Analysis of function in the absence of extant functional homologues: a case study using mesotheriid notoungulates (Mammalia)

Abstract

We use two approaches to test hypotheses regarding function in a group of extinct mammals (Family Mesotheriidae, Order Notoungulata) that lack any close extant relatives: a principle-derived paradigm method and empirically derived analog method. Metric and discrete morphological traits of mesotheriid postcranial elements are found to be consistent with the morphology predicted by a modified version of Hildebrand's paradigm for scratch diggers. Ratios of in-force to out-force lever arms based on skeletal elements indicate that the mesotheriids examined had limbs modified for high out-forces (i.e., they were “low geared”), consistent with the digging hypothesis. Other mesotheriid characters, such as cleft ungual phalanges, a curved olecranon, and a highly modified pelvis (with extra vertebrae incorporated into the sacrum and fusion between the ischium and the axial skeleton) are regarded as being functionally significant for digging and also occur in a variety of extant diggers. Analog methods indicate that mesotheriids share numerous traits common to a variety of extant diggers. Principal component analyses of postcranial elements indicate that mesotheriids consistently share morphometric space with larger extant fossorial mammals: aardvark, anteaters, wombats, and badger. Likewise, discriminant function analyses categorized mesotheriids as fossorial, though imperfectly analogous to the extant diggers analyzed. Thus, both theory-driven and empirically derived methods of estimating function in these extinct taxa support a digging hypothesis for the mesotheriids examined. Adaptations for digging in both the forelimb and sacropelvic functional complexes of mesotheriids provide independent support for the fossorial hypothesis.

Paleontology and Geochronology of the Deseadan (late Oligocene) of Moquegua, Perú

“…For now it will suffice to note a few characteristics that exclude it from the monophyletic Mesotheriinae, such as having a complete upper dentition and root formation of the premolars and M1. The other two taxa are known by postcranial elements, which are very similar to those of other mesotheriids (see Shockey et al, 2007). Where known, Trachytheriinae postcranials are similar to those of mesotheriine mesotheriids (Shockey et al, 2007).…”

“…They were generally sheep-sized animals and, where known, their postcranial skeletons were modified for strength, such that they have been characterized as digging animals (Shockey et al, 2007).…”

Estimación de la masa corporal en géneros del Orden Notoungulata

“…Alguno de los géneros especializados en el desarrollo de fuerza en la locomoción o soporte de la masa corporal comprenden a Thomashuxleya (Isotemnidae), Toxodon (Toxodontidae) e Interatherium (Interateriidae), este último posiblemente desarrollase fuerza en funciones accesorias a las citadas como puede ser la actividad cavadora (Elissamburu, 2007). Entre los Typotheria se ha propuesto además la capacidad de cavar para los mesotéridos (Shockey et al, 2007;Elissamburu, 2007) y los géneros de la familia Hegetotheriidae Paedotherium (Elissamburu, 2004;Elissamburu y Vizcaíno, 2005) y Hemihegetotherium Rovereto 1914(Elissamburu, 2007.…”

Ontogenetic and life history trait changes associated with convergent ecological specializations in extinct ungulate mammals

“…3C), although forests probably remained dominant until the Middle Miocene in South America (15,39), especially in the northern part (14). Pending more precise environmental data, it seems at least reasonable to assume that the acquisition of a more durable and rapidly efficient dentition allowed notoungulates to widen the spectrum of plants consumed (19,40,41) in being more tolerant toward abrasives, which partly led to their subsequent specializations. These innovations may also have allowed juvenile notoungulates to exploit more rapidly an abundant and readily available food type, which contributed to reducing the energetic cost of foraging (42).…”

“…Among these mammals, the extinct order of Notoungulata was highly diversified taxonomically and consisted of rodent-sized up to rhino-sized taxa (18)(19)(20). They exhibited very advanced dental specializations thought to be related to herbivory (21,22).…”