Specimen: NHMUK PV R 36710 * incomplete snout (maxillae* nasals* vomers).

Locality: Stonebarrow beach * Charmouth * Dorset * England * United Kingdom.

Horizon: Black Ven Mudstone Member* Charmouth Mudstone Formation* Lias Group.

Age: Late Sinemurian* Early Jurassic. Believed to have originated from the Asteroceras obtusum ammonite Biozone.

Description: The ‘Sinemurian snout’ (NHMUK PV R 36710; Fig. 1) was discovered in 2005 at Stonebarrow beach in Dorset * UK by Bernie Abbott of Uplyme* UK. The specimen was discovered by Mr Abbott on Stonebarrow beach directly underneath exposures of the Asteroceras obtusum ammonite Biozone. The snout was originally preserved in hard limestone* which is characteristic of the A. obtusum layer (M. Rigby* pers. obs.). This gives a Late Sinemurian age (Page 2010). The specimen was purchased from Mr Abbott by Martin Rigby* who donated it to the NHMUK in 2013. The maximum preserved length is 72.8 mm * with a maximum width of 39.1 mm and a maximum height of 20 mm * both at the caudal end of the specimen.

Maxilla: Only the mid-sections of the maxillae are preserved. Both the left and right maxillae preserve nine alveoli (Figs 1 * 2). The external surfaces of the maxillae are slightly convex rostrally* but there is a subtle convexity (possibly the rostral-most remnants of a longitudinal sulcus) on the caudal-part of both maxillae (Figs 1 * 2). The external surfaces have conspicuous ornamentation* composed of pits and grooves (Fig. 1). The ornamentation is of irregular length and positioning* although the more elongated grooves are closer to the maxillary tooth row and there are more pits towards the maxillonasal suture.

In lateral view* the ventral margin of the maxilla is straight* showing no evidence of festooning or any ventral deflection (Figs 1 * 2). Moreover* there are no reception pits for dentary teeth present on the lateral margins of the preserved maxillae or on the palatal surface. Given the intensity of the surface ornamentation* neurovascular foramina cannot be clearly discerned externally. Curiously* the alveoli are broadly exposed in lateral view. This is due to their distinct ventrolateral orientation. This is distinct from most known three-dimensionally preserved thalattosuchian rostra (e.g. Cuvier 1824 * Eudes-Deslongchamps 1867 –69* Hulke 1870 * Andrews 1913 * Antunes 1967 * Krebs 1968 * Gasparini and Dellapé 1976 * Buffetaut et al. 1981 * Buffetaut 1982b * Godefroit et al. 1995 * Hua and Atrops 1995* Vignaud 1998 * Deville 2004 * Pierce and Benton 2006* Buchy et al. 2007 * Jouve 2009 * Pol and Gasparini 2009* Young et al. 2012* 2013a* 2014* 2020* 2021* Herrera et al. 2013b * 2015 * Martin and Vincent 2013* Parrilla-Bel et al. 2013* Martin et al. 2015 * Barrientos-Lara et al. 2018 * Johnson et al. 2018 * 2019 * 2020a * 2020b * c* Ballell et al. 2019 * Foffa et al. 2019 * Hua 2020). The Suchodus durobrioensis Lydekker * 1890 specimen described by Wenz (1970: pl. 22* fig. A) also has the alveoli exposed in lateral view; however* there that exposure appears to be due to abrasion of the maxillary alveolar margins. In NHMUK PV R 36710* there is also abrasion of the maxillae* at their rostral and caudal ends (see Fig. 1B–E). However* there is no obvious sign of post-mortem damage in the middle of the element. As such* we consider the ventrolateral orientation of the maxillary alveoli to be a natural feature. Slight lateral exposure of the maxillary alveoli can be seen in the early diverging metriorhynchoid Eoneustes gaudryi (Collot* 1905; NHMUK PV R 3353)* although not as extreme as in NHMUK PV R 36710. Lateral exposure of the caudal maxillary alveoli is present in the holotype of ‘ Metriorhynchus ’ hastifer (Eudes-Deslongchamps* 1867) (MNHN.F RJN 119)* while there is variable lateral exposure of the caudal maxillary alveoli in Ŋalatosuchus superciliosus (see: Eudes-Deslongchamps 1867 –69* Wenz 1968).

