Abstract
BODY-MASS ESTIMATION OF SPARASSODONTA SPECIES (MAMMALIA, METATHERIA) OF SANTACRUCIAN AGE (EARLY MIOCENE) BASED ON CENTROID SIZES OF POSTCRANEAL ELEMENTS: PALAEOECOLOGICAL INFERENCES. Sparassodonts have been described as a group with a large body-mass diversity, but few works had focused on body mass estimations. In this contribution, we calculated the centroid size of the anterior view of the distal humerus, lateral view of the proximal ulna, and proximal articular view of the tibia. This data together with the known body-mass for many current predators (marsupials and placentals) allowed us to derive new allometric equations to estimate body size in the Sparassodonta. We used comparative analyses to quantify the effect of the phylogenetic structure. The relationships between body mass, centroid size, and locomotory habits were explored. The phylogenetic effect was significant for all regressions. In the humerus analysis, we found an important relationship between body mass and locomotory habit, suggesting adaptive causes. We used the ulna and tibia regressions to predict body mass, since they were less influenced by locomotory habits. Our equations produced the following masses: Arctodictis sinclari Marshall, 39.9 kg; Borhyaena tuberata Ameghino, 36.4 kg; Cladosictis patagonica Ameghino, 6.6 kg; Lycopsis longirostrus Marshall, 29.77 kg; Prothylacynus patagonicus Ameghino, 31.79 kg; Sipalocyon gracilis Ameghino, 2.11 kg; and Thylacosmilus atrox Riggs, 117.36 kg. For Borhyaena and Thylacosmilus, we obtained values higher than previously published masses, while for Pseudonotictis pusillus Ameghino, we have made the first body mass estimation of 1.17 kg. The results suggest that the guild of the santacrucian predators was structured by the body mass and other variables, such as diet and locomotion.
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