Revision of the Late Jurassic deep-water teleosauroid crocodylomorph Teleosaurus megarhinus Hulke, 1871 and evidence of pelagic adaptations in Teleosauroidea

Historical background

The holotype of Teleosaurus megarhinus (NHMUK PV OR 43086) was discovered in the winter of 1870 in the Kimmeridge Clay Formation strata at Kimmeridge Bay (Dorset), in England. J.C. Mansel-Pleydell (a Dorset antiquary, famous for his contribution to geology, botany and zoology) sent the specimen to J.W. Hulke to be described (Hulke, 1871; Delair, 1958). After making comparisons with other specimens, Hulke assigned the specimen to Teleosaurus (without supporting the decision) and proposed the specific designation megarhinus based on the ‘dilation of the terminal nostril’ that he considered as ‘greater than in any other Teleosaurus known to me’ (sic Hulke, 1871, pp. 442). Lydekker (1888) later referred the species to the genus Steneosaurus.

A skull from near Quercy (France) was referred to ‘Steneosaurus’ cf. megarhinus based on its total skull (premaxillary and maxillary) tooth count, and stratigraphic occurrence (Vignaud et al., 1993). Vignaud et al. (1993) also compared ‘S.’ megarhinus with other longirostrine teleosauroids from the Late Jurassic of Europe (Steneosaurus deslongchampsianus Lennier, 1887 and Aeolodon priscus Von Sömmerring, 1814), and considered ‘S.’ megarhinus to be a valid taxon, but they were only able to differentiate these three species based on tooth counts. Vignaud (1997) also noticed differences in the dentition (tooth count and crown shape proportions) between ‘Steneosaurus’ leedsi Andrews, 1909, and ‘S.’ megarhinus.

Pierce, Angielczyk & Rayfield (2009) hypothesised that ‘Steneosaurus’ megarhinus was a synonym of ‘Steneosaurus’ leedsi. Under the rules of the ICZN Code this would have resulted in ‘S.’ megarhinus being the senior subjective synonym, not ‘S.’ leedsi (contra Pierce, Angielczyk & Rayfield, 2009) who considered ‘S.’ leedsi as the senior synonym). It would also indicate a species that lasted for approximately 12 million years, and was morphologically distinct at both chronostratigraphic termini. However, the teleosauroid species diagnoses of Pierce, Angielczyk & Rayfield (2009) have been criticised as being diagnostic only to the generic level or using characters that describe all teleosauroids (Martin & Vincent, 2013, p. 194). Furthermore, Pierce, Angielczyk & Rayfield (2009) reported that S. megarhinus is known “from the Oxfordian-Kimmeridgian of England and Germany” (sic). However, contra Pierce, Angielczyk & Rayfield, (2009, p. 1067), we cannot find any mention of any German or Oxfordian specimens referred to ‘S.’ megarhinus, by Vignaud (1995) or anyone else.

The validity problem of ‘S.’ megarhinus is due to the use of overall upper jaw tooth total count as the sole means to differentiate the taxon from other teleosauroids (e.g.,  Vignaud et al., 1993; Pierce, Angielczyk & Rayfield, 2009). This has caused considerable confusion on the taxonomy of long snouted teleosauroids. Thus, it is not surprising that the validity of the species has been questioned. Instead, other significant features (e.g., the characteristic shape and extreme lateral expansion of the premaxilla, the arrangement of the premaxillary alveoli, and number of premaxillary alveoli) are rarely (if ever) considered in systematic studies of teleosauroids, until this study.

Only one phylogenetic analysis (Mueller-Töwe, 2006), so far, has included ‘S.’ megarhinus and recovered it as the sister taxon to Teleosaurus cadomensis Lamouroux, 1820 (the type species of Teleosaurus), based on prefrontal characters (Mueller-Töwe, 2006). This would be consistent with the initial placement of ‘S.’ megarhinus as a species of Teleosaurus, as originally proposed by Hulke. Subsequently, the validity and systematics of ‘Teleosaurus’ megarhinus have not been further investigated.

