Given such potentially high European atoposaurid diversity within a narrow geographic and temporal range, and a lack of taxonomic consensus, a full revision of atoposaurid systematics is overdue. Presented here is a re-assessment of specimens of Alligatorellus from the Late Jurassic of France and Germany in the first of a series of papers in which we will revise the taxonomy, systematics and phylogenetic relationships of Atoposauridae. We refer the German occurrence to a new species of Alligatorellus , providing a comprehensive re-description, and make detailed comparisons with the French type species. We also consider the taxonomic affinities of an additional German specimen described as Alligatorellus sp. ( Schwarz-Wings et al., 2011 ), and examine the osteoderm morphology of Alligatorellus , investigating its utility in atoposaurid systematics. Finally, we examine the taxonomy and validity of the contemporaneous, multispecific taxa Alligatorium and Atoposaurus , and discuss the diverse atoposaurid faunal composition of the Late Jurassic of western Europe.

Systematic Palaeontology

Note on taxonomy: Gervais (1871) did not designate a holotype specimen in his original description of Alligatorellus beaumonti. Wellnhofer (1971) elected MNHN 15639 as the holotype of A. beaumonti beaumonti. As this is one of the two specimens described by Gervais (1871), we follow Wellnhofer (1971) in considering MNHN 15639 to be the holotype for the genus and type species of Alligatorellus beaumonti.

Wellnhofer (1971, p. 144) provided the following diagnosis of Alligatorellus (translation adapted from Schwarz-Wings et al., 2011): (1) a large-sized atoposaurid (420–550 mm) with an acute-triangular skull and large orbits; (2) the supratemporal fossae are not internally fenestrated, and are connected to the orbit by a superficial furrow; (3) the nasal aperture is divided; (4) the tail is longer than half of the precaudal body length; (5) presence of a biserial osteoderm shield from the nuchal to the caudal region; (6) single osteoderms are sculpted; (7) presence of a lateral keel on the nuchal and dorsal osteoderms, whereas the caudal osteoderms bear a more medial keel; (8) ventral armour possesses two rows of scutes in the tail region; (9) the ventral scutes are oval and medially keeled.

Comments: In light of more recent atoposaurid discoveries and an improved understanding of their anatomy, much of Wellnhofer’s (1971) diagnosis requires revision. The first putative defining characteristic (1) is a feature that also describes the sizes of Alligatorium meyeri, A. franconicum, Montsecosuchus depereti, and Theriosuchus pusillus, and may in fact be an over-estimation of their size. The lack of internal fenestration (2) of the supratemporal fenestra is not seen in other atoposaurids, including Alligatorium, Montsecosuchus, and Theriosuchus, and is thus retained as a locally diagnostic feature. The division of the nasal aperture (3) is not visible in LMU 1937 I 26 as a result of crushing of the anterior-most portion of the snout, but is present in MNHN 15639. Regardless, this appears to be a feature shared by other atoposaurids including Theriosuchus pusillus (NHMUK PV OR48330) and Theriosuchus grandinaris (Lauprasert et al., 2011). The relative length of the tail (4) is a feature seen in other atoposaurids including Atoposaurus and Theriosuchus pusillus and appears to be widespread among Atoposauridae, as are characters (5) and (6). Indeed, osteoderm sculpting and a biserial osteodermal shield are present in Alligatorium, Montsecosuchus, and Theriosuchus. The presence, prominence, and position of a dorsal keel on the biserial osteoderms might be diagnostic at the generic level (7), although there are differences between the German and French specimens, as discussed below. The presence of a dual row of ventral osteoderms in the caudal region is also questionable (8), especially with respect to their morphology (9)—they are rarely and poorly preserved in the ventral region in both French and German specimens. It is probable that post-mortem flattening has re-arranged the paravertebral dorsal osteoderms, which, when viewed laterally, might easily be misinterpreted as belonging to a ventral series. Finally, it should be noted that in the referred specimen of A. beaumonti (MNHN 15638), the osteoderms are much less visible, with just a single noticeable row overlying the anterior caudal vertebrae, and possibly a single row concealed underneath the dorsal vertebrae.

