Necrosuchus ionensis, SIMPSON, 1937

Brochu, Christopher A., 2011, Phylogenetic relationships of Necrosuchus ionensis Simpson, 1937 and the early history of caimanines, Zoological Journal of the Linnean Society 163 (5), pp. 644-645 : 644-645

NECROSUCHUS IONENSIS SIMPSON, 1937

Holotype: AMNH 3219 , right dentary with associated cranial fragments and partial postcranial skeleton.

Occurrence: Salamanca Formation, Estancia Las Violetas, Chubut Province, Argentina. Palaeocene, Peligran South American Land Mammal Age (SALMA). Further stratigraphical and locality information are provided by Simpson (1937).

Emended diagnosis: Alligatorid crocodylian with long, slender descending processes of the exoccipitals extending ventrally nearly to the basioccipital tubera (shared with caimanines); slender dentary with at least 18 alveoli; first four dentary alveoli widely spaced; dentary symphysis extends back to a level just behind the fourth dentary alveolus (shared with crown caimanines); splenial bears slender anterior process extending almost to dentary symphysis.

Description: Fragments of the skull were identified in boxes of bone fragments associated with the jaw. These include a partial right quadrate and the ventral-most tip of the braincase. However incomplete these are, they nevertheless preserve derived features that allow assessment of its phylogenetic position.

The preserved quadrate fragment ( Fig. 1C–E ) includes the distal end of the ramus. The medial hemicondyle is depressed ventrally relative to the lateral hemicondyle and is not dorsoventrally expanded. The foramen aereum is located on the dorsal, and not the dorsomedial, surface of the ramus.

The braincase fragment ( Fig. 1A, B ) includes the basioccipital tubera and the base of the occipital condyle. The basisphenoid is not preserved, but its sutural surface with the basioccipital is preserved. On the left side, the ventral-most tip of the descending process of the exoccipital is preserved, revealing a slender structure that reaches nearly to the basioccipital tubera. The Eustachian foramina themselves are not preserved, but discontinuities in the ventral margin of the basioccipital indicate a median foramen at the ventral-most tip and lateral foramina dorsolateral to the median foramen.

The right dentary ( Fig. 2 ) is largely complete, although its posterior end is distorted and the anterior margin of the external mandibular fenestra is not preserved. At least 18 alveoli are preserved. The posterior-most alveolus is incomplete, as its medial wall would have been the unpreserved splenial. The third and fourth alveoli are not confluent. The dorsal surface of the dentary is concave between the enlarged fourth and 13th alveoli.

The dentary symphysis extended posteriorly to just behind the level of the fourth alveolus ( Fig. 2A, D ). The first four alveoli are spaced relatively widely apart, and the anterior margin of the dentary is orientated anteromedially, which would have given the articulated jaw an acute anterior end. The dorsal surface of the dentary is broad and smooth between the symphyseal surface and the tooth row.

The splenial is not preserved, but we can nonetheless see its attachment scar on the medial surface of the dentary ( Fig. 2D, E ). Simpson (1937) stated that the splenial entered the symphysis, but the anterior end of the splenial appears to have passed along the dorsal rim of the Meckelian groove and approached, but not actually reached, the symphysis itself. The anterior tip of the splenial was dorsal to the Meckelian groove.

Four procoelous dorsal vertebrae are preserved in articulation ( Fig. 3A ). The neural spines are rectangular in lateral view, and the transverse processes are missing. Neurocentral sutures are visible ( Fig. 3B ). At some point, someone numbered these vertebrae ‘D-5’ to ‘D-8.’ The basis for these identifications is not known, but the anterior-most of these preserves the base of a hypapophyseal keel. In living crocodylians, hypapophyses typically extend no further back than the tenth or 11th postaxial vertebra, and the first five or six dorsal vertebrae (as defined by the position of the parapophysis on the neural arch rather than the centrum) have hypapophyses that diminish in dorsoventral height posteriorly. The anterior-most vertebra (‘D-5’) may, indeed, be the fifth dorsal vertebra.

The last two dorsal vertebrae are also preserved in articulation with the anterior sacral vertebra ( Fig. 3C ). The sacral ribs have posterolaterally orientated distal facets for the ilium, and the anterior margins of the capitula lie sufficiently forward of the anterior margins of the tubercula to make the capitula visible in dorsal view. The posterior surface of the centrum lacks a hemispherical cotyle.

The intact left rib remains in articulation with the posterior sacral vertebra ( Fig. 3D ). Its articulation facet for the ilium is orientated laterally and slightly anteriorly. The anterior one-fifth of the facet is approximately circular in lateral view, and the remaining surface is dorsoventrally very thin. The anterior surface of the centrum is flat, and the posterior surface is deeply concave. As with the anterior sacral vertebra, all neurocentral and vertebrocostal sutures are visible.

