Did the frontal migrate to the nasal in pliosaurids? – part 3

Earlier here and here we looked at the possible migration of the frontals to the shape and position of the nasals in pliosaurs, those giant marine relatives of the plesiosaurs.

Figure 1. Although this pliosaur is missing the skull roofing bones, Sassoon et al. 2012 restored the nasals and frontals of BRSMG Cd6172 like so. Color added for clarity.

Figure 1. Although this pliosaur is missing the skull roofing bones, Sassoon et al. 2012 restored the nasals and frontals of BRSMG Cd6172 like so. Color added for clarity. Rather than show the central frontals fused to the parietals, as Sassoon et al. indicates, I have combined them with the lateral frontals (both in white) here. They also show the prefrontals contacting the postfrontals, but other specimens (see earlier posts) show a palpebral portion of the frontals extended laterally beyond them, despite prf/pof contact in some cases.

Sassoon et al. 2012 described the skull of Pliosaurus (BRSMG Cd6172). Although they report the skull roof is missing (Fig. 1) they restored and described the frontals and nasals like so:

“Missing elements. Skull roofing elements (nasals, prefrontals, frontals, lacrimals, postfrontals, postorbitals and epipterygoids) are not preserved in BRSMG Cd6172, and these are reconstructed from other pliosaurian species (Andrews 1897; Taylor and Cruickshank 1993; Fig. 2A–B). The absence of nasals in Plesiosauria was received wisdom for some time (Storrs 1991; Taylor and Cruickshank 1993; Carpenter 1997; see also Ketchum and Benson 2010), and lacrimals were also thought to be missing in Sauropterygia (Storrs 1991; Carpenter 1996; Rieppel 1998). Basal sauropterygians, such as Nothosaurus (Rieppel and Wild 1994, 1996) and Pistosaurus (Sues 1987), have reduced nasals, and it was suggested that these were either lost or fused with the frontals (Romer 1956) in derived sauropterygians. However, two of us (LFN, JS) have observed that BRSMG Cc332 does possess nasals (contra Taylor and Cruickshank 1993), which are also undoubtedly present in the Callovian Liopleurodon (Noe` 2001). Further, lacrimals also appear in both species (Druckenmiller and Russell 2008; Ketchum and Benson 2010). Therefore, the reconstructions of BRSMG Cd6172 (Fig. 2A–C) show both nasals and lacrimals. The presence of nasals and lacrimals, however, should not be surprising as pliosaurids retain a number of plesiomorphic characters  (Ketchum and Benson 2010), also present in BRSMG Cd6172, such as coronoids and suborbital fenestrae; the latter are not present in plesiosauroids or other sauropterygians (Druckenmiller 2002b).”

I did not know about this paper until this yesterday, after I had posted the earlier two blog posts. So, I consider this welcome vindication and confirmation of observation and interpretation.

References
Sassoon J, Noe LF and Benton MJ 2012. Cranial anatomy, Taxonomic implications and Palaeopathology of an Upper Jurassic pliosaur (Reptilia, Sauropterygia) from Westbury, Wiltshire, UK. Palaeontology 55(4):743-773.

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