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 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
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
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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
‘mess’ has been the work of countless generations of field geologists.
Outcrops here, cliff-sections there, have been studied and slowly
assembled into the jigsaw that is the geological structure of the
land. As a result, it is now possible to identify rocks of Mesozoic age
(belonging to the Triassic, Jurassic, and Cretaceous Periods) with
some accuracy in any country in the world. However, that is not
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.