A hundred years after the first excavations, the asphalt pools at Rancho La Brea, in Los Angeles, are still filling in details of life in the Pleistocene.
To a parched mammoth or giant sloth, they must have looked heaven-sent：mirror pools of water waiting to slake a thirst. Heaven, however, was not their progenitor, for the pools were traps. Below their siren surfaces lay not mud, but tar. For thousands of years they lured thirsty animals, and also hungry ones - carnivores attracted by the calls of those already stuck - to their doom.
Rancho La Brea (Tar Farm) as the plot became known millennia later when Hispanic colonists arrived, thus turned into the best source of fossils of its period yet discovered. The site, now engulfed by Los Angeles, has yielded millions of bones from dozens of vertebrate species, and also hundreds of species of invertebrates, since mass excavations began in 1913. And a century later, on October 30th, the Society of Vertebrate Palaeontology met in the city to discuss, among other things, the latest findings from the tar.
In geological terms, La Brea's fossils are recent. The oldest dates from 38,000 years ago, and the pits continued their lethal work until the city grew around them in the 20th century, so some are very recent indeed. Those of greatest interest, though, are of species that succumbed in a blip of extinctions about 12,000 years ago, when human beings arrived in the Americas. Why them, the question is, and not others?
Before people arrived, for example, the area that is now called the United States supported six species of cat. Now it supports but one, known, according to taste, as the cougar, puma or mountain lion. Ryan Haupt of the University of Wyoming and Larisa DeSantis of Vanderbilt University told the meeting that this selective extinction is probably explained by what the different animals ate. Picky eaters, in a parable worthy of Aesop, died out whereas the less fussy cougar lived on.
Mr Haupt and Dr DeSantis came to this conclusion by studying the microscopic patterns of wear on the teeth of fossil cougars, American lions and sabre-tooths (the last two being among the species that did not survive humanity's arrival). They knew, from looking at the teeth of modern animals such as hyenas, cheetahs and African lions, that meals consumed during the last few weeks of an animal's life leave telltale marks behind.
In modern cheetahs, which are exceptionally picky (they eat only tender meat and rarely use their slender teeth to break bones), those marks are smooth and linear. In hyenas, which consume pretty much everything, bones included, they are jagged and deep, and in African lions, which eat mostly tender meat, but occasionally break bones, they are intermediate. The researchers also knew from Dr DeSantis's earlier work that American lions had wear patterns like those of cheetahs whereas sabre-tooths had patterns indicating only a little more consumption of bone.
Cougars from La Brea were a mixed bunch. Some did, indeed, seem to have been finicky. But others were bone-crunchers of hyena-like proportions (and much more so than modern cougars). Clearly, they could adapt to circumstances, and that seems to have been what happened.
It is unlikely that people exterminated the other cats directly. Hunting such animals would have been suicidally brave. But humans did exterminate lots of herbivores that such cats would have preyed on. An ability to make maximum use of what remained probably explains the cougar's survival and others' demise.
How carnivores caught their prey is also shown by their bones, as Mairin Balisi and Caitlin Brown of the University of California, Los Angeles, explained to the meeting. They have constructed, by examining numerous specimens, maps of the injuries sustained by two extinct carnivores abundant in the tar - sabre-tooths and a large canid called the dire wolf - and thus illuminated the animals' hunting techniques.
Dire wolves, as Ms Balisi and Ms Brown show, took most punishment to their skulls. They interpret this as being because the wolves got kicked in the head by their prey at the end of a long chase when those prey had been cornered.
Most sabre-tooths injuries, by contrast, are to the lower back. This supports the idea that sabre-tooths ambushed their prey, catching them by surprise and leap on them. It is hard to kick the head of a cat weighing a quarter of a tonne if it has its paws on your back and is trying to drag you to the ground. On the other hand, the amount of twisting involved in doing this dragging would often have been injurious to a sabre-tooth's spine.
It was hunting technique, too, that explains one of tar pits' curiosities. As might be expected, lots of avian scavengers have been dug out of them：some 720 vultures, for example. But these are outnumbered by owls, which hunt only live prey and of which 1,103 have so far been found. That puzzled Kenneth Campbell, of the Los Angeles County Natural History Museum, because most owls hunt by night, when the tar has cooled and formed an un-sticky crust.
Closer analysis showed, however, that the two most common owls in the pits were the short-eared and the burrowing owls, both of which hunt by day. The third-most-common, the greater horned owl, though nocturnal, pounces from the air with such force that it would easily penetrate any crust, and thus come to a sad and sticky end - entombed for millennia until the palaeo-ornithologists of the 21st century disinterred it.
