Necrosuchus, SIMPSON, 1937

NECROSUCHUS SIMPSON, 1937

Emended diagnosis: A caimanine with the following combination of characters: differs from all other caimanines (except Purussaurus brasiliensis Barbosa-Rodrigues, 1892 and some specimens of Paleosuchus ) in having the 13th dentary alveolus as the largest immediately caudal to fourth; differs from Purussaurus in having a slender mandibular ramus and in not having the first four alveoli as the largest of the mandibular ramus; differs from Paleosuchus in having an atlantal rib without a thin lamina in the anterior end, and the posterior mandibular alveoli and teeth not lateromedially compressed.

Type specimen: Necrosuchus ionensis Simpson, 1937 .

Diagnosis: Same as for the genus, because it is the only species.

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

Occurrence: Salamanca Formation, Palaeocene of Argentina.

Description and comparisons

After being described originally ( Simpson, 1937), the holotype and only known specimen of N. ionensis was subjected to a detailed redescription by Brochu (2011), making a detailed assessment on the anatomy of this species unnecessary. However, given our reanalysis of the holotype and of what has been published since about N. ionensis , it is considered that its status as a valid species requires revision.

Two characteristics indicate that N. ionensis is a caimanine alligatoroid: the presence of a slender process ventral to the basioccipital tubera ( Brochu, 2011: Character 176, state 2; Fig. 1), which among Crocodylia is found only in caimanines, and is present in all Caimaninae taxa except Culebrasuchus ( Hastings et al., 2013) ; and the splenial being excluded from the mandibular symphysis, with the anterior tip of the splenial passing dorsal to the Meckelian groove ( Brochu, 2011: Character 54, state 2; Fig. 2A View Figure 2 ). This last feature is present in all caimanines except Globidentosuchus brachyrostris Scheyer, Aguilera, Delfino, Fortier, Carlini, Sánchez, Carrillo-Briceño, Quiroz & Sánchez-Villagra, 2013 , Gnatusuchus and E. itaboraiensis . The scapulocoracoid synchondrosis of the holotype seems to be closing (see Brochu, 2011; Fig. 3A, B View Figure 3 ), and given that the holotype was not an osteologically mature individual upon death ( Brochu, 2011), this possible early closure of the synchondrosis would be another feature to indicate that N. ionensis belongs to Caimaninae (see Brochu, 1995, 1997b: Character 24; equivalent to Brochu, 2011: Character 25). In fact, the species has been recovered consistently in the Caimaninae clade by phylogenetic analyses that have included it ( Brochu, 2011; Fortier et al., 2014; Hastings et al., 2016).

However, the differential diagnosis proposed by Brochu (2011) for N. ionensis requires revision. Two of the characters used (the presence of a slender process ventral to the basioccipital tubera, and the dentary symphysis extending back to a level immediately behind the fourth dentary alveolus) are respectively shared with caimanines and the taxa of the crown-group caimanines according to Brochu (2011). Two other characteristics are, according to Brochu (2011), shared with other taxa: the first four dentary alveoli being widely spaced from one another is a feature typically present in caimanine taxa, and the presence of ≥ 18 dentary alveoli is shared with taxa such as E. cavernensis , Caiman and Melanosuchus Gray, 1862 (see Brochu, 2011). Upon commenting on the last character, Brochu (2011) also notes that the dentary of N. ionensis is slender. A slender dentary can also be seen in several caimanines, such as Paleosuchus (GM Cidade personal observation), E. cavernensis , Centenariosuchus and Tsoabichi (see Simpson, 1933; Brochu, 2010; Hastings et al., 2013). Additionally, the possibility that the width of the dentary might be subject to ontogenetic or individual variations makes this character not useful for taxonomy.

