Pachyrhachis Research Articles

Reevaluation of the anatomy of the Cenomanian (Upper Cretaceous) hind-limbed marine fossil snakes Pachyrhachis, Haasiophis, and Eupodophis

ABSTRACT New anatomical observations and reinterpretations of previously identified structures have resulted in new taxonomic diagnoses for the fossil hind-limbed marine snakes Pachyrhachis problematicus, Eupodophis descouensi, and Haasiophis terrasanctus. Among the most important conclusions of our study are the following: Haasiophis and Eupodophis show no evidence of possessing a laterosphenoid; Pachyrhachis and Eupodophis do retain a jugal; Haasiophis, like Eupodophis, has chevron bones in the caudal region; Haasiophis has a large number of unfused intercentra along the anterior portion of the precloacal column; the dentary of Pachyrhachis has numerous mental foramina (at least four); Pachyrhachis has at least one sacral vertebra with unfused sacral ribs. To test the effect of our new observations on the phylogenetic relationships of snakes, we ran three phylogenetic analyses using alternative outgroups to polarize the character transformations. The ingroup consisted of all well-preserved fossil snakes from the Cretaceous, the madtsoiids, and taxa that are representative of all major groups of extant snakes. The analyses yielded a series of most parsimonious trees that placed Pachyrhachis, Eupodophis, and Haasiophis either as a series of stem taxa at the base of the radiation of snakes (two analysis), or as members of a clade of fossil snakes that are the sister group of all living alethinopidians (one analysis). SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

The Cenomanian: stage of hindlimbed snakes

Three with legs are known: Pachyrhachis problematicus, Haasiophis terrasanctus and Eupodophis descouensi. They have short posterior limbs but lack an anterior girdle and forelimbs. Moreover, Pachyophis woodwardi, Mesophis nopcsai and Simoliophis ssp. appear to be closely related to the hindlimbed taxa; consequently, although the presence of posterior limbs has not been demonstrated for these genera, it is presumed that they too were hindlimbed. All these snakes have been recovered only from the Cenomanian. Moreover, these six genera come from a restricted area (western Europe and northwesternmost Africa to the Middle East). This limited geographic range suggests that snakes might have originated in the Mediterranean part of the Tethys, but the restricted stratigraphical range remains unexplained.

On the phylogenetic relationships of the Cretaceous snakes with legs, with special reference to Pachyrhachis problematicus (Squamata, Serpentes)

ABSTRACT Pachyrhachis problematicus is an early Upper Cretaceous snake with legs from the Middle East. The taxon is involved in an increasingly controversial debate about the origin and higher-level interrelationships of snakes. Its status is problematic because it combines characters of advanced (macrostomatan) snakes with plesiomorphic squamate traits. Two competing hypotheses of relationships have consequently been proposed: (1) Pachyrhachis is the sister group of all other snakes, and links Serpentes with Mosasauroidea; and (2) Pachyrhachis is related to the advanced macrostomatan snakes and has no bearing on the issue of snake origins. In a recent paper published in this journal, Caldwell critically reviewed the status of Pachyrhachis as the sister-taxon of macrostomatan snakes, and concluded that Pachyrhachis is the sister-group of all other snakes instead. In the present paper, we review several aspects of character delimitations as proposed by Caldwell, and corroborate the macrostomatan affinities of Pachyrhachis.

Palaeoenvironment and palaeoecology of three Cretaceous snakes: Pachyophis, Pachyrhachis, and Dinilysia

The palaeoecology of three Late Cretaceous snakes is evaluated. Pachyophis woodwardi Nopcsa, 1923 and Pachyrhachis problematicus Haas, 1979, are Cenomanian in age and are found in carbonate rocks deposited in marine inter-reef basin environments of the European and African Tethys Sea. Dinilysia patagonica Woodward, 1901, Coniacian in age, is considered closely allied to living anilioid snakes, and is found in clastic rocks deposited in a terrestrial inter-dune basin environment in northern Patagonia, Argentina. All three snakes are known from well preserved and articulated specimens found in sediments where detailed sedimentological and taphonomic analyses are possible. Pachyophis and Pachyrhachis were laterally compressed, have pachyostotic ribs and vertebrae, and small, narrow heads. These two snakes are interpreted as aquatic predators living in and around the margins of reef mounds on a shallow water carbonate platform. Dinilysia was a large bodied snake with a relatively large head, and is interpreted here as a terrestrial predator that lived in a dry, interdune basin environment dominated by aeolian sedimentation. Sedimentary units preserve ichnological evidence of burrowing insects and rooting plants.