Cranial elements

Premaxillae

The premaxillae of Bathysuchus megarhinus are ladle-shaped elements with a strongly convex dorsal side and strongly concave ventral surface (Figs. 2–4). The premaxillae of the holotype (NHMUK PV OR 43086) each bear five alveoli (although the last one is difficult to see in ventral view due to poor preservation). Unfortunately, the P5 alveoli cannot be seen in DORCM G.05067i due to the poor preservation of both the premaxilla posterior to the P4 alveoli and the premaxilla-maxilla suture (Fig. 3). Such a high alveolar count is unusual amongst teleosauroids, being known only in B. megarhinus and P. multiscrobiculatus (MNHNL TU895, SMNS 9930), as well as having been reported in T. cadomensis (see Lamouroux, 1820; Eudes-Deslongchamps, 1869; Westphal, 1961; Westphal, 1962; Johnson et al., 2018), ‘Steneosaurus’ deslongchampsianus (Lennier, 1887; Godefroit, Vignaud & Lieger, 1995) new specimens referred to ‘Steneosaurus’ jugleri (Von Meyer, 1845; Schaefer, Püntener & Billon-Bruyat, 2018), but not Aeolodon priscus (contra Godefroit, Vignaud & Lieger, 1995). The distribution of the premaxillary alveoli differs in teleosauroids and has phylogenetic importance. In Bathysuchus, Aeolodon (MNHN.F.CNJ 78), Mycterosuchus (CAMSM J.1420), and ‘Steneosaurus’ jugleri (SCR011-406), in fact, the P1 and P2 alveoli are regularly circular/subcircular and laterally aligned. This differs in ‘Steneosaurus’ leedsi, ‘S.’ heberti, and Machimosaurini, where P2 is posterior and lateral compared to P1, giving the anterior premaxilla a tapering shape. In these taxa, the P1 and P2 alveoli are very close together, have an irregular (oval/teardrop shape) and in Lemmysuchus obtusidens they are separated by only a thin lamina (Johnson et al., 2017). In Bathysuchus, [and to a minor extent in Aeolodon (MNHN.F.CNJ 78), Mycterosuchus (NHMUK PV R 2617, CAMSM J.1420) and ‘Steneosaurus’ jugleri (SCR011-406)], the lateral margins of the premaxillae are strongly laterally expanded, so that the P3-P4 alveoli are anteroposteriorly aligned on a more lateral plane than the external margin of the P2 alveoli. Posterolateral to the P2 alveolus is a noticeable diastema (separating it from the P3 alveolus). The P3 and P4 alveoli are also well separated, and the P5 alveolus is small, positioned dorsally compared to P1-4 and laterally and posteriorly oriented. This morphology is unique in Bathysuchus megarhinus and is clearly visible on NHMUK PV OR 43086 and the LPP (Quercy) specimen (Figs. 2–4 and 9).

The external nares are well preserved in both NHMUK PV OR 43086 and DORCM G.05067i, but less so in the LPP specimen, where only the anterior portion is partially exposed from the matrix that fills the narial cavity. The anteromedial and posteromedial margins of the external nares are exceptionally bulbous, and project anteriorly and dorsally, respectively (Hulke, 1871) (Figs. 2F, 2L, 3E, 3J and 9). This gives the external nares a peculiar ‘8-shape’ in dorsal and anterior views (Figs. 2F, 2L, 3E, 3J and 9), which is not evident in any other teleosauroid besides Mycterosuchus nasutus (CAMSM J.1420, DF, pers. obs.). However, this area is too damaged in Aeolodon (MNHN.F.CNJ 78) to be confidently assessed (Figs. 9D–9E and 10). Overall, the external nares constitute a small length of the entire premaxillae: the portion of the premaxilla posterior to the external nares is more than 67% of its entire length, longer than in A. priscus (MNHN.F.CNJ 78), where it is approximately 60–65% (it is between 50–65% in the basal ‘S.’ gracilirostris and ‘S.’ leedsi). The anterior and posterior medial margins of the external nares are made by two bulbous projections of the premaxillae in the dorsal and anterior directions, respectively (Hulke, 1871) (Figs. 2F, 2L, 3E, 3J and 9D–9F). Faint ridges ornament the anterior margin of the anterior projection and the external nares of B. megarhinus (NHMUK PV OR 43086, DORCM G.050671i); these ridges are commonly present in other teleosauroids (e.g., Lemmysuchus obtusidens NHMUK PV R 3168).

