Diplocynodon hantoniensis, WITH OTHER

Rio, Jonathan P., Mannion, Philip D., Tschopp, Emanuel, Martin, Jeremy E. & Delfino, Massimo, 2020, Reappraisal of the morphology and phylogenetic relationships of the alligatoroid crocodylian Diplocynodon hantoniensis from the late Eocene of the United Kingdom, Zoological Journal of the Linnean Society 188, pp. 579-629 : 620-625

COMPARISONS OF D. HANTONIENSIS WITH OTHER DIPLOCYNODON SPECIES

Diplocynodon hantoniensis has a particularly broad rostrum, which among the known species of Diplocynodon is closest in shape to D. darwini , D. deponiae , D. elavericus and D. muelleri . In contrast, D. ratelii , D. ungeri and D. tormis have notably narrower, more pointed rostra. Furthermore, those taxa with a narrow rostrum appear to have a very modest canthus, which is absent in D. hantoniensis . The outline of the naris is square in D. hantoniensis , with straight, anteriorly diverging lateral margins. This contrasts with the strongly rounded narial margins seen in D. ratelii , D. ungeri and D. tormis . The presence of a deep notch situated lateral and slightly posterior to the external nares is characteristic of Alligator , but a similarly deep notch is observed in D. hantoniensis and D. tormis .

There is a large amount of inter- and intraspecific variation in cranial ornamentation in Diplocynodon . Diplocynodon hantoniensis , appears to be the only species with preorbital ridges (NHMUK OR 30392, CAMSM TN 907). These ridges are not as tall, narrow or elongate as those in some extant Crocodylus species, e.g. C. porosus Schneider, 1801 . A small step immediately anterior to the orbits, and crossing the frontals, is absent or very weakly developed in adult D. hantoniensis , as in D. ratelii (e.g. MNHN SG 539) and D. darwini (e.g. HLMD Me 7500). However, a step has been described in D. remensis ( Martin et al., 2014) and was figured in the description of D. tormis ( Buscalioni et al., 1992: fig. 2). A step was not described in D. muelleri , but figures in Piras & Buscalioni (2006: figs 2 and 3a) indicate its presence. The holotype of D. deponiae does not preserve the dorsal surface of the rostrum, and the best preserved referred specimens are broken along the frontal [e.g. SMF Me 2609, IRSNB R 261 ( Delfino & Smith, 2012)]. A juvenile specimen (HLMD-Be-147) apparently lacks a step and so it is inferred to be absent in adult D. deponiae too.

Mostspeciesof Diplocynodon exhibitanectopterygoid that contacts the walls of the posteriormost maxillary alveoli, which is the plesiomorphic condition in Crocodylia ( Brochu, 1997) ( Fig. 29C, D). Only D. deponiae ( Delfino & Smith, 2012) and D. tormis ( Buscalioni et al., 1992) seem to express the alternative character state, whereby the maxilla separates the posteriormost alveoli from the ectopterygoid ( Fig. 29B). Although D. hantoniensis also expresses the plesiomorphic condition, it shows a slight variation from other taxa. Here, the posteriormost maxillary alveolus is bordered medially by the ectopterygoid, but anteriorly the ectopterygoid narrows abruptly to an acute point, separated from the toothrow by the maxilla ( Fig. 29E). This differs from other Diplocynodon species, e.g. D. ratelii (MNHN SG 539), whereby the ectopterygoid narrows gradually to an acute point ( Fig. 29D). This condition resembles that seen in some Caiman crocodilus chiapasius (Bocourt, 1876) specimens (FMNH 73694, 73701, 73721) ( Fig. 29F), which do not show the typical alligatoroid condition. We regard this unique condition as a local autapomorphy of D. hantoniensis .

