Asiatosuchus, MOOK, 1940

Delfino, Massimo & Smith, Thierry, 2009, A reassessment of the morphology and taxonomic status of ‘ Crocodylus’ depressifrons Blainville, 1855 (Crocodylia, Crocodyloidea) based on the Early Eocene remains from Belgium, Zoological Journal of the Linnean Society 156 (1), pp. 140-167 : 141-144

ASIATOSUCHUS MOOK, 1940

The genus Asiatosuchus was erected by Mook (1940) on the basis of an incomplete lower jaw and a few skull fragments coming from the Middle Eocene of Mongolia (Irdin Manha Formation). According to Mook, Asiatosuchus should be characterized by being similar to Crocodylus , but with at least 17 teeth in each ramus of the lower jaw and with splenial bones not reaching the symphysis. The diagnosis of the type species, Asiatosuchus grangeri Mook (1940) , is as follows: ‘symphysis extending back to the level of the sixth mandibular teeth, the two rami of the mandible diverging at a moderately wide angle, dental row shorter than postdental portion of jaw, teeth stout and faintly striated, interfenestral plate flat, sutures of nasals with lacrimals considerably shorter than sutures with prefrontals’( Mook, 1940: 1). Steel (1973: 59) strengthened the genus definition as follows: ‘dentition comprising five premaxillary teeth, fourteen maxillary (of which the fifth is the largest), and at least seventeen (possibly up to twenty) mandibular elements. Palatine bones extending forward only as far as the anterior margin of the palatal vacuities. Splenial excluded from mandibular symphysis, which reaches back to the 6 th or 7 th dentary tooth’. Vasse (1993) suggested that the specific characters should be applied to the genus.

Such a list of characters includes some features which are shared, although not exclusively, by all the species ascribed to this genus in the following decades (such as the elongation of the dentary symphysis) and also features that are present only in some of the species (such as the exclusion of splenial from dentary symphysis) or that have a wide distribution among crocodylians (such as the number of premaxillary teeth).

Brochu (1997) added significant information to the morphology of A. grangeri , explicitly stating that the frontoparietal suture was completely excluded from the supratemporal fossae, and that the medial jugal foramen is small, as well as implicitly offering morphological information thanks to the character coding of this species [characters evidently assessed on remains not described by Mook (1940), as Mook does not mention, for example, any jugal].

The referral of fossil remains to the genus Asiatosuchus has usually been based mostly on the elongated dentary symphysis despite the presence of some discrepancies in the skull morphology. This is the case for two other species that have been described from the Palaeocene of Asia, Asiatosuchus volgensis ( Efimov & Yarkov, 1993) and Asiatosuchus zajsanicus Efimov, 1982 (later on referred by its author to the tomistomine genus Dollosuchus ; see Efimov, 1988; but see Brochu, 2007a, for the synonymy of Dollosuchus ). According to Angielczyk & Gingerich (1998: 185), the naming of these species is premature because of ‘the lack of any truly diagnostic material’.

In the absence of a thorough revision, the complex relationships among these taxa are mostly based on speculations so that Asiatosuchus has been defined as a ‘wastebasket taxon’ ( Angielczyk & Gingerich, 1998: 185). Vasse (1992, 1993) in his ‘bilan’ on the knowledge concerning this genus (he did not take into consideration A. zajsanicus and, of course, A. volgensis which was described later), accepted as valid only three species: the European A. depressifrons ( Blainville, 1855) , the Asian A. grangeri Mook, 1940 (the type of the genus) and A. nanlingensis Young, 1964 . He listed the following taxa as synonyms of A. depressifrons : ‘Crocodile des gravières de Castelnaudary’ (partim; Cuvier, 1824); Crocodylus coelorhinus Pomel, 1847 ; Crocodylus dodunni Gray, 1831 ; Pyrenodon sp. Dujardin, 1843 ; C. doduni Giebel, 1847 ; C. vicetinus Lioy, 1865 (sometimes misspelled in the literature as C. vicentinus ); Isselo-saurus dodunni Filhol, 1877 ; Diplocynodon haeckeli Seidlitz, 1917 ; Atacisaurus crassiproratus Astre, 1931 ; Asiatosuchus germanicus Berg, 1966 . According to Ortega, Buscalioni & Gasparini (1996), Atacisaurus crassiproratus is not a crocodylian and therefore it is not a synonym of Asiatosuchus depressifrons .