Revised diagnosis: Among currently recognised atoposaurids, Alligatorellus can be diagnosed based on the following unique combination of features and autapomorphies (highlighted with an asterisk): (1) rostrum unsculpted or substantially less so than cranial table; (2) cranial sculpting comprised of homogeneous shallow pitting; (3) absence of hypertrophied maxillary tooth 5, with homodont pseudocaniniform dentition; (4) frontal width between the orbits narrower than maximal width of nasals; (5*) broad frontal anterior process, not constricted; (6) absence of raised orbital or supratemporal rims; (7) unperforated supratemporal fenestra; (8*) anterior process of squamosal extends to the orbital margin; (9*) posterodorsal margin of parietals and squamosals completely covers dorsal occipital region; (10) smooth mandibular outer surface; (11) proportionally short first metatarsal; (12) dorsal surface of dorsal osteoderms completely sculpted, with parallel and straight anterior and posterior margins; (13*) dorsal osteoderms with longitudinal ridge along entire lateral margin; (14) caudal osteoderms with smooth, non-serrated edges.

Holotype: MNHN 15639, part and counterpart slabs preserving a near-complete, articulated skull and skeleton, missing the distal forelimb elements and part of the left hindlimb (Fig. 3).

Figure 3: (A) Line drawing of holotype specimen of Alligatorellus beaumonti (MNHN 15639) in dorsolateral view; (B) photograph of holotype specimen.

Referred specimen: MNHN 15638, part slab comprising a near-complete articulated skeleton, missing the distal-most caudal vertebrae and part of the left forelimb (Fig. 4).

Figure 4: (A) Line drawing of referred specimen of Alligatorellus beaumonti (MNHN 15638) in dorsoventral view; (B) photograph of referred specimen.

Locality and stratigraphic age: Cerin, Ain, eastern France; Kimmeridgian (Late Jurassic) (Wellnhofer, 1971).

Preservation of holotype: The specimen is dorsolaterally flattened and, on the part, the dorsal surface of the skull is embedded into matrix comprising grey lithographic limestone. This obscures both the lateral and ventral surfaces, and much of the mandible. Thirteen maxillary teeth are preserved. The complete, articulated axial skeleton is preserved, with the exception of the three posterior-most caudal vertebrae, and is overlain by a continuous sheath of parasagittal biserial osteoderms. At least eleven ribs are preserved in situ on the left hand side. A partial right scapula is the only preserved element of the pectoral girdle. The right forelimb is missing the proximal humerus and manus, and the left forelimb is disarticulated, lacking the manus. Some fragmentary pelvic elements remain, including both ilia. The left hindlimb is articulated but damaged, missing part of the femoral midshaft, the proximal tibia and fibula, and distal tarsals. The right hindlimb is articulated but missing both the proximal femur and the distal phalanx on digit I. The counterpart preserves two osteoderms and fragments of skull material embedded within the impressions. There is some dendritic mineral growth propagating from the skeleton.

Preservation of referred specimen: The entire skeleton is laterally flattened on a brick-red and grey slab of lithographic limestone. No counterpart is preserved. The skull is ventrolaterally flattened, exposing only the ventral and sinistral sides of the mandible, the ventrolateral portion of the skull, and nine maxillary teeth. The right forelimb is preserved only as an impression, as are the posterior-most caudal vertebrae. Otherwise, the entire axial skeleton is preserved, together with three ribs (and several rib impressions), and the left pectoral and pelvic girdles. Both hindlimbs are complete. A single row of osteoderms is preserved along the nuchal-dorsal series. The cervical vertebrae are recurved slightly posteriorly, and the posteroventrally deflected limbs give the impression of hanging loosely from the trunk.

Additional comments: Wellnhofer (1971) provided a detailed description of both specimens of Alligatorellus beaumonti. Here, we provide only a revised diagnosis as the basis for its taxonomic discrimination from the Bavarian specimens of Alligatorellus. Using linear morphometrics, Wellnhofer (1971) regarded the Cerin and Bavarian specimens to be of similar, adult ages, and largely based his justification for recognising two distinct taxa on the relatively smaller size of the Cerin specimens (which are approximately 50 mm shorter in total length). However, size and geographical distribution are not the only attributes demarcating the two as distinct taxa, as outlined below.

Revised diagnosis: Alligatorellus beaumonti can be diagnosed based on the following unique combination of characters and autapomorphies (highlighted with an asterisk): (1) smooth contact between maxilla and jugal (Fig. 6); (2*) frontal with unsculpted posterior and anterior portions; (3) surface of rostrum notably less sculpted than cranial table; (4) relatively large lateral temporal fenestra, approximately 30% the size of the orbit; (5*) medial longitudinal depression on posterior portion of nasal and anterior portion of frontal; (6*) frontal width between orbits narrower than nasals; (7) smooth and unsculpted region on anterior portion of squamosal nearing orbit and posterolateral process of squamosal; (8*) vertebral centra shape grades continuously posteriorly from cylindrical to elongate-spool; (9) secondary osteoderms in caudal series present; (12*) lateral ridge on sacral osteoderms forms an incipient posterior projection; (10) ratio of femur to tibia high (1.11).

