Why dinosaur fossils are rare

It is important, at the outset, for the reader to realize that the fossil record is incomplete and, perhaps more worryingly, decidedly patchy. The incompleteness is a product of the process of fossilization. Dinosaurs were all land-living (terrestrial) animals, which poses particular problems. To appreciate this, it is necessary first to consider the case of a shelled creature living in the sea, such as an oyster. In the shallow seas where oysters live today, their fossilization potential is quite high. They are living on, or attached to, the seabed and are subjected to a constant 'drizzle' of small particles (sediment), including decaying planktonic organisms, silt or mud, and sand grains. If an oyster should die, its soft tissues would rot or be scavenged by other organisms quite quickly and its hard shell would be gradually buried under fine sediment. Once buried, the shell has the potential to become a fossil as it becomes trapped under an increasingly thick layer of sediment. Over thousands or millions of years, the sediment in which the shell was

5. The meat-eating dinosaur Herrerasaurus

buried is gradually compressed to form a silty sandstone, and this may become cemented or lithified (literally, turned to stone) by the deposition of calcium carbonate (calcite) or silica (chert/flint) carried through the fabric of the rock by percolating water. For the fossil remains of the original oyster to be discovered, the deeply buried rock would need to be lifted, by earth movements, to form dry land, and then subjected to the normal processes of weathering and erosion.

Land-living creatures, by contrast, have a far lower probability of becoming fossilized. Any animal dying on land is likely, of course, to have its soft, fleshy remains scavenged and recycled; however, for such a creature to be preserved as a fossil it would need to be subject to some form of burial. In rare circumstances creatures may be buried rapidly in drifting dune sand, a mud-slide, under volcanic ash, or some by other catastrophic event. However, in the majority M of cases the remains of land animals need to be washed into a | nearby stream or river, and eventually find their way into a lake or £ seabed where the process of slow burial, leading to fossilization, can commence. In simple, probabilistic terms the pathway to fossilization for any land creature is that much longer, and fraught with greater hazard. Many animals that die on land are scavenged and their remains become entirely scattered so that even their hard parts are recycled into the biosphere; others have their skeletons scattered, so that only broken fragments actually complete the path to eventual burial, leaving tantalizing glimpses of creatures; only very rarely will major parts, or even whole skeletons, be preserved in their entirety.

So, logic dictates that dinosaur skeletons (as with any land-living animal) should be extremely rare and so they are, despite the impression sometimes given by the media.

The discovery of dinosaurs and their appearance within the fossil record is also a decidedly patchy business, for rather mundane reasons. Fossil preservation is, as we have just come to appreciate, a chance-laden, rather than design-driven, process. The discovery of fossils is similarly serendipitous in the sense that outcrops of rocks are not neatly arranged like the pages of a book to be sampled perhaps in sequence, or as fancy takes us.

The relatively brittle surface layers of the Earth (its crust, in geological terms) have been buckled, torn, and crumpled by huge geological forces acting over tens or hundreds of millions of years that have wrenched landmasses apart or crushed them together. As a result, the geological strata containing fossils have been broken, thrown up, and frequently destroyed completely by the process of erosion throughout geological time, and further confused by later periods of renewed sedimentation. What we, as palaeontologists, are left with is an extremely complex 'battlefield', pitted, cratered, and broken in a bewildering variety of ways. Disentangling this o

'mess' has been the work of countless generations of field geologists. 0 Outcrops here, cliff-sections there, have been studied and slowly | assembled into the jigsaw that is the geological structure of the Inland. As a result, it is now possible to identify rocks of Mesozoic age 1 (belonging to the Triassic, Jurassic, and Cretaceous Periods) with t some accuracy in any country in the world. However, that is not S sufficient to aid the search for dinosaurs. It is also necessary to disregard Mesozoic rocks laid down on the sea floor, such as the thick chalk deposits of the Cretaceous and the abundant limestones of the Jurassic. The best types of rocks to search in for dinosaur fossils are those that were laid down as shallow coastal or estuarine environments; these might have trapped the odd, bloated carcasses of land-living creatures washed out to sea. But best of all are river and lake sediments, environments that were physically much closer to the source of land creatures.

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