In palatal view* the maxillary shelves are unornamented* and there is no evidence of palatal grooves (= paired longitudinal sulci) (Figs 1 * 2). In metriorhynchoids and Early Jurassic teleosauroids these grooves are present on the palatines and maxillae (e.g. Eudes-Deslongchamps 1867 –69* Andrews 1913 * Pierce and Benton 2006* Parrilla-Bel et al. 2013* Foffa and Young 2014 * Barrientos-Lara et al. 2018 * Johnson et al. 2019 * 2020a * Aiglstorfer et al. 2020 * Hua 2020* Young et al. 2020 * 2021 * 2023)* while in Middle and Late Jurassic teleosauroids they become either vestigial or absent* such as in aeolodontine teleosaurids (e.g. Foffa et al. 2019 * Johnson et al. 2020a) and machimosaurin machimosaurids (e.g. Johnson et al. 2018 * 2020a).

The maxillary shelves forming the secondary bony palate are in a plane dorsal to the alveolar ventral margin (i.e. the secondary palate does not project ventrally relative to the tooth row; Figs 1 * 2). In most metriorhynchids the maxillary contributions to the secondary palate are convex* resulting in the ventral margin of the maxilla being ventral to the maxillary alveolar margins (e.g. see the three-dimensionally preserved specimens in: Eudes-Deslongchamps 1867 –69* Gasparini and Dellapé 1976 * Buchy et al. 2007 * Lepage et al. 2008 * Young et al. 2012* 2013a* 2020* 2021* 2023* Parrilla-Bel et al. 2013* Herrera et al. 2015 * Barrientos-Lara et al. 2018). Note* that in metriorhynchids the palatines project ventral to the tooth row even more greatly. In teleosauroids and early diverging metriorhynchoids the secondary bony palate is also ventral to the ventral margin of the tooth row* although this ventral offset is not as significant as in metriorhynchids* especially for the palatines (e.g. see: Eudes-Deslongchamps 1867 –69* Andrews 1913 * Antunes 1967 * Buffetaut et al. 1981 * Buffetaut 1982b * Li 1993 * Godefroit et al. 1995 * Vignaud 1998 * Pierce and Benton 2006* Lepage et al. 2008 * Jouve 2009 * Martin and Vincent 2013* Young et al. 2014 * 2023 * Johnson et al. 2018 * 2019 * 2020a * b* c* Schaefer et al. 2018 * Ballell et al. 2019 * Foffa et al. 2019 * Hua 2020). However* the early diverging metriorhynchoids Eoneustes and Zoneait may have a morphology approaching that of metriorhynchids (Collot 1905 * Mercier 1933 * Hua and Atrops 1995* Wilberg 2015). In the machimosaurid teleosauroid Machimosaurus * the middle maxillary palatal shelves remain almost horizontal (e.g. Krebs 1968 * Buffetaut 1982b * Martin and Vincent 2013* Young et al. 2014). Interestingly* there is a metriorhynchid that is also known to have concave maxillary palatal shelves (that is not the result of post-mortem deformation)* the Suchodus durobrioensis specimen described by Wenz (1970).

When observed in ventral view* the preserved alveoli range from approximately subcircular in shape to slightly oval-shaped (rostrocaudal length being the longest axis). The interalveolar spacing progressively narrows caudally* with the spacing becoming thin laminae between the caudal-most alveoli (laminae that are broken). Only one alveolus has an in situ tooth crown* the second rostral-most preserved right alveolus (Fig. 1B). Contained within is a small replacement tooth. Little of the crown can be discerned* other than it is small and strongly mediolaterally compressed. The apex is broken* the carinal morphologies cannot be seen* and the enamel surface morphology also cannot be described.

