Page 73 - Introduction to Paleobiology and The Fossil Record

60 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Table 3.1 Mineralized materials in protists, plants, and animals. The commonest occurrences are
indicated with XX, and lesser occurrences with X.
Aragonite Inorganic Organic

Carbonates Calcite Phosphates Silica Iron Chitin Cellulose Collagen Keratin
oxides
Prokaryotes XX X X X X
Algae XX XX X X XX
Higher plants X X X XX
Protozoa XX XX XX X X
Fungi X X X XX XX
Porifera X XX XX X XX
Cnidaria XX XX X X
Bryozoa XX XX X XX X
Brachiopoda XX XX XX X
Mollusca XX X X X X X X
Annelida XX XX XX X X XX
Arthropoda XX XX X X XX X
Echinodermata XX X X XX
Chordata X XX X X XX XX

ing carbon and oxygen to carbon dioxide and labile (likely to decay early) to highly decay-
water, according to this equation: resistant. Most soft parts of animals are made
from volatiles, forms of carbon that have
CH 2 O + O 2 → CO 2 + H 2 O molecular structures that break down readily.
Other organic carbons, termed refractories,
Microbial decay can also take place in anaer- are much less liable to break down, and these
obic conditions, that is, in the absence of include many plant tissues, such as cellulose.
oxygen, and in these cases nitrate, manganese The normal end result of scavenging and
dioxide, iron oxide or sulfate ions are neces- decay processes is a plant or animal carcass
sary to allow the decay to occur. stripped of all soft parts. In rare cases, some
The second set of factors controlling decay, of the soft tissues may survive, and these are
temperature and pH, may be the most impor- examples of exceptional preservation.
tant. High temperatures promote rapid decay.
Decay proceeds at normal high rates when the
pH is neutral, as is the case in most sediments, Exceptional preservation
because this creates ideal conditions for micro- There are many famous examples of excep-
bial respiration. Decay is slowed down by tional preservation (Table 3.2). Certain fossil-
conditions of unusual pH, such as those found bearing formations of different ages, termed
in peat swamps, which are acidic. Fossils pre- Lagerstätten, have produced hundreds of
served in peat or lignite (brown coal) may be remarkable fossil specimens, and in some
tanned, like leather, and many of the soft cases soft parts are preserved. In the most
tissues are preserved. Examples are the famous spectacular cases, soft tissues such as muscle,
Neolithic and younger “bog bodies” of north- which is composed of labile forms of organic
ern Europe, in which the skin and internal carbon, may be preserved. Usually, however,
organs are preserved, and silicifi ed fossils in only the rather more decay-resistant soft
the lignite of the Geiseltal deposit in Germany tissues, such as chitin and cellulose, are fos-
(Eocene) that show muscle fi bers and skin. silized. Plant and animal tissues decay in a
Decay depends, thirdly, on the nature of the sequence that depends on their volatile
organic carbon, which varies from highly content, and the process of decay can only be