Temnodontosaurus zetlandicus (Laboury & Bennion & Thuy & Weis & Fischer, 2022)

Laboury, Antoine, Bennion, Rebecca F., Thuy, Ben, Weis, Robert & Fischer, Valentin, 2022, Anatomy and phylogenetic relationships of Temnodontosaurus zetlandicus (Reptilia: Ichthyosauria), Zoological Journal of the Linnean Society 195, pp. 172-194 : 181-184

TEMNODONTOSAURUS CF. ZETLANDICUS

Referred specimen: MNHNL TU885, a partial skull from the Schistes Carton Formation, Lower Toarcian, Schouweiler, Grand Duchy of Luxembourg.

Comparative description of MNHNL TU885: The specimen MNHNL TU885 is, as preserved, 383 mm long and consists of a cranium lacking the snout and a fragmentary surangular ( Figs 5 , 6 ). Due to its preservation state, the orbits and temporal fenestrae are not fully delimited and the right vomer and the left palatine have been slightly displaced ( Fig. 6A, B ). The sutures between the nasals, frontals, right prefrontal and postfrontals are often difficult to discern ( Fig. 5C, D ). The parietal foramen is entirely preserved and appears unusually large, with a markedly raised rim; it is presently unclear whether this condition is pathological or not, as the parietal foramen is not located on the cranial midline. The postorbital region is only preserved on the left side and is anteroposteriorly long ( Fig. 5A, B ), measuring 99 mm. The basicranium is completely preserved ( Fig. 6C, D ), only lacking the left opisthotic. Hereafter, we compare salient features of MNHNL TU885 with early neoichthyosaurians.

This specimen shares many features with early neoichthyosaurians such as a frontal without a temporal process, a supraoccipital with a shallow ventral notch, a robust stapes with a prominent proximal head, a stout opisthotic with a short paroccipital process, a basioccipital with a well-demarcated extracondylar area, which is ventrally saddle-shaped and marked by the presence of a basal tubera, and a paired carotid foramen as suggested by the morphology of the parasphenoid, even if the posterior part mostly seems to be broken ( McGowan, 1973; Fischer et al., 2011; Martin et al., 2012; Marek et al., 2015; Moon & Kirton, 2016; Lomax et al., 2019; Miedema & Maxwell, 2019). In posterior view ( Fig. 6C, D ), the quadrate is a robust and large element that has a straight occipital lamella unlike T. azerguensis ( Martin et al., 2012) and T. crassimanus ( Swaby & Lomax, 2020) and with a massive and well-developed condyle as in species currently referred to as Temnodontosaurus for which the quadrate is preserved, with the exception of T. azerguensis ( Martin et al., 2012) . The postorbital region is well developed with a long, extensive quadratojugal, as in Temnodontosaurus spp. ( McGowan, 1974, 1994; Maisch & Hungerbühler, 2001; Swaby & Lomax, 2020) but unlike in other Early Jurassic parvipelvians (Maisch, 2001, 2008; Maisch & Matzke, 2003; Lomax & Massare, 2016; Maxwell & Cortés, 2020). The contribution of the supraoccipital to the dorsal margin of the foramen magnum is limited and nearly absent, as in Temnodontosaurus ( Maisch, 2002) . The frontal has a reduced, lanceolate dorsal exposure and forms the anterior margin of the parietal foramen, as in T. zetlandicus and T. trigonodon ( Maisch, 1998b) .

For all of these reasons, MNHNL TU885 is clearly assignable to Temnodontosaurus . The skull roof of MNHNL TU885 shares many similarities with that of T. zetlandicus ( Fig. 7 ), such as: (1) the bifurcated posterior processes of the nasal which partially covers the postfrontal which anteriorly appears in the medial and anterolateral processes; (2) the presence of a prominent mediolaterally oriented ridge on the postfrontal which forms the anterior margin of the supratemporal fenestra; and (3) a lanceolate frontal posteriorly covered by a slender forked process of the parietal. The skull roof of MNHNL TU885 also resembles that of T. trigonodon , but the presence of the bifurcated posterior processes of the nasal markedly differs from T. trigonodon ( Maisch, 1998b) . MNHNL TU885 is also similar in size and in cranial proportions to T. zetlandicus (see Table 1 ) and is here referred to as T. cf. zetlandicus .

SPECIES INQUIRENDA

Figures

Figure 5. Photograph and interpretation of the specimen MNHNL TU885 Temnodontosaurus cf. zetlandicus A, B, in right lateral view. C, D, in dorsal view.

Figure 6. Photograph and interpretation of the specimen MNHNL TU885 Temnodontosaurus cf.zetlandicus A, B, palate in ventral view. C, D, Basicranium in posterior view. Abbreviation: ECA, extracondylar area of the basioccipital.

Figure 7. Braincase comparison of the holotype of Temnodontosaurus zetlandicus comb. nov. (CAMSM J35176) (A) and the referred specimen of Temnodontosaurus cf. zetlandicus (MNHNL TU885) (B).