Finally back for some discussion of saber-tooth cats, nimravids, and barbourofelids. There are two complementary articles that appeared almost simultaneously regarding the shape of the saber-tooth skull. Skull shape was examined by mapping various landmarks on different species’ skulls and measuring the change in position of these landmarks compared to average values.
The first paper by Per Christiansen, who has done excellent work on various extinct species correlating skeletal structure and biomechanics, appears open-access in PLoS ONE.1 This paper was also covered by Greg Laden a few weeks ago. Christiansen’s study concentrates specifically on felids, and compares the saber-toothed cats to modern cats. Christiansen found that the saber-toothed skulls varied the most in shape along a dorso-ventral axis, with saber-toothed cats developing taller, shorter skulls.
Relative warp 2 is primarily related to dorsoventral skull shape, and specimens with lower warp scores have a dorsoventrally much taller and anteroposteriorly more compact skull, ventrally deflected glenoid fossa, greatly curved and anteroventrally compressed and dorsoventrally tall zygomatic arch, elevated facial portion of the skull, and abbreviated mid-section of the skull. They also have enlarged external nares and distinct posterior retraction of the infraorbital foramen, posteroventral deflection of the ventral orbital rim, and slightly smaller and dorsally deflected occipital condyles.
Modern cats varied along a different axis, with larger cats tending to possess skulls with longer muzzles, thicker zygomatic arches, and taller eye sockets. However, cats do not classify neatly as either “large cat” or “small cat”, but range along a continuum.
The saber-toothed cats as well are not simply classified as morphologically distinct from modern cats, since many early saber-toothed cats fall within the range of modern cat skull shape. The later, more derived machairodonts developed longer canines (the dirk-toothed morphology), and with longer canines skull shape changed to allow a larger gape. This resulted additionally in a decrease in bite strength. As mentioned before, the development of saber-teeth is thought to have led to a different means of prey capture, with a stabbing killing bite driven by powerful neck muscles delivered to pinned prey. This simultaneously decreases the chance of snapping the saber-teeth and decreases the necessary bite force, compared to the suffocating bites of modern cats that can take many minutes to kill.
The second paper is a similar study, but examines several species of nimravids and barbourofelids in addition to modern cats and saber-toothed cats.2 The authors, Slater and Valkenburgh, seem unconvinced of the argument for the movement of barbourofelids to their own family and include them with the nimravids. They compare the skull shapes of three groups: modern cats, modern cats and saber-toothed cats, and all felids plus nimravids and barbourofelids.
Like Christiansen, they note the changes in skull shape that result from increasing size in the modern cats, but also note that the puma (Puma concolor) has an unusual skull shape, more typical of that of the smaller cats than the large cats. The cheetahs (extant cheetah Acinonyx and the extinct American cheetah Miracinonyx) also have an unusual domed skull shape resulting from enlarged nasal passages to aid breathing. The authors note that the presence of this adaptation in the American cheetah, which is more closely related to the puma than the modern cheetah, lends support to the interpretation of the American cheetah as a fast-sprinting hunter.
Slater and Valkenburgh find a similar clustering of modern cats separately from derived saber-tooth cats, and find that nimravids and barbourofelids cluster with the saber-toothed cats, indicating convergent evolution of a similar skull shape with the evolution of longer canines. The barbourofelids examined were dirk-toothed and clustered with the derived saber-toothed cats, while most of the nimravids were scimitar-toothed and tended to cluster with the modern cats. This is a similar pattern to that observed by Christiansen, with the more derived, dirk-toothed species acquiring more drastic modifications in skull shape to accommodate the longer canines.
Per Christiansen has interpreted the clouded leopard Neofelis nebulosa in the past as a possible modern example of a cat in the early stages of saber-tooth cat divergence, since it possesses unusually long upper canines for its body size.3 However, Slater and Valkenburgh find that this species falls on the continuum midway between small cats and large cats in skull shape, and that the pantherines all possess a more saber-tooth-like skull shape. They suggest that the saber-tooth morphology requires a suite of characteristics, and unusually long canines are not sufficient, especially in the absence of lateral flattening of the canines. More research into the ecology of the clouded leopard will be required to determine if its canines demonstrate a new experiment in the saber-tooth morphology.
A final question asked by Slater and Valkenburgh is whether the traits present in saber-toothed carnivores develop as a mosaic, with different traits acquired at different times, or pleiotropically, with all traits emerging together. Slater and Valkenburgh take the extensive overlap in skull shape of conical-toothed and saber-toothed forms as evidence for mosaic evolution of the saber-toothed morphology. This suggests that the early evolution of scimitar-toothed forms did not initially require much change in hunting tactics, and thus must have evolved under a different selective pressure. The saber-teeth may have initially been used for threat displays in intraspecific competition, and then fallen under selection for a stabbing killing bite with longer canines and skull modifications to allow a wider gape.
The saber-tooth morphology is an unusual hypercarnivore body plan adapted for rapid kills, probably arising in an environment with relatively large prey animals and high competition from other predators. While this morphology has evolved independently in the felids, nimravids, barbourofelids, marsupials, and creodonts, little comparison of these groups to each other has been done. It appears that the saber-tooth morphology of the felids, nimravids, and barbourofelids evolved in a similar context, and comparison with the more distantly related groups to have evolved this morphology may find similarities in these as well.
- Christiansen, P. “Evolution of Skull and Mandible Shape in Cats (Carnivora: Felidae),” PLoS ONE 2008, 3, e2807. DOI: 10.1371/journal.pone.0002807
- Slater, G. J.; Valkenburgh, B. V. “Long in the tooth: evolution of sabertooth cat cranial shape,” Paleobiology 2008, 34, 403-419. DOI: 10.1666/07061.1
- Christiansen, P. “Sabertooth characters in the clouded leopard (Neofelis nebulosa Griffiths 1821),” Journal of Morphology 2006, 267, 1186-1198. DOI: 10.1002/jmor.10468