RADICAL SHIFTS IN THE EVOLUTION TOWARDS THE SABERTOOTH CRANIAL MORPHOLOGY

The sabertooth morphology has evolved independently in several different lineages. Besides hypertrophied maxillary canines, another shared characteristic of the sabertooth morphology, regardless of phylogeny, is a shortened distance between the craniomandibular joint and the upper carnassial. It has been hypothesized that the clouded leopard, Neofelis nebulosa, which has the longest canines of modern felids, might begin to show similar cranial morphologies as sabertooths. Here we test whether species with medium-sized canines, such as Neofelis, have an intermediary cranial form between modern felids and sabertooths. We took images of modern felids and nimravids, thylacosmilids, and machairodontids, all with varying canine lengths, and analyzed their skull morphology via landmarks using geometric morphometrics. We have found a bimodal distribution of skull morphologies consisting of species with short canines at one extreme and species with long canines at the other, and few species with an intermediary cranial form. This suggests that rather than a gradual shift towards the sabertooth morphotype as canine length increases, the evolution towards the sabertooth morphology may instead consist of a rapid and permanent shift, with few species maintaining an intermediary morphology. Neofelis falls within the skull morphology of other modern felids, and other species with medium-sized canines vary greatly in their morphologies between the two extremes.