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By Mauricio Antón, James O. Farlow
Indiana University PressCopyright © 2013 Mauricio Antón
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What Is a Sabertooth?
TODAY SABERTOOTHS ARE A FAMILIAR KIND OF EXTINCT CREATURES for scientists and for many laypersons, but in the early days of paleontology, not even scientists knew that such a thing as a sabertoothed predator had ever existed. Consequently, when early paleontologists first tried to make sense of fragmentary fossils of sabertooths, they attributed the remains to other, already known groups of animals. After all, those early discoveries were not of complete skeletons or even skulls, which would have revealed right away that the bizarre canines of sabertooth cats fit into an otherwise catlike skull and skeleton. Instead, the partial finds were like pieces of a jigsaw puzzle with no complete picture to refer to.
One of the first people faced with the task of interpreting sabertooth fossils was the nineteenth-century Danish naturalist P. Lund. In the 1830s, Lund devoted a lot of time and effort to exploring the caves of Lagoa Santa, in the Brazilian region of Minas Gerais. He had left Denmark in 1833 (at the age of thirty-two) to pursue botanical research in Brazil, but in 1834 he met his compatriot P. Claussen, a fossil collector who had worked in Argentina before coming to Brazil. Lund was immediately fascinated by fossils and paleontology, so he abandoned botany and moved to Lagoa Santa, then a village with fewer than five hundred inhabitants that was surrounded by numerous calcareous caverns, some of them rich in fossils. Many of the caves were actively exploited for saltpeter, with lots of fossils being destroyed in the process, so Lund set out to salvage as much material as possible (Paula Couto 1955; Cartelle 1994). With admirable dedication, he and his local assistants explored cave after cave, collecting over 12,000 fossils between 1834 and 1846. It was extremely hard work, but Lund's fascination for the extinct fauna of Brazil led him through all the difficulties he encountered, and whenever he found a rich fossil site, his imagination was set aflame. One of his peak achievements was to find the first remains of perhaps the most spectacular sabertooth cat, the Pleistocene felid Smilodon populator. Lund's first finds were just a few isolated pieces, and he thought they belonged to a hyena, naming the creature Hyaena neogaea in 1839. But in 1842, with the addition of a little more material, including a few more teeth and some foot bones, Lund – an adept follower of G. Cuvier, the father of comparative anatomy – soon realized that the predator actually belonged to the cat family.
Lund was convinced that the numerous bones of large mammals that he found in the caves had been dragged there by big predators, which retreated to their dark dens to feed at leisure. The identification of Smilodon as the dominant predator of its time rounded out the scenario in his mind:
Regarding its size, this unique extinct carnivore rivaled the largest known cats or bears; the size of its canines is very much larger than in any species of carnivore, living or fossil. Judging by the dimensions of its foot bones, its body must have been heavier than that of any of the living felines, including the lion.
It is evident that a carnivore of such size, armed with such formidable weapons, must have reaped abundant victims among the inhabitants of the ancient world. In fact, I found the remains of its prey in three different caverns, which included, without exception, great accumulations of bones of diverse animals, many of them of gigantic size ...
In view of the unusual form of the canines of this animal, I propose for its generic designation the name Smilodon ("tooth shaped like double-edged knife"). Its bloody deeds, whose memory still endures in the caves that served it as dens, doubtlessly qualify it for the specific name of populator, "he who brings devastation." (quoted in Paula Couto 1955:7–8)
Lund's account is a fitting introduction for an impressive animal that no human being had seen in over ten thousand years, and that no one would ever see alive again. While his interpretation of the origin of the fossil accumulations in the Lagoa Santa caves as the result of the activities of Smilodon is now thought to be not quite correct, his assessment of the size and strength of the newly discovered creature was soon confirmed by the appearance of more complete remains. In 1846 he could proudly write: "I now possess nearly all parts of the skeleton of this remarkable animal of the prehistoric world, mostly from different individuals" (quoted in Paula Couto 1955:7–8). During the few years following the initial description of Smilodon, the pace of discoveries quickened spectacularly. In 1843 a complete, amazingly well-preserved skeleton was found near Lujan, in Argentina, by the naturalist Francisco Javier Muñiz, and it was sent to the Museo de Ciencias of Buenos Aires. Around the same time, a complete skull was found by Claussen, Lund's friend, in a cave in Lagoa Santa, and sold for 2,000 francs to the Académie des Sciences de France, which in turn donated it to the Muséum national d'Histoire naturelle of Paris, where it can still be admired. A second, beautiful skeleton was found in the 1870s by a M. Larroque near Buenos Aires; it was obtained by the American Museum of Natural History of New York, where it remains on exhibit. The American paleontologist E. D. Cope published several drawings of parts of this skeleton in 1880, but the specimen remains mostly undescribed.
