FOSSIL TADPOLES: NEW EVIDENCE FOR THE EVOLUTION OF CALYPTOCEPHALELLA (ANURA, NEOBATRACHIA)

MUZZOPAPPA, PAULA

Neuquén, Provincia de Neuquén, Argentina

Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008

Universidad Nacional del Comahue

In recent years there has been an increasing interest in understanding the evolutionary role of embryonic development. The remains of an anuran tadpole from the Deseadan (Late Oligocene) Scarritt Pocket Locality, Chubut, Argentina, are herein studied and assigned to the extinct species Calyptocephalella canqueli Schaeffer. The latter has been recently recognized as a valid taxon (Muzzopappa and Báez, MS submitted) contrary to the opinion of Lynch (1971), who considered C. canqueli as a junior synonym of the living C. gayi. The well-preserved specimen consists of bony elements of the skull and of the postcranial skeleton. It is assigned to Calyptocephalella based on the general close resemblance ?e.g., the broad transverse processes of the first three vertebrae and the shape and ornamentation pattern of the frontoparietals? to the larvae of C. gayi. In addition, adults of C. canqueli are also present in this locality, which is the type locality of the species. The genus Calyptocephalella is an ancient inhabitant of southern South America, as evidenced by the conspicuous fossil record, which dates back at least to the earliest Cenozoic (Báez, 2000). The study of the fossil tadpole furnishes additional evidence for the validity of C. canqueli, although at the same time poses several questions regarding the ontogeny of this species and even the evolution of the genus. The tadpole has a well­-developed and ossified axial skeleton that contrasts with the comparatively tiny, but already ossified, hind limbs. The comparison with tadpoles of the living species with a similarly developed presacral column (Gosner stage 38) shows more ossified hind limbs and pelvic elements than those of the latter, making it difficult to stage the fossil tadpole. This discordance indicates a different ontogenetic trajectory of the two species and corroborates the validity of the fossil taxon, as it might indicate either an earlier onset of the ossification of the hind limbs or a higher ossification rate in C. canqueli than in the living species. This could be correlated with a shorter larval period in comparison with the living species, a possibility that is in agreement with the observation that the adults of C. canqueli resemble juvenile individuals of C. gayi (Muzzopappa and Báez, MS submitted). Changes in life-history, perhaps related to variation of the climatic conditions, might be involved in the evolution of genus.