Caudal Cranium of Thylacosmilus atrox (Mammalia, Metatheria, Sparassodonta), a South American Predaceous Sabertooth

No evidence for niche competition in the extinction of the South American saber-tooth species

The end of South American isolation during the Great American Biotic Interchange (GABI) promoted the contact between South and North American saber-tooth forms that evolved in isolation. This contact may have driven saber-tooth species to a competitive interaction, resulting in the extinction of the South American saber-tooth form. Here, we used paleoclimatic data to compare the climatic niche of the saber-tooth forms Thylacosmilus atrox (from South America), Smilodon fatalis, and Smilodon populator (both originally from North America). We evaluated niche width, overlap, and similarity to infer potential geographic distribution overlap and competition between these North and South American predators. To do so, we obtained the climatic variables from sites where occurrence fossil records were available. Our results suggest that T. atrox had a narrower climatic niche compared to Smilodon species. Although we found a significant climatic niche overlap and similarity between S. fatalis and T. atrox, it seems unlikely that both species have co-occurred. Low niche overlap and similarity between T. atrox and S. populator dismiss competitive interaction between them. Moreover, climatic niche and low tolerance for environmental changes may have been the cause of the South American saber-tooth extinction.

Seeing through the eyes of the sabertooth Thylacosmilus atrox (Metatheria, Sparassodonta)

The evolution of mammalian vision is difficult to study because the actual receptor organs-the eyes-are not preserved in the fossil record. Orbital orientation and size are the traditional proxies for inferring aspects of ocular function, such as stereoscopy. Adaptations for good stereopsis have evolved in living predaceous mammals, and it is reasonable to infer that fossil representatives would follow the same pattern. This applies to the sparassodonts, an extinct group of South American hypercarnivores related to marsupials, with one exception. In the sabertooth Thylacosmilus atrox, the bony orbits were notably divergent, like those of a cow or a horse, and thus radically differing from conditions in any other known mammalian predator. Orbital convergence alone, however, does not determine presence of stereopsis; frontation and verticality of the orbits also play a role. We show that the orbits of Thylacosmilus were frontated and verticalized in a way that favored some degree of stereopsis and compensated for limited convergence in orbital orientation. The forcing function behind these morphological tradeoffs was the extraordinary growth of its rootless canines, which affected skull shape in Thylacosmilus in numerous ways, including relative orbital displacement.

The evolution of mammalian brain size

Relative brain size has long been considered a reflection of cognitive capacities and has played a fundamental role in developing core theories in the life sciences. Yet, the notion that relative brain size validly represents selection on brain size relies on the untested assumptions that brain-body allometry is restrained to a stable scaling relationship across species and that any deviation from this slope is due to selection on brain size. Using the largest fossil and extant dataset yet assembled, we find that shifts in allometric slope underpin major transitions in mammalian evolution and are often primarily characterized by marked changes in body size. Our results reveal that the largest-brained mammals achieved large relative brain sizes by highly divergent paths. These findings prompt a reevaluation of the traditional paradigm of relative brain size and open new opportunities to improve our understanding of the genetic and developmental mechanisms that influence brain size.

An eye for a tooth: Thylacosmilus was not a marsupial "saber-tooth predator"

Background

Saber-toothed mammals, now all extinct, were cats or “cat-like” forms with enlarged, blade-like upper canines, proposed as specialists in taking large prey. During the last 66 Ma, the saber-tooth ecomorph has evolved convergently at least in five different mammalian lineages across both marsupials and placentals. Indeed, Thylacosmilus atrox, the so-called “marsupial saber-tooth,” is often considered as a classic example of convergence with placental saber-tooth cats such as Smilodon fatalis. However, despite its superficial similarity to saber-toothed placentals, T. atrox lacks many of the critical anatomical features related to their inferred predatory behavior—that of employing their enlarged canines in a killing head strike.

Methods

Here we follow a multi-proxy approach using canonical correspondence analysis of discrete traits, biomechanical models of skull function using Finite Element Analysis, and 3D dental microwear texture analysis of upper and lower postcanine teeth, to investigate the degree of evolutionary convergence between T. atrox and placental saber-tooths, including S. fatalis.

Results

Correspondence analysis shows that the craniodental features of T. atrox are divergent from those of placental saber-tooths. Biomechanical analyses indicate a superior ability of T. atrox to placental saber-tooths in pulling back with the canines, with the unique lateral ridge of the canines adding strength to this function. The dental microwear of T. atrox indicates a soft diet, resembling that of the meat-specializing cheetah, but its blunted gross dental wear is not indicative of shearing meat.

Conclusions

Our results indicate that despite its impressive canines, the “marsupial saber-tooth” was not the ecological analogue of placental saber-tooths, and likely did not use its canines to dispatch its prey. This oft-cited example of convergence requires reconsideration, and T. atrox may have had a unique type of ecology among mammals.