ‘‘reniform’’ ( Gilmore, 1928: 141); as it is a ventrally convex arch in posterior view, with a slight dorsal concavity. Even so, the occipital condyle is single (rather than trilobed or tripartite). This morphology is consistent with most extant anguids, and with Glyptosaurus sylvestris (Middle Eocene of western U.S.; see Sullivan, 1986) and Peltosaurus granulosus (the other glyptosaurines for which the occipital condyle is known) (e.g., AMNH FR 1710).

SUPRAOCCIPITAL: The supraoccipital is present in both specimens, but is best preserved in AMNH FR 5175. Dorsoventral compression and some mediolateral compression of the skull make contacts between the otooccipitals, prootics, and supraoccipital obscure. This is further complicated by the apparent partial fusion between the prootics and otooccipitals. On the right side of the braincase, the supraoccipital has moved somewhat dorsally with respect to the right prootic, revealing the straight posterior suture between them.

The general form of the supraoccipital appears to be consistent with that of other anguimorphs. A triangular articular surface for the processus ascendens is preserved anteromedially. The apex of this triangle faces posteriorly and is contiguous with a midline ridge that becomes progressively less pronounced posteriorly and terminates about two-thirds of the way to the dorsal margin of the foramen magnum.

PROOTIC: Prootics are visible only in AMNH FR 5175. Both cristae alaris prootica have been lost and the bullae housing the anterior part of the membranous labyrinth are broken, so that the canals themselves, which are infilled by matrix, are visible. The posterior processes, which anteriorly and anterodorsally overlie the paroccipital processes of the otooccipital, are also broken, so that their posterior extent is unknown. The right prootic is more complete than the left.

The trigeminal notch is a deep incisure of the prootic posteroventral to the housing of the anterior semicircular canals and the anterior part of the membranous labyrinth. No supratrigeminal ridge is present. The inferior prootic process appears to be complete and is square anteriorly with an anteroventrally projecting anterior margin. Its contact with the sphenoid is damaged. However, the robust prootic crest (crista prootica) is well preserved posterior to the level of the exposed part of the Vidian canal. Its lateral portion curves ventrally, forming a dorsal recess. A single facial foramen (through which cranial nerve VII exits the braincase) is present within this recess at a level just anterior to the spheno-occipital tubercle (fig. 1). The posteri- or process overlies the otooccipital posteriorly and is partly fused to it.

OTOOCCIPITAL: Adult and subadult squamates are usually characterized by the fusion of the exoccipital and the opisthotic. This compound structure is referred to as the otooccipital (following recent usage, e.g., Maisano, 2001; Maisano et al., 2002; Conrad, 2004; Bever et al., 2005a; Maisano et al., 2006). The opisthotic portions of the otooccipitals in both specimens are generally less complete than the exoccipital portions. Both paroccipital processes have been shorn off near the level of the foramen magnum in AMNH FR 5175. The left paroccipital process is more complete in AMNH FR 5168 and it was figured by Camp (1923) and Gilmore (1928) (see below) (fig. 2).

Most of the sutures between the otooccipitals and the basioccipital have been obliterated by co-ossification of the two bones, but the otooccipital contribution to the occipital condyle appears to be small relative to that of the basioccipital. The vagus foramen (communicating cranial nerve X) is well separated from a series of three hypoglossal foramina (for the exit of branches of cranial nerve XII). The crista tuberalis is damaged in both specimens, but its dorsal part is preserved in AMNH FR 5175 and is somewhat posterodorsally inclined. The occipital recess is well preserved and appears as a deep, narrow recess delimited posteriorly by the crista tuberalis, dorsally and anterodorsally by the crista interfenestralis, and anteroventrally by the swelling at the base of the spheno-occipital tubercle. Repreparation of AMNH FR 5175 has revealed the inner division of the occipital recess into the perilymphatic duct and the medial aperture of the recessus scala tympani (sensu Bever et al., 2005a; aquaeductus cochleae of Jollie, 1960 and round window niche of Wever, 1978). Dorsal to the crista interfenestralis, the fenestra ovalus opens posterolaterally.

The broken surface of the paroccipital process in AMNH FR 5168 has received some scrutiny in the past. Camp (1923) and Gilmore (1928) identified what they thought was a separate bony element of uncertain homology. Camp (1923) identifies this element as the paroccipital, but Gilmore (1928) suggests that it might be the same element as is present in some specimens of Lacerta viridis , citing Leydig as a source, but listing no specific study.

Dorsoventral compression of AMNH FR 5168 has dislocated many skull bones out of their natural articulation. The parietal and supraoccipital (tabulare of Gilmore, 1928) have been separated and the supraoccipital process of the parietal has been translated medially relative to the preserved part of the paroccipital process. Given this movement, it seems possible that the element in question might represent the posterior section of the posterior process of the prootic (the portion that typically anterolaterally overlies the paroccipital process).