1 1 



Although this problem potentially exists at all taxonomic levels, Arnold (1981) holds that 

 it may become more detrimental at the lower levels. As well, as exemplified in the study 

 of de Muizon (1982a), it may be difficult to identify enough shared derived features to 

 adequately establish any species level relationships; Arnold (1981) has suggested that the 

 frequency of synapomorphies likely decreases with decreasing taxonomic level (although 

 molecular data may be more immune to this problem). These practical problems may be 

 offset somewhat as most phocid genera are monotypic. Thus, except for the polytypic 

 genera Mirounga, Monachus, Phoca, and Pusa, any species differences will essentially 

 translate into generic differences. 



Compounding all these problems is evidence for one or more relatively recent adaptive 

 radiations among phocids. The case for the Phocini has been mentioned above, but Ray 

 (1976a) also indicates that the full modernization of the lobodontines and of the phocines 

 as a whole could have occurred no more than four million years ago, in response to climatic 

 deterioration and adaptation to high latitudes (see also Repenning et al. 1979). With such 

 a comparatively short time for differentiation, achieving full resolution within these groups 

 might be difficult. 



Ancestral affinities of the phocids 



An important historical problem influencing phocid phylogeny is the uncertainty regarding 

 phocid ancestry, a debate that underlies the controversy over whether the pinnipeds have 

 a single or a dual origin. With regard to this latter question, a clear dichotomy is evident 

 in the literature. Although the arctoid affinities of all pinnipeds are not in doubt (Flynn et 

 al. 1988), most morphological, biogeographical, and paleontological studies historically 

 favour a diphyletic origin for the pinnipeds, whereby the phocids are accorded a mustelid 

 (possibly lutrine) ancestry, while the remaining pinnipeds (the otarioids) display ursid 

 affinities (e.g., Flower 1869; Mivart 1885; McLaren 1960b; Hunt 1974; Ray 1976a; 

 Tedford 1976; de Muizon 1982a, Wozencraft 1989; Nojima 1990). In contrast, most 

 biomolecular and karyological studies support a monophyletic Pinnipedia of ursid ancestry, 

 with the phocids and otarioids being sister taxa (e.g., Sarich 1969a, 1969b, 1975, 1976; 

 Arnason 1974, 1977; Haslewood 1978; de Jong 1982; de Jong & Goodman 1982; Wayne 

 et al. 1989; Vrana et al. 1994; Arnason et al. 1995; Lento et al. 1995). The monophyly 

 hypothesis rests on the overall similarity between all pinnipeds in all aspects, including 

 those features representing adaptations to an aquatic existence. Proponents of the diphyly 

 hypothesis dismiss these latter features as being convergent [see especially Mitchell (1967) 

 and Repenning (1990); but see Wyss (1989) for a contrasting viewpoint], and emphasize 

 other, non-aquatically related, similarities between the appropriate taxa. An especially 

 strong argument for the diphyly camp rests with the different centres and timing of the 

 first appearance of the otarioids [North Pacific about 22 million years before present 

 (MYBP)] versus the phocids (North Atlantic about 15 MYBP) in the fossil record 

 (Repenning et al. 1979). The case for diphyly is also strengthened by the suggestion of 

 the fossil taxa Potamotherium, and possibly Semantor, as putative intermediates between 

 the phocids and their musteloid ancestors (Ray 1976a; Tedford 1976; de Muizon 1982a). 

 Recently, however, there has been increasing acceptance of a monophyletic Pinnipedia, 

 due not only to molecular work (see above), but also to numerous morphological studies 