Alligatorellus bavaricus Wellnhofer, 1971 Alligatorellus beaumonti bavaricus Wellnhofer, 1971

Note on taxonomy: Wellnhofer (1971) regarded LMU 1937 I 26 as the holotype of A. beaumonti bavaricus, and we elect this specimen as the holotype of A. bavaricus, which we re-rank from subspecies to species level.

Holotype specimen: LMU 1937 I 26 (Fig. 5).

Figure 5: (A) Line drawing of holotype specimen of Alligatorellus bavaricus (LMU 1937 I 26) in dorsolateral view; (B) photograph of holotype specimen.

Figure 6: Photograph and line drawing of the skull of the holotype specimen of Alligatorellus beaumonti (MNHN 15639) in dorsal aspect.

Referred specimen: Wellnhofer (1971) also described a second specimen of A. bavaricus, held in the private collection of E. Schöpfel. Based on the images and description provided by Wellnhofer (1971), we follow this referral. However, in view of the fact that this specimen remains in a private collection and is not publicly accessible, this referral is informal and is used only to draw attention to the existence of a second specimen.

Type locality and horizon: Solnhofen beds near Eichstätt, southeast Germany; early Tithonian (Late Jurassic, Hybonoticeras hybonotum zone; Wellnhofer, 1971).

Preservation: The specimen is a semi-three-dimensional body fossil preserved obliquely on a slab of Solnhofen ‘Plattenkalk’, and is fully articulated with its head dorsally recurved. As preserved, the spinal column is rod-like with a slight ventral flex, and the limbs are splayed out beneath the trunk. Trunk elements (posterior cervical and dorsal vertebrae, ribs, and osteoderms) are mostly damaged and crushed beyond recognition in an agglomeration, where there is a noticeable trace of soft tissue residue. Poor skeletal preservation means that the anterior-most vertebrae (atlas, axis, and anterior cervical vertebrae) are indistinguishable from one another. Only the eleven anterior-most dorsal paravertebral osteoderms are substantially preserved with a minor and variable degree of caudal imbrication. The next four osteoderms in the series are missing (anteriorly adjacent to the sacrum), but twenty five paired osteoderms are preserved along the tail. Poorly preserved ventral osteoderms are part of the agglomeration around the torso, and are present along the sacrum and tail. The ventral osteoderms terminate posteriorly at the same position as the dorsal series.

Etymology of species name: bavaricus, based on the area of the type locality, and also the sub-species name provided by Wellnhofer (1971) for this specimen.

Additional comments: The majority of the features Wellnhofer (1971) proposed in the original diagnosis of A. bavaricus characterise atoposaurids in general, or are more widespread within Atoposauridae. For example, an ‘acute-triangular skull with large orbit’ is a general feature seen in many crocodyliforms, including all known atoposaurids and bernissartiids, and the ‘biserial osteoderm shield from the nuchal to caudal region’ is found in the atoposaurids Theriosuchus (Owen, 1879) and Alligatorium (Wellnhofer, 1971), and may be synapomorphic for Atoposauridae.

Diagnosis: Alligatorellus bavaricus can be diagnosed based on the following unique combination of characters and autapomorphies (highlighted with an asterisk): (1*) extremely narrow and short skull (ratio of skull width to orbit length is 1.29; Fig. 7); (2*) posterior surface of nares longitudinally crenulated; (3) small, slit-shaped antorbital fenestra, enclosed by nasals; (4*) prominent transverse ridge defining frontal–parietal suture, medial to supratemporal fenestrae; (5) smooth posterior region of parietal dorsal surface; (6*) dorsal osteoderms with longitudinal medial ridge, becoming more laterally placed anteriorly; (7) isometric caudal osteoderm morphology; (8*) distinct ridge on proximodorsal edge of scapula; (9*) an extremely high humerus to ulna ratio of 1.45; (10*) an extremely low femur to tibia ratio of 1.04; (11*) an extremely low tibia to ulna ratio of 0.64; (12) metatarsals I–IV equidimensional.

Figure 7: Photograph and line drawing of the skull of the holotype specimen of Alligatorellus bavaricus (LMU 1937 I 26) in dorsolateral aspect.