Nasals: Only the rostral processes of the nasals are preserved* with the rostral-most tips of the processes not present (Figs 1* 2). The rostral margins of these processes are triangular in shape* with strongly converging lateral margins just like other thalattosuchians (e.g. Eudes-Deslongchamps 1867 –69* Fraas 1902 * Andrews 1913). The nasals are not fused* with a clear internasal suture visible. The external surfaces of the nasals are slightly convex rostrally and strongly ornamented. The ornamentation is composed of irregularly shaped and sized pits that become more prominent and more closely positioned in the caudal-half of the preserved elements.

Vomer: Interestingly* the vomer is exposed on the palatal surface* articulating with the maxillary palatal shelves* thereby contributing to the secondary bony palate (Figs 1 * 2). In palatal view* the vomer forms a triangular wedge at the midline* at the caudal-most section of the preserved rostrum. This wedge is approximately as long as two maxillary alveoli. This surface structure was confirmed to be part of the vomer through CT scanning as it is part of the fused ‘Y’-shaped element (see Supporting Information* Video SV2).

The vomer is a fused element* as has been previously described by Leeds (1908) and Andrews (1913) for the ‘ Metriorhynchus ’ brachyrhynchus (Eudes-Deslongchamps 1867) specimen NHMUK PV R 3700. In that metriorhynchid* the vomer articulates with the palatines along its ventral and lateral margins* except more caudally where it also articulates with the pterygoid laterally. A similar morphology was described by Wilberg (2015) for the early diverging metriorhynchoid Zoneait nargorum Wilberg 2015 * with the vomer being a single element that has a broad articulation ventrally with the palatines. In Zoneait the vomer changes shape* being ‘V’-shaped rostrally* and becoming progressively more ‘Y’-shaped caudally (Wilberg 2015). The ‘Y’-shaped vomerine morphology was also described by Wenz (1968) for metriorhynchid specimens from France. Through use of CT scanning* the same shape change seen in Zoneait was observed in the ‘Sinemurian snout’ (Supporting Information* Video SV2). However* here* the vomer articulates with the maxilla ventrally instead of the palatines. The vomer contributing to the external surface of the secondary bony palate is unusual amongst crocodyliforms. Amongst extant crocodylians exposure of the vomer in palatal view is known for the alligatorid Melanosuchus niger (Spix 1825) (e.g. Mook 1921 * Brochu 1999 * Bona et al. 2017)* and the gavialid Tomistoma schlegelii (e.g. Martin et al. 2019b).

Internal neurooascular canals: Like extant crocodylians and early diverging metriorhynchoids (Bowman et al. 2022)* the ‘Sinemurian snout’ (NHMUK PV R 36710) had three longitudinal neurovascular canals oriented caudorostrally within the rostrum* as revealed by the CT data (Figs 3 * 4). The major canal is the dorsal alveolar canal* which runs immediately dorsal to the alveoli (Figs 3 * 4). The second canal* which is ventrolateral to the dorsal alveolar canal and dorsal to the alveoli* supplied neurovasculature to the foramina along the rostrum external surface. The final canal* which is ventromedial to the alveoli* most likely supplied neurovasculature to the gingivae and palate. This differs from metriorhynchids* which only had two of the longitudinal canals—a distinct longitudinal canal for the vasculature supplying the external foramina is not present (Bowman et al. 2022). Based on extant crocodylians* these internal canals would have transmitted maxillary blood vessels and the trigeminal nerve branches (for recent overviews* see: Porter et al. 2016 * Lessner 2020 * Lessner and Holliday 2020).

Internal pneumatic and cancellous caoities: Within the maxilla* there are numerous cancellous osseous cavities (Fig 3 * 5). These cavities are elongated and have an irregular shape (best seen in Fig. 5A * C). There is no evidence of pneumatic cavities within the palatal shelves of the maxilla* as with other thalattosuchians (see: Cowgill et al. 2022).

The ‘Sinemurian snout’ (NHMUK PV R 36710) shares five phylogenetic characters that allow us to refer it to Thalattosuchia [based on the phylogenetic dataset of Young et al. (2024)]: the maxilla is conspicuously ornamented with pits and grooves; the diverticula of the paranasal sinus do not invade the maxilla surrounding the caudal-most alveoli; the palatal processes of the maxillae are apneumatic; the rostral-most margins of the nasals are triangular* with the lateral margins being strongly confluent rostrally (shared with Notosuchia); and the maxillary ventral margins are straight with no evidence of festooning (shared with Pholidosauridae * Dyrosauridae * and Gavialoidea).