Un nouveau serpent bipède du Cénomanien (Crétacé). Implications phylétiques

A new bipedal snake from the Cenomanian (Cretaceous). Phylogenetic implications. A new bipedal snake is described as Podophis descouensi gen. et sp. nov. Like Pachyrhachis problematicus, the only other bipedal snake, it comes from the Cenomanian of the Middle East. It provides data not revealed in Pachyrhachis and it confirms that bipedal snakes are basal Ophidia. It is not possible to assess whether these two bipedal snakes make up a monophyletic group (sister-taxon to the Serpentes) or a paraphyletic assemblage (stem group of the Serpentes).

The phylogenetic relationships of Pachyrhachis problematicus, and the evolution of limblessness in snakes (Lepidosauria, Squamata)

We have revisited the type material of Pachyrhachis problematicus, a fossil snake from the Cenomanian (Cretaceous) of the Middle East, and found a number of characters in need of revision with respect to its recent re-description. A new interpretation is given for several cranial characters. There is no identifiable regionalization of the presacral-vertebral column beyond that found in booid snakes. The identification of a free sacral rib in Pachyrhachis is more parsimoniously interpreted as the first lymphapophysis. It is questionable that the pelvic rudiment would have been suspended from the axial skeleton. We have also extensively reviewed the evidence that is believed to link Pachyrhachis, and with it all other snakes, to a mosasauroid ancestor. We were not able to corroborate this hypothesis, which involves characters related to the dentition, braincase morphology, and the intramandibular joint. Instead, re-analysis of the phylogenetic relationships of Pachyrhachis corroborates the hypothesis that it represents the sister-group of macrostomatan snakes, i.e., advanced snakes. Given that Pachyrhachis is nested within the hierarchy of snakes, it is entirely possible that this snake re-developed almost complete hind limbs. A less parsimonious hypothesis would be to consider the hind limbs to have been lost independently in the major basal clades of extant snakes (Scolecophidia, Anilioidea, and Macrostomata). The latter hypothesis is considered a serious possibility given the recently described findings on the mechanisms of limb development in Python, which could account for the highly variable pattern of hindlimb reduction found among basal snakes.

Anatomy and relationships of Pachyrhachis problematicus, a primitive snake with hindlimbs

The anatomy ofPachyrhachis problematicus, an elongate, limb–reduced squamate from the Upper Cretaceous of Israel, is described and evaluated in detail. Previously considered a snake–like ‘lizard’ of uncertain affinities, it is here shown to be the most primitive snake, and the sister–group to all other snakes.Pachyrhachisexhibits numerous derived characters uniting it with modern snakes (scolecophidians and alethinophidians): e.g. mobile premaxilla–maxilla articulation, braincase enclosed by frontals and parietals, sagittal parietal crest, absence of tympanic recess, single postdentary bone, over 140 presacral vertebrae, and complete loss of shoulder girdle and forelimb. However, it is more primitive than all modern snakes in retaining some strikingly primitive (lizard–like) features: presence of a jugal, squamosal, normal sacral attachment, and well–developed hindlimb composed of femur, tibia, fibula, and tarsals.Pachyrhachisprovides additional support for the hypothesis that snakes are most closely related to Cretaceous marine lizards (mosasauroids). Almost all of the derived characters proposed to unite snakes and mosasauroids are highly developed inPachyrhachis: the mobile mandibular symphysis, intramandibular joint, long and recurved pterygoid teeth, quadrate suspended by the supratemporal, loosely united pelvic elements (ilium, ischium, and pubis), and separate astragalus and calcaneum.

A snake with legs from the marine Cretaceous of the Middle East

Although snakes are descended from limbed squamates ('lizards'), all known snakes lack well developed legs and their nearest lizard relatives have yet to be identified1–4. Here we provide compelling evidence that the Cretaceous squamate Pachyrhachis problematicus, previously interpreted as a varanoid lizard5–7, is actually a primitive snake with a well developed pelvis and hindlimbs. Pachyrhachis is the sister-taxon of all other snakes. The skull exhibits most derived features of modern snakes, and the body is slender and elongated. But unlike other snakes, Pachyrhachis retains a well developed sacrum, pelvis and hindlimb (femur, tibia, fibula, tarsals). Pachyrhachis was marine, and provides additional support for mosasauroid–snake affinities.