Allometric scaling and heterochrony in Mesosaurids (Amniota, Sauropsida) from the Irati Formation (Permian of the Paraná Basin, Brazil)

ALLOMETRIC SCALING AND HETEROCHRONY IN MESOSAURIDS (AMNIOTA, SAUROPSIDA) FROM THE IRATI FORMATION (PERMIAN OF THE PARANÁ BASIN, BRAZIL)

MAURO J. CAVALCANTI ^*

Setor de Paleovertebrados, Departamento de Geologia e Paleontologia, Museu Nacional / UFRJ - 20940-040, Rio de Janeiro, RJ.

Presented by ALEXANDER W.A. KELLNER

Mesosaurids were aquatic to semi-aquatic sauropsids (a sister group to Reptilia) that appear as abundant elements of the fossil record from the Irati and Whitehill formations (Lower Permian) of the Paraná (Southern Brazil) and Karoo (South Africa) basins. In an attempt to investigate patterns of ontogentic and evolutionary allometry in this group, a study of allometric scaling was undertaken using a bivariate regression procedure. The material included 37 specimens from the Irati Formation belonging to three species: Mesosaurus tenuidens Gervais, 1864 (n = 20); Stereosternum tumidum Cope, 1886 (n = 13) and Brazilosaurus sanpauloensis Shikama and Ozaki, 1966 (n = 4) and are housed at Museu Nacional/UFRJ. Data published by Araújo (An Acad Bras Cienc 48: 91-116, 1976) were also included in the analysis. The measurements taken on each specimen comprised the length of the skull and of the neck.

Allometric coefficients were computed for each species using the reduced major axis (RMA) method, a Model II bivariate regression procedure adequate when neither variable can be regarded as independent and both are subject to error, thereby violating the assumptions of Model I regression analysis based on the least-squares method. Due to the very small sample sizes available for each species, a bootstrap resampling procedure was used for the computation of confidence limits for the RMA coefficients, using a variation of the algorithm provided by Plotnick (Syst Zool 38: 144-153, 1989). A BASIC program, adapted to IBM-PC microcomputers, was used in the calculations, and is available from the author upon request.

The analyzed variables showed positive allometry between Mesosaurus and Stereosternum, and negative allometry for Brazilosaurus. These results suggest an heterochronic shift as the explanation of major evolutionary changes in the morphology of this group and provide additional support for the current hypothesis of phylogenetic relationships among the mesosaurids, that place Brazilosaurus sanpauloensis as the most distinct taxon in the family Mesosauridae. — ( December 20, 2001 ).

Publication Dates

• Publication in this collection
  17 July 2002
• Date of issue
  June 2002