Polcyn, M. J., L. L. Jacobs, and A. Haber. 2005. A morphological model and CT Assessment of the skull of Pachyrhachis problematicus (Squamata, Serpentes), a 98 million year old snake with legs from the Middle East. Palaeontologia Electronica 8:26A:24p.

ABSTRACT

Pachyrhachis problematicus is a snake with well-developed hind limbs known from two specimens from the Cenomanian of the Middle East. One specimen has a complete and articulated, albeit crushed skull. The second specimen has a disarticulated skull crushed beneath the body. Pachyrhachis has recently been at the center of a debate on the origins and relationships of snakes, specifically whether Pachyrhachis is the sister taxon to all other snakes, or alternatively, a relatively advanced snake allied with boids and pythonids. Workers using the same specimens arrived at different interpretations of the morphology underlying the alternative hypotheses. The most complete skull was resin-embedded and acid-prepared for its original study nearly three decades ago, rendering the dorsal surface difficult to view using optical techniques, and thus significantly hampering later studies. This study utilizes CT scanning and computer reconstruction to test conflicting interpretations of morphology and thus provides a method to falsify alternative phylogenetic hypotheses. Pachyrhachis is found to possess an inclined quadrate with a well-developed stylohyal process and lacking a suprastapedial process, no squamosal, and a single mental foramen. The separation of exoccipitals above the foramen magnum cannot be demonstrated. There is no jugal. Confirmed morphology best supports the phylogenetic hypothesis that Pachyrhachis is a basal macrostomatan snake. Limb retention in a basal macrostomatan snake implies that loss of hind limbs occurred multiple times within Serpentes.

This specimen was collected from a limestone quarry at Ein Jabrud (Bed-Meir Formation, lowermost Cenomanian, mid-Cretaceous), 20 km north of Jerusalem, Israel. It was scanned by the University of Texas High-Resolution X-ray CT Facility for Dr. Lou Jacobs of Southern Methodist University and Dr. Mike Polcyn of the Institute for the Study of Earth and Man. Partial funding for image processing was provided by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin and a National Science Foundation Assembling the Tree of Life grant (EF-0334961), The Deep Scaly Project: Resolving Squamate Phylogeny using Genomic and Morphological Approaches, to Drs. Jacques Gauthier of Yale University, Maureen Kearney of the Field Museum, Mike Lee of the University of Adelaide, Jessie Maisano of The University of Texas at Austin, Tod Reeder of San Diego State University, Olivier Rieppel of the Field Museum, Jack Sites of Brigham Young University, and John Wiens of SUNY Stonybrook.

The specimen was scanned by Richard Ketcham and Cambria Denison on 19 September 1997 along the coronal axis for a total of 308 slices. Each slice was 0.24 mm thick, with an interslice spacing of 0.20 mm (for a slice overlap of 0.04 mm) and a field of reconstruction of 70 mm. A problem during scanning resulting in the missing slices seen in the accompanying images and animations.

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Digital animations of Pachyrhachis problematicus:

Comparison of dorsal and ventral CT reconstructions illustrating alignment of dorsal and ventral structures.

Animation sequence illustrating taphonomic affects on the preservation of Pachyrhachis problematicus (HUJ-PAL 3659).

Digital reconstruction of the quadrate of Pachyrhachis problematicus. Animation sequence showing extraction of region of interest, rotation of region of interest, removal of elements other than quadrate, removal of crushing of the quadrate, and rotation of restored quadrate.