陕西府谷古鳄类一新属

本文详细记述了陕西府谷下三叠统上部二马营组底部的一古鳄类化石.据其骨骼形态特征拟订为古鳄亚目(Proterosuchia)、引鳄科(Erythrosuchidae)的一新属种:石拐古城鳄(Guchengosuchus shiguaiensis gen. et sp. nov.),同引鳄科内已知属的对比,表明新属古城鳄与武氏鳄 Vjushkovia 的关系最为接近.

A NEW GENUS OF PROTEROSUCHIA FROM LOWER TRIASSIC OF SHAANXI, CHINA

A new proterosuchian remain was collected from the lower part of Ermaying Formation (Lower Triassic) of Fugu County, Shaanxi Province during the summer of 1986. It consists of incomplete skull, portion of lower jaw, 3 cervical, 6 dorsal and 3 tail vertebrae, right scapular and forelimbs (IVPP V8808). Evidently these materials were derived from the same individual because they are of proportionally the same sizes without any repeated bones and are buried in an area of 1m~2. Guchengosuchus shiguaiensis gen. et sp. nov. Diagnosis Medium-sized proterosuchian. Narrow and high skull with two antorbital fenestrae. Frontal expanded posteriorly. Postfrontal small. No parietal foramen. Postparietal with posterior point projection. Notch present at lower border of skull between premaxillaand maxilla. Maxilla teeth subthecodont, 14 in number. Palatal teeth absent. Retroarticularprocess well-developed. Cervical vertebrae elongated, centra strongly constricted with diapophysis supported by four lamellae, a small vertical lamella present beneath the lamella betweendiapophysis and postzygapophysis. Intercentra absent. Pectoral ribs three-headed. Scapularslender, forelimbs lightly constructed. Tail long. No dermal armor. According to the classification defined by Charig & Reig (1970), it seems to be unquestionable to accommodate Guchengosuchus in the suborder Proterosuchia as it clearly shows typical proterosuohian features. Charig and Reig distinguished erythrosuchids from proterosuchids by numerous characters (1970, see also Charig & Sues, 1976). Wherever those characters can be investigated they seem to indicate that Guchengosuchus shows more similarities to erythrosuchids than, to proterosuchids, such as: the presence of the second antorhital fenestra and the notch in lower border between premaxilla and maxilla, 14 maxillary teeth (much less than 29 maxillary teeth in proterosuchids), frontal expanded transversely at rear (different from reoctangular frontal in proterosuchids), postparietal projecting backwards as peg (postparietal projecting upwards in proterosuchids), small postfrontal and disappearance of parietal foramen and palatal teeth, specialized structure of cervical and dorsal vertebrae, pectoral three-headed rib and slender scapular (different from short and wide scapular in proterosuchids). However, the presence of posttemporal fenestra and the supinator process in humerus, and the slender paroccipital, not expanded distally in Guchengosuchus, show its resemblance with proterosuchids, but these features are also appeared in the early reptile, Eosuchia, which is considered to be closely related to proterosuchians (Gow 1975; Carroll, 1975), and here are interpreted to be synplesiomorphy. Therefore, it is reasonable to identify Guchengosuchus as one member of the family Erythrosuchidae. Up to now, there are 5 known genera in Erythrosuchidae: Garjinia, Erythrosuchus, Shansisuchus, Vjushkovia and the new genus Guchengosuchus. The presence of the second antorbital fenestra, pectoral three-headed rib and specialized cervical and dorsal vertebrae clearly distinguish Guchengosuchus from Garjinia and Erythrosuchus. Comparison with Shansisuchus, The appearance of the second antorbital fenestra easily combines Guchengosuchus and Shansisuchus together. However, similar structure also can be seen in late pseudosuchians such as in Luperosuchus and Saurosuchus of Rauisuchidae (Romer 1972, Sill 1974) and in Pseudohesperosuchus of Sphenosuchidae (Bonaparte 1967). Additionally, similar structure also exists in Dibothrosuchus, but it is considered to be the foramen for the fourth dentary tooth (Wu, 1986). Such wide presence of the second antorbital fenestra in archosaurs prevents the author from approaching the interpretation of the structure being the synapomorphy of Guchengosuchus and Shansisuchus, although it is only found in the two genera of Proterosuchia. Other differences between Guchengosuchus and Shansisuchus are noticeable. The ascending ramus of maxilla is broader and more anterior in Guchengosuchus than that in Shansisuchus. The postfrontal and interparietal of Guchengosuchus is smaller. Especially the elongated centrum with long lamellae of cervical in Guchengosuchus is very distinct from the very short cervical in Shansisuchus. The existence of pectoral three-headed ribs is also special in Guchengosuchus. All these do not support that the two genera have a close relationship. Comparison with Vjushkovia, Huene (1960) argued that the threeheaded rib is an important character distinct from the other proterosuchians, and this was reiterated by Young (1973). There has been considerable debate about whether the three-headed ribs do (or do not) exist widely in the proterosuchians (Tatarinov, 1961; Hughes, 1963; Charig & Reig, 1970; Thulborn, 1979). The discovery of Guchengosuchus demonstrates that rite three-headed ribs actually include two types: cervical and pectoral ones, representing different structures and functions: the three heads of cervical ribs are in different directions and may effectively stiffen the neck region, which are widely present in archosaurs and also can be seen in the extant crocodiles; whereas the three heads of Pectoral ribs are arranged in a straight line and all articulated with vertebrae in the pectoral region, which are practically movable. Evidently, the three-headed ribs in Vjushkovia described by both Huene (1960) and Young (1973) are the pectoral ones. Chasmatosaurus and Shansisuchus virtually show no three-headed ribs at all. No evidence confirms that the pectoral three-headed ribs exist in any other proterosuchians except Vjushkovia and Guchengosuchus. Thus, the pectoral three-headed rib is interpreted to be the synapomorphy of Vjushkovia and Guchengosuchus. Also the cranial and postcranial structures of the two genera are very comparable, such as their small postfrontal and very similar forelimbs and vertebrae. Therefore, Guchengosuchus and Vjushkovia are considered to be closely related genera in the known proterosuchians.