But the rest of the vertebrate is soft tissue (and in many organisms there are NO hard parts),
and so these are only preserved in rare instances.
Bone (like shell and wood) is not solid material, but porous. Pore space is occupied by organic
material in life. Upon death, organic material begins to decay.
In order for bones and teeth to become fossilized (turned into a fossil):
Animal must die (in the case of bones) or lose teeth
Body must be buried by sediment before decay, weathering, scavengers, etc.,
destroy the remains
The vast majority of living things wind up inside other living things (i.e., are eaten or
decayed). Only a tiny fraction are buried.
Environment of deposition becomes important. High energy environments (like river channels) bury quickly, but are
likely to destroy smaller bodies. Low energy environments (lakes, lagoons, etc.) might preserve small corpses, but are not
quick enough to bury large animals before they decay/are scavenged.
Larger bodies can be covered by rivers at flood stage:
VAST majority of fossils are broken up bones or teeth. A small fraction are complete isolated bones or teeth. A smaller
fraction still are a few bones in articulation (still connected). A very small fraction are
nearly complete skeletons.
The study of burial and fossilization is called taphonomy. There are various modes
of preservation after the bone is buried:
simple burial, some weathering. Organic material may be lost (but see below), but original hard parts
are all still present with nothing added. Relative rare in dinosaur fossils, especially as one gets further
back in time.
Recrystallization: very common in calcitic fossils, but not so common in vertebrate bone.
After burial, calcite crystals reorder and grow into each other. Original mineralogy
remains, but structure is lost.
Impressions of dinosaur skin
can form if the body was pressed into the mud before either decay set in or the mud hardened
Different organisms have different potential for fossilization:
Hard parts vs. no hard parts
Single hard parts (e.g., gastropods & cephalopods) vs. two hard parts (e.g.,
brachiopods & bivalves) vs. many well-connected parts (e.g., arthropods & echinoderms)
vs. many parts connected only by soft tissue (e.g., vertebrates)
Microscopic to sediment-sized to immense
Lived in erosive environments (e.g., mountains) vs. depositional environments
Lived in accessible vs. inaccessible environments (e.g., lowlands and continental
shelves vs. deep oceanic basins)