However* ‘Sinemurian snout’ cannot be referred to the unnamed Plagiophthalmosuchus + Neothalattosuchia subclade* as it lacks the following two phylogenetic characters: the dorsal alveolar canals caudally start off being medial to the maxillary alveoli* and shift to a dorsomedial position (in NHMUK PV R 36710 the dorsal alveolar canals are dorsomedial to the alveolar when adjacent to the nasal bones* compared to other thalattosuchians we should see evidence of the ‘S’-shape in this region of the rostrum; see: Pierce et al. 2017 * Bowman et al. 2022 * Wilberg et al. 2022); and the presence of palatal grooves on the surface of the bony secondary palate. As such* ‘Sinemurian snout’ must be a non-neothalattosuchian thalattosuchian. Unfortunately* we cannot compare ‘Sinemurian snout’ to the holotype of Turnersuchus hingleyae Wilberg et al. * 2023* as they do not have any overlapping elements. While both taxa are known from the Charmouth Mudstone Formation* they are not known from the same locality or horizon* with ‘Sinemurian snout’ known from the late Sinemurian Black Ven Mudstone Member (considered to be from the Asteroceras obtusum ammonite Zone)* while Turnersuchus hingleyae is known from the Early Pliensbachian Stonebarrow Marl Member (considered to be from Uptonia jamesoni ammonite Zone; Wilberg et al. 2023) (with a temporal gap of at least 2 million years).

The only pre-Toarcian mesoeucrocodylian that has the cranial rostrum preserved is the holotype of Calsoyasuchus oalliceps Tykoski et al. * 2002 (which is Sinemurian or Pliensbachian in age). The ‘Sinemurian snout’ differs from Calsoyasuchus by:

1. Having nasal rostral processes that are triangular in dorsal view (i.e. rostral-most margins converge) rather than being (sub)parallel* as in Calsoyasuchus (and most neosuchians).

2. No evidence of a midline ridge on the maxillary secondary palate (although this may only be present in the rostral-most maxilla in Calsoyasuchus).

3. No evidence of festooning or any other ventral maxillary deflection* unlike Calsoyasuchus (and most neosuchians* except most Tethysuchia and Gavialoidea)

4. The maxillary alveoli are broadly exposed in lateral view* unlike in Calsoyasuchus where they are more ventrally oriented.

5. The maxillary alveoli are either subcircular in shape or are oval-shaped with the rostrocaudal axis being the longest* whereas in Calsoyasuchus when alveoli are oval-shaped the mediolateral axis is the longest.

6. The dorsal surface of the rostrum rises gently* rather than the noticeable curvature of the rostrum dorsal margin in Calsoyasuchus.

7. Lacks the extensive pneumatic cavities within the rostrum present in Calsoyasuchus and other members of Neosuchia (as well as Notosuchia). In these taxa the maxillary shelves forming the secondary bony palate are pneumatized* as are the maxillae surrounding the caudal-most alveoli (e.g. Tykoski et al. 2002 * Cowgill et al. 2022).

8. Vomer is exposed on the palatal surface of the secondary bony palate articulating with the maxillary palatal shelves in ‘Sinemurian snout’* whereas it is not in Calsoyasuchus.

Therefore* we conclude that NHMUK PV R 36 710 is a non-neothalattosuchian thalattosuchian with a unique combination of rostral characteristics not before seen in any known crocodylomorph. As the specimen is not from the same locality or horizon as the holotype of Turnersuchus hingleyae * and the two specimens do not share any overlapping elements* there is no a priori reason to presuppose that they pertain to the same taxon (although such a possibility cannot be precluded). However* as we describe specimens below that could belong to an unknown species of metriorhynchoid* the biodiversity of thalattosuchians from the Charmouth Mudstone Formation (and the Sinemurian and Pliensbachian in general) remains exceptionally poorly understood.