Leposternon microcephalum occurs in Brazil, Bolivia, Paraguay, Uruguay, and northern Argentina. Leposternon is a member of Amphisbaenia, a lineage (160 species) of mostly limbless burrowing squamates. The amphisbaenian skull classically has been difficult to study due to its small size (this skull measures just 14 mm in length) and largely closed construction, but high-resolution X-ray CT offers a solution to these problems.
There are four major amphisbaenian groups: Bipedidae, the only one to have forelimbs; Amphisbaenidae, the most diverse (149 species) and widespread (to which Leposternon belongs; see also Anops kingii, Loveridgea ionidesii and Geocalamus acutus); Trogonophidae, whose members use an oscillating excavation pattern (see Diplometopon zarudnyi); and Rhineuridae, represented by numerous fossils (see Rhineura hatcherii) but only one extant species. Amphisbaenians occur in northern and sub-saharan Africa, southwest Asia, the Mediterranean, South America east of the Andes, the West Indies, western Mexico, Baja California, the southeasternmost United States, and Cuba. They are generally poorly represented in collections, and little is known of their life history, because of their burrowing lifestyle.
There are four basic amphisbaenian head morphotypes, each of which appears to correspond to a different burrowing mode: 'round-headed', 'shovel-headed', 'spade-headed', and 'keel-headed'. Leposternon represents the 'spade-headed' morphotype; this is readily apparent when compared to the 'keel-headed' Geocalamus acutus.
Amphisbaenians, like other squamates, have paired evertible hemipenes, a transverse cloacal slit, and shed their skin in its entirety. They differ in that they have a highly modified skull architecture, a unique modification of the ear called the extracolumella, and skin with annuli (rings, like a worm -- hence the common name).
It is difficult to discern exactly where amphisbaenians fit in the squamate tree, as even the earliest-known fossil representatives already exhibit the highly derived cranial morphology seen in living forms. Phylogenetic analyses based on morphology tend to place amphisbaenians with the other two major limbless squamate clades -- snakes and dibamids; however, this may be due to convergence of characters correlated with a burrowing lifestyle rather than ancestry.
About the Species
This specimen was reportedly collected from the vicinity of Fazenda Ipanema, Varnhagem, Sao Paulo, Brazil by Lewis E. Long 27 September 1951. It was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Nathan Kley of SUNY Stonybrook and Dr. Adam Summers of the University of California, Irvine. Funding for scanning was provided by a National Science Foundation grant (IBN-0317155) to Dr. Summers and by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin. Funding for image processing was also provided by the Digital Libraries Initiative grant to Dr. Rowe.
About this Specimen
The specimen was scanned by Richard Ketcham on 11 June 2003 along the coronal axis for a total of 810 slices, each slice 0.0277 mm thick with an interslice spacing of 0.0277 mm. The original scan included the skull and first three vertebrae; only those slices containing the skull (slices 001-584) are shown here.
Gans, C. 1971. Studies on amphisbaenians (Amphisbaenia, Reptilia). 4. A review of the amphisbaenid genus Leposternon. American Museum of Natural History Bulletin 144:379-464.
Gans, C. 1978. The characteristics and affinities of the Amphisbaenia. Transactions of the Zoological Society of London 34:347-416.
Gans, C., and E. G. Wever. 1972. The ear and hearing in Amphisbaenia (Reptilia). Journal of Experimental Zoology 179:17-34.
Kearney, M. In press. Systematics and evolution of the Amphisbaenia (Reptilia: Squamata) based on morphological evidence from fossil and living forms. Herpetological Monographs.
Zangerl, R. 1944. Contributions to the osteology of the skull of the Amphisbaenidae. American Midland Naturalist 3:417-445.
Amphisbaenidae page from the EMBL Reptile Database