The premaxillae in both Dorset specimens (NHMUK PV OR 43086 and DORCM G.05067i) are laterally expanded (such that they extend considerably more laterally than the anterior maxilla lateral margins) in line with the P3-P4 alveoli, and are also strongly ventrally deflected (Figs. 2–4 and 9F). The lateral expansion of the premaxillae cause P3-P4 to be positioned on a plane lateral to the rest of the premaxillary and maxillary alveoli. Due to the ventral deflection, the P1-P3 alveolar margin is also situated on a ventral plane compared to the remaining snout dentition, including the P4 alveoli (which is more dorsally and slightly posteriorly oriented) (Figs. 2C, 2I, 3C, 3H, 4C, 4G and 9). The lateral premaxillary expansion is not uncommon in teleosauroids within the subclade including ‘Steneosaurus’ brevior and Mycterosuchus nasutus (Fig. 9). Yet, it is noticeably more extreme in Bathysuchus megarhinus than in the other taxa. It is possible that this condition has been slightly exaggerated by the preservation of the specimens. However, we see no reason why a dorsoventral compaction would result in moving the premaxillae downwards rather than buckling it on the same level as the maxillae (as it can be seen in Mycterosuchus nasutus, ‘Steneosaurus’ leedsi and other dorsoventrally flattened specimens of the Oxford Clay Formation) (Fig. 9). Notably the difference between the latter two taxa is still visible regardless of diagenetic flattening. Perhaps more convincingly, NHMUK PV OR 43086 and DORCM G.05067i are not significantly dorsoventrally distorted, as can be assessed by the well-preserved oval shape of their rostrum cross-section (Figs. 2–3). Finally, this set of features (that also affect the orientation and shape of the external nares) is present in the well preserved and undistorted LPP specimen, which is the strongest evidence that they are genuine (Fig. 4).

As reported by Hulke (1871), the P4 alveoli are the largest alveoli in the premaxillae, and the P1 and P5 alveoli are the smallest (Figs. 2–4). In dorsal view, the premaxillae contact the maxillae via a slightly interdigitating, ‘V-shaped’ suture that reaches level to the M3 alveoli, or slightly posterior. In ventral view, the same suture has a straight anterior margin, creating a sub-square profile with the anterior-most side reaching in between the P3 and P4 alveoli (Figs. 2C, 2I, 3C, 3H, 4C and 4G). The ornamentation of the dorsal surface of the premaxillae is weak and shallow, as in A. priscus (MNHN.F.CNJ 78), and considerably less pronounced than in M. nasutus (NHMUK PV R 3577, and CAMSM J.1420), ‘S.’ brevior (NHMUK PV OR 14781) and taxa within Machimosaurini (e.g., Lemmysuchus obtusidens NHMUK PV R 3168; Machimosaurus buffetauti V1600Bo) (Figs. 2–4 and 11).

As well as NHMUK PV OR 43086 and DORCM G.05067i, the LPP specimen shows the combination of characteristic premaxillary and maxillary features of Bathysuchus: (1) five premaxillary alveoli (with the P5 being small and posteriorly oriented); (2) extreme lateral expansion of the premaxillae; (3) the premaxilla posterior to the external nares is over 67% of its entire length; (4) the P1 and P2 alveoli are laterally aligned and do not form a couplet; (5) the P3-P4 alveoli are anteroposteriorly aligned more laterally than the P1-P2; (6) P1-P3 alveoli are situated on a ventral plane compared to the rest of the premaxillary alveoli; (7) faint, shallow ornamentation on the premaxillae. As the interpremaxillary septum is slightly broken/covered by matrix in the LPP specimen, we cannot assess whether the external nares were ‘8-shaped’ with bulbous anteromedial and posteromedial margins. They were, nevertheless, anterior-dorsally oriented and shorter than their width.

Methods

We conducted a phylogenetic analysis to test the evolutionary relationships of Bathysuchus megarhinus gen. nov. within Thalattosuchia, using a modified version of the dataset published by Ösi et al. (2018), which is continuously being updated, as it forms the foundation of the ongoing Crocodylomorph SuperMatrix Project. The dataset was first presented in Ristevski et al. (2018); however, it has been extensively updated subsequently (see Ösi et al., 2018 for full details). All data are summarised in Supplementary data files.

