Fearful scimitars

Sabertooth by Mauricio Anton (2013) Bloomington. Indiana University Press. 243 pp.

This is the natural history of an ecological niche: an ambush predator that captures prey by a throw and pin, with powerful forelimbs and a long, strong back, followed by a bite to the neck, using greatly elongated canine teeth, made possible by jaw adaptations that permit a huge gape and a long, strong neck. The bite results in bleed out and death of the victim. It is argued that this leads to quicker and safer kills than the suffocation method employed by modern big cats on large prey. The author does a remarkable job of tracing the paleontology of successive groups of species that have occupied this role since the Permian, although there seem to have been no dinosaurs that did so.

The illustrations of fossils, reconstructed animals and landscapes are beautifully done, in loving detail. The painstaking anatomical analyses to show how these killing machines worked are clear and persuasive, at least to a non-expert. There are interesting accounts of the constantly changing nomenclature of the fossils.

It would be useful to see similar accounts of modern large cats, hyaenids and other carnivores to get a sense of how the sabertooths fit into the big picture of carnivore evolution and why it is they went extinct instead of the others.

The extinction happened several times, as the successive groups of sabretooths disappeared, and they were not quickly replaced. Sometimes millions of years seem to have elapsed before a new lineage appeared to fill the niche. Indeed, sometimes there were no large felids of any kind for extended periods, if we can trust the fossil record. At other times, a new group may have out-competed the current occupants of the sabertooth niche. One might say that there is room at the top of the food web, but it is a precarious position that isn’t always filled.

Perhaps the most recent occupants of the niche went extinct with the megafauna of the old and new worlds, but could they have been done in by more efficient pantherids? Was their technique only suitable for very specific types of prey that got replaced by more wary and evasive herbivores or did vegetation change doom their hunting methods? Did their very specialized anatomy and techniques simply run out of room for improvement against ever more challenging prey? Anton thinks that their very specialized niche may have left them relatively more vulnerable to shifts in prey abundance. This would be a major factor in the late Pleistocene, along with competition from lions and humans, among others. As the least flexible group, they may have been the first to go.

Does it mean there’s an unoccupied niche now, or are those habitats and resources simply gone?

What other convergences in form and behavior has natural selection produced across time and biogeographic realms? We all know at least a few ecological equivalents, like pangolins and armadillos, or moles, marsupial moles and mole crickets. There’s an interesting one involving modern beetles’ and early rodents’ mandibles, but it isn’t clear what the functional significance is (John Acorn in American Entomologist, Summer 2014, p 128). Thanks to Mauricio Anton for presenting this story so beautifully.

Image: Smilodon fatalis – National Park Service

New world history

A Natural History of the New World. Ecology and Evolution of Plants in the Americas, by Alan Graham. 2011. Chicago. University of Chicago Press. 387 pp.
This is an ecological and evolutionary story acted on the stage of two continents from the close of the Mesozoic to the recent. South America starts out isolated or nearly so, while North America begins joined to Eurasia across the proto-Atlantic. It ends with two continents joined by a narrow isthmus and a sporadic connection to Siberia across the Bering Sea. During this time as the Americas override the Pacific plates, a series of great mountain ranges form along the western edges of both continents, altering the directions of rivers and radically altering the climate of the continental interiors. Late in the period, the shift towards glacial climates turns what were temperate climates under a polar insolation regime into boreal forest and tundra, with deciduous forests to the south and new dry ecosystems in the arid west.
It is a dramatic story, with a shifting cast of characters, most impressively the higher angiosperms and the radiating mammals responding to each other as well as struggling among themselves to dominate under the shifting conditions. The rise of groups like the grasses and the ungulates with their associated carnivores are among the most visible and dramatic developments, if not quantitatively as significant as the insects and fungi, which changed much less over the same time. This is a vexed question; megafauna and keystone species enthusiasts on the side of top-down regulation and ecosystem engineering, those of us who look at energy flux and nutrient cycles as keys to ecological processes and who see microbes as the dominant force, alongside plants, on the bottom-up side. Probably both views are right some of the time. No doubt, though, that climate and geology – lithosphere, atmosphere, hydrosphere – are the ultimate regulators, although the biosphere’s impact on carbon cycling is also significant.
This is a very detailed book, giving an account of dozens of types of communities both in the past and the present vegetation of the Americas. It also describes the phases of development step by step, tracking the geologic changes and the shifting vegetation as revealed by pollen and macro fossils. The author is a noted paleobotanist at the Missouri Botanic Garden. There are good photos, maps and graphs of changing temperatures over the epoch. Truly a history of nature.
Graham also has excellent chapters on the techniques of paleoclimate reconstruction and the collection and interpretation of fossils. The text is also a wonderful travelogue, full of historical and prehistorical anecdotes, and covering the modern biogeography of the new world as well as the story of how it came to be as it is.