Mayulestes ferox, a borhyaenoid (Metatheria, Mammalia) from the early Palaeocene of Bolivia. Phylogenetic and paleobiologic implications

Mayulestes ferox is a borhyaenoid marsupial from the early Palaeocene of Tiupampa (Bolivia). The holotype and only known specimen is a partial skeleton which is described and discussed below. Mayulestes ferox is a member of the family Mayulestidae, a taxon which also includes the species Allqokirus australis from the same locality and age, but which is only known by a few isolated molars. Mayulestes and Allqokirus are the two oldest known borhyaenoids. Mayulestes differs from Allqokirus in the morphology and proportions of its molars. A major feature of the molars of both genera is the reduction of the entoconid which is regarded here as a synapomorphy of the Mayulestidae. Mayulestes has the plesiomorphic marsupial dental formula (I5/i4; C1/c1; P3/p3; M4/m4) and its molar morphology approaches the plesiomorphic marsupial cheek tooth pattern. Mayulestes ferox does not have a tympanic process of the alisphenoid, a structure whose presence is generally regarded as a marsupial synapomorphy. Comparison with other borhyaenoid taxa indicates that the lack of tympanic process of the alisphenoid is in fact a plesiomorphic character state for the superfamily, and it is suggested that this feature appeared several times during marsupial evolution. The ear region of Mayulestes bears a conspicuous medial process of the squamosal and there is a shallow cavity (the roof of the alisphenoid sinus) between the foramen ovale and the glenoid cavity, excavated within the squamosal anteriorly, the periotic posteriorly, and the alisphenoid between. The contribution of the squamosal to the roof of the alisphenoid sinus is regarded as the key synapomorphy of the borhyaenoids. Other borhyaenoid synapomorphies are: the loss of the prootic canal, the reduction and the loss of the anterolateral process of the maxilla, and the probable loss of epipubic bones. The postcranial skeleton of Mayulestes is represented by twenty complete or partial vertebrae, a few ribs and most major limb bones. A comparison with living didelphids, Pucadelphys, other borhyaenoids, and several arboreal (or probably arboreal) mammals such as sciurids, tupaiids, procyonids, multituberculates morganucodontids, triconodontids, and Henkelotherium reveals that many features of the postcranial skeleton of Mayulestes are indicative of arboreality. These traits are: probable prehensility of the tail; posterodorsally extended posterodorsal angle of the scapula; anteriorly and distally projected acromion; low tubercles of the humerus; circular shape of the head of the humerus; large size of the epicondyloid ridge and distomedially protruding medial epicondyle of the humerus; deep flexor fossa on the medial side of the olecranon of the ulna; morphology of the McV; great mobility of the hip attested by the shallowness of the acetabulum and the strong development of the femoral trochanters; sigmoid shape of the tibia and morphology of its distal articulation; shape and orientation of the ectal facet of the calcaneum; large size of the peroneal process and, at last, transversely compressed tuber calcanei. Several other features (size of the neural spine and transverse process of the lumbar vertebrae; morphology of the zygapophyses of the last thoracics and lumbar vertebrae; long, anteriorly bent olecranon of the ulna; eversion of the iliac wing; relative depth of the femoral trochlea; flattened distal epiphysis of the tibia; great length of the tuber calcanei) indicate that Mayulestes was a relatively agile, scansorial animal capable of bounding. Mayulestes is regarded as a partially arboreal predaceous mammal capable of bounding and of some relatively fast but short runs. Mayulestes was certainly fairly agile and could have had an ecological niche close to that of weasels or martens, although more arboreal than the former. Several arboreal features of Mayulestes are also found in Pucadelphys, a didelphid marsupial from the same locality. Consequently, this genus is also regarded as partially arboreal, although to a lesser extent than Mayulestes. The fact that the two oldest skeletons of American marsupials denote arboreal habits reinforces the hypothesis that arboreality is probably a symplesiomorphy within marsupials.