Evolution of Caniforms during the cat-gap

It has been suggested by some that as a result of the cat gap caniforms (dog-like species including canids, bears, weasels, and other related taxons) evolved a more carnivorous and hypercarnivorous adaption to fill ecological niches that would otherwise have been filled by cats.[7] This conclusion, however, is disputed.[8] Canid diversity did increase during the cat gap period, but this diversity is not indicative that the diversity was due to the cat gap. Hypercarnivore feliforms (felids and nimravids) occupied an area that canids did not and where felids, nimravids, and hypercarnivorous creodonts are found. Hypercarnivorous canids were present before the disappearance of the nimravids. Following the extinction of nimravids, only three new taxa originated, two of which were relatively small in body size. Disparity increased during the cat gap even with the extinction of the hypercarnivorous extremes. This was due to the extinction of morphological intermediates, and because carnivorans began to occupy hypocarnivorous (non-meat-specialist) morphospace for the first time in North America. Procyonids began to arrive in North America in the early Miocene, and "modern" ursids arrived in the late Miocene. Extinct lineages of Ursidae were present in North America from the late Eocene through the Miocene, and Amphicyonid (Bear-dogs) were present during this period as well, but they occupied a morphospace generally shared with canids and not in close proximity to the ursids.[8] “ During or just prior to this "cat gap," numerous caniform species evolve catlike features indicative of hypercarnivory, such as reduced snouts, somewhat enlarged canines, and fairly extreme reduction of their crushing molars. In North America the first caniform group of moderate body size to move in the direction of hypercarnivory were the endemic hesperocyonine canids, with three genera (Parenhydrocyon, Enhydrocyon, and Mesocyon), ran

ing in size from jackals to small coyotes, appearing in the early Arikareean (circa 28 MYA). Notably, these three evolved alongside the last hyaenodont and the remaining three nimravids, two of which were puma-sized. The small hypercarnivorous canids were soon joined by and ultimately replaced by numerous species from other families which also had evolved more specialized meat-eating teeth and skulls. These included at least three larger genera of similarly adapted amphicyonids, one endemic (Daphoenodon) and two from the Old World (Temnocyon and Mammocyon), a leopard-sized mustelid (Megalictis) as well as two hypercarnivorous bears, the hemicyonines Cephalogale and Phoberocyon.[7] ” However, other paleontologists take issue with this conclusions: “ It has been suggested that canids evolved hypercarnivorous morphologies because feliforms were absent during this period (the ‘cat-gap’’, 26–16 Ma). The data presented here do not support this hypothesis. In the calculated morphospace... Canids never occupy the area of morphospace in which felids, nimravids, and hypercarnivorous creodonts are found. More pertinent to the issue at hand, however, is that most of these hypercarnivorous canids were present before the disappearance of the nimravids, and all went extinct before the appearance of felids....There was a progressive and marked decrease in hypercarnivorous forms during the ‘‘cat-gap.’’ 28–20 Ma are characterized by above average extinction intensities and below average origination intensities. 20 Ma was marked by an increase in origination intensity, and 18 Ma showed a decrease in extinction intensity and a large increase in origination intensity. Nonetheless, despite increased origination intensities and decreased extinction intensities near the end of the ‘‘cat-gap’’ (20–16 Ma), there was still no substantial invasion of hypercarnivorous morphospace until the immigration of felids into North America."