5 



INTRODUCTION 



As a group, the true or earless seals (Mammalia: Carnivora: Phocidae) present one of the 

 more interesting puzzles in mammalian systematics. The roughly century-old debate on 

 the position of the phocids within the carnivores (and especially their placement relative 

 to the remaining pinnipeds) has attracted consistent attention, but the internal relationships 

 of the group remain reasonably poorly studied to this day. About the only point of universal 

 agreement is that the phocids are a natural, distinct group. It remains for an all-encom- 

 passing study employing a suitably rigorous methodology (such as cladistic analysis) to 

 attempt to resolve the points of contention or uncertainty in phocid systematics. 



Characterization of the Phocidae 



The phocid seals have been referred to as being among the most specialized of carnivores 

 (Wyss 1988a). Like all pinnipeds, the phocids are amphibious and are characterized by 

 many features that can be interpreted as adaptations to an aquatic environment. These 

 range from a fusiform, streamlined body shape and flippers that enhance aquatic 

 locomotion, to the many specializations of the inner ear required for efficient underwater 

 hearing (see Repenning 1972; de Muizon 1982a), to a simplified homodont dentition to 

 help capture their slippery aquatic prey (see Chapskii 1955a). However, they are clearly 

 distinguished from the remaining pinnipeds (and especially the sea lions and fur seals) by 

 features denoting a greater adaptation to the aquatic environment: the lack of a protruding 

 external pinna (as in the walrus as well), their generally superior diving ability (Costa 

 1993), and their reliance on the hind limbs for aquatic locomotion. In fact, the 

 modifications associated with this last point are so great as to define perhaps the most 

 definitive phocid characteristic, the inability to turn the hind limbs forward to support the 

 weight of the body on land. Thus, the phocids are restricted on land to some form of 

 crawling locomotion: inchworm-like movements (with or without assistance from the 

 flippers), a modified "swimming" type of locomotion, and/or rolling and sliding 

 (O'Gorman 1963; Ridgway 1972; King 1983). 



The phocids inhabit both the northern and southern hemispheres, although they are largely 

 restricted to the polar and sub-polar regions. The limits of their distribution seem to be 

 marked by the 20°C summer isotherm, with only the monk seals (Monachus spp.) breaking 

 this rule of thumb to inhabit tropical climes (Davies 1958a; McLaren 1960a; King 1964). 

 The phocids are the only pinnipeds to inhabit Antarctica year-round, with several species 

 being largely tied to the ice along the continent (see King 1968). One curiosity of phocids 

 among pinnipeds is their ability to survive in estuarine and freshwater habitats (King 

 1983), allowing for the existence of many populations or entire species in land-locked 

 lakes (Doutt 1942; Davies 1958b; King 1983). 



The phocids show a tremendous diversity in size, spanning from the largest to among the 

 smallest of all pinnipeds. Smallest of all phocids are the ringed seals (Pusa spp.) which 

 average about 1.4 m nose-to-tail length, while the largest is the male southern elephant 

 seal (Mirounga leonina) which spans four to five metres in length and can weigh up to 

 3.6 tonnes (King 1983). 



