Ontogeny of the skull of Sotalia guianensis (Cetartiodactyla: Delphinidae) on the Northern coast of Brazil

ABSTRACT This study aimed to investigate the ontogenetic development of the skull in the Guiana dolphin (Sotalia guianensis Van Bénéden, 1864 ) through the comparison of 29 morphological non-metrical characters together with the estimated age of 54 specimens derived from four areas on the northern coast of Brazil. The state of each character was associated with the age of each specimen to assess the formation of bone structures, and the degree of fusion of the sutures in each state of the character studied. Nineteen qualitative morphological characters presenting state ontogenetic variations have been analyzed statistically. The analysis depicted that the characters associated with the parietal-frontal suture, parietosupraoccipital, parietoexoccipital, supraoccipital-exoccipital and basioccipital-pterygoid varied according with the age. The results suggest that the physical maturity of the cranium was more premature in the specimens from the Northern Coast, suggesting a geographical adjustment of the species, or a biological response to an intense pressure of by-catch in fisheries over decades on the populations.摘要 本研究以圭亚那海豚（Sotalia guianensis Van Bénéden, 1864）为研究对象，通过对比29项非度量形态特征，并结合巴西北部沿海四个区域采集的54份标本的估算年龄，探究其颅骨的个体发育过程。研究将每一项性状的状态与对应标本的年龄进行关联，以评估骨骼结构的形成进程以及所研究性状各状态下的骨缝愈合程度。本研究对19项存在性状状态个体发育变异的定性形态特征开展了统计学分析。结果显示，与顶骨-额骨缝、顶骨-上枕骨缝、顶骨-外枕骨缝、上枕骨-外枕骨缝以及基枕骨-翼骨缝相关的性状随年龄发生显著变化。研究结果表明，巴西北部沿海的标本其颅骨生理成熟度更早出现，这提示该物种存在地理适应性，或是数十年来渔业副渔获物高强度捕捞压力下种群产生的生物学响应。