Differential diagnosis to A. beaumonti: Alligatorellus bavaricus can be distinguished from A. beaumonti based on possessing the following features: (1) proportionally larger orbits; (2) longitudinal crenulations on the posterior external surface of the nares; (3) a diminutive antorbital fenestra; (4) frontals proportionally wider between orbits than nasals; (5) prominent transverse ridge defining the frontal–parietal suture on the cranial table; (6) lack of posterolateral squamosal process; (7) medially-placed dorsal keels on dorsal osteoderms; (8) osteoderm shapes are isometric down length of body; (9) humerus proportionally longer than ulna (1.45 to 1.12); (10) higher ratio of humerus to femur length (0.89 to 0.75).

The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “http://zoobank.org/”. The LSID for this publication is: urn:lsid:zoobank.org:pub:B7CC4367-4203-4AED-8C30-2D7E4E71665D. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.

Description and comparisons of Alligatorellus bavaricus The following description is solely of the type specimen LMU 1937 I 26 but, based on the images presented in Wellnhofer (1971), the referred specimen does not appear to differ in any notable way. Elements of the skull of the type are fully fused, and vertebrae display complete neurocentral fusion, implying that this specimen of Alligatorellus had reached a mature stage of growth (Joffe, 1967). Measurements are provided in Table S1. Skull: Observations of the skull are restricted to the dorsal and right-lateral surfaces. These external surfaces display a moderate degree of sculpting, although to a lesser extent than that of Theriosuchus (Owen, 1879; Brinkmann, 1992; Wu, Sues & Brinkman, 1996; Schwarz & Salisbury, 2005) and Alligatorium (Wellnhofer, 1971). The skull has an acute-triangular morphology (platyrostral) in dorsal view, typical of atoposaurids, with concave lateral margins along the relatively short snout. The intramandibular angle (defined as the angle between the lateral extremities of the cranial table and the distal snout tip, in dorsal aspect) is slightly greater (37°) than that of Theriosuchus (30–32°). Several teeth are preserved in situ, and are peg-like (pseudocaniniform), pointed and possess apicobasally and mesiodistally oriented, parallel striations. None of the teeth appear to be serrated, and in general aspect they are indistinguishable from the teeth observed in the Cerin specimens of Alligatorellus beaumonti. Alligatorellus bavaricus may possess one more maxillary tooth than the French species, although this is difficult to confidently assess due to the mode of preservation. The dentition of Theriosuchus (Owen, 1879; Joffe, 1967; Brinkmann, 1992; Martin, Rabi & Csiki, 2010) is substantially different in that it is heterodont. No palatal elements are visible, and aspects of the anatomy of the premaxilla, maxilla, nasals and external nares are difficult to discern due to dorsal flattening into the matrix and mandible, and because of the absence of the distal snout tip. The occipital region of the skull is also obscured by matrix and crushed, granular bone fragments, which probably represent the anterior-most elements of the axial skeleton. There is a ventrolateral notch between the premaxilla and maxilla but, unlike in Theriosuchus ibericus (Brinkmann, 1992) and Theriosuchus sympiestodon (Martin, Rabi & Csiki, 2010), this is not occupied by an enlarged tooth. The paired nasals contribute to the external nares via a sagittal anterior projection, as in Alligatorellus beaumonti, Alligatorium meyeri, and Theriosuchus pusillus. Wellnhofer (1971) regarded this feature as diagnostic of Alligatorellus. However, it may be a synapomorphy of all atoposaurids: in other crocodyliforms with divided external nares, this division is formed by a sagittal projection of the premaxillae, e.g., the metriorhynchid Maledictosuchus (Parilla-Bel et al., 2013), whereas the external nares are fully open or only partially divided posteriorly in eusuchians (e.g., Delfino et al., 2008). A pair of small, slit-like antorbital fenestrae are present and are entirely enclosed by the nasals, a feature absent in A. beaumonti, but present within all specimens of Theriosuchus for which the snout is preserved; as such we consider this feature to be locally diagnostic of A. bavaricus within non-Theriosuchus atoposaurids. The dorsal surface of the nasals is sculpted by faint longitudinal crenulations, a feature unique within Atoposauridae, but also present in the goniopholidid Eutretauranosuchus delfsi (Smith et al., 2010; Pritchard et al., 2013). As such, this feature is considered a local autapomorphy of A. bavaricus. Posterior to the external nares, the lateral margins of the nasals are straight, contrasting with the concave margins observed in A. beaumonti. The dorsolaterally facing orbits are large with respect to the cranium, occupying about one third of the total cranial length and the majority of the skull width. This is comparable to Atoposaurus oberndorferi but distinct from A. beaumonti, in which the orbits occupy one quarter of the skull length. The relatively large size of the orbits might represent retention of a paedomorphic characteristic (Joffe, 1967). A large amount of secondary calcite growth is present within the orbit, obscuring much