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The remarkable convergence of skull shape in crocodilians and toothed whales - PubMed

  • ️Sun Jan 01 2017

The remarkable convergence of skull shape in crocodilians and toothed whales

Matthew R McCurry et al. Proc Biol Sci. 2017.

Abstract

The striking resemblance of long-snouted aquatic mammals and reptiles has long been considered an example of morphological convergence, yet the true cause of this similarity remains untested. We addressed this deficit through three-dimensional morphometric analysis of the full diversity of crocodilian and toothed whale (Odontoceti) skull shapes. Our focus on biomechanically important aspects of shape allowed us to overcome difficulties involved in comparing mammals and reptiles, which have fundamental differences in the number and position of skull bones. We examined whether diet, habitat and prey size correlated with skull shape using phylogenetically informed statistical procedures. Crocodilians and toothed whales have a similar range of skull shapes, varying from extremely short and broad to extremely elongate. This spectrum of shapes represented more of the total variation in our dataset than between phylogenetic groups. The most elongate species (river dolphins and gharials) are extremely convergent in skull shape, clustering outside of the range of the other taxa. Our results suggest the remarkable convergence between long-snouted river dolphins and gharials is driven by diet rather than physical factors intrinsic to riverine environments. Despite diverging approximately 288 million years ago, crocodilians and odontocetes have evolved a remarkably similar morphological solution to feeding on similar prey.

Keywords: Crocodilia; Odontoceti; crania; ecomorphology; feeding; rostra.

© 2017 The Author(s).

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Figures

Figure 1.
Figure 1.

Lateral (left) and dorsal (right) views of representative elongate morphotypes: (a) Pontoporia blainvillei, (b) Gavialis gangeticus, and representative robust morphotypes: (c) Orcinus orca and (d) Osteolaemus tetraspis. The crania are scaled to the same length.

Figure 2.
Figure 2.

Landmark locations on (a) odontocete Feresa attenuata and (b) crocodilian Crocodylus moreletii. Note: there is a landmark placed at the centre of the jaw articular surface that is obscured in the Crocodylus moreletii diagram.

Figure 3.
Figure 3.

Morphological variation in the cranium. The diagrams show theoretical morphologies in dorsal and lateral views created by morphing a crocodilian (Mecistops cataphractus, above) and an odontocete (Tursiops truncatus, below) to the minimum and maximum of each of the first three principal components.

Figure 4.
Figure 4.

Morphological variation in the mandible. The diagrams show theoretical morphologies in dorsal and lateral views created by morphing a crocodilian (Mecistops cataphractus, above) and an odontocete (Delphinus delphis, below) to the minimum and maximum of each of the first three principal components.

Figure 5.
Figure 5.

Phylomorphospace plots showing the ecomorphology of crocodilians and odontocetes for cranium (left) and mandible (right). (a) Habitat classes, (b) diet classes and (c) maximum relative prey size classes (percentage of predator length). In all plots odontocetes are represented as circles and crocodilians as squares. Gharials and river dolphins are shown as open symbols. Taxa of interest are labelled with abbreviated taxonomic names. Abbreviations: P.ga: Platanista gangetica, I.ge: Inia geoffrensis, P.bl: Pontoporia blainvillei, L.ve: Lipotes vexillifer, G.ga: Gavialis gangeticus, T.sc: Tomistoma schlegelii, O.br: Orcaella brevirostris, S.fl: Sotalia fluviatilis.

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