The pectoral girdle is represented by parts of both scapulocoracoids, both humeri, both radii and ulnae, and the right radiale ( Fig. 4A–J). The left scapulocoracoid is best preserved, and the coracoid is nearly complete. It bears a circular foramen anterior to the glenoid fossa and a mediolaterally thin, anteroposteriorly flared ventral blade. Neither scapular blade is preserved, and the remaining scapular bases preserve rugose attachment scars for the triceps longus lateralis muscle ( Meers, 2003). The deltoid crest along the anterolateral side of the base is thin. On both scapulae, thin laminae extend from the ventrolateral and ventromedial margins across the joint toward the coracoid ( Fig. 5A, B ). This is very similar to the condition found in extant crocodylians as the scapulocoracoid synchondrosis closes ( Brochu, 1995; Fig. 5C ), and I interpret this as evidence that the synchondrosis was in the process of closing in AMNH 3219.

The preserved forelimb bones are largely congruent with homologues in most extant crocodylians. The ventromedial margins of both humeral deltopectoral crests are damaged ( Fig. 4D, E), and whether they were concave or emerged smoothly from the humeral head cannot be determined. The olecranon process of the ulna is mediolaterally broad ( Fig. 4K).

A thin, rectangular bone is interpreted as an incomplete interclavicle ( Fig. 4K). It is slightly thicker along the midline, but because the specimen is compressed, whether it was flat along its length or flexed anteriorly is unknown. It bears an expansion at what I interpret to be its anterior end. Remarkably, the sternal ribs appear to have been preserved, suggesting that they were calcified prior to the animal’s death.

Both pelvic girdles are preserved ( Fig. 4L–Q). The right ilium is distorted, but the shape of the iliac blade appears to be preserved on the left side ( Fig. 4M), revealing a dorsoventrally low structure. This is consistent with the unusually thin articulation facet on the posterior sacral rib. The blade shows no constrictions toward its posterior tip. The pubic blades are less symmetrical than in most living crocodylians and extend further medially than laterally.

Preserved hindlimb elements consist of a complete left femur and tibia, the proximal end of the left fibula, and an articulated right foot preserving the astragalus, calcaneum, distal tarsals, and the second, third, and fourth metatarsals ( Fig. 4R–V). These are consistent with corresponding structures in most extant crocodylians.

Several osteoderms are preserved with the holotype ( Fig. 6 ). They were not found in articulation, and so the arrangement of osteoderms on the dorsal and nuchal shields is unknown. Most have thin, smooth articulation facets along the anterior margin and sutural margins medially and laterally. None has a sutural margin perpendicular to these, suggesting that none comes from a ventral osteoderm comprised of paired anterior–posterior ossifications. Most have midline keels, and most are square or nearly square in outline. At least one – a lateral osteoderm with only one sutural margin, presumably from the dorsal shield – has what appears to be a compound keel ( Fig. 6B ), but most keels are single thin crests ( Fig. 6A ).

Comparisons

The dorsal position of the foramen aereum on the quadrate ramus ( Fig. 1D ) is a derived condition found in alligatoroids ( Norell et al., 1994; Brochu, 1999). In the plesiomorphic condition, found in nearly all other crocodylians for which the quadrate is known, the foramen aereum is located on the dorsomedial surface of the quadrate ramus. This strongly suggests that Necrosuchus is an alligatoroid.

A long, slender descending process of the exoccipital lateral to the basioccipital ( Fig. 1a ) is typical of caimanines. The descending process typically extends to a point about half of the distance from the occipital condyle and the ventral-most tip of the basioccipital, but the process extends further in caimanines, sometimes contacting (and forming part of) the basioccipital tubera ( Brochu, 1999). Gavialoids also have relatively long exoccipital descending processes, but in this case the process is anteroposteriorly broad ( Brochu, 2004b) and dissimilar to the structure seen in Necrosuchus .

The symphyseal region of the dentary is reminiscent of those of extant caimans ( Fig. 7a, b ). The dentaries of most globidontans curve broadly as they approach the symphysis and meet each other at a high angle, sometimes nearly perpendicular to the sagittal plane. As a result, the articulated lower jaw adopts the typical U-shaped outline in dorsal view that is seen in modern Alligator . In contrast, the dentaries of most caimans intersect each other at a lower angle acute to the sagittal plane. Simpson (1937: 1) was probably referring to this by including ‘mandible pointed anteriorly, narrow across symphysis, and not noticeably expanded at fourth tooth’ in the original diagnosis for Necrosuchus . This is outwardly similar to the condition found in several crocodyloids, including most living Crocodylus . This is not true for all caimans; the mandibles have a broader U-shape in Mourasuchus ( Langston, 1965; Bocquetin & Filho, 1990) and Tsoabichi ( Brochu, 2010) . The anterior ends of the dentaries are not known in Eocaiman cavernensis ( Simpson, 1933) , but the preserved portions are suggestive of a broader intersection.

