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ancestors (see also de Muizon 1982a). As well, the Bering Strait may have been 

 infrequently open around this time (Hopkins 1967). However, a more severe obstacle to 

 a migration through the Arctic basin is its colder climate, which would presumably hinder 

 the progression of the pagophobic phocid ancestors (Scheffer 1967; Ray 1976a). In 

 contrast, the Central American Seaway seems to be a more likely option. By all accounts, 

 it was wide open throughout much of the late Oligocene to early Miocene and beyond 

 (Davies 1958a; Berggren & Hollister 1974; Ray 1976a; de Muizon 1982a), and the use 

 of this route does allow the phocid ancestors to maintain their warm water affinities in 

 agreement with the supposed warm water origin of the phocids in the North Atlantic. 

 Although the utter lack of any phocoid fossils in the reasonably well known Oligocene to 

 Pleistocene fauna of the Pacific coast of North America has been used to argue against a 

 common Pacific origin (Barnes & Mitchell 1975; Ray 1976a), the acceptance of a 

 desmatophocid sister group to the phocids does much to ameliorate this. The various 

 desmatophocid genera are known to have existed around the time of the first appearance 

 of the phocids and their ranges extended at least to south central California for certain 

 species of both Allodesmus and Pinnarctidion (Mitchell 1975; Repenning 1975; Barnes 

 1979). Thus, the phocids may represent an offshoot of one of these lineages that migrated 

 through the Central American Seaway into the North Atlantic, as was later also held to 

 have been done by the ancestors of the modern walrus (Repenning 1975; Repenning & 

 Tedford 1977; Repenning et al. 1979). 



Within the phocids (accepting the minimal position that the North Atlantic served as the 

 common dispersal site of the family), the terminal position of Monachus spp. within the 

 lobodontines demands either a northward re-invasion by this genus from some southern 

 hemisphere locale, or for multiple southward invasions by the lobodontines. There is some 

 precedent for either a northward re-invasion (albeit slight) by, or a more southerly exten- 

 sion of, the Monachus lineage. Hendey (1972) holds for a slight northward "re-invasion" 

 by the ancestors of M. tropicalis, while M. monachus has been reported as far south as 

 Senegal in historical times (Hendey 1972). As well, fossil allies of Monachus (i.e., fossil 

 Monachini) have been reported from Peru (de Muizon 1982a), although they more likely 

 represent an unsuccessful colonizing population. The case for Mirounga spp. is equivocal 

 here, and does not speak for southward migrations on either the Pacific (e.g., Ray 1976a; 

 de Muizon 1982a) or Atlantic coast (e.g., Hendey 1972) of South America (but see below). 

 The case for putative northern re-invasions becomes stronger if one examines the timing 

 of parturition among the monachines. Among phocids, parturition typically occurs during 

 the spring of their respective hemispheres (i.e., the beginning versus the end of the calendar 

 year for the northern and southern hemispheres respectively). Curiously, however, the 

 monachines Mirounga angustirostris, Monachus monachus, and Monachus tropicalis 

 possess autumnal pupping times that coincide, in absolute terms, with those of the truly 

 southern hemisphere monachines (Allen 1887; McLaren 1966; Hendey 1972; Bonner 

 1981; Kenyon 1981b; King 1983; also references in Hayssen et al. 1993). [Most European 

 populations of Halichoerus also possess an autumnal pupping time (McLaren 1966; 

 Bonner 1981; King 1983; also references in Hayssen et al. 1993), but this shift has been 

 attributed to competition with Phoca vitulina for pupping sites (McLaren 1966).] The 