The current dataset consists of 140 crocodylomorph OTUs (70 of which are thalattosuchians, including 18 teleosauroids, seven basal metriorhynchoids and 42 metriorhynchids) scored for 456 characters. Of these 456 characters, 25 characters representing morphoclines were treated as ordered (see Data S1) and Postosuchus kirkpatricki (Chatterjee, 1985) was used as the outgroup taxon. The differences between our analyses and those presented by Ösi et al. (2018) are: (1) the rescoring of B. megarhinus, M. nasutus and A. priscus; (2) the rescoring of the Chinese teleosauroid (IVPP V 10098) OTU; and (3) a re-organisation of the character list, with the addition of 3 new characters (Ch. 31, 274, 275), deletion of one (former Ch. 42), and inclusion of two new anatomical sections (palaeoneuroanatomy and craniomandibular pneumaticity). The character scoring for B. megarhinus was based on first-hand examination of the holotype by DF, MMJ and MTY, as well as first-hand examination of the referred DORCM specimen Bathysuchus by DF and the LPP specimen by MMJ. Due to the poor preservation and incompleteness of these specimens, B. megarhinus was scored for 159 out of 456 characters (35.0%).

The protocol used to analyse the dataset is the same adopted by Ösi et al. (2018), and it is described in detail in Data S1.

Comparisons with other teleosauroids

Bathysuchus megarhinus shares a number of characters with other teleosauroids, most notably with a handful of long-snouted taxa (e.g., Mycterosuchus nasutus and Aeolodon priscus, ‘Steneosaurus’ jugleri). As mentioned in the description, the high premaxillary alveolar count (five) is unusual, and is only seen in B. megarhinus (NHMUK PV OR 43086, DORCM G.05067i, LPP specimen), P. multiscrobiculatus (MNHNL TU895, SMNS 9930 MMJ pers. obs.) and T. cadomensis (see Lamouroux, 1820; Westphal, 1962); ‘Steneosaurus’ deslongchampsianus (Godefroit, Vignaud & Lieger, 1995; Savalle, 1876) and ‘Steneosaurus’ jugleri (Schaefer, Püntener & Billon-Bruyat, 2018), but not Aeolodon priscus (contra Godefroit, Vignaud & Lieger, 1995). The peculiar premaxillary alveolar distribution of the P1-P2 and the P3-P4 alveoli are characteristic of both B. megarhinus (NHMUK PV OR 43086, LPP specimen, DORCM G.050671i) and M. nasutus (CAMSM J.1420 DF, pers. obs.), which is one of a number of features unique to these taxa (Fig. 9). Well-developed anterior premaxillary projections seen in B. megarhinus (NHMUK PV OR 43086 and DORCM G.050671i—but not in LPP specimen, as its narial cavities are infilled with matrix) are also present in M. nasutus (CAMSM J.1420, DF pers. obs.) (Figs. 2–3). This feature is considerably less clear in other teleosauroids, including closely related taxa such as Aeolodon priscus (MNHN.F.CNJ 78), but also ‘Steneosaurus’ leedsi (NHMUK PV R 3806) and the Chinese teleosauroid (IVPP V 10098). Bathysuchus megarhinus shares three premaxillary features in with P. multiscrobiculatus (SMNS 9930, MNHNL TU895), ‘Steneosaurus’ brevior (NHMUK PV OR 14781), Mycterosuchus nasutus (NHMUK PV R 2617; CAMSM J.1420), the Chinese teleosauroid (IVPP V 10098), and ‘Steneosaurus’ jugleri.

(1)

The anterolateral margins of the premaxillae are strongly anteroventrally deflected (Figs. 2–4 and 9). In other teleosauroids (e.g., ‘S.’ leedsi NHMUK PV R 3806) the anterior and anterolateral premaxillary margins are either not anteroventrally deflected, or not as strongly;

(2)

The anterior external nares face anteriorly (or anterodorsally) (Figs. 2–4 and 9). In other teleosauroids (e.g., ‘S.’ leedsi NHMUK PV R 3806) the external nares face mainly dorsally;

(3)

The premaxilla is laterally expanded, in line with the P3-P4 alveoli.

In other teleosauroids (e.g., ‘S.’ leedsi NHMUK PV R 3806, S. edwardsi NHMUK PV R 3701, Machimosaurini) (Young & Steel, 2014) the lateral expansion and the ventral deflection are not as clear, although we noticed that this feature may be present in Mycterosuchus nasutus (NHMUK PV R 3577; CAMSM J.1420), Aeolodon priscus (MNHN.F CNJ 78) and ‘Steneosaurus’ jugleri (SCR011-406). Furthermore, in Aeolodon the premaxillae are laterally expanded and may be ventrally deflected, but both features may be concealed by the diagenetic deformation of the specimens (Fig. 9). Nevertheless, even allowing for deformation the extent of these features do not reach the extreme morphology present in every B. megarhinus specimen.