of the internal cranial morphology. The right lateral temporal fenestra is deep and arcuate in cross-sectional morphology, but largely obscured as a result of the crushing of the skull. It is separated from the orbit by a mediolaterally-oriented postorbital bar, which descends steeply into the posterolateral internal margin of the orbit. The lateral temporal fenestra is similar in size to the dorsally located supratemporal fenestra, and is approximately a quarter of the size of the external opening of the orbit. The frontals are mediolaterally concave, to a slightly greater degree than the parietals, and become extremely thin at the orbital margin, lacking the elevated orbital rims seen in Theriosuchus (Owen, 1879). Compared to the nasals, they are relatively wide with respect to the frontals in A. beaumonti. The anterior frontal ramus extends slightly beyond the anterior tip of the prefrontal, a feature which we consider to be a local autapomorphy because of its absence in other atoposaurids, but that is present in some other non-eusuchian neosuchians, including Eutretauranosuchus delfsi (Pritchard et al., 2013) and Pholidosaurus purbeckensis (Salisbury, 2002; Montefeltro et al., 2013). The anterior contacts between the frontals, prefrontals and lacrimals are largely obscured, as is the overall morphology of these pre-orbital elements. However, the majority of the anterior margin of the orbits comprises a deep and thick wedge of bone that descends as a vertical sheet into the orbit, forming a distinctive anterodorsal brow. The maxilla contributes extensively to the ventral margin of the orbit, with the contact between the maxilla and the lacrimal becoming indiscernible more anteriorly as a result of the mode of preservation. The jugal occupies half of the ventral margin of the orbit, posterior to the maxilla. Palpebrals were either absent or are not preserved, but appear to be present in the anterior orbit of Alligatorellus beaumonti. Posterior to the orbits, the dorsal surface of the skull is mildly sculpted by anisotropic and heterogeneously spaced pits that are similar to Alligatorellus beaumonti, but are less prominent than those seen in Theriosuchus and Alligatorium. In contrast, this surface is smooth and unsculpted in Atoposaurus (Wellnhofer, 1971; J Tennant, pers. obs., 2013). It is plausible that the heterogeneous degree of cranial sculpting seen in atoposaurids including Alligatorellus and Montsecosuchus is useful in distinguishing specimens at the species level. Between the supratemporal fenestrae is a prominent mediolateral ridge defining the suture between the frontal and parietal, a feature we consider diagnostic of A. bavaricus. The anterior parietal is not sculpted where it contacts the frontals, unlike A. beaumonti where the whole cranial table (excluding the frontals) is homogeneously sculpted with small circular pits. The squamosal is homogeneously sculpted, as with the parietal, with a dorsally convex dorsal surface and orthogonal lateral and posterior margins, differing from Theriosuchus pusillus which has a smooth posterolateral process (Owen, 1879; J Tennant, pers. obs., 2013). The cranial table is mostly flat, as is the case in most other atoposaurids, with the exception of the slightly domed structure that characterises Montsecosuchus (Buscalioni & Sanz, 1990a), and possibly Atoposaurus. The anterolateral portion of the squamosal is sharply pointed and curves posteromedially around the supratemporal fenestra. Here, it is initially gently arcuate, then becomes straight as it contacts the parasagittally-directed and straight medial edge. This gives the squamosal an overall distorted rhombohedral shape in dorsal aspect. The majority of the dorsomedial margin of the squamosal contributes to the supratemporal fenestra, with the lateral portion obscuring most of the ventrally-placed quadrate and quadratojugal. The posterolateral process of the squamosal is greatly reduced compared to other atoposaurids, in which it generally tapers to a point, and is therefore considered to be a local autapomorphy of A. bavaricus, being similarly present in other basal neosuchians such as Amphicotylus lucasii (Mook, 1942). In Alligatorellus beaumonti, there is no development of the posterolateral process, the posterior edge instead being slightly anterolaterally directed. Between the supratemporal fenestrae, the paired, rectangular parietals are as mediolaterally wide as the frontals between the orbits. The parietals contribute to the posteromedial margin of the supratemporal fenestra, but the relationship with the postorbitals is difficult to see due to post-mortem damage. However, the postorbital bar is present and weakly developed, possessing a superficial furrow connecting the orbit and the supratemporal fenestra. The frontal only contributes to the supratemporal fenestra at its anteromedial edge. Here, the frontal and parietal form a lateral wedge, which thins laterally into the postorbital bar. The posterior portion of the dorsal surface of the parietal is smooth, a feature otherwise only found in Atoposaurus, although in that taxon the skull is entirely unsculpted (Wellnhofer, 1971; J Tennant, pers. obs., 2013); as such, we consider this heterogeneous pattern of cranial sculpting to be autapomorphic for A. bavaricus. The