SYSTEMATIC PALAEONTOLOGY

Order ICHTHYOSAURIA De Blainville, 1835
Family OPHTHALMOSAURIDAE Motani, 1999
Genus PLATYPTERYGIUS Huene, 1922

1922 Myopterygius; Huene, p. 98.

1922 Platypterygius; Huene, p. 99.

1927 Myobradypterygius; Huene, p. 27.

1985 Simbirskiasaurus; Ochev and Efimov, p. 76.

1997 Plutoniosaurus; Efimov, p. 77.

1998 Longirostria; Arkhangelsky, p. 66.

1998 Pervushovisaurus; Arkhangelsky, p. 66.

1998 Tenuirostria; Arkhangelsky, p. 66.

Type Species – Platypterygius platydactylus (Broili, 1907)

Diagnosis– Modified from McGowan (1972) and McGowan and Motani (2003) diagnosis of the genus, as pertaining to the forelimb. Humerus longer than wide with large dorsal and ventral trochanters; two distal facets for radius and ulna; radial facet smaller than ulnar facet; radial facet obliquely set so that its anterior margin is proximal to its posterior margin; an extra zeugopodial element anterior to the radius and a pisiform element, occasionally in contact with humerus via small facets; ulna wider (anteroposteriorly) than radius; intermedium does not contact humerus; phalanges rectangular; no digital bifurcation.

Referred Specimen. Platypterygius sp. DMNH 11843.

Occurrence. Fossil material was collected by Lee and Mendy Duchouquette, and Keven Bridges from the Grayson Marl (Late Cretaceous: Early Cenomanian) of Tarrant County, Texas, and donated to the Museum of Nature and Science. Locality information and correspondence is on file at the Museum of Nature and Science.

Description

Cranial Elements. The parietal is a rectangular shaped element anteriorly. Its sides diverge posteriorly to form the supratemporal process (Figure 1.1-1.2). The postfrontal is mostly complete (Figure 1.3-1.4). The anterior portion is dorsoventrally compressed and fan shaped with a crenulated margin. It is crescent shaped lateromedially, contributing to the lateral margin of the temporal fenestra. Posteriorly, the postfrontal is strut like and transversely compressed, forming a ridge on the ventral surface anteriorly. The quadrate is a concentric bone with a ventrally prominent articular condyle (Figure 2.1-2.2). The fragmentary frontal has a highly corrugated ventral surface (Figure 2.3-2.4).

The teeth are stout, conical, and slightly curved (Figure 3.1-3.2). The enamel extends one-third to one-half of the total crown length. The enamel exhibits fine regularly spaced longitudinal ridges. Below the base of the crowns, the teeth bulge slightly and are subrectangular in cross-section with coarser longitudinal ridges. A single tooth is preserved within a fragment of maxilla. The tooth is set into a dental groove without ankylosis, indicating an aulacodont implantation.

Vertebrae. The atlas-axis complex shows complete fusion, with the atlantal intercentrum fused with the atlas. There is no indication of sutures between the elements. Presacral vertebrae are discoidal, with an average height/length ratio of approximately 3.0 (Table 1). Position of rib facets indicate vertebrae ranging from the cervical to sacral regions. No neural arches are preserved.

Pectoral girdle. The coracoid is a broad plate-like element that is slightly wider (mediolaterally) than long (Figure 4). The articular margins are thicker and rugose. There is no indication of an anterior emargination of the coracoid.

Humerus. The proximal end of the right humerus is crushed anteroposteriorly (Figure 5). The distal end preserves all four facets. The left humerus is a short and robust element that is wider (anteroposteriorly) distally than proximally with an un-constricted shaft (Figure 6). This difference in width is most likely due to deformation during burial. Crest-like dorsal and ventral trochanters are restricted to the proximal portion of the shaft and decrease in size distally. There are four distinct articular facets on the distal surface. The two larger middle facets articulate with the radius and ulna, with the radial facet being concave and smaller than the ulnar facet. The posterior facet is half as small as either the radial or ulnar facet and articulates with the pisiform element. The smaller anterior facet articulates with the extra zeugopodial element.

Zeugopodium. The zeugopodium consists of four elements (Figure 6). The pisiform is rectangular in shape being longer than wide. It articulates with the humerus, ulna, and metacarpal five, but does not make contact with the ulnare. The ulna is both longer and wider than the radius. Its entire proximal surface articulates with the humerus. It is slightly concave on its distal surface and rounded on the antero-distal corner. The radius is roughly pentagonal and tapers posteriorly. It is obliquely set so that its anterior edge is proximal to its posterior edge. The anterior surface is concave and articulates with the anterior zeugopodial element. The extra zeugopodial element is rounded. Its proximal surface tapers slightly to fit into a small facet on the distal surface of the humerus.

Autopodium. The radiale is rectangular and lies distal to the radius. The intermedium is pentagonal and does not make contact with the humerus. It articulates between the radius' postero-distal corner and the ulna's antero-distal corner. The intermedium articulates with a single digit distally. The ulnare tapers anteriorly and lies distal to the ulna. Metacarpal five lies posterior to the ulnare and distal to the pisiform. The distal carpals and metacarpals, as well as the phalanges, are all rectangular in shape and do not form a mosaic pattern. The forelimb has only five digits preserved, but there are facets along both margins of the forelimb suggesting the presence of additional digits. There is no indication of digital bifurcation. The longest digit present preserves nine phalanges. The distal end of the forelimb is not present.