GEOGRAPHIC-VARIATION AND SEXUAL DIMORPHISM IN THE SKULL OF THE DUSKY DOLPHIN, LAGENORHYNCHUS-OBSCURUS (GRAY, 1828)

Variation in skulls of 415 dusky dolphins, Lagenorhynchus obscurus, wa s studied based on 37 standard cranial measurements and meristic varia bles and 28 non-metrical characters (NMC) by using both bivariate and multivariate analyses. Geographic variation was analysed in mature sku lls from central Peru (N=189), Chile (N=22), New Zealand (N=47) and so uthwestern Africa (N=40). Advanced fusion in the frontal-supraoccipita l suture is the most reliable (95% efficiency) cranial criterion of se xual maturity; distal fusion in premaxillary and maxillary cannot be u sed. Sexual dimorphism, verified with t-tests and analysis of covarian ce, was encountered in only 6 of 37 measurements and in none of the NM C characters. Although skull size is the same, males have a wider rost rum and a longer temporal fossa than females. Analyses of covariance a nd t-tests indicated highly significant differences between geographic groups. A discriminant analysis successfully classified 96.5%, 91.7%, and 90.9% of skulls from respectively the Southeast Pacific (Peru and Chile pooled), New Zealand, and SW Africa. Such differentiation is co nsidered of subspecific level. All methods indicated higher morphologi cal similarity within the Peru/Chile and SW Africa/New Zealand grouped pairs than between the pairs. Most strikingly, skulls from New Zealan d and SW Africa are on average 3.1 cm (8.5%) shorter than these from P eru and Chile, concordant with differences in body length. New Zealand specimens differ from SW African animals by smaller tooth size and a greater number of teeth. Nine cranial variables revealed significant v ariation between dusky dolphins from central Peru and northern Chile, but the degree of heterogeneity should be re-evaluated with a larger s ample from Chile. NMC characters confirmed the trends mentioned but re solving power was limited ; their use is not recommended as the princi pal method for subspecific discrimination. The Peruvian dusky dolphin is thought to represent the most ancestral (plesiomorph) form from whi ch the others were derived through dispersion via. the east-flowing We stwind Drift.