The teeth of B. megarhinus are unusual, as the enamel ridges do not continue onto the apical region (Fig. 7). This feature has not been observed in any other described teleosauroid, in which the enamel ridges are more densely packed and reach the apex (e.g., M. nasutus NHMUK PV R 3577, CAMSM J.1420; A. priscus MNHN.F CNJ 78; ‘S.’ bollensis MNHNL TU799) (Andrews, 1913). However, it is worth noting that it has been observed in an undescribed MNHN teleosauroid (one tooth in association with a partial skull) and NHMW 1884 (from the ‘Lias’ of Germany), which is labelled as ‘Teleosaurus’ (however, this tooth is laterally compressed, with discontinuous ridges that are more prominent than in Bathysuchus) (MMJ pers. obs.). The same feature is also visible in one tooth referred to ‘Steneosaurus’ jugleri (TCH005-151) that is still wanting description (Schaefer, Püntener & Billon-Bruyat, 2018).

Reduced ornamentation and possible pelagic adaptations in Teleosauroidea

The premaxillary and maxillary dorsal and lateral surfaces of B. megarhinus (NHMUK PV OR 43086, DORCM G.05067i) are particularly weakly ornamented, which is similar to the condition in the Chinese teleosauroid (IVPP V 10098), A. priscus (MNHN.F CNJ 78), and ‘Steneosaurus’ jugleri (SCR011-406) (Figs. 5–8 and 10) (Schaefer, Püntener & Billon-Bruyat, 2018). This differs from most other teleosauroids, both closely related taxa and more distant relatives, that retain the plesiomorphic condition of strongly ornamented rostrum and skull roof (Fig. 11) (e.g., M. nasutus NHMUK PV R 3577, CAMSM J.1420; ‘S.’ brevior NHMUK PV OR 14781; L. obtusidens NHMUK PV R 3168).

The shape and arrangement of osteoderm pits are similar in B. megarhinus (DORCM G.050671i), A. priscus (MNHN.F.CNJ 78) (Figs. 8, 10 and 11), and ‘Steneosaurus’ jugleri (SCR010-312) (see Schaefer, Püntener & Billon-Bruyat, 2018). These pits are large, but few in number and well separated from one another when compared to other teleosauroids (Fig. 11). However, there is one notable difference between the osteoderms of B. megarhinus and A. priscus: the thoracic osteoderms of B. megarhinus are not keeled, whereas longitudinal keels are present on all osteoderms in A. priscus (MNHN.F.CNJ 78), even the cervical ones (Figs. 8 and 10).

The ornamentation of the dorsal-sacral osteoderms of B. megarhinus (DORCM G.05067i) radically differs from the irregular, reticular pattern seen in M. nasutus (NHMUK PV R 3577, CAMSM J.1420) (Fig. 11). The pronounced reduction in dermatocranial and osteoderm ornamentation are characters shared between B. megarhinus and A. priscus, and are unique to these two species within Teleosauroidea (Figs. 8, 10 and 11). This contrast is striking considering that the heavily ornamented Mycterosuchus nasutus is sister taxon to Aeolodon + Bathysuchus.

The shift from highly ornamented dermal bone to low levels of ornamentation (or no ornamentation) is characteristic of the shift from amphibious to pelagic forms in Crocodylomorpha (Clarac et al., 2017). Clarac et al. (2017) outlined a possible mechanism for the increase in bone ornamentation in amphibious pseudosuchians, as a way to increase their basking efficiency. As dermatocranial and osteoderm ornamentation is highly vascularised, the overlying soft tissue can drive heat radiation to the bones. We can therefore hypothesise that there was a thermoregulatory shift in the lineage from Mycterosuchus nasutus to Aeolodon + Bathysuchus. Between the Callovian taxon M. nasutus, and the late Kimmeridgian-early Tithonian Aeolodon + Bathysuchus subclade there was a reduction in the size and thickness of their osteoderms, as well as a pronounced reduction in their dermatocranial and osteodermal ornamentation. We here hypothesise that this was in response to living in different habitats: heavily ornamented forms in semi-aquatic coastal environments and weakly ornamented forms in a more pelagic habitat, in which basking was less important (or perhaps impossible). A similar reduction in dermatocranial and osteodermal ornamentation is also seen in Metriorhynchidae, which are well established as fully pelagic thalattosuchians (Fraas, 1902; Andrews, 1913; Fernández & Gasparini, 2008; Herrera, Fernández & Gasparini, 2013; Herrera et al., 2017).

