Two new plesiosaur species and new data on Brachauchenius

The most recent Journal of Vertebrate Palaeontology (Volume 27(1)) contains three new plesiosaur papers. A short communication by Ben Kear (Kear 2007, p. 241-246) clarifies the taxonomy of what has become a very confusing taxon – Eromangasaurus (Kear 2005). The confusion originated because two separate researchers (Ben Kear and Sven Sachs) simultaneously published separate descriptions and names for the same specimen. The officially settled name of this taxon is Eromangasaurus australis (Sachs 2005) Kear (2005).

The other two articles form parts 1 and 2 of a review of plesiosaurs from the Upper Cretaceous Tropic Shale of southern Utah by Albright et al.

The first part Albright et al. (2007a) (p.31-40) describes new material pertaining to Brachauchenius lucasi. Interestingly, I was surprised to see an infamous historical blunder rear its head. In 1957 Tarlo described an unusual pliosaur scapula and introduced the new genus Stretosaurus based on its unique morphology. Despite this interpretation being perpetuated in later papers (Tarlo 1959, Tarlo 1960). Tarlo (who later changed his name to Halstead) pointed out that this unique scapula was in fact a perfectly normal pliosaur ilium (Halstead 1989). In their new paper, Albright et al. (2007a) present two possible reconstructions for the partial scapula of Brachauchenius, one based on Liopleurodon, the other based on the ‘Stretosaurus’ ilium (Fig. 1). I’m surprised this error made it through the peer review process.

Fig 1. The two interpretations proposed by Albright et al for the partial scapula of Brachauchenius. ‘A’ is based in the ilium of ‘Stretosaurus’, erroneously identified as a scapula by Tarlo (1957). From Albright et al. (2007a).

It is also possible that both interpretations of the pelvic girdle (Fig. 2) given by Albright et al are erroneous. Comparison with other specimens of Brachauchenius overlooked by the authors (Fig. 3) and other pliosaurs (Figs. 4, 5) show that the pelvic girdle is always greatly elongate in pliosaurs. Although it is not possible to be certain without observing the specimen first hand, it appears that the orientation of at least some of the elements in Fig. 2  are open to reinterpretaion. Likely, the margin interpreteted as the obturator foramen is the lateral margin of the pubis. It is also possible that the pubis and coracoid have been muddled up.

Fig. 2. The two proposed interpretations of one half of the pelvic girdle of Brachauchenius. From Albright et al. (2007a)

Fig 3. Pubis of a specimen of Brachauchenius sp., as figured by Hampe (2005). Anterior towards the top – the orientation is confident because the specimen is articulated. This paper was overlooked by Albright et al. (2007a)

Fig 4. Pelvic girdle of Liopleurodon, showing the greatly elongated form typical of pliosaurs. from Andrews (1913)

Fig 5. Pelvic girdle of Simoletes, again showing the greatly elongated form typical of pliosaurs. from Andrews (1913)

The second paper in this two-parter From Albright et al. (2007b) (p. 41-58) introduces two new genera and species (Palmula quadratus and Eopolycotylus rankini), plus two new subfamilies within polycotylid plesiosaurs – Palmulainae and Polycotylinae. I may comment on this in the future.