In the second half of the nineteenth century, remains of Smilodon from the Pleistocene were also discovered in North America, although the fossils there were scantier – but that situation would change. In 1875 Major H. Hancock, then owner of the Rancho la Brea property in Los Angeles County, presented a Smilodon canine tooth, recovered from the asphalt pits on his property, to Professor W. Denton of the Boston Society of Natural History. In spite of the impression that this gift made, it took twenty-five more years for that society to join forces with other institutions and organize the first scientific investigations at La Brea. Then, during the first decade of the twentieth century, an enormous amount of fossil mammal bones were recovered from the site (Harris and Jefferson 1985). Among these were literally thousands of bones of Smilodon, representing hundreds of individuals of the North American species S. fatalis, closely related to but distinct from its South American cousin.
Thus in the early twentieth century, Smilodon was a well-established element of the known prehistoric world. It was not the first sabertooth to be known to science – that privilege apparently belongs to the genus Megantereon, ironically mistaken for a bear by Cuvier. But Smilodon was the first genus to be known from reasonably complete fossils; it remained the best-known one for many decades; and it still includes the biggest and most spectacular species recorded.
So What Is a Sabertooth?
Smilodon is such a spectacular fossil that it has become quite popular, and it was even named the state fossil of California. It is also one of the few fossil mammals to appear repeatedly in cartoons and movies, and as a result a popular image of sabertooths has taken shape: they are perceived as big cats, armed with impressively long fangs (the sabers), and they are often seen in the vicinity of cavemen and mammoths. This image has some truth in it, but it is certainly not the whole truth. Like many sabertooths, Smilodon differed from modern big felids in being very robust, with stocky and enormously muscular limbs, a long and strong neck, and a short tail like that of a lynx or bobcat. A study of its skeleton reveals many other, more subtle differences that, taken as a whole, point to a hunting style differing in several ways from that of modern cats.
The identification of the concept "sabertooth" with Smilodon has inevitably masked one of the main facts of sabertooth history: diversity. This book deals with sabertoothed predators, a broader concept than that of the sabertoothed cats. While the latter indeed belonged to the cat family (including animals like Smilodon itself and closely related genera such as Megantereon and Homotherium), many sabertooths did not, and some of them would look quite uncatlike to a modern observer. Some really were big and heavier than any living cat, but others were smallish creatures, scarcely larger than your average house cat (figure 1.1). Some were contemporary with early humans and mammoths, but much of the evolutionary history of sabertooths took place long before mammoths, humans, or even our earliest hominid ancestors existed.
How, then, can we define the sabertooths? Briefly, we can say that they were a group of fossil vertebrates, most – but not all – of them mammals; they were all predators; and they all possessed a set of anatomical features that define them as sabertooths, including the presence of elongated upper canine teeth and other adaptations in the skull that allowed them to open their jaws in the huge gapes necessary to bite with such enormous teeth. But this is a rather complex definition, and its various parts need to be discussed in more detail.
First of all, sabertooths are fossils. All of them became extinct so long ago that no human being ever saw one alive in historical times – to our knowledge, at least – so everything we know about them is based on their fossilized remains. Claims of modern sightings of sabertooth cats in South America and Africa remain completely unsubstantiated. And although some zoologists have suggested that the living clouded leopard of South Asia qualifies as an incipient form of sabertooth because of its long canines, the similarities between sabertooths and this extant felid are limited and rather superficial.
The history of sabertooths spans an enormous length of geological time. Although the latest species, such as the famous Smilodon fatalis from Rancho la Brea, disappeared "only" ten thousand years ago, the earliest mammalian sabertooths lived some 50 million years ago (or Ma) in the Eocene of North America. Even earlier, the gorgonopsians, a group of so-called mammal-like reptiles with many sabertooth features, lived in the Permian period, long before true mammals or even dinosaurs evolved, thus stretching the history of sabertooths, in the broad sense, as far back as 250 Ma (figure 1.2).
