Page 75 - Introduction to Paleobiology and The Fossil Record

62 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

eralization occurs very early, probably within
hours of death, and may preserve highly labile
shelly fossils
structures such as muscle fi bers (Fig. 3.3b), as
Decay minimum maximum lignified cellulose well as more refractory tissues such as cellu-
lose and chitin. The commonest mode of min-
cellulose
eralization of soft tissues is by the formation
chitin
of mineral coats of phosphate, carbonate or
tissue imprints
pyrite, often by the action of bacteria (Box
mineralized muscle
3.1). The mineral coat preserves an exact
replica of the soft tissues that decay away
early late
Mineralization completely. The third mode of soft tissue min-
eralization is the formation of tissue casts
Figure 3.2 The relative rates of decay and during early stages of sediment compaction.
mineralization determine the kinds of tissues that Examples of tissue casts include siliceous and
may be preserved. At minimum decay rate and calcareous nodules that preserve the form
with very early mineralization, highly labile of the organism and prevent it from being
muscle tissues may be preserved. When decay flattened or dissolved.

has gone to a maximum, and when The mode of accumulation of fossils also
mineralization occurs late, all that is left are the determines the nature of fossil Lagerstätten.
non-organic tissues such as shells. (Based on Fossil assemblages may be produced by con-
Allison 1988.)
centration, the gathering together of remains
by normal processes of sedimentary transport
and sorting to form fossil-packed horizons
halted by mineralization (Fig. 3.2). In the (see p. 65), or by conservation, the fossiliza-
process of fossilization, then, it is possible to tion of plant and animal remains in ways that
think of a race between rates of decay and avoid scavenging, decay and diagenetic
rates of pre-burial mineralization: the point of destruction (Fig. 3.5). Exceptionally preserved
intersection of those rates determines the fossil assemblages are produced mainly by
quality of preservation of any particular processes of conservation. Certain sedimen-
fossil. tary regimes, in the sea or in lakes, are stag-
Early mineralization of soft tissues may be nant, where sediments are usually anoxic, and
achieved in pyrite, phosphate or carbonate, are devoid of animals that might scavenge
depending on three factors: (i) rate of burial; carcasses. In other situations, termed obru-
(ii) organic content; and (iii) salinity (Fig. tion deposits, sedimentation rates are so rapid
3.3a). Physical and chemical effects, such as that carcasses are buried virtually instantly,
these, that occur after burial, are termed dia- and this may occur in rapidly migrating river
genesis. Early diagenetic pyritization (Fig. channels or at delta fronts and other situa-
3.3b) of soft parts is favored by rapid burial, tions where mass flows of sediment are depos-

low organic content and the presence of sul- ited. Some unusual conditions of instant
fates in the sediment. Early diagenetic phos- preservation are termed conservation traps.
phatization (Fig. 3.3c) requires a low rate of These include amber, fossilized resin that
burial and a high organic content. Soft-part oozes through tree bark, and may trap insects,
preservation in carbonates (Fig. 3.3d) is and tar pits and peat beds where plants and
favored by rapid burial in organic-rich sedi- animals sink in and their carcasses may be
ments; at low salinity levels, siderite is depos- preserved nearly completely.
ited, and at high salinity levels, carbonate is
laid down in the form of calcite. In rare cases,
decay and mineralization do not occur, when Breakage and transport
the organism is instantly encased and pre- The hard parts left after scavenging and decay
served in a medium such as amber (Fig. 3.3e) have taken their toll may simply be buried

or asphalt. without further modification, or they may be
Mineralization of soft tissues occurs in broken and transported. There are several
three ways. Rarely, soft tissues may be replaced processes of breakage (Fig. 3.6), some physi-
in detail, or replicated, by phosphates. Permin- cal (disarticulation, fragmentation, abrasion)