Abstract
Derived sabercats had craniomandibular morphologies that in many respects were highly different from those of extant felids, and this has often been interpreted functionally as adaptations for predation at extreme gape angles with hypertrophied upper canines. It is unknown how much of this was a result of intraspecific postnatal ontogeny, since juveniles of sabercats are rare and no quantitative study has been made of craniomandibular ontogeny. Postnatal ontogenetic craniomandibular shape changes in two morphologically derived sabercats, Smilodon fatalis and S. populator, were analysed using geometric morphometrics and compared to three species of extant pantherines, the jaguar, tiger, and Sunda clouded leopard. Ontogenetic shape changes in Smilodon usually involved the same areas of the cranium and mandible as in extant pantherines, and large-scale modularization was similar, suggesting that such may have been the case for all felids, since it followed the same trends previously observed in other mammals. However, in other respects Smilodon differed from extant pantherines. Their crania underwent much greater and more localised ontogenetic shape changes than did the mandibles, whereas crania and mandibles of extant pantherines underwent smaller, fewer and less localised shape changes. Ontogenetic shape changes in the two species of Smilodon are largely similar, but differences are also present, notably those which may be tied to the presence of larger upper canines in S. populator. Several of the specialized cranial characters differentiating adult Smilodon from extant felids in a functional context, which are usually regarded as evolutionary adaptations for achieving high gape angles, are ontogenetic, and in several instances ontogeny appears to recapitulate phylogeny to some extent. No such ontogenetic evolutionary adaptive changes were found in the extant pantherines. Evolution in morphologically derived sabercats involved greater cranial ontogenetic changes than among extant felids, resulting in greatly modified adult craniomandibular morphologies.
Highlights
Felids are some of the most anatomically specialized of all mammals for vertebrate predation, and the extinct sabertoothed felids (Felidae: Machairodontinae) included some of the most craniodentally specialized of all mammalian carnivores
Net ontogenetic shape changes in the crania of Smilodon are much greater than in extant pantherines, as indicated by vector analysis and the nature of the warp grids of net shape changes (Fig. 6); this is corroborated by much larger Procrustes Distances between juveniles and adults in Smilodon compared to extant pantherines
These two morphological differences from other felids have previously been regarded as key evolutionary adaptations for gaping at high angles to facilitate biting with hypertrophied upper canines [2,9,13,14], and it is here demonstrated that they are ontogenetic changes
Summary
Felids are some of the most anatomically specialized of all mammals for vertebrate predation, and the extinct sabertoothed felids (Felidae: Machairodontinae) included some of the most craniodentally specialized of all mammalian carnivores. Smilodon were the quintessential ‘‘sabertoothed tigers’’, and craniodentally, they differed markedly from all extant felids, having, among others, greatly elongate and lateromedially flattened upper canines and greatly reduced (incisiform) lower canines; a tall, compact cranial shape; an enormously developed mastoid process and a greatly reduced paroccipital process, implying great enhancement of the cranial flexor musculature; shorter and more massive zygomatic arches; prognatheous incisors; lowered glenoid joint; and a mandible with a rectangular and straight horizontal ramus, verticalized mandibular symphysis, greatly reduced coronoid process, laterally rotated lower carnassials, and a deflected retroarticular process [2,9,10,11,12,13,14,15,16,17] Their bite forces relative to body size were lower than among extant large felids [18,19,20,21], and their mandibular morphology indicated significant differences in predatory behaviour from extant felids [10,21,22]. The prognatheous incisors are thought to have facilitated carcass dismembering with very large upper canines, and perhaps as an auxiliary anchor point during predatory biting [2,10,11]
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