BARBOUROFELIDAE

Therrien, François, 2005, Feeding behaviour and bite force of sabretoothed predators, Zoological Journal of the Linnean Society 145 (3), pp. 393-426 : 407-409

BARBOUROFELIDAE , CARNIVORA

Barbourofelids undoubtedly evolved some of the most extreme sabretooth adaptations ( Schultz, Schultz & Martin, 1970; Naples & Martin, 2000). They are found in early through late Miocene deposits of North America, Europe, Asia and Africa ( Fig. 1 ) ( Martin, 1989, 1998a; Peigné et al., 2000; Morales et al., 2001; Morlo et al., 2004). As mentioned above, barbourofelids were once considered to form a monophyletic clade with nimravids, but they are now considered more closely related to felids. Barbourofelids are represented by dirk-toothed predators exclusively ( Barbourofelis whitfordi , B. morrisi , B. loveorum and B. fricki are studied here).

Mandibular force profiles

No dichotomy of mandibular force profiles occurs in barbourofelids, reflecting the sole occurrence of dirktoothed ecomorphs in this lineage ( Fig. 8 ). The dorsoventral and labiolingual force profiles show that force values are greater at P 3 P 4 than at the carnassial, an observation apparently contradicting beam theory, which states that bite force should decrease with increasing distance from the articulation (see Material and methods above). These perplexing results indicate that a significant change in mandibular morphology occurs near the cheek teeth. Indeed, the mandibular corpus of barbourofelids is labially inclined at the cheek teeth to insure proper occlusion with the upper carnassial. This inclination places the cheek teeth ‘on a sort of lateral “island” away from the ramus’ ( Schultz et al., 1970: 11), P 3 being situated farther laterally than M 1. Consequently, the labiolingual diameter of the corpus increases at each interdental gap (see Schultz et al., 1970: fig. 5), producing the observed anterior increase in bending force.

Although strong in absolute terms, the mandibular symphysis of barbourofelids appears to be subequal or only slightly stronger than at the cheek teeth ( Fig. 8 ), a condition reminiscent of the canid profile ( Fig. 4) (see Therrien, 2005). However, this similarity is solely an artefact of the labial rotation of the ramus described above, which inflates the force values at the cheek teeth. When compared to the dorsoventral and labiolingual force values behind the carnassial (where the ramus is not rotated), the symphyseal region is seen to be stronger than the mandibular corpus, a condition also typical of dirk-toothed nimravids. The relative mandibular force profile (Zx / Zy) reflects the fact that the barbourofelid mandible is effectively wider near the cheek teeth relative to the postcarnassial region, the Zx / Zy values being lower at the P 3 P 4 and P 4 M 1 interdental gaps ( Fig. 8 ). At the canine, the mandible is strongly dorsoventrally buttressed, attaining Zx / Zy canine values of 2.77 or higher.

Interpretation

Barbourofelids have been described as having extremely robust mandibular rami ( Schultz et al., 1970), a fact clearly supported by the biomechanical models presented here. When comparison of Zx / L -values is made posterior to M 1, barbourofelids are shown to have possessed more powerful bites than nimravids and extant felids of similar mandibular length ( Table 1). For example, B. morrisi is shown to have had a bite force 44% superior to the similar-sized Hoplophoneus dakotensis , 66% superior to the similarsized Panthera pardus , and equal to P. tigris , a larger felid (28%-longer mandible). Barbourofelis fricki , the largest barbourofelid, is estimated to have been able to generate a bite 63% more powerful than P. leo .

As was the case in dirk-toothed nimravids, the dorsoventrally and labiolingually strong mandibular symphysis of barbourofelids demonstrates that important loads were exerted on the anterior extremity of the mandible, indicating that a powerful canine bite was used to subdue prey.

The Zx / Zy profiles of barbourofelids are atypical for dirk-toothed predators. Although the mandibular ramus increases in depth anteriorly ( Schultz et al., 1970), as in hoplophoneines (see above), this fact is obscured in the biomechanical models by the lateral rotation of the ramus near the cheek teeth, effectively decreasing the Zx / Zy values at P 3 P 4 and P 4 M 1 ( Fig. 8 ). Nevertheless, those Zx / Zy values are still within the range of modern felids ( Fig. 4), indicating an adaptation for slicing flesh (Therrien, 2005), and the anteriorly increasing depth of the ramus suggests that loads are better constrained within the sagittal plane towards the symphysis.

As was the case for dirk-toothed nimravids, the Zx / Zy canine values of barbourofelids are significantly higher than those of scimitar-toothed nimravids, sug-

Dorsoventral force Labiolingual force Relative force (Log Zx/L) (Log Zy/L) (Zx/Zy)

Barbourofelis whitfordi (L = 13.01 cm)

gesting that the dirk-toothed barbourofelids could more efficiently immobilize their prey. Anyonge (1996) included barbourofelids in his study of extinct predator locomotor behaviour and obtained enlightening results. By comparing the brachial index (maximum length of the radius/maximum length of the humerus) and crural index (maximum length of the tibia/maximum length of the femur) of extinct predators to those of modern carnivores, Anyonge (1996: fig. 3) discovered that while the crural index of the leopard-sized B. morrisi was similar to that of the lion-sized Smilodon fatalis , ambush predators and ambulators (i.e. bears), its brachial index was lower than that of the machairodont and ambush predators, being closer to the index of ambulators. Because lower intralimb indices (shorter distal element relative to proximal element) indicate a greater power output at the limb extremity, due to a shorter out-lever arm ( Hildebrand et al., 1985; Hildebrand, 1995), Anyonge’s (1996) results suggest that the forelimbs of the small barbourofelid had the same biomechanical advantage or leverage as those of the larger S. fatalis while its hindlimbs were biomechanically capable of generating a relatively greater force. Similarly, Baskin (in press) calculated brachial and crural indices for B. loveorum (BI = 0.72, CI = 0.75) and B. fricki (BI = 0.72, CI = 0.65) and discovered that they are even smaller than those of S. fatalis (BI = 0.82, CI = 0.84). These results suggest that the power output of the limbs of barbourofelids may have been greater than in S. fatalis! Therefore , the dirk-toothed barbourofelids could have easily subdued prey with their powerful forelimbs, thus limiting torsional stresses induced by struggling prey, prior to delivering a powerful canine bite to rapidly kill it.

Figures

Figure 1. Phylogeny and temporal range of sabretooth genera studied. Ischyrosmilus gracilis (sensu Churcher, 1984) is included in the Smilodon lineage (probably equivalent to S. gracilis). Although this taxon was not studied, Megantereon is presented to complete the Smilodontini tribe. Based on Goin & Pascual (1987), Bryant (1996b), Turner & Antón (1997), Gunnell (1998), Peigné (2003), and Morlo et al. (2004).

Figure 8. Mandibular force profiles of barbourofelids. Values are presented for the canine, P3P4, and post-M1 interdental gaps. Zx/L-values at the canine are subequal or only slightly higher than those at post-M1, due to the labial rotation of the ramus. No increase in dorsoventral buttressing occurs in the vicinity of the cheek teeth, indicating that the mandibular ramus is adapted for slicing meat exclusively. The Zx/Zycanine values are much higher than those of extant felids, suggesting that prey was restrained prior to the sabre bite.