The last character included in the diagnosis of Brochu (2011) is the splenial bearing a slender anterior process that extends almost to the dentary symphysis. As previously mentioned, the splenial of N. ionensis does not participate in the symphysis, and the anterior tip of the splenial passes dorsal to the Meckelian groove, which is a common character among caimanines ( Brochu, 2011; Hastings et al., 2013; Cidade et al., 2017; Fig. 2A View Figure 2 ), but how close the splenial gets to the symphysis is variable in at least two extant caimanines: C. crocodilus and C. latirostris . Some specimens of C. crocodilus exhibit splenials whose anterior tip is close to the symphysis (AMNH R 43291, AMNH R 137179, FMNH 69817, FMNH 69821, FMNH 69824, FMNH 69825, FMNH 69831 and FMNH 69842; Fig. 2B View Figure 2 ), whereas in others the anterior tip is more distant (FMNH 69819, FMNH 69832, FMNH 69854, FMNH 69855, FMNH 69865, FMNH 73700 and MN 1031; Fig. 2D View Figure 2 ). The same difference is observed for C. latirostris , in which some specimens exhibit the anterior tip of the splenial close to the symphysis (MN 1255, MN 2078, MN 69, MN 1257 and MN 2395; Fig. 2C View Figure 2 ), whereas in others the anterior tip is more distant (MN 1041, MACN 30566 and MCT 156-RR). The specimens that exhibit the splenial anterior tip more distant from the symphysis are juveniles or subadults, which raises the possibility of an ontogenetic variation in this character. However, detailed studies about the relationship between the anterior tip of the splenial and the mandibular symphysis in extant caimanines are lacking. Nevertheless, the presence of the anterior tip of the splenial close to the symphysis in C. crocodilus and C. latirostris and the variation seen in those species make this character not recommendable to be used in taxonomy, at least for the time being, until detailed ontogenetic studies eventually reveal otherwise.

However, there is one character that differs in N. ionensis from most other caimanines: the 13th dentary alveolus as the largest immediately caudal to the fourth dentary alveolus ( Fig. 4B View Figure 4 ), which had already been noted by Simpson (1937) and which fits N. ionensis into state 0 (the 13th or the 14th dentary alveolus as the largest immediately caudal to the fourth) of Character 51 of Brochu (2011). In most caimanines, the largest dentary alveolus immediately caudal to the fourth is either the 11th or the 12th (state 2 of the same character): C. crocodilus , C. latirostris , C. yacare , Centenariosuchus , Melanosuchus and Paleosuchus (even though there is individual variation in this last genus, as detailed below; see Fig. 4 View Figure 4 ). Other caimanines have the 13th or the 14th alveolus as the largest, together with a series of large alveoli behind them (state 1): C. brevirostris and G. brachyrostris .

In other taxa of Caimaninae, a series of large posterior alveoli starts with a large 12th alveolus ( C. wannlangstoni and Kuttanacaiman ). In N. ionensis , the 14th alveolus is only slightly smaller than the 13th, but the alveoli posterior to it are progressively slightly smaller instead. In No. stromeri , the largest alveoli are the 15th and 16th. In Mourasuchus , the first to the fifth alveoli are the largest of the tooth row, after which the alveoli become progressively smaller (see Langston, 1965). At least two Purussaurus specimens [ Pu. brasiliensis specimens DGM 527-R (see Price, 1967) and UFAC-4559 (GM Cidade personal observation)] exhibit the 13th alveolus as the largest; most alveoli are not preserved in the holotype of Purussaurus neivensis (Mook, 1941) , while in the holotype of Purussaurus mirandai Aguilera, Riff & Bocquentin-Villanueva, 2006 the alveoli posterior to the fourth become progressively smaller ( Aguilera et al., 2006). However, N. ionensis is markedly distinct from Purussaurus by overall mandibular morphology. In the latter, the mandibles are remarkably massive in accordance with the large size of Purussaurus (see Langston, 1965; Aguilera et al., 2006; Aureliano et al., 2015) and because the first four alveoli are the largest of the dentary in Purussaurus (see Barbosa-Rodrigues, 1892; Price, 1967; Aguilera et al., 2006; Salas-Gismondi et al., 2015), whereas in Necrosuchus (see Simpson, 1937: fig. 3) and most caimanines (the other exception being Mourasuchus , as noted above), one or more alveoli behind the tenth alveolous either approaches or overcomes the size of the first and fourth alveoli, which are usually the largest between the first four.