There is further evidence for a more pelagic lifestyle in the post-cranium of A. priscus (unfortunately the post-cranium of B. megarhinus is largely unknown). The largest A. priscus specimen, MNHN.F.CNJ 78, has proportionally very small dorsal osteoderms, proportionally short tibiae, and reduced forelimbs that have become more flipper-like. These reduced limb measurements contrast with almost all other teleosauroids with known postcranial skeletons (Fig. 13) (Data S2). To quantitatively demonstrate this, we selected two limb measurements to examine. The first is a ratio of the forelimb and hindlimb propodials (humerus: femur or H:F). This was chosen as a proxy for the relative reduction of the forelimb, as few metriorhynchid specimens have associated forelimb stylopodial elements, and even fewer a preserved manus. Within Metriorhynchidae the morphofunctional changes that transformed the forelimb into a hydrofoil-like structure resulted in it being greatly reduced in relative size compared to the hindlimb (e.g., Fraas, 1902). Secondly, we used the tibia: femur (T:F) ratio, because within Crocodylomorpha the tibia is proportionally reduced in size relative to the femur in numerous lagoonal and pelagic taxa (Fraas, 1902; Andrews, 1913; Wu, Russell & Cumbaa, 2001; Schwarz, Frey & Martin, 2006; Buscalioni et al., 2011). We produced and examined two distinct datasets including thalattosuchians, and extant crocodylomorphs (H:F—26 teleosauroids; 10 metriorhynchoids; 39 extant specimens; and T:F—31 teleosauroids; 11 metriorhynchoids; 39 extant specimens) (Data S2). The measurements of extant specimens were selected from Iijima, Kubo & Kobayashi (2018) dataset, while thalattosuchian measurements were personally taken by DF, MTY or MJ or come from Mueller-Töwe (2006) (Data S2).

The first intriguing feature is that the H:F and T:F linear equations of teleosauroids, metriorhynchids and extant species noticeably differ. This further highlights that the body-plans of these three groups where distinctly different, as the linear equations of basicranial length-total body length and femoral length-total body length are already known to have noticeably differed between them (see Young et al., 2011; Young et al., 2016). Overall, relative to extant crocodylians, thalattosuchians had proportionally: longer skulls relative to total body length, shorter femoral lengths relative to total body length, shorter humeral lengths relative to femoral length, and shorter tibial lengths relative to femoral length. This suggests that thalattosuchians had—compared to modern crocodylians—proportionally larger skulls, shorter femora, smaller forelimbs, and shorter tibiae, perhaps in relation to a marine lifestyle. This contention is supported by the fully pelagic metriorhynchids having even shorter humeri and tibiae relative to the femora than teleosauroids (Fig. 13).

When we examine the H:F ratio, Aeolodon priscus (H:F ∼0.36) falls very close to the range of pelagic metriorhynchids (H:F ∼0.20–0.34), and outside the range of any other known teleosauroids (H:F ∼0.38–0.76) (Fig. 13). In comparison, in extant crocodylians the H:F ratio is considerably higher (H:F ∼0.76–0.97), as Crocodylus, Alligator, Gavialis, Mecistops, Caiman, Melanosuchus, Paleosuchus, Osteolaemus, and Tomistoma have humeri and femoral lengths closer to subequal. The Toarcian teleosauroids have a H:F ratio range of 0.43–0.76, whereas the Callovian species have a range of 0.38–0.52, suggesting a generalised trend towards shortening the humeri in Teleosauroidea across time (with the Late Jurassic Aeolodon priscus being the most extreme known example of this trend).

For the T:F ratio, Aeolodon priscus (H:F ∼0.40–0.46) is intermediate between the ranges of pelagic metriorhynchids (H:F ∼0.27–0.43) and other known teleosauroids (H:F ∼0.41–0.70) (Fig. 13). In comparison, in extant crocodylians the T:F ratio is again higher (T:F 0.67–0.81). The Toarcian teleosauroids have a T:F ratio range of ∼0.48–0.70, the Callovian species have a range of 0.43–0.51, and the three known Kimmeridgian-early Tithonian specimens complete enough to include in our dataset have a range of 0.40–0.46. Once again, there is a generalised trend towards shortening the tibia over time in Teleosauroidea.

