Structural and physiologic remarks on fossil giant xenarthrans (Mammalia)

VIZCAÍNO, SERGIO FABIÁN; BARGO, MARÍA SUSANA; TOLEDO, NESTOR; CASSINI, GUILLERMO HERNÁN

Punta del Este -Uruguay

Congreso; 9th International Congress of Vertebrate Morphology; 2010

Universidad de La República - International Society of Vetebrate Morphologyst

Beginning in the Pliocene (~5 My), xenarthrans evolved a large diversity of gigantic forms (ground sloths and glyptodonts) that dominated the large herbivore guild in South America. Probably due to strong phylogenetic constraints in a singular biogeographic scenario, gigantism was attained in part by retaining certain features of smaller forms and incorporating some features of only distantly related gigantic forms. Sauropods and elephants, the largest extinct and extant terrestrial tetrapods, share morphologic features usually considered as adaptations to large size: essentially vertical limbs and straight, relatively robust long bones with terminal articular surfaces and increased sectional supporting areas (no marrow cavity). As elephants have low metabolic rates relative to size, they require proportionately less food than smaller mammals; as is also expected for sauropods. In general, ground sloths and glyptodonts retain flexion between the stylopodium and zeugopodium, and between the former and the girdle. The ulna possesses a long olecranon aligned with the diaphysis, indicating a non-terminal proximal articular surface and a high input lever arm for extension of the antebrachium. Cross-sectional supporting areas are maximized, being solid, and attained strength indicators equivalent to or higher than those of any modern large mammal. Xenarthrans also have less dental occlusal surface area available for triturating food than other placental mammals of similar sizes, which may be related to the intrinsic low basal metabolic rates within the clade. This implies that giant xenarthrans had much lower energetic requirements and needed much lower food intake.