Tucked away in a suburb of Los Angeles, amongst the sprawl of a modern concrete and steel infrastructure, is a link with North America’s prehistoric past. Rancho la Brea is one of the most famous fossil locations in the world – and with good reason. An astonishing one million bones have been recovered from the tar pits since 1906 representing an incredible 231 vertebrate species (“Brea” is Spanish for tar). The age of these specimens’ span thousands of years but the oldest specimens can be carbon dated to around the fifty thousand year old mark with a margin of error of around five thousand years either way.
Carnivores make up the vast majority of animals recovered. Over 4000 specimens of dire wolves, 2000 specimens of sabre-toothed cats head the carnivorous throng and there are also bears, foxes, coyotes, weasels, badgers and, of course, Panthera atrox – the American lion. The most obvious question is why did so many carnivores become trapped in the tar?
Rancho la Brea is generally considered to be a predator trap. Unsuspecting animals such as mastodons, elephants, horses, bison and antelope may have become mired in the tar pools and their cries of distress, in turn, would have attracted predators eager for an easy meal but actually found themselves to also be trapped in the sticky tar. An interesting aside to this is that a lot of the carnivores are very young, very old or crippled and the sabre-tooths, in particular, often display chronic pathologies such as fused vertebrae around the pelvic region. This could be interpreted that such injured, old or inexperienced animals were drawn to the apparent easy pickings of the tar pools with often fatal consequences.
Although a predator trap makes perfect sense in this instance, there is still no hard and fast evidence to back it up. And yet all the circumstantial evidence screams that it is a trap and, for me, it is almost certainly the most sensible explanation (I use the present tense because even today the tar is still claiming unsuspecting victims). Possible predator traps are not confined to recent times, however, and they have often been suggested to explain concentrations of carnivores in the Mesozoic. Chief amongst these is the Cleveland-Lloyd Dinosaur Quarry (CLDQ).
The CLDQ is located 30 miles south of Price, Utah and was first worked in 1929 by a field crew from the University of Utah. The quarry has been incredibly productive ever since and over 12,000 bones have been recovered and, like Ranch la Brea, the predominant fossils recovered are those of carnivores – especially Allosaurus fragilis. Over 70% of all the bones here are from this one taxon alone (Foster 2007) and Gates (2005) suggests at least 46 individual allosaurs are represented although that number is sure to increase (if it has not already done so).
The initial thoughts, again, were that animals became mired, this time in mud at a dwindling water hole, attracting large numbers of carnivores who in turn became, themselves, trapped. Of course this appears the most obvious explanation and, coincidentally, around 80% of the Allosaurus individuals are juveniles, again mirroring a lot of the fauna at Ranch la Brea. Unfortunately, these things are never that simple.
The bones are found in a mudstone of the Brushy Basin Member of the Morrison Formation and are indicative of a floodplain ephemeral-pond deposition. They are found in a horizontal or semi-horizontal position which is of interest since similar quarries, such as the Howe Quarry, have exposed limb bones in an upright position which is highly suggestive of a miring scenario.
Another proposal to explain the unusually high proportion of predators in the quarry is that it was a drought-induced assemblage – a theory proposed by Terry Gates (2005). At some water holes today, during times of drought, it is a documented fact that, in Africa, predators have been known to prevail over the dwindling water source safe in the knowledge that any herbivore approaching the hole for a drink was almost certainly desperate, near death or just plain mad.
There is a possibility that the allosaurs may have behaved in a similar fashion. Like today’s extant predators, they may have been reluctant to leave their only source of water and, if the prey animals had all been consumed in the immediate area, the carnivores would certainly have been susceptible to starvation or disease. This could also account for the high percentage of juveniles in the quarry since they would have been more vulnerable.
Similarly, there is the suggestion that animals that died around the dwindling water hole would have been scavenged by the predators who in turn would have succumbed to disease, especially botulism, since rotting carcasses are full of such nasties. In such a scenario, large numbers of allosaurs could have easily died (eg see Varricchio 1995).
Of course there are counter arguments. It is very difficult to recognise drought and disease in the fossil record. There is the thought that, in a situation such as this, that carcasses may have been spread over a much wider area – it’s hard to believe that so many animals would congregate in a relatively small area and there are sauropod remains in this quarry. But if it was a dwindling water source then why not?
Other things of note are predation marks on the bones – not only on prey animals but on the allosaur bones themselves. Foster (2007) also reports that many foot bones are chewed by carnivores but this is not so easily explained. So there are still many questions to be answered about the CLDQ regardless of what theory you subscribe to. Of course there are other examples of quarries with a high predator prey ratio such as the Albertosaurus bone bed (featured many times in this blog) and Ghost Ranch but they too attract comparable discussion and their taphonomic origins remain debatable.
Foster, J.R. 2007. Jurassic West: The Dinosaurs of the Morrison Formation and Their World: pp 93-95. Indiana University Press, Bloomington, Indiana.
Gates, T.A. 2005. The Late Jurassic Cleveland-Lloyd Dinosaur Quarry as a drought-induced assemblage. Palaios 20:363-375.
Varricchio, D.J. 1995. Taphonomy of Jack's Birthday Site, a diverse dinosaur bonebed from the Upper Cretaceous Two Medicine Formation of Montana. Palaeogeography, Palaeoclimatology, Palaeoecology 114:297-323