Second, all sabertooths were synapsids – that is, they belonged to the large group of vertebrates that includes both the mammals and the mammal-like reptiles. In anatomical terms, the skull of synapsids is characterized by the possession of a single cranial aperture behind the orbits (or eye sockets), hence their name, which means "a single opening." In mammals, that opening corresponds to the large area in the side of the skull where the temporalis muscle attaches to the parietal bone. In contrast, the diapsids – a large group of vertebrates that includes the dinosaurs, crocodiles, and birds – originally had two openings behind the orbits. The more familiar-looking of sabertooths were all mammals, and during the last 50 million years there were quite a lot of species of sabertooths, belonging to several unrelated families, orders, and even infraclasses of mammals. Sabertoothed predators include the following groups:
1. The gorgonopsians, an extinct group of Permian therapsids, or mammal-like reptiles.
2. Thylacosmilid sabertooths, an extinct family of South American predaceous marsupials.
3. Machaeroidine sabertooths, members of the Creodonta, an extinct order of meat-eating placental mammals related to but different from the true carnivorans (members of the order Carnivora).
4. Nimravid sabertooths, members of the extinct carnivoran family Nimravidae.
5. Barbourofelid sabertooths, members of the extinct carnivoran family Barbourofelidae.
6. Felid sabertooths, or true sabertooth cats, extinct members of the living carnivoran family Felidae.
The last three families were members of the Feliformia, a major division of the order Carnivora that includes the modern cats, civets, mongooses, and hyenas.
It is not impossible that nonsynapsid sabertooths could evolve, but it is improbable, and certainly we haven't found any in the fossil record. Carnivorous dinosaurs such as the allosaurs, as well as modern varanid lizards, have sharp, flattened and serrated teeth that resemble the namesake canines of sabertooths to a remarkable degree, but otherwise the animals are just too different to be regarded as sabertooths.
Third, all sabertooths were predators. Apart from their fearsome-looking upper fangs, the shape of the rest of the dentition of sabertooths shows that they fed almost exclusively on meat. They were "hypercarnivores," meaning that their dentitions were so specialized for cutting meat that they had lost most of their ability to deal with other food items, such as bone or vegetable matter. Many modern mammalian predators, including wolves and bears, are more versatile, with dentitions that are suited to crushing bones, crunching insects, and processing vegetable matter. These animals possess molar and premolar teeth with different shapes adapted to those functions, in addition to the scissor-like, meat-cutting teeth known as the carnassials. Other modern carnivores – including all of the cats and many members of the weasel family, or mustelids – are true hypercarnivores (figure 1.3). But sabertooths went one step further in their predatory specialization. Therapsid sabertooths were obviously predatory too, but – unlike other sabertooths – they lacked any substantial dentition behind their canines, and they would have swallowed large chunks of meat whole, in truly reptilian fashion.
Other groups of mammals have developed elongated upper canines, more or less like sabers in appearance, but their function is related to display, defense, intraspecific aggression, or the manipulation of food items, rather than the hunting of large prey. Therefore, these creatures – including the extant musk deer, chevrotains, and walruses, as well as the bizarre, vaguely rhino-like uintatheres of the American Eocene – do not qualify as sabertooths.
In the fourth place, all sabertooths share a series of morphological modifications in their skulls and skeletons. Of course, the most obvious feature is the elongation of their upper canines, and most of the other modifications are related to the use of the canines. These modifications, which can be defined collectively as the "sabertooth complex," affected articulations and muscle insertion areas in the mandible, skull, neck, back, and limbs, and they varied considerably among different saber-toothed animals. No two groups share all the features making up the sabertooth complex, but all have at least several of them in common, and in some cases the list of shared features is strikingly long.
One word that often occurs in the literature about the sabertooths is "machairodont," which means "knife-toothed" and derives from the same root as the genus name Machairodus, referring to a felid sabertooth of the Miocene. The term "machairodont" is often used in a general way for all things sabertooth and can be either a noun or an adjective, so what we call here "sabertooth features" can be also termed "machairodont features." However, the term "machairodontine" is more specifically applied to the sabertooth subfamily Machairodontinae within the cat family Felidae. The reader should be prepared to be patient with the occasional ambiguity of some scientific terms, which can easily confuse the layperson.
The fact that sabertooths were not a single group of animals, but a series of totally unrelated groups that developed similar adaptations independently, constitutes a dramatic example of a phenomenon known as convergent evolution. In spite of changing views about the precise mechanisms of evolution, there is a general consensus among specialists that the process starts within organisms, whose genomes are subject to more or less random genetic changes or mutations, and then continues as the environment allows individuals that carry beneficial or at least neutral changes to survive and reproduce, while individuals with maladaptive changes quickly die off.
Excerpted from Sabertooth by Mauricio Antón, James O. Farlow. Copyright © 2013 Mauricio Antón. Excerpted by permission of Indiana University Press.
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