Evolution of skull and mandible shape in cats (Carnivora: Felidae)

Abstract

The felid family consists of two major subgroups, the sabretoothed and the feline cats, to which all extant species belong, and are the most anatomically derived of all carnivores for predation on large prey with a precision killing bite. There has been much controversy and uncertainty about why the skulls and mandibles of sabretoothed and feline cats evolved to become so anatomically divergent, but previous models have focused on single characters and no unifying hypothesis of evolutionary shape changes has been formulated. Here I show that the shape of the skull and mandible in derived sabrecats occupy entirely different positions within overall morphospace from feline cats, and that the evolution of skull and mandible shape has followed very different paths in the two subgroups. When normalised for body-size differences, evolution of bite forces differ markedly in the two groups, and are much lower in derived sabrecats, and they show a significant relationship with size and cranial shape, whereas no such relationship is present in feline cats. Evolution of skull and mandible shape in modern cats has been governed by the need for uniform powerful biting irrespective of body size, whereas in sabrecats, shape evolution was governed by selective pressures for efficient predation with hypertrophied upper canines at high gape angles, and bite forces were secondary and became progressively weaker during sabrecat evolution. The current study emphasises combinations of new techniques for morphological shape analysis and biomechanical studies to formulate evolutionary hypotheses for difficult groups.

Figure 1. The 22 (cranial) and 17 (mandibular) morphologically homologous landmarks used in the analysis of felid craniomandibular shape.

Skull and mandible of a puma (Puma concolor; ♂; CN3435) illustrating the various landmarks. Landmarks on skull are: apex of supraoccipital (1); dorsoventral extent of occipital condyle (2, 3); transition of horizontal temporal bridge and occiput (4); centre of acoustic meatus (5); posterior extent of zygomatic arch (squamous portion) (6); ventral (7) and dorsal (8) squamous and jugal suture of zygomatic arch; ventral sutural connection of jugal to maxilla (9); ventro-arboreal extent of orbital foramen (10); anteroposterior extent of P⁴ (11–12), P³ (12–13), and C¹ (14–15) along gumline; arboreal extent of premaxilla at alveolar margin of I³ (16); apex of nasal (17); dorsal nasal-maxilla suture (18); apex of skull at postorbital frontal process (19); apex of skull at coronal suture (20); ventral palatine-pterygoid suture (21); centre of infraorbital foramen (22). Landmarks on mandible are: centre of mandibular condyle (1); anteroposterior extent of basal portion of coronoid process (2–3); apex of coronoid process (4); anteroposterior extent (5–6) and ventral deflection of angular process; anterior extent of mandibular fossa (7); length of M₁ (8–10), P₄ (10–12), and P₃ (12–14); dorsoventral depth of horizontal ramus posterior to M₁ (8–9), P₄ (10–11), and posterior (12–13) and anterior (14–15) to P₃; anteroposterior diameter of C₁ (16–17). Scale bar equals 5 cm.

Figure 2. Skull and mandible shapes in cats as illustrated by 22 (cranium) and 17 (mandible) landmarks.

(A) Scatter plots of relative warps 1 and 2 for shape changes in the skulls of felids, along with morphological standards at the axis apices. Relative warps 1 and 2 summarize 40.1% and 20.0%, respectively, of sample variation in the analysis. (B) Scatter plots of relative warps 1 and 2 for shape changes in the mandibles of felids, , along with morphological standards at the axis apices. Relative warps 1 and 2 summarize 50.7% and 18.2%, respectively, of sample variation in the analysis. Symbols: Open circles, non-pantherine (“small”) felids: 1, Acinonyx jubatus; 2, Caracal caracal; 3, Catopuma temmincki; 4, Felis chaus; 5, Felis silvestris; 6, Leopardus pardalis; 7, Leopardus tigrina; 8, Leopardus wiedii; 9, Leptailurus serval; 10, Lynx canadensis; 11, Lynx lynx; 12, Oncifelis geoffroyi; 13, Pardofelis marmorata; 14, Prionailurus bengalensis; 15, Prionailurus planiceps; 16, Prionailurus viverrinus; 17, Puma concolor. Open squares, pantherine felids: 1, Neofelis diardi; 2, Neofelis nebulosa; 3, Panthera leo; 4, Panthera onca; 5, Panthera pardus; 6, Panthera tigris; 7, Panthera uncia. Closed squares, sabertoothed felids: 1, Dinofelis barlowi; 2, Epimachairodus giganteus; 3, Homotherium crenatidens; 4, Homotherium serum; 5, Machairodus aphanistus (mandible only); 6, Megantereon cultridens; 7, Paramachairodus ogygia; 8, Smilodon fatalis; 9, Smilodon populator.

Figure 3. Bite force quotients against skull shape in felids.

(A) Plots of bite force quotients at the canine (BFQ_canine) against relative warps 1 and 2 in modern felids. Bite force quotients are entirely uncoupled from skull shape on both relative warps, and the regression lines are not even significant at the 90% level. (B) Plots of bite force quotients at the canine (BFQ_canine) against relative warps 1 and 2 in extinct sabretoothed felids. Bite force quotients are significantly coupled to skull shape, although small sample size precludes assumptions of significance of the regression line at the 5% level for relative warp 2, but the regression is significant at the 10% level. Symbols as in Figure 1. Regression lines are interspecific Reduced Major Axis regression; for regression coefficients, see Table 1.