lateral and ventral surfaces of the skull are largely obscured by the displaced and crushed mandible, and the preserved orientation of the skeleton. The mandible is not visible ventral or anterior to the orbit, and is largely obscured posteriorly. It has been slightly dorsally displaced into the ventrolateral portion of the right-lateral face of the skull. The mandible broadens posteriorly both mediolaterally and dorsoventrally, developing a lateral shelf as it flares out beneath the lateral temporal fenestra, possibly at the position at which the mandibular fenestra would have been situated. The ventral margin of the mandible curves medially and substantially thins mediolaterally at its posterior extremity, where it forms an acute and recurved process, the posterior margin of which is gently concave and slightly set back from the posterior edge of the cranial table. Axial skeleton: One of the most striking features of atoposaurids is that the tail length is greater than the length of the torso, and comprises approximately one-half of the total length of the skeleton. In Alligatorellus bavaricus there are seven cervical (including the axis and atlas) and fifteen dorsal vertebrae (note that Wellnhofer (1971) observed only seventeen presacral vertebrae, using osteoderm count as a proxy). These vertebrae are mostly indistinguishable from one another, but their presence is estimated based on their associated dorsal paravertebral osteoderms which, along with the poor preservation of the trunk region, largely obscure the morphology of the vertebral column. As noted by Wellnhofer (1971), three sacral vertebrae seem to be present, but their preservation means that this cannot be determined with any certainty, with all elements crushed beyond distinction. If correctly determined, sacral count might be a distinguishing feature between A. bavaricus and A. beaumonti, with the latter only having two sacral vertebrae, but variation in sacral count is difficult to discern in atoposaurids due to poor preservation of the axial skeleton in specimens of Alligatorellus. There are around forty caudal vertebrae, although the precise number is difficult to determine, with the distal-most two or three absent, as indicated by impressions. Much of the caudal vertebral series is variably covered in matrix and fixing glue, obscuring most of the morphological detail and intervertebral articulations. In the central caudal series, a melange composed of dorsal and ventral paravertebral osteoderms obscures much of the anatomical detail. Only the first four caudal vertebrae can be used to observe any of the anatomy from a right-lateral perspective. It is unknown whether the vertebrae were procoelous, as in Theriosuchus and eusuchians (e.g., Pol & Gasparini, 2009), or amphicoelous. The dorsal osteoderms occur in a biserial row from the anterior-most cervical vertebrae to about the mid-point of the caudal series, a feature that characterises all unambiguous atoposaurids, with the exception of Atoposaurus, and that is also absent in the putative atoposaurid Karatausuchus (Efimov, 1976; Storrs & Efimov, 2000). The osteoderms of A. bavaricus are imbricated along their entire length, and there is no ‘peg and socket’ articulation as described in two scutes assigned to Theriosuchus pusillus (Owen, 1879; Schwarz-Wings et al., 2011) and in Theriosuchus guimarotae (Schwarz & Salisbury, 2005). The osteoderms of A. bavaricus are rounded, and the lateral edges are predominantly convex, with one or two being marginally concave. There is a central longitudinal ridge on the dorsal surface of osteoderms of A. bavaricus, similar to some of the caudal osteoderms in Theriosuchus, but contrasting with Alligatorium meyeri and other atoposaurids. The degree of sculpting on the osteoderm dorsal surfaces increases posteriorly, as does the prominence of the longitudinal keel which shifts to a slightly medial position from an initially more central position, unlike Alligatorellus beaumonti in which it is consistently laterally placed as a distinct shelf. The lateral and medial edges of the osteoderms are smooth and either straight or convex, and the straight anterior and posterior margins are parallel. The morphology of the ventral osteoderm series is very similar, where visible, but with more prominent longitudinal ridges in the more posterior elements. There is no visible morphological heterogeneity in the nuchal and sacral osteoderms, contrasting with Alligatorellus beaumonti in which this feature is highly distinctive. It is unknown whether the ventral series are paired or not in A. bavaricus, as the ventral portion of the skeleton is mostly unobservable. Other minor axial elements are partially visible beside the osteoderms. Two thoracic ribs are preserved embedded within the trunk melange. They are gently arcuate in their overall morphology, and not preserved in situ. There are several other rib elements more anterior to these and just ventral to the anterior-most osteoderms, but they are largely obscured by the overlying matrix and axial elements. Three posteroventrally directed chevrons are in situ with their proximal caudal vertebrae, positioned just posterior to the only visible three-dimensionally preserved vertebrae. Pectoral girdle: Only the right scapula is preserved, and is