Variação osteológica e desenvolvimento ontogenético das espécies do gênero Sotalia (Cetacea, Delphinidae)

Abstract

Despite important morphological studies for both species of the genus Sotalia, knowledge on the skeleton as a whole is still limited, when considering inter and intraspecific, and geographic and sexual ontogenic variation. In order to contribute to a better understanding of the osteology, and to provide subsidy for conservation, 536 syncrania and 257 partial and complete postcranial skeletons of S. guianensis from various locations in South America (Brazil, Suriname, Venezuela and Colombia) were analyzed, in addition to 34 syncrania and 39 postcranial skeletons of S. fluviatilis (tucuxi) from the Amazon basin. The skeletons were evaluated in relation to non- metric (degree of fusion of sutures and formation of bone structures) and metric characters (mandible, scapula, pectoral fin, sternum, vertebrae and hyoid apparatus). In addition, meristic data were collected, including the number of vertebrae, vertebral and sternal ribs. In order to analyse the chronological sequence of bone fusion, information on 165 specimens were compared with their corresponding ages. It was observed that S. fluviatilis showed delay in the fusion of some cranial and postcranial sutures when compared to S. guianensis. The slower development in S. fluviatilis, the general shape of the skull with less projections and crests, late sexual maturity and the occurrence of a generally open lacerate anterior foramen in adults, classify this species as pedomorphic. In the postcranial skeleton, morphological differences were observed between both species, regarding the measurements of the radius, ulna, basihyal, thyrohyal and stylohyal. In order to analyse the geographical and sexual variation, 253 skulls of adult S. guianensis from various locations in South America were analyzed, together with 22 skull specimens of S. fluviatilis. They were observed in dorsal, lateral and ventral views, using geometric morphometry as a tool. It was noted that populations of S. guianensis from the Southeast and South of Brazil showed morphological similarities, which indicates a probable genetic flow between individuals of these regions. The population from the northern region, on the other hand, showed a tendency to diverge from other marine populations. Meanwhile the north-eastern population showed to be more plastic and, with the exception of the dorsal view, it appears to be an intermediate form between populations from the North and Southeast/South. According to the deformations generated by geometric morphometry, S. fluviatilis differs from S. guianensis as it presents a slightly compressed skull in the dorsoventral direction and the occipital condyles are displaced ventrally. In addition, the nasal and premaxilla are displaced posteriorly towards the supraoccipital crest. The pterygoid bones are further separated and the entire basal cranial region that involves the palatines, pterygoid and basioccipital bones also seem to be displaced posteriorly in S. fluviatilis. Sexual dimorphism was observed in the marine population of the Northern region (Amapá state, Suriname, Venezuela and Colombia), with females presenting nasal bones displaced posteriorly, similarly to the fluvial species. In relation to the body size, it was observed that marine individuals of the Northern region do not differ statistically from S. fluviatilis. It therefore confirms that animals of this region show a reduced body size when compared to other marine specimens from other sites of the Brazilian coast. And finally, 43 complete and partial skeletons of S. fluviatilis were evaluated in order to seek traumatic, morphologic and pathologic variations. Fractures were the most frequent alterations, occurring in various regions of the skeleton such as ribs, hyoid apparatus, neural and transverse processes of the vertebrae and scapula. Three individuals were observed with ankylosis between the cervical vertebrae and two individuals with morphological alterations (elongated haemal arch in the craniocaudal direction and scapula with flat cranial edge). The only observed pathology was an osteomyelitis case in the left dentary, the first record of osteomyelitis on this species.