Ichthyosauria Blainville, 1835
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publication ID |
https://doi.org/ 10.4202/app.00062.2014 |
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persistent identifier |
https://treatment.plazi.org/id/03CD87A7-BD5C-FF8E-55B4-F92F83FFF9D7 |
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treatment provided by |
Felipe |
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scientific name |
Ichthyosauria Blainville, 1835 |
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Order Ichthyosauria Blainville, 1835 Family Shastasauridae ? Merriam, 1902
Fig. 2 View Fig .
Material. — NMW95.61 View Materials G.1 (radius) was collected in January 1995 as a fallen block on the beach about 1100 m south of the lifeboat station at Penarth in south Wales (Ordnance Survey National Grid Reference ST 18665 70107). We attribute the specimen, based on the appearance of the lithology and on the currently known ranges of the invertebrate assemblage preserved in the matrix, to the Blue Lias Formation and more precisely to the Psiloceras planorbis Biozone (Hettangian) .
Comparative description and affinities. —NMW95.61G.1 is an exceptionally large radius; it is nearly as tall as wide with a quadratic outline ( Fig. 2 View Fig ; Table 1). The anterior margin of the bone is slightly convex, whereas its posterior margin has a broad concavity, being emarginated at midshaft. Fragments of the fused ulna are restricted both proximally and distally from the emargination, implying the presence of a large interosseous opening positioned between the radius and the ulna and extending for most of their proximo-distal length, but excluded from the distal margin of the humerus. Such a large interosseous opening is not
Proximo-distal maximum length 24.8
Antero-posterior length at mid-shaft 22.2
Proximal articular facet length 22
Proximal articular facet maximum width 8
Distal articular facet length 24.5
Distal articular facet maximum width 6.5
observed among parvipelvians, which possess adjoining radii and ulnae, but pierced by a small circular foramen in the earliest parvipelvians Hudsonelpidia brevirostris ( McGowan 1995) and Macgowania janiceps ( McGowan 1996) and sometimes observed in the early neoichthyosaurians Leptonectes tenuirostris and Temnodontosaurus burgundiae (Godefroit 1992; McGowan 1996). On the other hand, a large epipodial opening is a common feature of non-parvipelvian Triassic ichthyosaurs: the forms with tentative affinities Cymbospondylus buchseri ( Sander 1989) and Californosaurus perrini ( Merriam 1902) ; the mixosaurids Barracudasaurus maotaiensis (Jiang et al. 2005) , Mixosaurus cornalianus ( Motani 1998) , M. panxianensis ( Jiang et al. 2006) , and Phalarodon atavus ( Liu et al. 2013) ; and also among shastasaurids, Shonisaurus sikanniensis ( Nicholls and Manabe 2004) , Callawaya neoscapularis ( McGowan 1994), Shastasaurus spp. (Callaway and Massare 1989), and Shonisaurus popularis Kosch 1990 ). The presence of such an epipodial opening was interpreted by Callaway and Massare (1989) and Maisch and Matzke (2000) as a primitive feature observed in all Triassic genera, although Besanosaurus leptorhynchus appears to be an exception (Dal Sasso and Pinna 1996).
However, the overall outline of NMW95.61G.1 is dissimilar to many of the Triassic forms cited above, in which the radius is longer than wide and the anterior margin is not convex but notched as in some mixosaurids ( Jiang et al. 2006; Liu et al. 2013). On the other hand, NMW95.61G.1 is quadratic and presents an even convex anterior margin. In this regard, the genus Cymbospondylus also has a radius longer than wide with C. buchseri displaying an anterior notch ( Sander 1989) or without in C. petrinus ( Merriam 1908) . In some other Triassic ichthyosaurs, the radius is about twice as wide as it is high (e.g., Shastasaurus osmonti [ Motani 1998] = S. pacificus according to McGowan and Motani 2003). Nevertheless, the morphology of NMW95.61G.1 closely resembles that of the genus Shonisaurus as illustrated in McGowan and Motani (1999), and especially that of Shonisaurus sikanniensis ( Nicholls and Manabe 2004) , with the radius and ulna forming a single unit as evidenced by the fused remains of the ulna) and in presenting an extensive epipodial opening and a convex leading edge. Another taxon with a similar shaped and sized radius is the shastasaurid Himalayasaurus tibetensis ( Motani et al. 1999) .
In light of its morphology, this radius is most like that of a shastasaurid ichthyosaur, especially the genus Shonisaurus . However, it should be noted that in the absence of further skeletal remains, this taxonomic attribution remains tentative.
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