The cotylar fossa is not a common synapomorphy to link afrotherian mammals and south american native ungulates.

LORENTE, M.; GELFO J. N.; BOND M.; LOPEZ G. M.; KRAMARZ, A.; REGUERO, M.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Museo de Ciencias Naturales Universidad Nacional de San Juan

The cotylar fossa was defined as an anterior extension of the medial malleolar facet of the astragalus that deepens into a cup and receives the condylar articular surface developed on the tibial medial malleolus (MacPhee, 1994). This character was considered as a reliable afrotherian synapomorphy (Tabuce et al., 2007), and later as a synapomorphy shared by most Afrotheria, Notoungulata, Astrapotheria and Xenungulata (Agnolin and Chimento, 2011). Here we analyze tarsal remains of representatives from all South American native ungulate groups, and compare them with extant afrotherians. The state of cotylar fossa of the malleolar facet is extremely variable among Notoungulata. Is absent in basal taxa as Pleurostylodon (Isotemnidae), and in Homalodotherium (Homalodotheriidae). In Miocochilius, Protypotherium and Interatherium (Interatheriidae) the medial malleolar facet is vertical. In contrast, in Nesodon (Toxodontidae) the neck of the astragalus is extremely reduced and the head is almost in contact with the trochlea, the malleolar facet is well developed and extends over the head forming a depression comparable to a cotylar fossa. In Astrapotheria and Pyrotheria the astragalar morphology is modified since the more graviportal lifestyle. In Parastrapotherium and Liarthrus (Astrapotheriidae) there is a structure comparable to a cotylar fossa, but is absent in the astragali assigned to the basal astrapotheriid Trigonostylops (Trigonostylopidae). In Pyrotherium (Pyrotheriidae) there is a long and thin medial facet without a cotylar fossa. In Xenungulata, Carodnia (Carodniidae) has a prominent posteromedial protuberance of the astragalus, a broad and flat head, somewhat medially oriented, which together forms a medial groove in the talus, but is improbable that this could be homologous to a cotylar fossa. In Macrauchenia (Litopterna, Macraucheniidae) the malleolous of the tibia is not extended over the medial side of the astragalus and no cotylar fossa is present. However in South American “condylarths”, considered as not related to afrotherian mammals (Agnolin and Chimento, 2011), the astragali presumably assigned to Lamegoia, Paulacoutoia and Didolodus (Didolodontidae) have a well developed cotylar fossa. Summarizing, the cotylar fossa is absent in most native ungulates groups or in their stem taxa. The cotylar fossa could not be considered as a character to link native ungulates with afrotherians. In fact, this character was convergently developed in non-afrotherians, as in primates, macropodid marsupials, laurasic “condylarths” as Meniscotherium and some dinocerates as Probathyopsis.