Deja vu: the evolution of feeding morphologies in the Carnivora

Abstract

The fossil record of the order Carnivora extends back at least 60 million years and documents a remarkable history of adaptive radiation characterized by the repeated, independent evolution of similar feeding morphologies in distinct clades. Within the order, convergence is apparent in the iterative appearance of a variety of ecomorphs, including cat-like, hyena-like, and wolf-like hypercarnivores, as well as a variety of less carnivorous forms, such as foxes, raccoons, and ursids. The iteration of similar forms has multiple causes. First, there are a limited number of ways to ecologically partition the carnivore niche, and second, the material properties of animal tissues (muscle, skin, bone) have not changed over the Cenozoic. Consequently, similar craniodental adaptations for feeding on different proportions of animal versus plant tissues evolve repeatedly. The extent of convergence in craniodental form can be striking, affecting skull proportions and overall shape, as well as dental morphology. The tendency to evolve highly convergent ecomorphs is most apparent among feeding extremes, such as sabertooths and bone-crackers where performance requirements tend to be more acute. A survey of the fossil record indicates that large hypercarnivores evolve frequently, often in response to ecological opportunity afforded by the decline or extinction of previously dominant hypercarnivorous taxa. While the evolution of large size and carnivory may be favored at the individual level, it can lead to a macroevolutionary ratchet, wherein dietary specialization and reduced population densities result in a greater vulnerability to extinction. As a result of these opposing forces, the fossil record of Carnivora is dominated by successive clades of hypercarnivores that diversify and decline, only to be replaced by new hypercarnivorous clades. This has produced a marvelous set of natural experiments in the evolution of similar ecomorphs, each of which start from phylogenetically and morphologically unique positions.

Ecomorphology of Carnivora challenges convergent evolution

“…Feeding adaptations were functionally identified according to previously suggested categorisations that were likewise applied to test for morphological convergence in the skull and the mandible of Carnivora (Van Valkenburgh 1991, 2007Figueirido et al 2011Figueirido et al , 2013.…”

“…Although this diversity unfolds across a number of different ecomorphotypes, most carnivorans are basically meat eaters that share a common digestive physiology (Clauss et al 2010). As such, it is not surprising that patterns of iterative evolution are reported over and over in describing the evolution of carnivore skull and dental morphologies (Van Valkenburgh 1991, 1999, 2007. The main factor sought to explain carnivoran skull shape differentiation is the adaptation to different feeding habits including hypercarnivory, herbivory and hard food consumption (Ewer 1973;Van Valkenburgh 1989, 1991Holliday and Steppan 2004;Raia 2004;Goswami 2006;Figueirido et al 2010Figueirido et al , 2011Figueirido et al , 2013.…”

Deciduous dentition and dental eruption of Hyainailouroidea (Hyaenodonta, “Creodonta,” Placentalia, Mammalia)

“…Apterodontines combine a key morphological feature of hypercarnivores, the reduction or absence of metaconids on the lower molars, with features of more generalist carnivores such as well-developed talonids on the lower molars, divergent metacones and paracones, and mesiodistally abbreviated metastyles on the upper molars (Van Valkenburgh, 2007) complicating dietary and phylogenetic inferences (Van Valen 1966;Szalay, 1967;Grohé et al, 2012). Recent phylogenetic studies (Borths et al, 2016;Borths and Seiffert, 2017) incorporate morphological features of the cranial vault , suggesting Apterodontinae as the sister clade of Hyainailourinae.…”

New carnivoraforms (Mammalia) from the middle Eocene of California, USA, and comments on the taxonomic status of Miacis gracilis

“…So far as can be discerned from their dental morphology, most of the middle-Eocene carnivoraforms of southern California are small to mediumsized mesocarnivores (sensu Van Valkenburgh, 2007;see e.g., Tomiya, 2011). Although Tapocyon deviates from the rest of carnivoraforms in increased carnivory as indicated by its reduced m1 talonid basin and vestigial second molars (cf.…”