Friday, 30 October 2015

A World In The Tar Pits: California before Humans

California is known for many things; Hollywood, Silicon valley, and more recently unprecedented droughts. What it is less known for however, at least to the general public, is the vast diversity of life that once lived there. Located in California lie the La Brea tar pits, which are, as the name suggests, a group of tar pits, that have been trapping animals for the last 40,000 years, giving us an incredibly detailed view into the paleofauna of the region. Perhaps the best site for late Pleistocene animals in the world, it certainly contains some of the most spectacular creatures. From giant ground sloths and saber toothed cats to mammoths and mastodons, prehistoric California had one of the most diverse megafaunal assemblages in the world, far outmatching that of even Africa, both today and in the Pleistocene. In this post we will be travelling back 11,000 years, shortly after the end of the ice age, and right before the arrival of the first humans.

Paleoenvironment of La Brea, by Charles Knight. The animals in the foreground are the sabertooth Smilodon, Harlan's ground sloth, and the giant vulture Teratornis. In the background can be seen a herd of Columbian mammoths. 

The Americas, north and south, had the richest megafaunas in the world. California was in many ways one of the crowning jewels, its fauna impressive even when compared to the rest of the continent. While Europe and southern Asia only have 1 endemic species of elephant each, the Asian elephant (Elephas maximus) and European elephant (Elephas antiquus) respectively, La Brea preserves two Proboscidean genera, the American mastodon (Mammut americanum), and the Columbian mammoth (Mammuthus columbi). Of these, only the latter was actually a true elephant, while the former belonged to the group mammutidae, an early off-shoot of the elephant lineage. Interestingly, the two had quite different ecologies. While the Columbian mammoth was a mixed grazer and browser, much like the extant elephants, the American mastodon was a specialized browser, and would probably have been less common in the arid conditions of California. As the largest animals in their environment, adults would have been almost entirely free from predation, but their young may have been major prey items of the many carnivores of the time. 

The second largest animals living in California were the ground sloths. Extant sloths are small and relatively sluggish animals, quite innocuous in appearance and behavior, but their recently extinct relatives were anything but. The largest of the ground sloths were larger than modern day elephants, and while the species found in La Brea were not quite that large, the biggest, Jefferson's ground sloth, was still larger than a rhinceros, and even the smallest, the Shasta ground sloth, was still the size of a Brown bear. They may have filled ecological niches similar to those taken by the aforementioned rhinos in the old world, as the Americas were oddly devoid of any rhinoceros species during the Pleistocene, the last having died out in the late Miocene. Together, the sloths and proboscideans would have been some of the most important keystone species in their habitat, ecosystem engineers capable of exerting great force upon their environments. They would have opened up dense tree stands, but also fertilized the ground and helped spread seeds, resulting in more abundant but disparate woodlands. 

Broad-headed bison, Bison latifrons, by Roman Uchytel

While elephants and giant sloths may be particularly impressive, they would not have been the most abundant animals in pre-human California. As in Africa, the bulk of the region's megafauna would have consisted of comparatively smaller grazers, which would have formed great herds and congregations. In many respects, prehistoric California, like most of the ancient world, would have resembled the great wilderlands of Africa in many significant respects. Instead of antelopes, the Americas have pronghorns, which today persist through only a single species, but were represented by 12 different species back in the late Pleistocene. In the place of zebras were 2 separate species of horses, giraffes were replaced by giant camels, wildebeests by bison, and hippos by tapirs. All of these groups still survive today in some regions of the world, but the great diversity of lineages has been lost. While there now is only 1 species of bison in North America, the appropriately named American bison (Bison bison), La Brea preserves 2, the enormous, long-horned Bison latifrons, and the "ancient bison", Bison antiquus, the ancestor of the American Bison, and therefore technically still extant. 

There were 2 species of horses in California, the Mexican horse (Equus conversidens), and the Western horse (Equus occidentalis). Similar in size and shape to extant wild equines, they would probably have lived equally similar lifestyles. As with zebras today and tarpans before their extinction, the horses of California were one of the most common prey items for many predators, due to their relative lack of defenses compared to other, more imposing herbivores. While the original American horses are now completely extinct, the genus Equus was unintentionally reintroduced by the Spanish in the 17th century, and persist to today. These animals are commonly known as Mustangs, and while they are indeed wild living, they are most commonly referred to as "feral" animals due to their domestic ancestry, arbitrary as this definition may be. 

Smilodon and Columbian mammoths, by Mauricio Anton

The last aspect of prehistoric California that must be touched on is the predators. As with its herbivore guild, ancient California had an unusually high diversity of carnivores. There were several animals that have persisted to today, such as the Grey wolf, Coyote and Cougar. Even these however differed from the ones today, and were generally larger than their modern counterparts, as competition was greater and prey far more plentiful. Probably the most famous predator found in La Brea was the sabertooth Smilodon, specifically the species S. fatalis. A large and robust animal, it exceeded any extant big cats in size, and seems to have been by far one of the most abundant and successful predators. It was not however the largest, that title goes to the American lion. Far larger than extant African lions, it dwarfed even Smilodon, and would have been the unrivaled apex predator of its environment. Despite the huge size difference however, it was nevertheless the same species as those still found in the old world, Panthera leo. The difference in scale may have been a result of Bergman's Rule, which says that animals in colder regions typically have larger body size than equivalent species further south.

Another sabertooth inhabited California 11,000 years ago, namely the Scimitar cat of the genus Homotherium. Slightly smaller but more gracile than Smilodon, it seems to have been more of a pursuit predator, capable of hunting faster prey than its slower relatives. Given its smaller size and more diminutive nature however, it would probably have been a frequent victim of kill-stealing by other, more powerful predators. The last big cat that must be mentioned when discussing Pleistocene North America is Miracinonyx, commonly called the American cheetah. Despite its name, it was not in fact a true cheetah at all, nor even a particularly close relative, but instead a case of convergent evolution, where two lineages evolve down similar paths.

Miracinonyx hunting an extinct species of Pronghorn, also by Mauricio Anton

The ancient world revealed in the La Brea tar pits is gone now, but from a geological perspective only very recently. Every single species currently inhabiting the regions around the tar pits once shared their habitats with the now extinct giants, and some even show signs of having co-evolved with them. The pronghorn is by far the fastest animal in North America, far quicker than the speediest extant predator, the wolf. They are not however too fast to escape the extinct American cheetah. Perpetually running from the ghosts of vanished predators, these relics of ancient ecological interactions give us a glimpse at the past, in a way perhaps more real than any bones we could dig up. They show us that the past was indeed a real place, that these animals really were alive once, and that they played an important role in the ecosystems of their time. As a last note to end the story of the tar pits, around 10,000 years ago, bones of two new animals appear in the tar. Humans and domestic Dogs. Shortly after, the bones of Sabertooths, Ground sloths, Mastodons, and all of the other magnificent giants suddenly disappear. Climate change played no role here, the true perpetrator is obvious.

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