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The fidelity of preservation of Insects from the Crato Formation (Lower Cretaceous) ofBrazil

Student thesis: Doctoral Thesis

  • Nathan Barling
The Early Cretaceous Nova Olinda Member (Crato Formation) of Brazil boasts the most exceptionally well-preserved non-amber Mesozoic fossil insects. In this project these insect fossils are comprehensively studied. Their fidelity of preservation is investigated, the mechanisms that allowed for it are modelled, and the palaeoenvironments that they lived in are hypothesised.

The Nova Olinda Member fossil insects have a broad range of preservational fidelities. At their lowest-fidelity, they are fragmented low-relief ‘scrappy’ traces. At their highest-fidelity, they are complete, fully-articulated, high-relief specimens with submicron-scale replication of both external and internal morphology. Cuticular structures (setae, scales, ommatidia, etc.) are sometimes replicated to the submicron-scale via nano-crystalline impregnation of the epicuticle. Internal labile soft-tissues (genitals, guts, tracheal system, etc.) are replicated with high fidelity by globular encrustations and impregnations. The remaining tissues are obliterated by pseudomorphed pseudoframboids (or pseudoframboid-like aggregates), which also protected the carcass from compaction. Globular/granular fabrics generated by decay are proposed based on their consistent occurrence with particularly decay-prone tissues. Artefacts of preparation/curation (cracks, punctures, etc.) are distinguished based on their occurrence without associated mineralogical fabrics.

Statistical analyses are undertaken to quantify the preservational fidelity of the Nova Olinda Member fossil insects and identify taphonomic trends. Although the collection analysed is biased towards members of Orthoptera and Blattodea, results indicate that taxonomy has no control over preservational fidelity. As no other Lagerstätten has been quantitively analysed in this manner, qualitative comparisons are undertaken. Eleven other Lagerstätten are examined, revealing that none yielded fossils as well-preserved as the Nova Olinda Member. In all cases where fossils are examined, they either have their micron-scale morphology obliterated by coarse mineral growth, are compacted to compressions, or are encrusted by irremovable microfossils.

The chemistry and preservational fabrics of the Nova Olinda Member fossil insects are analysed to determine the mechanisms of their exceptional preservation. The fossils are largely preserved in goethite, which pseudomorphs pyrite. The pyrite has two distinct fabrics: a nano-crystalline impregnation of the epicuticle and a coarser pseudoframboidal infill of the remaining carcass. Precipitation of both of these fabrics was stimulated by the metabolic activities of a sulphate reducing microbial mat, with the framboids only forming once pyrite had reached supersaturation. The globular encrustations/impregnations of internal labile soft tissues are a result of apatite precipitation brought about by ‘minor’ decay. This decay created ‘active sites’ for mineralisation and liberated ions which, combined with dissolved porewater ions, allowed for calcium phosphate precipitation. These fabrics were deposited in a specific order that prevented compaction and allowed for submicron-scale preservation of tissues: Firstly, internal labile tissues were impregnated and encrusted in apatite. Secondly, the epicuticle was impregnated by nano-crystalline pyrite. Thirdly, pyrite overgrew/obliterated the remaining tissues as pseudoframboids. This process is presented in a novel multi-step preservational model.

To understand the biases and controls on the preservation of these fossil insects, their environmental preferences (and modes-of-life where applicable) are comprehensively analysed and compared to modern relatives. The modern environmental preference of each family (or order) reported from the Nova Olinda Member are grouped into simplified environments and tabulated. The data are analysed and three hypothetical regions proposed in a novel palaeoenvironmental reconstruction. These include: an arid scrubland around the palaeolake, a humid fluvio-deltaic region, and a distant forested region. Hypothetical ecological niches are also reconstructed, with xerophytic plants and arthropods, as well as burrowing arthropods, living in the arid scrubland, aquatic/semi-aquatic arthropods living in marginal ‘tongues’ of freshwater at the palaeoshore, and a further fluvial/deltaic freshwater niche inhabited by aquatic/semi-aquatic/riparian plants and arthropods. These reconstructions reveal that the articulated insect fossils were transported to the site of deposition alive or shortly after death by seasonal small-scale flash-floods (caused by seasonal rains). These flashflood events allowed for insects to be preserved indiscriminate of taxon, size, robustness, or mode-of-life. Finally, a new taxon of fossil wasp is described and its systematics discussed.

