Differential influences of allometry, phylogeny and environment on the rostral shape diversity of extinct South American notoungulates

Abstract

Understanding the mechanisms responsible for phenotypic diversification, and the associated underlying constraints and ecological factors represents a central issue in evolutionary biology. Mammals present a wide variety of sizes and shapes, and are characterized by a high number of morphological convergences that are hypothesized to reflect similar environmental pressures. Extinct South American notoungulates evolved in isolation from northern mammalian faunas in highly disparate environments. They present a wide array of skeletal phenotypes and convergences, such as ever-growing dentition. Here, we focused on the origins of the rostral diversity of notoungulates by quantifying the shape of 26 genera using three-dimensional geometric morphometric analysis. We tested the influence of allometry and phylogeny on rostral shape and evaluated rates of evolutionary change in the different clades. We found strong allometric and phylogenetic signals concerning the rostral shape of notoungulates. Despite convergent forms, we observed a diffuse diversification of rostral shape, with no significant evidence of influence by large-scaled environmental variation. This contrasts with the increase in dental crown height that occurred in four late-diverging families in response to similar environmental pressures. These results illustrate the importance of considering both biological components and evolutionary rates to better understand some aspects of phenotypic diversity.

Sciuromorphy outside rodents reveals an ecomorphological convergence between squirrels and extinct South American ungulates

“…The anatomical study of pachyrukhines reveals that they display a mosaic of traits, combining plesiomorphic characteristics, present in ungulates, with others, more derived and convergent with rodents. Interestingly, the informative osteological features analyzed here were well known since the late nineteenth century 23 , and have been used in recent systematic and evolutionary studies of notoungulates 20,22,26,31 . However, they had never been analyzed in a detailed morpho-functional way, like the one proposed here.…”

“…It has been proposed that a markedly extended zygomatic plate on the ventral and anterior aspects of the anterior root of the zygomatic arch would have arisen at least twice independently in Typotherioidea, in the common ancestor of mesotheriine mesotheriids and hegetotheriids 18,20,22 . The morphological analysis of typotherioids and the available descriptions (Supplementary Method; see refs.…”

“…Beyond the fact that the zygomatic plate of typotherioids is at first sight similar to that of sciuromorph rodents, as was earlier stated by Patterson 25 for some mesotheriids (see also refs. 22,32 versus ref. 36 ), the remaining contributions that described these notoungulates made reference to rodent-like morphologies in a broad sense, or compared them exclusively with caviomorph rodents, lagomorphs, hyracoids, and/or small ungulates.…”

“…The first major diversification moment in which typotherioids became taxonomically and ecologically diverse was at the Eocene−Oligocene transition 18–20 . The families Mesotheriidae and Hegetotheriidae, recorded from the Early Oligocene, convergently acquired rodent- or rabbit-like morphologies 20–22 , presenting hypsodont dentition with chisel-like incisors, wide rostrum with diastemata, and several others cranial and postcranial features 18,19,22–29 . Among typotherioids, the group which more closely resemble rodents, by morphology and size, are the Pachyrukhinae, a Hegetotheriidae subfamily probably also recorded from the early Oligocene 30 and whose biochron extends to Late Pliocene 26,31 .…”

“…These studies lacked sciuromorph rodents as comparative models or even proposed alternative interpretations of the rostral anatomy 34 . As was stated by Gomes Rodrigues et al 22 , there exists a necessity of more detailed studies of the morpho-functional feeding ecology of the masticatory apparatus to solve the relationship between the evolution of the rostral morphology of typotherioids and notoungulates and the potential drivers such as ecology and environmental conditions. According to these authors, this relationship cannot be understood from the data available so far.…”