Simpson (1937) included the presence of 18 dentary alveoli in his diagnosis of Necrosuchus . I cannot reject the possibility that additional alveoli were present because the dentary is damaged toward the posterior end of the tooth row, but the space available for additional alveoli is short, and a count greater than 20 is unlikely. This is within the range of variation for E. cavernensis and extant Caiman and Melanosuchus , but it distinguishes Necrosuchus from Paleosuchus , which uniformly has more than 20 dentary alveoli ( Wermuth, 1953).

Simpson (1937) relied on variation in alveolar diameter along the dentary to draw his conclusion that Necrosuchus was related to ‘ Leidyosuchus ’. (In fact, most of the comparisons are between Necrosuchus and species now regarded as Borealosuchus .) However, Simpson also noted similarities between Necrosuchus and caimans, in particular with the relative enlargement of the 13th or 14th dentary alveolus. This was included in his diagnosis for Necrosuchus . Alligatorines and stem alligatoroids, by contrast, share the plesiomorphic condition of a relative enlargement of the 11th or 12th alveolus ( Brochu, 2004a). Some of the specimens that Simpson measured appear to have been atypical; the pattern reported for Jacare sclerops (= Caiman crocodilus ) shows an enlarged 11th alveolus, but the 13th or 14th alveoli are enlarged in that species (pers. observ.). Simpson also reported a specimen of Borealosuchus sternbergii with an enlarged 13th alveolus, but the 11th or 12th is enlarged in Borealosuchus (pers. observ.).

There are additional similarities between Necrosuchus and other caimans. The alveoli in the concave portion of the dentary between the fourth and tenth alveoli are comparatively larger and more widely spaced than in basal alligatorines. The anterior-most four alveoli are more widely spaced from each other in Necrosuchus and caimans than in most other alligatoroids.

The postcranial skeleton of Necrosuchus argues against a relationship with Borealosuchus . The limb bones of Necrosuchus are not as long and slender as in Borealosuchus ( Brochu, 1997) , and the ilium of Necrosuchus lacks the discrete prominent anterior process found plesiomorphically in Borealosuchus . Langston (2008) reports a prominent anterior process on the ilium of Mourasuchus , but the condition in Mourasuchus is dissimilar from that in Borealosuchus and more like what is seen in other alligatoroids.

The scapulocoracoid synchondrosis of AMNH 3219 appears to be closing ( Fig. 5 ). Closure is seen in other living crocodylians, but usually very late in ontogeny. This condition is rarely observed in most crocodylian ·

species, but it occurs at earlier ontogenetic stages in extant caimans ( Brochu, 1995). That the sacral and dorsal neurocentral sutures are still open ( Fig. 3B ) suggests that AMNH 3219 was not a fully mature individual, and incipient closure of the scapulocoracoid synchondrosis at a relatively early ontogenetic stage is a similarity shared between Necrosuchus and living caimans.

The posterior blade of the left ilium ( Fig. 4M) is comparatively low compared with the condition seen in most alligatoroids. A low iliac blade with a modest posterior constriction is, however, seen in living caimans ( Brochu, 1999).

Another caimanine – E. palaeocenicus Bona, 2007 – has been described from the Salamanca Formation of Chubut Province, Argentina. The holotype is a partial lower jaw from a locality south-west of Estancia Las Violetas that can be clearly distinguished from N. ionensis . The articulated rami form a broad U, and the fifth to ninth alveoli are comparatively smaller, more closely spaced, and set within a deeper concavity on the dorsal surface of the dentary. Bona (2007) stated that the dentary symphysis extends to a level just behind the fifth alveolus, but the published figures suggest the symphysis might have been somewhat longer.

Another putative caiman, Notocaiman stromeri Rusconi, 1937 , has also been described from the Palaeocene of Patagonia. In this case, the material is limited to the anterior end of a right dentary. Derived states linking it with Caimaninae , or even Alligatoroidea, are lacking; but there is a broad resemblance to the anterior end of the dentary of Necrosuchus ( Fig. 7C ), although the symphysis is comparatively longer, extending to the level of the sixth alveolus.

Material possibly pertaining to Necrosuchus

Kuhn (1933) described fragmentary crocodyliform material, including cranial remains and vertebrae, from Punta Peligro, about 50 km south-east of the type locality of N. ionensis . The specimens derive upturned orbital margins and being nearly planar between the orbits. There is no evidence for a ridge (‘spectacle’) anterior to the orbits, but the specimens are incomplete. The frontoparietal suture on these specimens is anteriorly convex, not linear (as in modern caimanines). More intriguing are incomplete parietals, one of which Kuhn (1933) referred to Leidyosuchus , that suggest little, if any, dorsal exposure of the supraoccipital on the skull table.

