Abstract
BackgroundThe study of postnatal ontogeny can provide insights into evolution by offering an understanding of how growth trajectories have evolved resulting in adult morphological disparity. The Ursus lineage is a good subject for studying cranial and mandibular shape and size variation in relation to postnatal ontogeny and phylogeny because it is at the same time not diverse but the species exhibit different feeding ecologies. Cranial and mandibular shapes of Ursus arctos (brown bear), U. maritimus (polar bear), U. americanus (American black bear), and the extinct U. spelaeus (cave bear) were examined, using a three-dimensional geometric morphometric approach. Additionally, ontogenetic series of crania and mandibles of U. arctos and U. spelaeus ranging from newborns to senile age were sampled.ResultsThe distribution of specimens in morphospace allowed to distinguish species and age classes and the ontogenetic trajectories U. arctos and U. spelaeus were found to be more similar than expected by chance. Cranial shape changes during ontogeny are largely size related whereas the evolution of cranial shape disparity in this clade appears to be more influenced by dietary adaptation than by size and phylogeny. The different feeding ecologies are reflected in different cranial and mandibular shapes among species.ConclusionsThe cranial and mandibular shape disparity in the Ursus lineage appears to be more influenced by adaptation to diet than by size or phylogeny. In contrast, the cranial and mandibular shape changes during postnatal ontogeny in U. arctos and U. spelaeus are probably largely size related. The patterns of morphospace occupation of the cranium and the mandible in adults and through ontogeny are different.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0521-z) contains supplementary material, which is available to authorized users.
Highlights
The study of postnatal ontogeny can provide insights into evolution by offering an understanding of how growth trajectories have evolved resulting in adult morphological disparity
We investigate intra- and interspecific cranial shape changes in extant U. arctos, U. maritimus, and U. americanus and compare the shape changes and life history traits to the extinct relative, U. spelaeus, in order to trace and understand the evolutionary skull shape change
PC1 is associated with a set of transformations that separates the juvenile U. arctos and U. spelaeus from adult U. spelaeus; the adults from the extant species are centered on PC1 (Fig. 3a & Additional file 2: Figure A1)
Summary
The study of postnatal ontogeny can provide insights into evolution by offering an understanding of how growth trajectories have evolved resulting in adult morphological disparity. Cranial and mandibular shapes of Ursus arctos (brown bear), U. maritimus (polar bear), U. americanus (American black bear), and the extinct U. spelaeus (cave bear) were examined, using a three-dimensional geometric morphometric approach. Three-dimensional (3D) GM are applied for the first time on species of the genus Ursus, a lineage that is not diverse and contains herbivorous, omnivorous, and carnivorous species This clearly constitutes an advantage of the present study, as shape change among relatively few closely related species and these feeding ecologies can be investigated. We investigate intra- and interspecific cranial shape changes in extant U. arctos, U. maritimus, and U. americanus and compare the shape changes and life history traits to the extinct relative, U. spelaeus (cave bear), in order to trace and understand the evolutionary skull shape change. Fossils harbour the difficulties of incomplete and insufficient sampling, it is worth to include them into a study of skull growth trajectories, since they, even if fragmentary, shed light on the evolution of observed changes and the generation of extant phenotypic disparity
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