ICHTHYOSAURUS ACUTIROSTRIS OWEN, 1840

In the past, T. zetlandicus has been regarded as a junior synonym of ‘ Ichthyosaurus ’ acutirostris mainly because both came from the Lower Toarcian of Whitby (UK), without considering differences in cranial shape (McGowan, 1974; McGowan & Motani, 2003; Maisch, 2010). The current taxonomic placement of ‘ I.’ acutirostris is still controversial and has widely fluctuated overtime as this species has been classified in numerous genera (McGowan, 1974; Maisch & Matzke, 2000; McGowan & Motani, 2003; Maisch, 2010). According to Maisch (2010), this species might represent a novel genus of Early Jurassic ichthyosaurian. Considering that the referred material from Whitby requires a complete revision, possibly representing a variety of taxa (Maisch, 2010), focusing on the holotype seems adequate for the time being. However, this holotype (NHMUK PV OR 14553) (Fig. 8) was thought to be lost but finally relocated in the ichthyosaur collections about 20 years ago (Chapman & Doyle, 2002; Lomax, 2019), in a damaged state since the anterior part of the rostrum and the basal part of the right forefin are now missing. Even more problematic, the specimen is de facto unavailable for an undefined period of time (S. Chapman, pers. comm., August 2021) making the comparison even more challenging.

The right forefin of NHMUK PV OR 14553 is complete and composed of more than 25 elements on the longest digit (Fig. 8A); however, the authenticity of this trait has been questioned (Chapman & Doyle, 2002; Maisch, 2010; Lomax, 2019). If genuine, its length would represent an apomorphy since the number of elements in the longest digit in many early neoichthyosaurians does not exceed 20 (Motani, 1999a; Swaby & Lomax, 2020). Moreover, this condition looks more similar to Stenopterygius uniter (von Wurstemberger 1876), which has an elongated forefin (Maxwell, 2012). The left side of the skull is poorly preserved (Fig. 8), which restricts our comparisons with the holotype of T. zetlandicus (CAMSM J35176). Nevertheless, some remaining elements allow to differentiate the two specimens. Firstly, the most noticeable difference concerns the nasal. In T. zetlandicus this structure anteriorly ends as far as the maxilla and is not anterodorsally extended (Figs 2, 3A, B), whereas in ‘ I. ’ acutirostris, the nasal, even if it is incomplete, seems to anteriorly end further than the maxilla (Fig. 8). This condition in ‘ I. ’ acutirostris is more similar to the rest of the Early Jurassic parvipelvians (Maisch, 2008; Maxwell et al., 2012; Lomax & Massare, 2016; Maxwell & Cortés, 2020) with the exception of Suevoleviathan (Maisch, 2001). The morphology of the jugal also seems to differ in that the posterior extremity would not be notched (Fig. 8B). In NHMUK PV OR 14553, the postfrontal does not bear a prominent ridge on the anterior margin of the supratemporal fenestra (Fig. 8B), which is a distinctive feature of T. zetlandicus. Concerning cranial dimensions, the length of the postorbital region is larger compared to the diameter of the orbit than in T. zetlandicus (~0.8 for ‘ I. ’ acutirostris vs. ~0.5 for T. zetlandicus; see Supporting Information, Table S1) even if the skull appears to be smaller in ‘ I.’ acutirostris.

The taxonomic decisions in this paper require the assessment of the possible influence of ontogeny in driving the differences we observe between the holotype of ‘ I.’ acutirostris, which is small, and the holotype of T. zetlandicus. To do so, we assess the ontogenetic stage of the holotype of ‘ I.’ acutirostris by analysing the relative diameter of the sclerotic ring and the sclerotic aperture. This analysis has been used in the past to segregate juveniles (and supposed deep divers) from adults in neoichthyosaurians (Fernández et al., 2005; Fischer et al., 2013). Indeed, the orbit is nearly completely filled by the sclerotic ring in juveniles and deep divers, whereas in nondeep diving adult forms, the sclerotic rings tend to occupy a smaller area within the orbit. Our measurements (taking into account that the postorbital has been displaced in NHMUK PV OR 14553) place of the holotype of ‘ I ’. acutirostris well within the adult ontogenetic stage (Fig. 11). According to these results, the differences in size and morphology between the holotype of ‘ I.’ acutirostris and the holotype of T. zetlandicus cannot be regarded as driven by osteological immaturity.

Our morphological comparison, albeit limited, indicates that the incorporation of T. zetlandicus as a junior synonym of ‘ I.’ acutirostris is untenable. The other material referred to ‘ I.’ acutirostris needs a thorough reinvestigation once the material is accessible again (Maisch, 2010; Swaby & Lomax, 2020). Like Maisch (2010), we suggest removing ‘ I.’ acutirostris from Temnodontosaurus and placing it as species inquirenda, as Ichthyosaurus acutirostris according the initial assignment given by Owen (1840).