Molecular Preservation in Graptolites - Biopolymers: A Molecular Paleontology Approach - page 144

Chapter 9
Molecular Preservation in Graptolites
Abstract Graptolites are important fossils in the Lower Palaeozoic. Preserved
graptolite periderm consists dominantly of an aliphatic polymer, immune to base
hydrolysis. It contains no protein even though its structure, and chemical analyses
of the periderm of the living relative Rhabdopleura , indicate that it was originally
collagen. This anomaly was previously interpreted as the result of replacement by
macromolecular material from the surrounding sediment. New analyses suggest
that the aliphatic composition of graptolite periderm refl ects direct incorporation of
lipids from the organism itself by in situ polymerization.
Keywords Graptolite • Protein • Kerogen • Aliphatic • In situ polymerization
Graptolites are the dominant component within many Lower Palaeozoic fossil
planktic assemblages, and may be enormously abundant within organic-rich hemi-
pelagic mudstone facies. This, combined with their widespread distribution and
rapid evolution, accounts for their importance in biostratigraphy. Graptolites lacked
a biomineralized skeleton. Similarities to the periderm of closely related living
pterobranchs such as Rhabdopleura (Briggs et al. 1995 and references therein) and
Cephalodiscus indicate that the periderm of graptolites was composed originally of
collagen. This confi rms interpretations of its composition based on the structure of
the periderm as revealed by transmission electron microscopy (Towe and Urbanek
1972 ; Crowther and Rickards 1977 ; Crowther 1981 ).
The skeleton of graptolites often provided a locus for the precipitation of authi-
genic minerals such as clays, which may be altered to chlorite (Underwood 1992 ).
Pyrite may preserve the 3-dimensional morphology by growing as an infi ll
(Underwood and Bottrell 1994 ). In many cases, however, the graptolite periderm is
preserved as organic material (Bustin et al. 1989 ; Briggs et al. 1995 ), and graptolites
can be released from carbonates by dissolving the matrix. Carbonaceous traces of
the stolon of graptoloid graptolites have been reported (Loydell et al. 2004 ) but
evidence of the morphology of the zooids rarely survives and relies on authigenic
mineralization (Rickards et al. 1991 ).