Philip R. Wilby

The role of the calcium carbonate-calcium phosphate switch in the mineralization of soft-bodied fossils

Authigenic minerals play an important role in the preservation of most soft-bodied fossils. The greatest detail is preserved in apatite (calcium phosphate) but its precipitation is usually inhibited by the high concentrations of HCO3- in aqueous settings. Nonetheless, investigations of soft-bodied biotas have revealed very early authigenic calcite crystal bundles in close association with phosphatized soft-tissues. This demonstrates that the geochemical controls on soft-tissue mineralization are dynamic and act on a very local scale. Direct comparisons with experimental results permit the conditions of fossilization to be inferred.

Phosphatization of soft-tissue in experiments and fossils

Soft-tissues phosphatized in laboratory experiments closely resemble fossil phosphatized soft-tissues, indicating that similar processes were involved. The smaller the aggregations of calcium phosphate particles precipitated the greater the fidelity of morphological preservation. The highest fidelity occurs where the bacteria themselves are not replicated even though precipitation is bacterially induced. While extensive phosphatization of larger carcasses, however, may necessitate the build-up of concentrations in the sediment beforehand, this is not the case for phosphatization of small quantities of soft-tissue. Mineralization of soft-tissue in the laboratory is not ‘instant’ but may take several weeks, or even months if decay is inhibited. The precipitation of associated calcium carbonate is controlled by shifts in pH in response to the decay process.

Role of microbial mats in the fossilization of soft tissues

It has been speculated that microbial mats are an important agent in the fossilization of soft tissues, particularly when apatite (Ca 5 [CO 3 , PO 4 ] 3 [OH, F]) is involved. This has been tested by chemical analyses of the Jurassic limestones of Cerin, France, where phosphatized soft tissues are abundant and are associated with unequivocal microbial mats. The sedimentary distribution of P, K, and Fe following deposition was controlled by the presence of the mats. P concentrations in the mats may approach 2.5 times those elsewhere in the sediment. The highest P concentrations correlate with the occurrence of phosphatized soft tissues. This is the first analytical evidence to demonstrate a fundamental role for microbial mats in the preservation of soft-bodied fossils.

Direct chemical evidence for eumelanin pigment from the Jurassic period

Melanin is a ubiquitous biological pigment found in bacteria, fungi, plants, and animals. It has a diverse range of ecological and biochemical functions, including display, evasion, photoprotection, detoxification, and metal scavenging. To date, evidence of melanin in fossil organisms has relied entirely on indirect morphological and chemical analyses. Here, we apply direct chemical techniques to categorically demonstrate the preservation of eumelanin in two > 160 Ma Jurassic cephalopod ink sacs and to confirm its chemical similarity to the ink of the modern cephalopod, Sepia officinalis. Identification and characterization of degradation-resistant melanin may provide insights into its diverse roles in ancient organisms.

Mineralization of soft-bodied invertebrates in a Jurassic metalliferous deposit

The Jurassic marine biota of La Voulte-sur-Rhone, France, is characterized by three-dimensional preservation of soft-bodied animals and their internal organs. In contrast to other soft-bodied fossils, those from La Voulte are preserved in an unusual suite of minerals, dominated by apatite, calcite, gypsum, barite, and pyrite with accessory Cu, Pb, and Zn sulfides. These are thought to be sedimentary in origin. Fossilization occurred rapidly enough to be influenced by tissue composition and involved a diagenetic sequence: apatite → calcite ± gypsum → pyrite ± chalcopyrite → galena. The La Voulte fossils reveal the role of apatite as a “template” for calcification and pyritization in soft-tissue preservation.

A rich Ediacaran assemblage from eastern Avalonia: Evidence of early widespread diversity in the deep ocean

The Avalon Assemblage (Ediacaran, late Neoproterozoic) provides some of the oldest evidence of diverse macroscopic life and underpins current understanding of the early evolution of epibenthic communities. However, its overall diversity and provincial variability are poorly constrained and are based largely on biotas preserved in Newfoundland, Canada. We report coeval high-diversity biotas from Charnwood Forest, UK, which share at least 60% of their genera in common with ones in Newfoundland. This indicates that substantial taxonomic exchange took place between different regions of Avalonia, probably facilitated by ocean currents, and suggests that a diverse deepwater biota may already have been widespread at the time. Contrasts in the relative abundance of prostrate versus erect taxa likely record differential sensitivity to physical environmental parameters (hydrodynamic regime, substrate) and highlight their significance in controlling community structure.

