Page 79 - Introduction to Paleobiology and The Fossil Record

66 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

disarticulation

(a)

fragmentation

(b)

abrasion

(c)

bioerosion

magnified section
(d) of shell
corrosion and
dissolution
loss of
ridges
(e) loss of thin edge
burial

flattening

(f)
diagenesis

bivalve shell
(aragonite) (calcite)
(g)
Figure 3.6 Processes of breakage and diagenesis of fossils. Dead organisms may be disarticulated (a) or
fragmented (b) by scavenging or transport, abraded (c) by physical movement, bioeroded (d) by borers,
or corroded and dissolved (e) by solution in the sediment. After burial, specimens may be fl attened (f)
by the weight of sediment above, or various forms of chemical diagenesis, such as the replacement of
aragonite by calcite (g) may take place.

ones), the energy of currents and grain size works of fine borings by chemical dissolution

of surrounding sedimentary particles (large of the calcareous shell material. This process
grains abrade skeletal elements more rapidly continues after death, and some fossil shells
than small grains), and the length of exposure are riddled with borings that may remove
to the processes of abrasion. more than half of the mineral material of any
In certain circumstances shells, bones and single specimen. Other boring organisms eat
wood may undergo bioerosion, the removal their way into logs, and heavily modify the
of skeletal materials by boring organisms such internal structure.
as sponges, algae and bivalves (Fig. 3.6d). Before and after burial, skeletal materials
Minute boring sponges and algae operate are commonly corroded and dissolved by
even while their hosts are alive, creating net- chemical action (Fig. 3.6e). The minerals