Abstract
The remarkable elongated upper canines of extinct sabretoothed carnivorous mammals have been the subject of considerable speculation on their adaptive function, but the absence of living analogues prevents any direct inference about their evolution. We analysed scaling relationships of the upper canines of 20 sabretoothed feliform carnivores (Nimravidae, Barbourofelidae, Machairodontinae), representing both dirk-toothed and scimitar-toothed sabretooth ecomorphs, and 33 non-sabretoothed felids in relation to body size in order to characterize and identify the evolutionary processes driving their development, using the scaling relationships of carnassial teeth in both groups as a control. Carnassials display isometric allometry in both sabretooths and non-sabretooths, supporting their close relationship with meat-slicing, whereas the upper canines of both groups display positive allometry with body size. Whereas there is no statistical difference in allometry of upper canine height between dirk-toothed and scimitar-toothed sabretooth ecomorphs, the significantly stronger positive allometry of upper canine height shown by sabretooths as a whole compared to non-sabretooths reveals that different processes drove canine evolution in these groups. Although sabretoothed canines must still have been effective for prey capture and processing by hypercarnivorous predators, canine morphology in these extinct carnivores was likely to have been driven to a greater extent by sexual selection than in non-sabretooths. Scaling relationships therefore indicate the probable importance of sexual selection in the evolution of the hypertrophied sabretooth anterior dentition.
Highlights
The elongated upper canines of sabretoothed carnivores represent a classic example of a highly specialized morphological trait that has evolved repeatedly in the past, but the function of which is still not fully understood due to a lack of close analogues in living species
Research into allometry has produced several different models to explain scaling relationships that are not reliant on sexual selection [26,27,28,29], all of these models predict scaling exponents less than one, and so are unlikely to be relevant to our findings
These results suggest that sexual selection rather than natural selection may have contributed to upper canine evolution across the Feliformia
Summary
The elongated upper canines of sabretoothed carnivores represent a classic example of a highly specialized morphological trait that has evolved repeatedly in the past, but the function of which is still not fully understood due to a lack of close analogues in living species. Sabretoothed feliforms were not a morphologically homogenous group but instead consisted of three different recognised ecomorphs: (1) dirk-tooths (e.g. Barbourofelis, Eusmilus, Hoplophoneus, Megantereon, Paramachairodus, Smilodon), characterized by very elongated and laterally compressed canines, relatively shorter incisors, a long sagittal crest, and a robust, almost bear-like body shape; (2) scimitar-tooths (e.g. Dinictis, Homotherium, Ischyrosmilus, Machairodus, Nimravides, Nimravus), characterized by shorter, less compressed and often coarsely serrated canines, longer incisors, a shorter sagittal crest/temporalis musculature associated with relatively weaker bite force, and a more felidlike body shape; and (3) Xenosmilus hodsonae, which had a robust body and increased sagittal crest but relatively short canines [17,18] These ecomorphs are thought to be associated with different hunting strategies (ambush predation in dirk-tooths versus cursorial predation in scimitar-tooths [17]), and it is possible that different evolutionary pressures may have acted on the upper canines of each ecomorph, further complicating interpretation of the function of the ‘generalized’ sabretoothed morphotype
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