250
Journal of Paleontology 92(2):240–253
lateral surface of the retroarticular process (Knutsen et al., 2012) that is absent in P. almanzaensis n. sp. Sassoon et al. (2012) studied two specimens (BRSMG
Cc332 and Cd6172) from the lower Kimmeridge Clay, West- bury, Wiltshire, and referred them to Pliosaurus sp. Later Benson et al. (2013) named the species P. westburyensis Benson et al., 2013 based on BRSMG Cc332, and P. carpenteri Benson et al., 2013 based on BRSMG Cd6172. These species coincide with P. almanzaensis n. sp. because they show more than nine pairs of mandibular symphyseal alveoli. However, in both P. carpenteri and P. westburyensis, the angular does not penetrate in the symphysis in ventral view, whereas it does so in P. almanzaensis n. sp. Pliosaurus patagonicus and P. rossicus each have a short
symphysis with only six alveoli, differing from P. almanzaensis n. sp., which has at least nine symphyseal alveoli (Novozhilov, 1964, fig. 329; Halstead, 1971, text-fig. 1; Storrs et al., 2000, fig. 11.1B, C). Pliosaurus almanzaensis n. sp. differs from P. patagonicus in the following characters: (1) the symphysis of P. almanzaensis n. sp. is relatively more flattened than that of P. patagonicus; (2) the distance between the coronoid eminence and the posteriormost alveoli of P. patagonicus is equivalent to the length of the last fourteen alveoli, whereas in P. almanzaensis n. sp., this distance is shorter, approximately nine alveoli; and (3) the angular does not participate in the mandibular symphysis in P. patagonicus, but it does participate in P. almanzaensis n. sp. Thus, the validity of Pliosaurus almanzaensis n. sp. from
southwestern Gondwana is supported by two autapomorphies: (1) the angular entering into the mandibular symphysis and the occipital condyle without notochordal pit or grooves, and (2) by a combination of trihedral teeth, the symphysis with more than nine pairs of alveoli, the total number of alveoli approximately +26, and the parasphenoid without a ventral keel.
Discussion
The systematics of Pliosauridae is complex and under constant revision (e.g. Druckenmiller and Russell, 2008; Ketchum and Benson, 2010, 2011;Knutsen, 2012; Sassoon et al., 2012; Benson et al., 2013; Fischer et al., 2015). However, at least the definition of the genus Pliosaurus is now reaching a consensus. Both Knutsen (2012) andBenson et al. (2013) indicated the importance of the presence of trihedral or subtrihedral teeth. Fischer et al. (2015) also recognized the presence of trihedral teeth in the basal brachauchenine Makhaira rossica Fischer et al., 2015, but in this case the trihedral teeth show distinctive serrated carinae. The recent revisions of Pliosaurus by Knutsen (2012)
and Benson et al. (2013) generated a new diagnosis of the genus. The diagnosis given by Benson et al. (2013) includes seven features. Among these is the trihedral to subtrihedral (in P. kevani) teeth with a flat, smooth labial surface and convex lingual surface with longitudinal enameled ridges, a character also pointed by previous authors (e.g., Noè, 2001; Sassoon et al., 2012). MOZ 3728P shows trihedral teeth with crowns triangular in cross section, and thus, it is referred to Pliosaurus. The diagnosis of Pliosaurus given by Benson et al. (2013) considered six additional characters in addition to tooth morphology. Five of these cannot be observed in MOZ 3728P.
Only the third character, “occipital condyle lacking notochordal pit, but scored by several, irregularly-arranged grooves” (Benson et al., 2013, p. 4), could be scored. MOZ 3728P effectively lacks a notochordal pit but no irregular grooves are observed, although the condylar surface shows some irregularities. Within the genus Pliosaurus there are also some systematic
issues regarding the validity of the species included. Knutsen (2012) recognized four valid species of Pliosaurus plus four others of questionable validity, and, following Noè et al. (2004), remarked that the number of pairs of alveoli for functional teeth in the mandibular symphysis is a relevant character. Later, Benson et al. (2013) confirmed most of the conclusions of Knutsen (2012), but retained P. rossicus as a valid species and added three new species. Additionally, the issues about the validity of the classic P. macromerus (Philips, 1871) and P. brachyspondylus (Owen, 1840), both from the upper Kimmeridgian of England have not been solved. Both species are based on type material that is currently considered undiagnostic (Knutsen et al., 2012; Benson et al., 2013). Knutsen (2012) pro- posed the designation of neotypes for these two species but to date the request has not been submitted to the commission of the ICNZ. Therefore, they are here considered nomina dubia following Benson et al. (2013). The more recently nominated Pliosaurus species, P. patagonicus, was added by Gasparini and O’Gorman (2014). This is the general background that we follow here. The species considered valid are listed in Table 1.
Phylogenetic results.—The reduced consensus indicates some interesting relationships obscured in the complete consensus. First, Pliosaurus is recovered as a monophyletic group, including all Pliosaurus species considered other than ‘P. andrewsi,’ a result previously obtained by Benson et al. (2013). The position of ‘P. andrewsi’ outside the Pliosaurus clade is related to the absence of trihedral teeth (character 139, 0) and other features that differ from those recorded for other Pliosaurus species, such as mandibular glenoid fossa just posterior to the occipital condyle (character 10, 1), midpoint of the posterior interpterigoid vacuity posterior to anterior margin of the subtemporal fossa (character 104, 0), and anterior cervical neural spines curved posterodorsally (character 157, 0). There- fore ‘P. andrewsi’ needs referral to another genus. The results regarding the Pliosaurus clade indicate that the
plesiomorphic state of character 63 (condyle surface) is the lack a well-marked notochordal pit (which differs from those in Gallardosaurus Gasparini, 2009 and Liopleurodon), but also the lack of grooves mentioned by Benson et al. (2013) as diagnostic feature of Pliosaurus. Additionally, the topology indicates that the plesiomorphic state of character 83 (parasphenoid ventral surface) is the absence of a distinctive ventral keel on the parasphenoid, a feature shared byP. almanzaensis n. sp. and P. wesburyensis.
Basicranium comparisons.—MOZ 3728P shows the brasicra- nial elements, usually covered by the skull roof or poorly pre- served in other specimens, and thus enables improvement of the knowledge of this element in Pliosaurus. The occipital condyle of Pliosaurus almanzaensis n. sp. lacks a notochordal pit as in Kronosaurus spp., but differs from
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