Sabertooth

4. Sabertooths as Living Predators

157 4 Sabertooths as Living Predators All that has survived of the sabertooths are their fossilized bones, but they once were living creatures, and the aim of the science of paleobiology is to infer from their fossils as much as possible about their ways of life. But can we really do more than just imagine how the sabertooths moved, hunted, and interacted? Actually, if we know how to look, fossil bones can yield a surprising amount of information. Using a variety of methodological tools including functional morphology, comparative anatomy, dissection, and three-dimensional imaging, it is possible to get a remarkably rich picture of the once living creatures. The process is complex, and just like forensic scientists using the available evidence to solve a crime, we have to seek a balance between intuition and common sense. The first step in this process is to reconstruct the anatomy of the sabertooth from the inside out, starting with its skeleton, posture, and proportions, and continuing with the musculature and the rest of the soft tissues, including the skin and even coat patterns. After that, the next step is to set the reconstructed creature in motion, inferring from the physical traits of its locomotor system the likely gaits and athletic abilities of each different sabertooth species: running, climbing, wrestling down prey, and so forth. Cranial structures associated with the brain and nerves provide information about coordination and sensory development. Combining all these data with the information about prey species and characteristics of the paleoenvironments, we can build hypotheses about the sabertooths ’ hunting methods, which will be enriched by data about injuries and trauma, often associated with hunting accidents. Data on development and sexual dimorphism can give clues about family life and social structure, rounding out the picture of sabertooth lifestyles. Vertebrate paleontology has a lot to do with puzzle solving. Given the nature of most fossil sites, as discussed in chapter 2, the majority of the material available to paleontologists consists of separate, often broken, bones. It is thus easy to imagine the wonder of paleontologists when a complete, articulated skeleton comes to light. Such exceptional specimens instantly become the standards we can use in putting together the pieces of fragmentary finds of the same or related species. In the case of mammalian sabertooths, we probably have reasonably complete skeletons for fewer than twenty out of more than fifty recorded species. Reconstruction Sabertooth 158 Due to the scarcity of complete individuals, the work of reconstruction often starts by completing the missing parts of a skeleton. This task involves the reconstruction of the morphology of unknown parts and the scaling of pieces that belong to individuals of different size. Many sabertooth species varied considerably in individual size, which has to be accounted for during reconstruction. The relative lengths of the long bones of the limbs are essential in order to reconstruct body proportions, but there is an added problem: the longer the bone, the greater its chances of being broken before fossilizing. Museum collections abound in complete bones of the ankle or wrist of carnivores–small, squarish objects that survive well the processes of fossilization, but most of the long bones are broken. In these cases it is necessary to calculate ratios of length to width based on the proportions of comparable complete bones, to estimate how much of the bone’s original length is missing. For some species, genera, and even families, we lack relevant parts of their skeletons altogether. The history of sabertooths has plenty of such frustrating blanks. The American creodont sabertooth Apataelurus, the most derived genus in its family, is known from a single jaw, and in more than a century of excavations after its initial discovery, the extensive Eocene deposits of the Uintan still refuse to yield a single additional bone. Similarly, Eusmilus sicarius, the most spectacular nimravid sabertooth from North America, is known from cranial and mandibular remains only. These are extreme cases, but even when the blanks in a fossil animal’s anatomy are comparatively small, we need solid criteria to reconstruct the missing parts. Besides knowledge of vertebrate anatomy, in many cases we need to resort to phylogenetic information, as will become clear below. Phylogeny Whether we are filling in some missing vertebrae of a sabertooth skeleton or deciding about a coat pattern for a life restoration, we are reconstructing unpreserved attributes of the fossils, and our first step is to refer to other taxa, as closely related as possible to the fossil, as models for...