The affinities of Asiatosuchus are not limited to Asian and European material: Berg (1966) and Vasse (1993) underlined the apparent close relationships between the Asian–European Asiatosuchus and some taxa from the middle Eocene Bridger Formation in Wyoming: ‘ Crocodilus ’ affinis Marsh, 1871 (see Norell & Storrs, 1986, for a discussion of the synonyms of this taxon) and ‘ Crocodilus ’ clavis Cope, 1872, but both Berg and Vasse discussed the diagnosis of the genus Asiatosuchus on the basis of characters that in most cases can be now considered as devoid of any phylogenetic relevance.

If some of the proposed synonymies can be accepted simply on the basis of educated guesswork (i.e. most of the 19 th century species), others are still debatable: issues still being discussed include the synonymy between A. depressifrons and A. germanicus , and the possible inclusion of the North American taxa into the genus Asiatosuchus .

Brochu (1997), emphasizing the presence of frontals participating in supratemporal fenestrae and of splenials slightly participating in the symphysis in A. germanicus as an opposite condition to that shown by A. depressifrons , maintained as separate these two taxa. As for the relationships of the North American taxa mentioned above, he reported that the ‘ C. ’ affinis morphology of splenials and frontals, as well as the size of the medial jugal foramen, is congruent with that of the type species of Asiatosuchus , A. grangeri , and that, as suggested by Berg (1966), the inclusion of ‘ C. ’ affinis in Asiatosuchus could be reasonable but better material of A. grangeri should be found to support such a referral.

At present, character codings for the following species are currently available: ‘ C. ’ affinis, A. depressifrons , A. grangeri , and A. germanicus (based on the remains from Messel) (see, among others, Brochu, 2007b). Because the type of A. depressifrons is heavily altered and therefore of little help, the character coding now available for this taxon has been realised on the basis of some practically unpublished putative A. depressifrons remains coming from Orp-le-Grand in Belgium (IRSNB IG 9875 and 9912); such coding has been variously named in the literature as Dormaal crocodyloid, cf. Crocodylus depressifrons , European basal crocodyloid (?= ‘ Crocodylus ’ depressifrons ), or Belgian crocodyloid ( Brochu, 1997, 1999, 2000, 2003, 2007a, b; Jouve, 2004; Delfino, Piras & Smith, 2005; Piras et al., 2007; Vélez-Juarbe, Brochu & Santos, 2007).

The cladistic analysis based on these character codings fails to recognize Asiatosuchus as a monophyletic group ( Brochu, 1997, 1999, 2000, 2001b, 2003, 2007a, b; Brochu & Gingerich, 2000; Jouve, 2004; Delfino et al., 2005; Piras et al., 2007; Vélez-Juarbe et al., 2007; for different matrixes including only ‘ Crocodylus ’ affinis and A. germanicus see Salisbury & Willis, 1996; Salisbury et al., 2006) and the cladogram topology can be simplified as follows: A. germanicus is a very basal crocodyloid, and ‘ C. ’ affinis, A. depressifrons , and A. grangeri form a polytomy with more derived crocodyloids. As a result of such nonmonophyletic grouping, in the following sections each species, except the type one, will be referred to its description genus encircled by inverted comas to indicate a provisional status.

ABBREVIATIONS

Anatomical abbreviations a, alveolus; a.c, axis centrum; a.h, axial hypapophysis; a.i, atlas intercentrum; an, angular; a.n.a, atlas neural arch; a.n.s, axis neural spine; ar, articular; bo, basioccipital; c.r, choanal rim; d, dentary; di, diastema; e.n, external naris; eo, exoccpital; e.s.a, ectopterygoid sutural area; f, frontal; f.a, foramen aerum; f.i, foramen incisivum; f.m, foramen magum; f.v, foramen vagi; f.V, foramen Vth cranial nerve; f.XII, foramen XIIth cranial nerve; it.f, infratemporal fenestra; j, jugal; l, lacrimal; l.c.f, lateral carotid foramen; m, maxilla; n, nasal; o, orbit; o.c: occipital condyle; o.p, occlusal pit; od.p, odontoid process; p, parietal; pf, prefrontal; pm, premaxilla; po, postorbital; q, quadrate; qj, quadratojugal; qj.s, quadratojugal spine; sa, surangular; sb.f.r, suborbital fenestra rim; so, supraoccipital; sp, splenial; sq, squamosal; st.f, supratemporal fenestra; sy, symphysis; t.c, temporal canal.

Institutional abbreviations

DIIA R-RS, Richard Smith’s collection, Bruxelles; IRSNB, Institut royal des Sciences naturelles de Belgique, Bruxelles; MNHN, Muséum National d’Histoire Naturelle, Paris.