fragmented at both ends, including both the glenoid fossa and coracoidal contact. It is bow shaped, with a distinct dorsoventral contraction and mediolateral thickening into a compressed cylindroid at mid-length. The dorsal surface becomes thin and sharp anteriorly, culminating in a broad and deep, basin-like medial depression, contrasting with Montsecosuchus depereti in which the entire element is flat (Buscalioni & Sanz, 1990a). The proximodorsal edge overhangs this depression, a feature not observed in other atoposaurids, and is considered to be a diagnostic feature of A. bavaricus. Posteriorly, the scapula flares out in a similar fashion to the anterior blade, but the distal portion is mostly absent, so the complete morphology is unknown. A posteroventral process projects out from the posterior blade, twisting from the ventral surface into a short, thickened rod. Forelimbs: The right forelimb is nearly complete, with an articulated humerus, radius and ulna, but the manus is crushed. The proximal third of the humerus is also crushed, with the external cortices of the exposed shaft removed, revealing the internal bone. The humerus expands slightly proximally, and the shaft is straight and broader mediolaterally than anteroposteriorly. The morphology of the deltopectoral crest cannot be determined. The radial condyle is broad and directed anteriorly. The distal articular surface of the humerus is strongly rugose, and oriented at 40° to the long axis of the shaft. The shaft is relatively straight, similar to more advanced neosuchians such as Shamosuchus (Pol, Turner & Norell, 2009). The anterior intercondylar groove is not visible, but the supracondylar fossa forms a deep posterior furrow, terminating a short distance up the shaft, and is bound medially by the relatively weaker ulnar condyle, the morphology of which is mostly obscured. The external surfaces of the condylar heads are smooth. The humerus is slightly shorter than that of A. beaumonti, but the radius is proportionally longer. The stylopod to zeugopod ratio in both limbs is proportionally lower than in all other atoposaurids, a feature that we consider diagnostic of A. bavaricus. The radius is slightly longer than the more robust ulna, the two resting against each other without twisting sharply; as such the respective proximal and distal articular surfaces have long axes in the same orientation. The radius is gently longitudinally arcuate in its proximal third, conforming to the gentle curvature of the distal ulnar shaft. The radial head is mediolaterally expanded, and is about two-thirds the size of the ulnar head it rests against. The ulnar head is damaged, and the radial head and the associated humeral condyle actually appear quite mismatched in size, suggesting a large volume of cartilage or muscle attachment at this joint, also emphasised by the heavily rugose articular surface. The lateral part of the radial shaft thins to about 70% of its width and becomes ridge-like at around two-thirds of its length. The ulnar shaft is equidimensional through its entire length, and finishes with a triangular-shaped distal articular surface. The carpus cannot be fully observed. Little of the left forelimb is preserved: the distal humerus is crushed, with the proximal ulna and entire radius missing, preserved only as impressions. However, aspects of the morphology of the carpus can be observed. The radiale is long and slender, with expanded proximal and distal ends, much like A. beaumonti in which the elements are well-preserved in the holotype. The ulnare is slightly shorter, with a stronger mediolateral compression of the shaft, and overall more gracile morphology. In A. beaumonti, the ulnare has a proximal groove on the lateral surface, terminating at 80% of the length of the element, but whether this is present in A. bavaricus cannot be determined. However, the ulnare in A. bavaricus is not ‘hatchet shaped’ as in A. beaumonti or the specimen assigned to Alligatorellus sp. by Schwarz-Wings et al. (2011). Furthermore, the radiale in A. beaumonti is larger than the ulnare, distinguishing the two species of Alligatorellus. All additional carpal elements in A. bavaricus are crushed to the point where their morphology cannot be meaningfully observed. The entire manus is bent backwards, indicated by its impression and in a similar manner to the pedal orientations. All of the elements are highly distorted and crushed, with only moderate lateral compression indicated by the slight crushing of the more gracile elements. Pelvic girdle: Only fragments of the pelvic girdle are preserved. The ilium forms an elongated S-shape in dorsal view, and is thickened anteriorly. Much of the morphology is obscured by the orientation of the specimen on the rock slab, but the postacetabular process appears to be fenestrated at its tip (although this might be a post-mortem artefact), greatly thickened, and leads into a deep and broad acetabulum. An element just below this on the slab is one of the pubes. Much of the morphology is again obscured by the orientation in which it is embedded in the matrix. The proximal head is expanded into a broad wedge-shape and twists slightly to become oblique to the stouter distal end, which is more circular in cross section. The proximal portion of the shaft is transversely