In all extant alligatorids, the squamosal and parietal have a large sutural contact on the posterior wall of the supratemporal fenestra, anterior to the orbitotemporal canal ( Brochu, 1997) ( Fig. 30A). In contrast, extant Crocodylus , Tomistoma and Gavialis express an alternative condition, in which the quadrate widely separates the squamosal and parietal, forming the floor of the orbitotemporal foramen ( Fig. 30B). Diplocynodon ratelii possesses an intermediate condition, in which the squamosal approaches the parietal, but a narrow wedge of the quadrate separates them ( Brochu, 1997) ( Fig. 30C). Previously, this character was scored as unknown in D. hantoniensis (Brochu et al., 2012; Martin et al., 2014), but close inspection of the walls of the supratemporal fenestra in NHMUK OR 30393 reveals that the intermediate condition is also present in D. hantoniensis ( Fig. 30D). This condition is now recorded in almost all Diplocynodon species, except D. ungeri , in which it is unknown, and D. elavericus ( Martin, 2010) , which seems to express the derived alligatorid condition. However, in the latter species, the supratemporal fenestra is poorly preserved. Outside of Diplocynodon , this condition is known only in Brachychampsa montana , so this feature could be a local autapomorphy of Diplocynodon .

Flexure of the ectopterygoid–pterygoid suture is retained into adulthood in D. hantoniensis ( Fig. 8A). This feature was previously regarded as an unambiguous synapomorphy of crown caimans ( Brochu, 1999) ( Fig. 8B). However, it appears to occur more widely, as it is present in Allodaposuchus precedens ( Martin et al., 2016) , Thoracosaurus macrorhynchus de Blainville, 1855 [based on character scores in Brochu (1999)] and possibly in D. deponiae ( Delfino & Smith, 2012) . In the latter case, Delfino & Smith (2012) expressed some doubt that IRSNB 261, which formed the basis of their description, was a mature individual of D. deponiae . As a result, we cannot be confident that ectopterygoid–pteryoid flexure is retained into adulthood in D. deponiae , and thus we re-scored this character as missing in this taxon. All other species of Diplocynodon lack the ectopterygoid– pterygoid flexure. The condition in D. hantoniensis can only be confidently observed in a large isolated pair of pterygoids (NHMUK OR 30251). Here, the condition is slightly different to that of Caiman, which has a large lateral process of the pterygoid immediately posterior to the suborbital fenestra. In contrast, D. hantoniensis also has an acute process of the ectopterygoid in the pterygoid, which we consider an autapomorphy of the species ( Fig. 8A).

The choanae of D. hantoniensis are heart-shaped, resulting from a posterior projection of the pterygoid, which aligns with the recessed choanal septum ( Figs 6B, 31A). Furthermore, they are ornamented with an anterolaterally directed ridge on each side. This ridge is even developed in juvenile specimens, becoming a prominent ventrally directed lamina bound by a sulcus at maturity.Heart-shaped choanae with anterolaterally developed ridges are also present in D. remensis ( Martin et al., 2014: fig 6K) and, tentatively, in D. ratelii (MNHN SG 539) ( Fig. 31D). The lateral walls of the choana are broken off in the latter specimen, but deep sulci similar to D. hantoniensis are suggestive of a similar ridge. Wu et al. (2001) also described ridges lateral to the choana in Leidyosuchus canadensis Lambe, 1907 , although restricted to some of the largest individuals. This contrasts with the condition in D. hantoniensis , in which they appear throughout ontogeny. The ridges in D. hantoniensis , D. remensis and D. ratelii appear homologous to those of several caimanines ( Figs 31B, F), although the morphology is clearly different. In extant Caiman ( Fig. 31B), these ridges are shorter, restricted to the posterolateral choanal margins and commonly folded medially into the choana. In Paleosuchus ( Fig. 31F), ridges can encircle the choana and become almost completely everted. The ridges are also similar to those developed in Alligator ( Fig. 31C), but in that taxon they form a near continuous posterior wall to the choana and do not extend very far anteriorly.

