and partially created by deliberate digging for bones. It had been realised for a long time prior to the twentieth century that there were bones in considerable numbers present in the tar pits, but they had been assumed to be those of stray cattle that had wandered in to the tar and become stuck. It is true that animals wandered onto the unsupportive and sticky surface, making the mistake of thinking it was a watering hole, what had not been realised was that these wandering animals were not generally domesticated cattle. It would have been an easy mistake to make for wildlife to imagine the surface was solid as it would accumulate twigs and leaves and form pools on the surface when it rains. This would also attract night flying aquatic insects, such as water beetles that would similarly become trapped in the sticky tar. This is a well‐known phenomenon, where newly made roads with an apparently wet surface will attract night flying aquatic insects when there is a bright moon.
Once in the tar, escape is extremely difficult for the trapped animal, even if they were to escape, the mammalian behaviour of licking fur would render the animal sick due to the toxic compounds in the tar. It is similarly thought that the large numbers of carnivores which are present are due to them being attracted by the plight of the struggling trapped animal. The two predominating carnivorous species that were trapped in La Brea tar were the Sabre Toothed Cat, Smilodon fatalis (Figure 1.1), which is the second commonest skeletal remains of any sort recovered from the tar pits, and the Dire Wolf, Canis dirus. There are more than 400 skulls of Canis dirus on display at La Brea which have been recovered from the tar.
The lack of preserved soft parts in the tar pits has allowed speculation regarding the coat colour of species of Smilodon. They have been represented as plain‐coated or spotted, either of which would be possible. The coat colour of modern felids seems to be broadly dependent on the preferred terrain in which they live, but since there are exceptions to this, it becomes impossible to be sure of the coat in these species.
The formation of these tar deposits starts with a natural seepage of oil from underground reservoirs, as it reaches the surface, the lighter fractions evaporate or are used as an energy supply by some of the microorganisms present, leaving the heavy tar behind. Long after it was known that tar pits contained animal remains, it was not understood why only the skeletons remain. Part of the answer is quite prosaic, it seems that it takes a long time for the corpse to sink, quite long enough for decay to take a considerable hold on the soft parts. Besides this, the tar has residual solvents in it which disrupt the lipids in the body. Lipids are a group of organic molecules less related to each other by their chemical structure as by them being soluble in non‐polar solvents such as benzene and ether. The other major cellular components, proteins, are not soluble in non‐polar solvents, and it is these that would be decayed by fungi and bacteria or scavenged by small insect such as flies.
Figure 1.1 Smilodon fatalis (californicus) skull from La Brea Asphalt, Upper Pleistocene Rancho La Brea tar pits, Los Angeles, California, USA. Staining due to the tar renders the bone permanently discoloured.
Source: Photo. James St John, Creative Commons, generic.
Although preservation of ancient material, plant or animal, by encapsulation can result in very high resolution remains, the most usual way of thinking about preserved remains is as inclusions within rock. This process requires considerable changes in chemical structure and composition, a process described as taphonomy. The final outcome of taphonomy in the most frequently considered situations of fossilisation may appear to be the same, that is leaving a permanent record set in stone, but it takes little time investigating various fossils to see that the mineral nature of fossils can be radically different. This is most notably so when comparing fossils from different areas, as the colours vary quite widely. These colour variations reflect different mineral compositions within the final product of fossilisation, which of course is a reflection of the mineral composition of the rock in which the fossil was formed.
In broad terms and very simple terms, fossilisation resulting in a stone product requires rapid sedimentation of material which will eventually bind in a cement‐like fashion to become rock. The details can, of course, vary enormously from site to site, but in broad terms it always starts with sedimentation. This is one of the reasons that it is generally considered that fossilisation only takes place in shallow seas, lakes or shallow slow rivers and very wet swamp land.
Slow rivers and swamp land are often associated with floodplains, which also accumulate remains washed down stream and silt to cover them. The converse conditions are not so conducive, that is, fossilisation would not normally take place in dry, arid, conditions. This inevitably has some implication for the types of fossils which are most frequently found. Aquatic species will naturally form the bulk of fossilised material, but all species need water to drink and watering holes that attract grazing livestock also attract carnivores, both to drink and as an easy way to gain access to prey species.
A large part of the reason that fossils are not ‘dry found’ is that although mummification through desiccation is an excellent way of preserving mortal remains, we have to consider the length of time they can survive. The longevity of a mineralised fossil plays very well when considered against survival of mummified remains that are simply desiccated. In dry conditions, scavengers and recycling organisms will be active in breaking down bodies that contain nutrients and valuable mineral resources. Even when desiccated, there are a number of invertebrates that can use the material as a source of food. Assuming that burial takes place and the situation is one in which decay and scavenging do not occur, there is still a major long‐term problem of physical stability. In dry conditions, there will generally only be loose compaction of the overlying material, and this implies that there is insufficient mechanical stability of the substrate to guarantee survival of mummified remains. Shifting substrates can be a problem with standard models of fossilisation, unless the fossil is rapidly compacted and incorporated as part of the rock. By contrast, dry mummified remains in loose material, although dry and preserved, will be shifted about and abraded very quickly to dust by the surrounding material. This is very much associated with the time scales which have to be considered when thinking about the age of fossils and the aeons over which they have survived. Taking time to try and comprehend these immense time scales when compared to the age of mankind and associated civilisations is worth the effort, even though it is extremely difficult to appreciate the length of time involved.
It has been suggested that by comparing modern ecosystems with the fossil record, it may be possible to determine the biodiversity and species numbers in extinct ecosystems. By making a range of assumptions, based on ecosystem complexity, it is also possible to estimate the rate at which organisms leave visible traces. Modern studies would indicate that comparable ecosystems, such as rainforests in South America and West Africa, have the same broad biomass divided up into the same numbers of species and individual organisms. By implication, it would seem reasonable that comparable extinct ecosystems would have comparable numbers of species and comparable sizes of populations to modern ecosystems. Needless to say, the species would be radically different, but there would still be primary producers and an energy pyramid leading to the apex predators. Using these broad assumptions we can estimate that, depending upon conditions, anything from 0 to 70% of an ecology can become fossilised, with an average of 30% of biota leaving a trace of some kind. It is a very wide range, which is a reflection of the uncertainty of these sorts of estimates. From these numbers it should be self‐evident that most organisms don't leave any trace at all. If there was little or no recycling of organic remains, in our modern forests and woodlands we would, for example, be wading through the annually discarded antlers of deer.
One of the reasons for this low fossilisation rate, besides the unsuitable terrain for the process to take place, is the recycling of biological