Unfortunately, one cannot refer any of the isolated frontals or parietals to either Necrosuchus or Eocaiman . Both procoelous and amphicoelous presacral vertebrae are known from the Salamanca Formation at Punta Peligro (pers. observ.), demonstrating that at least one non-eusuchian crocodyliform – perhaps a sebecid – was also present in the fauna. We do not know which cranial remains correspond to which vertebrae.

from approximately correlative levels of the Salamanca Formation, and although Simpson (1937) suspected a close affinity with Necrosuchus , he stopped short of making a formal referral. Additional specimens from Punta Peligro have since been found and are housed in the MACN collections.

A partial dentary figured by Kuhn (1933: plate 1, fig. 1A) suggests a more broadly concave anterior end, more closely resembling that of Eocaiman than Necrosuchus , but direct comparisons are needed. Bona (2007) referred additional fragmentary mandibular material from Punta Peligro to E. palaeocenicus , but one of these specimens, MACN CH 1916 ( Fig. 8 ), is a fragment of a right dentary that resembles N. ionensis in three respects – the lateral margin suggests an acute intersection between dentaries, the dentary symphysis extends no further back than the fourth alveolus, and the scar for the splenial indicates a slender anterior terminus dorsal to the Meckelian groove approaching, but not quite reaching, the dentary symphysis. The anterior end is not preserved, so the actual configuration of the symphysis is unknown; moreover, the extent of the symphysis in living crocodylians can vary somewhat in living species (pers. observ.).

Isolated frontals ( Kuhn, 1933) suggest exclusion of the frontal from the supratemporal fenestrae, but differ from those of living caimans in lacking

Figures

Figure 1. AMNH 3219, holotype, Necrosuchus ionensis Simpson, 1937, cranial fragments. A, basioccipital, posterior view. B, basioccipital, anterior view. C, right quadrate ramus, ventral view. D, right quadrate ramus, dorsal view. E, right quadrate ramus, condylar view. Abbreviations: exo, exoccipital; fae, foramen aereum; leu, lateral eustachian foramen; lhc, lateral hemicondyle of quadrate; meu, median eustachian foramen; mhc, medial hemicondyle of quadrate; n, notch on dorsal surface of quadrate ramus. Scale bar = 1 cm.

Figure 2. AMNH 3219, holotype, Necrosuchus ionensis Simpson, 1937, right dentary in dorsal (A), lateral (B), and medial (C) views. D and E are close-up images and line interpretations of the symphyseal region in medial view. Abbreviations: d4, fourth dentary alveolus; d13, 13th dentary alveolus; ds, dentary symphysis; mg, Meckelian groove; sp, articulation surface for splenial on dentary. Scale bars = 1 cm.

Figure 3. AMNH 3219, holotype, Necrosuchus ionensis Simpson, 1937, axial skeletal elements. A, anterior dorsal vertebrae, right lateral view; B, close-up of neurocentral suture of ‘D-7,’ left lateral view; C, posterior-most two dorsal and sacral vertebrae, dorsal view (anterior to right); D, posterior sacral vertebra and rib, left lateral view. Abbreviations: h, base of hypapophysis; if, iliac facet on sacral rib; ncs, neurocentral suture; sr, sacral rib. Scale bars = 1 cm.

Figure 5. Left scapulocoracoid of AMNH 3219, holotype, Necrosuchus ionensis Simpson 1937, in lateral (A) and medial (B) view. C, UF 39062, Caiman crocodilus, left scapulocoracoid, medial view. Abbreviations: cf, coracoid foramen; sc, scapula. Scale bars = 1 cm.

Figure 6. Osteoderms associated with AMNH 3219, holotype, Necrosuchus ionensis Simpson 1937. Scale bar = 1 cm.

Figure 7. Symphyseal region of right dentary, dorsal view. A, AMNH 3219, holotype, Necrosuchus ionensis Simpson 1937. B, FMNH 69871, Paleosuchus trigonatus (left dentary, image reversed). C, Notocaiman stromeri, line interpretation based on illustration in Rusconi (1937). Scale bars = 1 cm.

Figure 8. MACN CH 1916, fragment of right dentary, Punta Peligro locality, Salamanca Formation (Lower Palaeocene), Argentina. Abbreviations: d4, fourth dentary alveolus; ds, dentary symphysis; mg, Meckelian groove; sp, articulation surface for splenial on dentary. Scale bar = 1 cm.