THE POTENTIAL FOR NEW PHYLOGENETIC CHARACTERS FROM THE POSTCRANIA

Postcranial character sampling is low in Crocodylia, as is the case for all of Crocodylomorpha ( Godoy et al., 2016; Mannion et al., 2019). For example, in the data matrix used here, just 46 of 187 characters (25%) represent the postcranial skeleton. The dearth of postcranial characters has been based on the assumption that the crocodylian postcranial skeleton is morphologically conservative, being less phylogenetically informative than cranial and mandibular components (see: Godoy et al., 2016). However, recent studies of axial and appendicular skeletal components in extant crocodylians have demonstrated that there are broad differences in crocodylian postcrania, which have yet to be incorporated into phylogenetic studies ( Chamero et al., 2013; Iijima et al., 2018). Furthermore, recent studies of notosuchian ( Pol et al., 2012; Godoy et al., 2016), basal neosuchian ( Martin et al., 2016) and crocodylian ( Iijima & Kobayashi, 2019) anatomy have started to demonstrate that there is a potentially richer suite of postcranial characters, and that these might impact upon crocodylomorph phylogenetic relationships.

The quantity and quality of preserved postcranial material belonging to Diplocynodon hantoniensis affords comparisons with extant and fossil taxa, and the identification of potential postcranial characters. In particular, the appendicular material is especially well preserved and represented by several ontogenetic stages, allowing us to distinguish between interspecific and ontogenetic variation. In addition to the recognition of numerous muscle scars (see Description), below we briefly outline a number of potential new characters pertaining to the appendicular skeleton.

Only two humeral characters appear regularly in phylogenetic analyses of crown group crocodylians. These pertain to the shape of the deltopectoral crest and the presence of a separate or single common insertion for M. teres major and M. dorsalis scapulae (Brochu et al., 2012: C27 and C28). In addition, character 36 in the matrix used herein describes the slenderness and relative length of the forelimb and hindlimb. Observations of D. hantoniensis and other crocodylians have led us to recognize two other potentially significant features in the humerus.

Stein et al. (2012) recognized differences in humeral torsion between the early Eocene Australian mekosuchine, Kambara , with that of Crocodylus johnstoni Krefft, 1873 and C. porosus . Humeral torsion describes the dorsoventral offset of the proximal and distal extremities (i.e. the metaphysis and epiphysis) of the humerus, which is most noticeable in medial view. A high degree of torsion results in a strongly sigmoidal outline of the humerus ( Fig. 32A), whereas a low degree of torsion results in a relatively straight outline ( Fig. 32C). Stein et al. (2012) noted that humeral torsion appears to be associated with a rotation of the lateral margin of the proximal extremity and resultant medial rotation of the deltopectoral crest (i.e. axial rotation of the proximal extremity). This results in movement of the apex of the deltopectoral crest from the lateral edge, to the centre of the humerus, when viewed ventrally ( Fig. 32B). This also exposes the ventral surface of the proximal extremity in medial view.Whereas these two features appear to be linked to one another in Crocodylus and Kambara , an examination of a larger sample of crocodylians reveals that these features are not always associated. For example, D. hantoniensis exhibits a high degree of dorsoventral offset of the proximal and distal humeral extremities, without the axial rotation of the proximal end( Fig.32E,F).This might be the plesiomophic condition in Crocodylia, as it is observed in the stem brevirostrine Borealosuchus sternbergii (Gilmore, 1910) (USNM 6533) ( Fig. 32G, H) and the basal crocodyloid Asiatosuchus germanicus Berg, 1966 (SMF Me 1801) ( Fig. 32I, J). The same condition is also observed in a number of other alligatoroids, e.g. D. ratelii (MNHN SG 628), Alligator (AMNH 71621) and Caiman (AMNH 97300). In contrast, Gavialis appears to lack significant dorsoventral offset of the proximal and distal extremities.When viewed medially, the distal half of the humerus of Gavialis is almost straight ( Fig. 32K). However, the proximal end, beginning around the level of the deltopectoral crest, shows extreme axial rotation ( Fig. 32L).

A comparison of the humeri of C. porosus at different ontogenetic stages reveals variation in the second character (axial rotation of the proximal extremity) ( Fig. 33A, B). Humeral torsion appears relatively consistent in this growth series ( Fig. 33C, D), but the juvenile specimens show less axial rotation of the proximal extremity than adult specimens. As a result, the latter should be compared only between adult specimens.