In some specimens of both species of Paleosuchus , the 13th alveolus is either larger or of the same size as the 12th and the 11th ( Pa. palpebrosus : AMNH R 137170, AMNH R 137174, AMNH R 145071, AMNH R 93812, FMNH 69874, MCT 291-RR; Pa. trigonatus : MN 65, MN 2491, AMNH R 129259, AMNH R 129260, AMNH R 66391 and USNM 234047; Fig. 4A View Figure 4 ), thus varying from the standard in the genus ( Fig. 4C View Figure 4 ), which is state 2 of Character 51 of Brochu (2011). This alone raises the possibility that Necrosuchus can be considered a Paleosuchus specimen, but Necrosuchus differs from the latter in other characters: the atlantal rib of Necrosuchus lacks the thin laminae in the anterior end that is present in Paleosuchus (see Brochu, 2011, in the scoring of Character 7); the dorsal margin of the iliac blade of Necrosuchus is rounded, with a modest dorsal indentation ( Brochu, 2011: Character 34, state 1; Fig. 5A View Figure 5 ), similar to Caiman (e.g. C. crocodilus ; Fig. 5B View Figure 5 ) but different from Paleosuchus , in which the dorsal margin of the iliac blade is narrow, with a dorsal indentation ( Brochu, 2011: Character 34, state 3; Fig. 5C View Figure 5 ); and, most notably, the posterior alveoli and teeth of Paleosuchus are lateromedially compressed, whereas those of Necrosuchus are circular ( Brochu, 2011: Character 79; Fig. 4 View Figure 4 ).

The alveolar pattern of the dentary of the fossil caimanine T. greenriverensis has some similarities with that of Necrosuchus . The alveolar counting of T. greenriverensis is not known, because the only significantly complete dentaries (those of the holotype, TMM 42509-1; see Brochu, 2010) have some anterior alveoli missing or fragmented. However, the posterior alveoli of the dentary exhibit two large alveoli followed by progressively slightly smaller ones, a morphology also observed in Necrosuchus . Nevertheless, comparisons between the two taxa based on the specimens currently known are problematic, because Necrosuchus preserves only four alveoli posterior to the two largest posterior alveoli ( Fig. 4B View Figure 4 ), whereas the holotype of Tsoabichi preserves from six to seven (see Brochu, 2010: fig. 1). Additionally, the placement of Necrosuchus in or close to the Jacarea clade, together with the placement of Tsoabichi as a member of the sister clade of Paleosuchus in the phylogenetic analysis of this paper, argues against a proximity between these two taxa.

Additionally, N. ionensis differs from Eocaiman in having the dentary at the level of the first and fourth teeth at the same level as at the 11th and 12th teeth, whereas in Eocaiman the dentary at the first level is lower than at the second ( Pinheiro et al., 2013: Character 124). It also differs from E. itaboraiensis , because in that species the splenial participates in the mandibular symphysis ( Pinheiro et al., 2013). Necrosuchus also differs from Gnatusuchus owing to the presence of an extensive mandibular symphysis and a ‘shovel-like’ process in the anterior portion of the mandible in Gnatusuchus , aside from the participation of the splenial in the mandibular symphysis in Gnatusuchus (see Salas-Gismondi et al., 2015). From Culebrasuchus , Necrosuchus differs in having the dentary slightly curved between the fourth and tenth alveoli, whereas the same portion of the dentary in Culebrasuchus is linear (see Brochu, 2011: Character 50; Hastings et al., 2013); additionally, the external mandibular fenestra in Necrosuchus is small, whereas in Culebrasuchus it is large (see Brochu, 2011: Character 63; Hastings et al., 2013). Furthermore, the exoccipital sends slender process ventrally to the basioccipital tubera in Necrosuchus . In Culebrasuchus , the processes are absent and the exoccipitals are located exclusively dorsal to the basioccipital tubera (see Brochu, 2011: Character 176; Hastings et al., 2013). Comparisons between Necrosuchus and Pr. peligrensis are limited because the only bone present in both species is the quadrate, which exhibits no systematically relevant differences between them.