Dromomeron sp.

Sarıgül, Volkan, 2016, New basal dinosauromorph records from the Dockum Group of Texas, USA, Palaeontologia Electronica 19, pp. 1-16 : 9-11

Dromomeron sp.

Figures 4 , 11

Material. Proximal end of right femur ( TTUP 12539X) from the Headquarters South locality ( MOTT 3898) ( Figure 4 ).

Description and remarks. The head (or capitulum) of the proximal femur is broken but its hooked morphology of a typical lagerpetid is still traceable ( Figure 11 ). The posteromedial tuber and adjacent trochanteric fossa (facies articularis antitrochanterica) are distinct. Right below the broken capitulum, a very faint emargination is present on the anterolateral side. The femoral shaft bears no anterior trochanter or a trochanteric shelf; the absence of which was previously noted for D. romeri and also for the smaller specimens of D. gregorii . Unfortunately, the inadequate preservation prevents a species-level assignment for this specimen.

DINOSAUROMORPHA Benton, 1985 DINOSAURIFORMES Novas, 1992 Gen et sp. indet.

Figures 4 , 12-13

Material. Left fibula ( TTU-P 10546) and distal end of left fibula ( TTU-P 19803) from the Boren Quarry ( MOTT 3869) ( Figure 4 ).

Description and remarks. TTU-P10546 represents a complete and mediolaterally compressed left fibula ( Figure 12 ). The proximal articulation is elongated in the anteroposterior direction and slightly altered around the edges. The anterior end of the proximal portion tapers smoothly with the missing tip and slightly curved anteromedially. The lateral corner of the posterior end is projected posterolaterally. The shaft is straight, slender, transversely compressed and considerably damaged where the attachment site for the M. iliofibularis was to be detected. The anterior and lateral borders of the distal end are rounded, whereas the posterior corner is expanded posteroventrally and becomes a prominent process. The reflection of this process on the distal surface is a small pyramidal tuberosity. There are two attachment surfaces detected on the distal surface. The lateral attachment surface is concave, extending from the posterior side towards the slightly raised medial side. This helical morphology of the lateral attachment surface probably conforms to the unevenly raised articular facet formed by astragalus and calcaneum. The medial attachment surface is located on the posteromedial side of the distal surface and bears a slight bevel expressed on the anteromedial portion. This is visible in both medial and distal views, probably articulating with the lateral edge of the ascending process of the astragalus.

The proximal end and shaft of TTU-P10546 have the typical plesiomorphic dinosauromorph morphology (e.g., Langer, 2004; Nesbitt, 2011), and the main indicative of its affinity is probably the configuration of the distal end, which differs substantially from the lagerpetid and dinosaurian condition. The lagerpetid distal part of fibula is documented only in Lagerpeton chanarensis ( Sereno and Arcucci, 1993) , where both anterior and posterior ends of the convex distal articular surface are elongated and tapering over the tibia and astragalus, respectively. In Silesaurus opolensis and in basal saurischians, on the other hand, the distal surface of the fibula is flattened, and it is widened in anteroposterior (or anteromedial-posterolateral) axis with a distinct posterior tuber (e.g., Novas, 1993; Bonaparte et al., 1999; Langer, 2003; Dzik, 2003, figure 13C; Sereno et al., 2013). The basal ornithischian condition is even more peculiar; the fibula is reduced to a rod, where the distal portion is significantly twisted and fused to the tibia and astragalocalcaneum (e.g., Butler, 2005; Butler et al., 2010; Norman et al., 2011). Pisanosaurus mertii somewhat differs from the typical ornithischian condition by lacking the distal torsion and yielding an expanded distal end ( Bonaparte, 1976; Norman et al., 2004).

The distal portion of the fibula of Marasuchus lilloensis offers the best comparison for TTUP10546. Following the original description of Sereno and Arcucci (1994), the distal portion of the M. lilloensis fibula possesses an elliptical distal surface that is anteroposteriorly concave but mediolaterally convex with a distinct bevel on the medial side. A prominent tuber is present on the posterior corner, as described for TTU-P10546. In both taxa, the enlargement of the posterior tuberosity forms an asymmetrical distal end uniquely among ornithodirans ( Nesbitt, 2011, character 345), differing from the posterodistal inclination on the flattened lateral border of the distal fibula seen in some basal saurischians like Eoraptor lunensis and Herrerasaurus ischigualastenesis ( Novas, 1993; Sereno et al., 2013). However, the medial edge is more rounded and the subdivision of the articular facets, if present, is less conspicuous in the distal part of the M. lilloensis fibula ( Sereno and Arcucci, 1994, figure 12A). There is also a notable size difference between the two fibulae; the maximum length of the M. lilloensis fibula is about 7 cm ( Sereno and Arcucci, 1994, table 5), whereas TTUP10546 is more than three times longer (24 cm).

Since only the distal end of TTU-P19803 is preserved, the overall morphology and size is similar to those of TTU-P10546; only slightly more compressed mediolaterally ( Figure 13 ). The lateral and anterior borders are rounded; a prominent process occurs on the posterior side that is followed by a small pyramidal tuberosity on the distal surface. The lateral and medial articulation surfaces seen in TTU-P10546 are not discernable in TTUP19803; the tarsal articulation surfaces are represented by a single concave surface that is raised towards the medial side. This pattern is more compatible to that observed for M. lilloensis ( Sereno and Arcucci, 1994, figure 12A).

Figures

FIGURE 4. Complete listing of hitherto published Dockum non-dinosaurian dinosauromorph specimens from the Dockum Group of Texas with voucher information and representing elements, and their stratigraphic distribution. Lithologic thicknesses are not to scale.

FIGURE 11. Dromomeron sp. (TTU-P12539X), proximal end of right femur in 1, anterior view; 2, medial view; 3, posterior view; 4, lateral view; 5, proximal view. Abbreviations: fh, femoral head (note that the femoral head is broken); pmt, posteromedial tuber; tf, trochanteric fossa; ve, ventral emargination. Hatching indicates the damaged areas. Arrow indicates anterior side.

FIGURE 12. Dinosauriformes gen. and sp. indet. (TTU-P10546), left fibula in 1, anterior view; 2, lateral view; 3, posterior view; 4, medial view; 5, proximal view; 6, distal view. Abbreviations: bs, beveled surface; da, damaged anterior part; dm, damaged posterior part; lf, lateral facet; mf, medial facet; pp, posterior process; py, pyramidal process. Arrow indicates anterior side.

FIGURE 13. Dinosauriformes gen. and sp. indet. (TTU-P19803), distal end of left fibula in 1, anterior view; 2, lateral view; 3, posterior view; 4, medial view; 5, distal view. Abbreviations: as, articular surface for astragalus and calcaneum; bs, beveled surface; pp, posterior process; py, pyramidal process. Hatching indicates the damaged areas. Arrow indicates anterior side.