Taphonomy of pigmentary colours in fossil insects

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Álvarez Armada, Nidia
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University College Cork
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Extant insects are renowned for the striking colouration displayed in their cuticles. Although fossil insects are abundant and the patternings in their cuticles highly resemble those found in extant insects, these are commonly monotonal, especially in those insects preserved across deep geological time. Colouration mechanisms in extant insects are majorly a combination of colour producing molecules called pigments and occasionally colour producing structures, which can be present in combination, but colouration mechanisms are largely unknown in their counterpart fossil taxa. Here, an experimental approach is utilized to elucidate the taphonomy of pigmentary colours in fossil insects. Decay and tumbling experiments are combined to explore environmental, biological and compositional biases constraining the potential for fossilisation of cuticles in various extant taxa. In addition, the trace elemental signatures embedded in patterned cuticles of extant taxa are characterized and compared with trace elemental signatures in fossil taxa. These analyses indicate that the fossilisation potential for insect cuticle is greatly diminished for those regions of the cuticle where the sclerotisation molecule, melanin, is not present. Environmental conditions also play an important role in the potential for fossilisation, where the increase of alkalinity in the media accelerates the process of internal decay and aids in the proliferation of decomposing bacteria, especially on those regions of the cuticle where melanin is absent. Physical decay of cuticles appears to be affected by the distribution of the pigment components throughout the cuticle; the rate of physical decay differs in cuticles with variables degrees of patterning, and those cuticles without heterogeneous distribution of pigments resist decay related to transport for longer periods of time. Trace elemental chemistry is taxonomically constrained in extant taxa at order and family levels. At genera level, these broad taxonomical trends are overprinted by a strong pigment-specific signal. The trace element chemistry in fossil insects is controlled by both taxonomy and depositional context, indicating some diagenetic overprinting. Regions of the fossilized cuticles displaying different tones contain distinct suites of elements, which suggests that there is a preservation of a pigmentary component in the chemical signal of fossil insects. Revealing biases in the fossil record of insects assist to the interpretation of fossil insect assemblages, the conditions of the environment of deposition and any biological constraints in the preservation potential of fossil insects. Trace elemental chemistry of fossil insects aids in the interpretation of original colours of fossil insects, and consequently informs on the models of evolution of colour and its ecological functions in insects through deep time.
Fossil colour , Insects , Pigments , Fossil preservation , XRF , GCMS , LCMS , Decay experiments , Transport , Fossil chemistry , Organic chemistry , Inorganic chemistry , Insect cuticle , Colour patterning , Trace elements
Álvarez Armada, N. 2020. Taphonomy of pigmentary colours in fossil insects. PhD Thesis, University College Cork.