Interestingly, these temporal trends independently occurred in the two subclades of Teleosauroidea. The basal Toarcian teleosauroid Steneosaurus gracilirostris has high ratios (H:F = 0.685, T:F = 0.687). In the ‘T’-subclade, the H:F ratio decreases throughout the Jurassic: Toarcian 0.683–0.761 (Platysuchus), Callovian 0.506–0.528 (Mycterosuchus), and Kimmeridgian 0.36 (Aeolodon). In the ‘S’-subclade, the H:F ratio similarly decreases: Toarcian 0.436–0.640 (S. bollensis), and Callovian 0.384–0.479 (S. leedsi, S. edwardsi and Lemmysuchus). Similarly, for the T:F ratio, the ‘T’-subclade lowers through the Jurassic: Toarcian 0.610–0.697 (Platysuchus), Callovian 0.500–0.510 (Mycterosuchus), and Kimmeridgian 0.397–0.464 (Aeolodon). This also occurs in the ‘S’-subclade: 0.481–0.643 (S. bollensis), Callovian 0.434–0.511 (S. leedsi, S. edwardsi and Lemmysuchus), and Kimmeridgian 0.412 (Machimosaurus mosae).

Overall three independent lines of evidence—reduction of ornamentation, modification of the appendicular skeleton, and their recovery in deeper-water sediments—hint that by the Late Jurassic, some teleosauroids were beginning to evolve a more pelagic lifestyle. All three lines of evidence are present in A. priscus, whereas two of them are clear in B. megarhinus (reduced ornamentation, recovery in deeper-water sediments; the relevant postcranial bones to examine for limb reduction are not preserved in any known specimen). As these two taxa are united in a clade, we hypothesize that this lineage of teleosauroids evolved from nearshore, lagoonal forms and entered deeper waters, modifying their skeletons as they did so.

Interestingly, this pattern may be a response to the deepening of waters in the Late Jurassic. Foffa, Young & Brusatte (2018) showed that teleosauroids and other shallow water taxa underwent a morphological and species diversity decline in the Jurassic Sub-Boreal Seaway (JSBS) across the Middle-Late Jurassic boundary, in concert with deepening of local and global sea-levels. In fact, the whole JSBS marine reptile assemblage and ecological niches changed in concert with changing habitats. The diverse array of Middle Jurassic shallow water taxa (longirostrine pliosaurids, teleosauroids, metriorhynchines) declined, and was replaced by an assemblage better suited to higher sea-levels (Pliosaurus, geosaurine and ophthalmosaurid radiations) (Young et al., 2012; Benson & Druckenmiller, 2014; Foffa, Young & Brusatte, 2018). Accordingly, the unique body plan of Bathysuchus and Aeolodon lineage, could be the result of habitat change, and the attempt of a divergent group of teleosauroids to adapt to a new type of environment.

Nevertheless, to the extent of our knowledge, teleosauroids never became fully pelagic completing the land-to-sea transition, unlike their relatives the metriorhynchoids. The latter are a textbook example of a secondary adaptation to an aquatic lifestyle, as witnessed by the numerous osteological (e.g., enlarged skull relative to body length, hypocercal tail, modified limb proportions, paddle-like forelimbs, streamlined bodies, complete loss of osteoderms), and soft tissue adaptations (e.g., enlarged nasal exocrine glands, hypertrophied cranial venous systems, simplified and reduced endocranial sinuses) (e.g., Fraas, 1902; Andrews, 1913; Young et al., 2011; Brusatte et al., 2016; Herrera, Leardi & Fernández, 2018). While the Bathysuchus and Aeolodon subclade only shared a few of those adaptations, it still represents an instance of parallel evolution. This demonstrates that different thalattosuchian lineages were dynamically changing their anatomy, lifestyles and habitats during the Jurassic. This may have more generally characterized Mesozoic marine reptile faunas—with the repeated evolution of different types of morphologies and lifestyles in concert with habitat changes (e.g., Benson, 2013; Neenan et al., 2017). Finally, more complete specimens (especially from the late Tithonian and Cretaceous) and digital segmentation of the neuroanatomy of these taxa may hold the key to validating or disproving our hypothesis that they were becoming increasingly adapted to a pelagic lifestyle, and offer insights on the mechanisms of secondary land-to-deep-water transitions in tetrapods.