Page 76 - Introduction to Paleobiology and The Fossil Record

TAPHONOMY AND THE QUALITY OF THE FOSSIL RECORD 63

preservation
of soft parts in
pyritization carbonate
high

(d)
(b) Rate of
burial

low phosphatization
low high
Organic
(a) content

(e) (c)
Figure 3.3 The conditions for exceptional preservation. (a) The rate of burial and organic content are
key controls on the nature of mineralization of organic matter in fossils. Pyritization (high rate of
burial, low organic content) may preserve entirely soft-bodied worms, as in an example from the Early
Devonian Hunsrückschiefer of Germany (b). Phosphatization (low rate of burial, high organic content)
may preserve trilobite limbs such as this example of Agnostides from the Cambrian of Sweden (c). Soft
parts may be preserved in carbonate (high rate of burial, high organic content), such as polyps in a
colonial coral, Favosites, from the Early Silurian of Canada (d). If decay never starts, small animals
may be preserved organically and without loss of material, such as a fly in amber from the Early

Tertiary of the Baltic region (e). (a, based on Allison 1988; b, courtesy of Phil Wilby; c–e, courtesy of
Derek Briggs.)

and some chemical (bioerosion, corrosion and noids, where the ligaments holding the sepa-
dissolution). rate skeletal elements together decay rapidly.
Skeletons that are made from several parts In trilobites and vertebrates, normal aerobic
may become disarticulated, separated into or anaerobic bacterial decay may take weeks
their component parts. For example, the mul- or months to remove all connective tissues.
tielement skeletons of armored worms and Skeletons may also become fragmented,
vertebrates may be broken up by scavengers that is, individual shells, bones or pieces of
and by wave and current activity on the seabed woody tissue break up into smaller pieces
(Fig. 3.6a). Disarticulation happens only after (Fig. 3.6b), usually along lines of weakness.
the scavenging or decay of connective tissues Fragmentation may be caused by predators
that hold the skeleton together. This may and scavengers such as hyenas that break
occur within a few hours in the case of cri- bones, or such as crabs that use their claws to