Ubiquitous Burgess Shale–style “clay templates” in low-grade metamorphic mudrocks

Despite the Burgess Shale’s (British Columbia, Canada) paleobiological importance, there is little consensus regarding its taphonomy. Its organic fossils are preserved as compressions associated with phyllosilicate fi lms (“clay templates”). Debate focuses on whether these templates were fundamental in exceptional preservation or if they formed in metamorphism, meaning that it is important to establish the timing of their formation relative to decay. An early diagenetic origin has been proposed based on anatomy-specifi c variations in their composition, purportedly refl ecting contrasts in decay. However, we demonstrate that these films bear a remarkable similarity to those that occur on organic fossils in graptolitic mudrocks and form as a normal product of low-grade metamorphism. Such phyllosilicates may also occur within voids created by volume loss in maturation, a process that may have aided their formation. In bedding-plane assemblages from graptolitic mudrocks, different taxa are associated with distinct phyllosilicates. This likely refl ects stepwise maturation of their constituent kerogens in an evolving hydrothermal fl uid, with different hyllosilicates forming as each taxon progressively underwent maturation. These observations provide an analogue for the distribution and composition of phyllosilicates on Burgess Shale fossils, which we interpret as refl ecting variations in the maturation of their constituent tissues. Thus, their clay templates seem unremarkable, forming too late to account for exceptional preservation.

Fossil fish stomachs: A microenvironment for exceptional preservation

Exceptionally preserved fossil fish from the Cretaceous of north east Brazil contain well preserved arthropods and small fish in the alimentary tract. The fish are preserved in early diagenetic concretions and often retain their original three dimensional morphology. A significant number of specimens have soft tissues preserved, including part of the alimentary tract, musculature and gills. In many specimens the stomach retains elements of the fishes’ last meal, including a variety of smaller fishes and arthropods. Although many are disarticulated, a significant number of specimens within the gut remain articulated. Many specimens display exceptional anatomical detail despite having been devoured by predators. Although arthropods can be found outside of the body cavity of the fish, such specimens are not usually so well preserved.

Syntectonic monazite in low-grade mudrocks: a potential geochronometer for cleavage formation?

Monazite paragenesis during low-grade metamorphism is poorly understood. The first unequivocal instance of the mineral having crystallized synchronously with compressional cleavage formation is reported. Compositionally, these monazites resemble diagenetic forms but differ texturally by displaying syntectonic microfabrics. Their predominant association with organic-walled fossils implies that organic material may play an important role in controlling the behaviour of light rare earth elements (LREE) during low-grade metamorphism, probably by releasing diagenetically sequestered LREE and organically bound phosphorus during thermal maturation (devolatilization), and/or by promoting a favourable environment for monazite precipitation. Potentially, such monazites could provide a powerful geochronometer for cleavage formation.

Stratigraphical and palaeoecological importance of Caradoc (Upper Ordovician) graptolites from the Cardigan area, southwest Wales

Graptolites from more than 60 horizons in the basinal Caradoc succession of southwest Wales, between Fishguard and Cardigan, allow recognition of the multidens , clingani and linearis biozones. The biostratigraphy permits recognition of major differences in the sedimentary rock-sequence north and south of structures associated with the Fishguard–Cardigan Fault Belt. The Penyraber Mudstone Formation, disconformably overlying the Fishguard Volcanic Group (Llanvirn), is partly of multidens Biozone age. It is succeeded south of the Newport Sands Fault by the Cwm yr Eglwys Mudstone Formation of clingani to linearis biozones age. North of the fault the Cwm yr Eglwys Mudstone Formation is replaced laterally by the northwards-thickening, sandstone turbidite-dominated Dinas Island Formation ( clingani and linearis biozones). Graptolite stratigraphical distribution indicates that Dicranograptus clingani occurs only rarely within the caudatus Subzone of the clingani Biozone and that Climacograptus antiquus s.l . also does not range above the lower part of the clingani Biozone. The first occurrence of Dicellograptus morrisi , within the upper clingani Biozone, confirms its value as a marker for the morrisi Subzone, and is associated with the first occurrences of Diplacanthograptus dorotheus and Normalograptus minimus . Dicellograptus flexuosus , used to indicate the morrisi Subzone elsewhere, occurs throughout the clingani Biozone in the Cardigan area. The linearis Biozone is recognized by Climacograptus tubuliferus . Oxic bottom conditions in early and early mid-Caradoc times largely precluded the influx of, or preservation of, graptolite faunas in the Penyraber Mudstone Formation. Anoxic mudstones of the Cwm yr Eglwys Mudstone and Dinas Island formations preserve graptolite assemblages of 21 and 26 species, signalling strong open marine influences which persisted in this area until late Caradoc times. This contrasts with the shelfal faunas in the Whitland area (south Pembrokeshire), where the late Caradoc is dominated by low-diversity Normalograptus- dominated assemblages.