Lipid Incorporation During Experimental Decay of Arthropods - Biopolymers: A Molecular Paleontology Approach - page 114

demonstrated that lipids are also necessary for the diagenetic alteration of leaf
components to an aliphatic composition in fossil leaves (Gupta et al. 2007d ).
Similarly, Versteegh et al. ( 2004 ) conducted heating experiments with vegetable oil
demonstrating that lipids were capable of forming aliphatic polymer abiologically.
The living arthropods investigated here do not contain any resistant non-
hydrolysable aliphatic biopolymer such as the algaenan or cutan found in some
green algae and plants. Hence, the aliphatic component has been incorporated dur-
ing decay and decomposition of the shrimp cuticle. However, such an aliphatic
component was not observed in the incubated scorpion and cockroach cuticle.
Thermodesorption of the decayed shrimp cuticle also revealed the aliphatic
component indicating that it is not an artifact of chemical pre-treatment but an
actual component of the macromolecule.
Scanning electron and general morphological analysis reveal extensive decay of
the shrimp cuticle from the early stages of the experiment, while the cockroach and
scorpion remained relatively unaltered. Decomposition of chitin and proteins in the
shrimp cuticle likely yielded sugars and amino acids. Thus the more rapid decay of
this shrimp cuticle could have generated an intermediate component that was prone
to bind lipids thereby promoting a progressive transformation to an aliphatic
composition.
The chain length of the aliphatic component of the decayed shrimp cuticle
extends up to C 24 (Fig. 6.2b, c ). The samples were pyrolysed after lipid extraction
without hydrolysis while the solvent extraction ensures that the residue we analyzed
is macromolecular (as noted in Gupta et al. 2006c ). The distribution of fatty acids in
modern shrimp ranges from C 12 to C 24 (Krzeczkowski 1970 ), including saturated
and unsaturated fatty acids (occurring as free fatty acids or bound in membrane),
consistent with the upper chain length that was found within the neoformed macro-
molecule. Thus, it is likely that lipids that are originally functionalized (e.g. unsatu-
rated and saturated fatty acids in this experiment) are a component of the
macromolecule. Further, the chain length of the aliphatic component in fossil shrimp
does not exceed C 24 (Baas et al. 1995 ; Gupta et. al. 2008b ), presumably refl ecting
the maximum chain length of the lipid available for incorporation from the organ-
ism. Analysis of 191 Da mass chromatograms did not reveal any bacterially derived
hopanoids and the n -alkyl chain length remained at C 24 during the course of the
experiment without any change in distribution, likely refl ecting the maximum chain
length of the fatty acid incorporated from the decaying carcass. Despite these obser-
vations, a minor bacterial contribution to the n -alkyl component cannot be ruled out
completely. The lack of lipid incorporation into the scorpion and cockroach cuticles
(both of which are much thicker and more robust than the shrimp cuticle) during the
course of the experiment presumably refl ects their resistance to decay as clearly
demonstrated using electron microscopy. Indeed, analysis of fossils from the
Oligocene at Enspel, Germany revealed that beetles retain a chitin-protein signature
but fl ies, which have a thinner more decay-prone cuticle, are transformed to an ali-
phatic composition (Stankiewicz et al. 1997 ). Shrimp cuticle does not have the
waxy epicuticle that is present in the cockroach and scorpion; waxes offer addi-
tional protection from microbial attack.