Cervical ribs and cleft arches in Brazilian Sotalia guianensis: contribution to comparative studies of skeletal morphology in northern and southeastern specimens

The Guiana dolphin, Sotalia guianensis (Van Beneden, 1864), is a delphinid species found along the Brazilian coast. Morphological and genetic data point to the existence of different biological populations of the species within this portion of the range. The present paper compares the frequencies of developmental anomalies in two skeletal collections from geographically distant areas. The vertebral column of 43 specimens from Marajo Bay, located on the Para State coast, and 20 specimens from the central-north coast of the Rio de Janeiro State were analyzed. The only anomalies found were cervical ribs and cleft neural arches, both located on the seventh cervical (C7) vertebra. Cervical ribs were found in 12% of the Para specimens and in 45% of the Rio de Janeiro specimens; according to the Fisher’s Exact Test the difference was significant at a confidence level of α = 0.01, thus confirming that the series belong to different populations. The frequencies of cervical ribs observed in the present study were also compared to the frequencies of the same anomaly found in S. guianensis from the Ceara and Santa Catarina states, Brazil, and in tucuxi ( S. fluviatilis ) from the Brazilian Amazon. Although the results can be partially explained by demographic and evolutionary models for Sotalia , they may be affected by differences in the composition of the collections and in diagnostic methods/criteria used by different authors. Regarding the presence of cervical ribs, Amazon tucuxi were significantly different from the Rio de Janeiro and Para series, as well as other marine series, as previously stated in the literature. The difference in the frequency of cleft neural arches in specimens from Rio de Janeiro and Para was also significant, but no comparison could be conducted with other series due to lack of published material. A metric technique to discriminate transverse processes with and without fused cervical ribs is proposed as a methodological contribution for diagnosing the described anomaly.