Cope's rule and the evolution of body size in Pinnipedimorpha (Mammalia: Carnivora)

Abstract

Cope's rule describes the evolutionary trend for animal lineages to increase in body size over time. In this study, we tested the validity of Cope's rule for a marine mammal clade, the Pinnipedimorpha, which includes the extinct Desmatophocidae, and extant Phocidae (earless seals), Otariidae (fur seals and sea lions), and Odobenidae (walruses). We tested for the presence of Cope's rule by compiling a large dataset of body size data for extant and fossil pinnipeds and then examined how body size evolved through time. We found that there was a positive relationship between geologic age and body size. However, this trend is the result of differences between early assemblages of small-bodied pinnipeds (Oligocene to early Miocene) and later assemblages (middle Miocene to Pliocene) for which species exhibited greater size diversity. No significant differences were found between the number of increases or decreases in body size within Pinnipedimorpha or within specific pinniped clades. This suggests that the pinniped body size increase was driven by passive diversification into vacant niche space, with the common ancestor of Pinnipedimorpha occurring near the minimum adult body size possible for a marine mammal. Based upon the above results, the evolutionary history of pinnipeds does not follow Cope's rule.

A dwarf walrus from the Miocene of Baja California Sur, Mexico

“…[7], greatly contrasting with that of coeaval odobenids and even more with the slightly younger Pontolis magnus (figure 4 and table 3). Interestingly, this mid–late Miocene dwarfism seems to mirror the pattern seen in the southeastern Pacific with the occurrence of a dwarf seal in assemblages dominated by mid–large phocids [34], and the occurrence of several small phocids in the North Sea and Paratethys [35].…”

“…In odobenids this increase seems to have been possible due to a combination of factors, such as extinction of desmatophocids, increased marine productivity and exploitation of other feeding niches (e.g. benthic feeding) [3,4,7,32]. Nanodobenus arandai seems to be the exception to this trend.…”

“…Identified as an odobenid based on the presence of post-canine teeth that are bulbous, longer than high and with smooth enamel. Smallest known odobenid, with a body length estimated at 1.65 m (based on [7]; table 3); further characterized by the following unique combination of mandibular characters: mandibular symphysis with a subtriangular outline (narrower posteroventrally), symphysis short (shared with Proneotherium repenningi , Neotherium mirum , Kamtschatarctos sinelnikovae and Pontolis magnus ), genial tuberosity extending well below ventral margin of ramus (shared with Pelagiarctos spp., Archaeodobenus akamatsui , Imagotaria downsi , P. magnus , Dusignathus spp. and Ontocetus emmonsi ), and located posterior to p2 (shared with P. magnus , Dusignathus spp., Gomphotaria pugnax and O. emmonsi ).…”

“…The new taxon is characterized by its small body size, representing a case of dwarfism in a group that is otherwise characterized by an increase in body size over time [3,7]. …”

First Record of Postcranial Bones in Devinophoca emryi (Carnivora, Phocidae, Devinophocinae)

“…Additionally, recent work examining the evolution of the body size of pinnipeds reveals that odobenids and otariids had generally smaller-sized ancestors, while the so-called "limited fossil record" of Phocidae prevents a defi nitive answer, according to Churchill et al (2014). Although fossil cranial remains of seals are limited compared to otariids and odobenids, countless recent publications have described small sized fossil seals from both sides of the Atlantic Ocean.…”