flattened and sub-elliptical in cross-section, and has a strongly rugose surface, partially obscured by an overlying displaced rib. There is a fan-shaped structure situated anterior to the ilium, which we interpret as a fragment of the anteriorly displaced ischium. The distal end is thin and gently convex, with a slightly crenulated distal extremity. Gentle striations from the distal end are directed towards the transversely thickened shaft, which increases in breadth more proximally on the dorsal margin and has a more slender ventral margin. The proximal end is hidden underneath the skeleton so that the remaining morphology cannot be observed. Hindlimbs: Overall, the hindlimbs are about 1.4 times the length of the forelimbs. The right hindlimb is mostly complete with a laterally flattened tarsus and pes. The femur is missing from the left hindlimb (although it is possibly hidden underneath the skeleton), and the tibia and fibula are both crushed. The left pes is well-preserved, with partially crushed tarsal and pedal elements. The femur is the most robust limb bone of the skeleton, and is morphologically similar to the ulna, being gently sigmoidal down the length of the shaft. The femoral head is moderately expanded and equidimensional to the distal end of the femur. The femoral head grades smoothly into the posteriorly placed fourth trochanter, which is weakly developed, ridge-like, and distally thickened, terminating at one-sixth of the length from the proximal end. Adjacent to this, on the lateral surface, there is an accompanying groove for attachment of the femoral-pelvic musculature. The distal end of the right femur is damaged and fractured, and the distal condylar morphology cannot therefore be determined. The left tibia and fibula are mostly concealed within the slab and underneath other bones, and only the straight shafts are exposed. The lateral surfaces of both elements from the right hindlimb are fully exposed, and demonstrate that they are equal in length to the femur. Both ends of the tibial shaft are anteroposteriorly compressed, with the distal end slightly more so. The proximal portion of the tibia is slightly posteriorly deflected, but to a lesser degree than in Alligatorellus beaumonti. The tibial shaft becomes slightly anteroposteriorly expanded at mid-length. Distally, the lateral margin of the tibia thins anteroposteriorly, culminating in a sharp ridge at the distal end, and resulting in a triangular cross-section. The proximal half of the fibula is gently twisted to accommodate the mid-tibial expansion, and articulates with the posterior face of the proximal head of the tibia. As a result of the fully articulated nature of the tibia and fibula, the morphology of the proximal and distal articular surfaces is obscured. Furthermore, the distal end of the fibula is damaged. In lateral view, the fibula is much more slender than the tibia, and has a more circular cross section than the elliptical to triangular tibia. The astragalus is not visible in either hindlimb. The calcaneum is present, but is obscured by matrix and glue. On the right hindlimb, metatarsals I–III and part of metatarsal IV are preserved, as well as a poorly preserved, vestigial fifth metatarsal that is less than one-third the length of the other four metatarsals. Their long axes are parallel to one another, with the proximal and distal ends resting against each other. The nature of the distal articulations is obscured. The left pes is preserved in an oblique view, and provides a better perspective of the metatarsal morphology, although metatarsal V is not visible. The tarsal phalangeal formula, as stated by Wellnhofer (1971), is 2-3-4-4-(1). The metatarsals are long, gracile, and transversely expanded at their proximal ends with an overall similar morphology to one another. Their distal ends have been slightly anteroposteriorly compressed, and the straight shafts all have an elliptical cross-section. On the left pes, the proximal tip of metatarsal I is obscured beneath metatarsals II–IV but, where visible, the metatarsal is anteroposteriorly compressed, and twists anteromedially towards its distal end, at which point it thickens and broadens into a sub-oval cross section. The distal articular surface of metatarsal I is only partially visible; this rugose surface curves medially to occupy the distal-most edge of the medial surface. Metatarsal II is slightly longer than metatarsal I, with a mediolaterally compressed proximal end, and a ventral surface that forms a thin ridge. Metatarsal II gradually thickens distally, and the shaft twists in a similar manner to metatarsal I, but instead the ventromedial edge becomes more prominent as a ridge, bounding the medial edge of a small distal depression on the ventral surface. The distal end of metatarsal II is convex, and the articular surface is obscured. Most of metatarsal III, except for the shaft, is obscured, with the shaft appearing to be as long as metatarsal II but thickened to a lesser degree distally. Metatarsal III is slightly more gracile than the others. The sharpness of the proximoventral ridge is also less apparent in metatarsal III. Metatarsal IV is mostly obscured, but has a straighter, less twisted shaft that is more continuously oval in cross-sectional morphology than the metatarsals.