Aims:
1. To investigate the fidelity of preservation of the Nova Olinda Member fossil insects by examining which tissues are preserved, quantifying how well-preserved they are, identifying if any taxonomic trends are present (using statistical analyses), and qualitatively comparing them to other Lagerstätten.
2. To determine the replacing minerals and their fabrics through elemental, chemical, and textural analyses, as well as produce a model for the process of fossilisation.
3. To investigate the taxonomic diversity of the Nova Olinda Member insect fauna and present a novel palaeoenvironmental reconstruction based on modern insect preferences.

Scope:
As this project is multidisciplinary, clear boundaries are established for each discipline to maintain achievable goals.
1. Entomology: Insects are the most diverse macroscopic organisms and entomology, particularly its systematics, is an extremely broad and dynamic topic, and cannot be examined comprehensively in this project. General insect anatomy and the diversity and ecology of several families are examined in this thesis. The systematics of Hymenoptera are explored in more detail during the description of a new taxa.
2. Taphonomy: Taphonomic analysis is restricted to controls affecting insects (and other terrestrial arthropods) in laminated limestones and is discussed in generalised terms rather than the specifics of each specimen.
3. Statistics: Taphonomic data is subject to explorative cluster and principal coordinate analyses to determine trends between insect groups or ecological roles. Both R-mode and Q-mode cluster analyses are undertaken.
4. Geochemistry: Iron sulphide geochemistry is described, including controls and phase pathways, although is restricted to the minerals and fabrics observed in Nova Olinda Member insect fossils.
5. Sedimentology: Descriptions of sedimentological features identified during this project are restricted to the laminated limestones of the Nova Olinda Member. Only brief descriptions of other well-known Lagerstätten are otherwise presented here, based on published data.
6. Palaeontology: Palaeontology is inherently multidisciplinary, and this project combines several other aspects of palaeontology. Entomological, taphonomic, and sedimentological data are examined to produce palaeoenvironmental and ecological reconstructions.
7. Energy-dispersive X-ray spectroscopy: Energy-dispersive X-ray analyses in this project have specific limitations. Prior agreements required most specimens be double Au-Pd coated. In some cases, this results in partially obscured spectra, with C also excluded from most spectra due to the use of a C filament. To preserve insect morphology, no specimens are polished, leading to topographic artefacts in some analyses. Other analyses are undertaken under time constraints, resulting in low counts. Finally, only Nova Olinda Member fossils are subject to energy-dispersive X-ray analysis, as examining the eleven other fossil Lagerstätten is beyond the scope of this project. Instead, mineralogical identifications in other Lagerstätten are based on published data.

Contribution:
Several key contributions to knowledge are made during this project:
1. Recording and presenting examples of exceptional preservation of Nova Olinda Member fossil insects, highlighting the preservation of tissues that are previously unreported on both the macro- and micro-scale. (Published in Cretaceous Research).
2. Recording and explaining the various mineralogical textures that replace Nova Olinda Member fossil insect tissues.
3. Creating a novel six-step taphonomic model for the mineralisation of Nova Olinda Member insects based on chemical analyses, textural observations, and the current understanding of iron sulphide geochemistry.
4. Presenting a novel adaptation of the ‘resin-transfer technique’, allowing its use for three-dimensional arthropod fossils in semi-porous carbonate sediments.
5. Creating a multi-stage transport model for the Nova Olinda Member insects, including different palaeoenvironments, different starting conditions, and how these affect an insect carcass.
6. Presenting a revised palaeoenvironmental reconstruction of the Nova Olinda Member hinterland, including the identification of multiple environments and seasonal climatic
conditions, supported by entomological, taphonomic, and sedimentological data.
7. Describing a new taxon of fossil wasp (Hymenoptera). (Published in Cretaceous Research).
Original languageEnglish
Supervisors/Advisors
Award dateAug 2018

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