161 



186) posterior process on plantar aspect of astragalus: 0 = absent; 1 - present (de Muizon 

 1982a). 



The condition of the phocid astragalus is unique among mammals. Firstly, the astragalus 

 possesses a strong posterior process causing it and the calcaneum to assume roughly equal 

 sizes (Howell 1928; de Muizon 1982a; pers. obs.; see character #185). Secondly, the 

 plantar surface of this posterior process is marked by a distinct groove for the passage of 

 the flexor hallucis longus tendon (see the following character). In phocids, this tendon is 

 better developed than it is in other mammals and plays a key role in their swimming 

 locomotion. In fact, the hypertrophy of this tendon is responsible for another diagnostic 

 feature of the phocids, that of being unable to turn the hind feet forward while on land 

 (Howell 1928; de Muizon 1982a; King 1983). As indicated, the development of a posterior 

 process is synapomorphic for phocids, and is possessed by all extant species (including 

 Monachus tropicalis). 



187) depth of groove on plantar aspect of posterior process of astragalus: 0 = groove 

 absent; 1 = shallow; 2 = moderate; 3 = deep; 9 = posterior process absent (de Muizon 

 1982a). 



As indicated by the previous character, the groove on the plantar aspect of the astragalus 

 is unique to the phocids (Howell 1928; de Muizon 1982a; King 1983). However, this 

 groove is not equally developed among the phocids, ranging from shallow to deep, and 

 even virtually absent in many monachines (pers. obs.). The differential expression of this 

 feature in the two phocid subfamilies renders the ancestral state for the phocids uncertain. 

 The phocines are characterized ancestrally by a deep groove. This is reduced to a shallow 

 groove in Phoca spp. and Pusa spp., before the primitive phocine morphology is regained 

 in Erignathus, Histriophoca, and Pagophilus. The monachines are defined by a lack of 

 the groove, although most taxa within this subfamily are polymorphic between this state 

 and states for grooves of various depths. 



188) length of metatarsal III relative to remaining metatarsals (shape of posterior flipper 

 margin): 0 = metatarsal III longest; 1 = metatarsal III intermediate; 2 = metatarsal III 

 subequal or slightly shorter; 3 = metatarsal III distinctly shorter (King 1966; Wyss 1988a). 

 All phocids are characterized by a shortening of the third metatarsal relative to the 

 remaining metatarsals, with the reduction tending to be more extreme in the monachines 

 than in the phocines (King 1966; Wyss 1988a). However, rather than view this as a 

 synapomorphy of the monachines, Wyss (1988a) was more inclined to view shortening as 

 primitive for the phocids, with the phocines partially reversing to approach the typical 

 carnivore pattern (states 0 or 1). This was supported by the observation that Cystophora 

 does not group with the phocines, but instead displays a monachine degree of reduction 

 (Wyss 1988a). A reduced third metatarsal is not present in the otarioids, where all 

 metatarsals are of about equal length (Wyss 1988a). Together with the relative reduction 

 of all bones of the third digit of the hind foot (King 1983), the shortening of the third 

 metatarsal also has coincident effects on the shape of the posterior flipper margin as a 

 whole. All phocids possess a concave outline to the posterior flipper to some degree, and, 

 again, it is much more marked in the monachines and the phocines Cystophora and 

 Histriophoca (King 1983; Wyss 1988a). Phocines, and Erignathus in particular, tend 



