"Stick 'n' peel": Explaining unusual patterns of disarticulation and loss of completeness in fossil vertebrates

Few fossil vertebrate skeletons are complete and fully articulated. Various taphonomic processes reduce the skeletal fidelity of decaying carcasses, the effects of most of which are reasonably well understood. Some fossil vertebrates, however, exhibit patterns of disarticulation and loss of completeness that are difficult to explain. Such skeletons are one of two variants. They are incomplete, often markedly so, but the preserved parts are highly articulated. Alternatively, they are complete, or nearly so, but articulation varies markedly between parts of the body. A characteristic feature is the absence of skeletal elements that, on the basis of their larger size and/or greater density, would be predicted to be present. Here we erect a model, termed “stick ‘n’ peel”, that explains how these distinctive patterns originate. The model emphasizes the role of decay products, especially fluids released from the carcass while resting on the sediment surface. These fluids permeate the sediment below and around the carcass. As a result, skeletal elements on the downward facing side of the carcass become adhered to the sediment surface, and are less likely to be remobilized as a result of current activity than others. The pattern of articulation and, especially, completeness is thus not what would be predicted on the basis of the size, shape and density of the skeletal elements. The effects of stick ‘n’ peel are difficult to predict a priori. Stick ‘n’ peel has been identified in vertebrate fossils in lacustrine and marine settings and is likely to be a common feature of the taphonomic history of many vertebrate assemblages. Specimens becoming adhered to the substrate may also explain the preservation in situ of the multi-element skeletons of invertebrates such as echinoderms, and integumentary structures such as hair and feathers in exceptionally preserved fossils

Taphonomic variation within a Middle Triassic fossil lagerstätte (Cassina beds, Meride Limestone) at Monte San Giorgio

Middle Triassic sediments at Monte San Giorgio, a UNESCO heritage site on the Switzerland-Italy border, have famously sourced well-preserved vertebrate fossils. Taphonomic studies of reptiles in the Besano Formation and in the Cava inferiore, Cava superiore and Cassina beds of the Meride Limestone, and fish in the Besano Formation have already elucidated changes in palaeoenvironment through time in sediments that predominantly comprise black shale and carbonate lithologies. Saurichthys skeletons from the Ladinian Cassina beds were scored for articulation and completeness, with the resulting data sorted to test for variation among layers and lithologies in the 2.7-m thick section. The greatest abundance of skeletons was found in finely laminated sediments, a reduced number in event bed sediments and an absence in volcanic ash deposits. No particular bed showed better or worse preservation. The final state of a skeleton entering the fossil record was the result of the mode of deposition and rate of sedimentation with the additional influence of features at the sediment surface, notably microbial mat growth and its ability to stick carcasses to substrate. Keywords Saurichthys · Middle Triassic · Meride Limestone · Taphonomy · Microbial ma

Taphonomic analysis of Saurichthys from two stratigraphic horizons in the Middle Triassic of Monte San Giorgio, Switzerland

Excavations of Middle Triassic strata at Monte San Giorgio (Switzerland–Italy) have famously recovered a range of reptiles whose affinities, morphology and preservation have been extensively described. The locality has also yielded a contemporaneous and equally diverse fish fauna that has yet to be described to the same degree. To address this imbalance, a taphonomic study was undertaken using the actinopterygian Saurichthys, a relatively abundant taxon found in the Besano Formation (latest Anisian– earliest Ladinian) and Cassina Beds of the Meride Formation (early Ladinian). Specimens from each horizon were scored for articulation and completeness across ten anatomical units, the resulting datasets being used to determine a taphonomic model and investigate preservational variation through time. Saurichthys showing moderate to high articulation and high completeness occur in both horizons but states of low articulation and moderate completeness are only present in the Besano Formation. The same feature is apparent in corresponding unit plots, suggesting different environmental conditions were present during deposition of each horizon, specifically those that reduced articulation and completeness during a prolonged residence on the sediment surface

A rediscovered Lower Jurassic ichthyosaur skeleton possibly from the Strawberry Bank Lagerstätte, Somerset, UK

An almost complete ichthyosaur from the historically significant collection of fossils amassed by Somerset geologist, Charles Moore, is described. Available information suggests that it was collected from the Lower Lias of Somerset, UK. However, inspection of the surrounding matrix, which seems to be a nodule, indicates that the specimen may instead hail from the Upper Lias (Toarcian) Strawberry Bank Lagerstätte of Ilminster, Somerset. The specimen had previously been identified as Leptonectes tenuirostris. Herein, further analysis of the skull and forefin, reveals closer affinities to Stenopterygius triscissus, to which the specimen is tentatively assigned. Therefore, the specimen represents one of the most complete ichthyosaurs collected from the Strawberry Bank Lagerstätte

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“Stick ‘n’ peel”: how unusual patterns of disarticulation and loss of completeness in fossil vertebrates originate as a result of carcasses adhering to the substrate during decay

International Meeting on Taphonomy and Fossilization (8º. 2017. Viena)The taphonomic histories of fossil vertebrate skeletons can be both complex and difficult to resolve, even if only examples from exceptional biotas (Konservat Lagerstätten) are considered. Typically, the fidelity with which skeletons are preserved in exceptional biotas is “good to excellent” - even in cases where the non-biomineralised tissues have decayed completely. Crucially, it is, however, unusual that for any biota as a whole, all skeletons are complete and fully articulated. At least a minority, - and often the majority - of taxa within an assemblage show some loss of completeness and articulation.UCD School of Earth Sciences, University College Dublin, IrlandaPaläontologisches Institut und Museum der Universität, SuizaMuseo Geominero, Instituto Geológico y Minero de España, EspañaSchool of Biological, Earth and Environmental Science, University College Cork, IrlandaSouth African Heritage Resources Agency, Archaeology, Palaeontology and Meteorites Unit, SudáfricaPeer reviewe

“Stick ‘n’ peel”: Explaining unusual patterns of disarticulation and loss of completeness in fossil vertebrates

Few fossil vertebrate skeletons are complete and fully articulated. Various taphonomic processes reduce the skeletal fidelity of decaying carcasses, the effects of most of which are reasonably well understood. Some fossil vertebrates, however, exhibit patterns of disarticulation and loss of completeness that are difficult to explain. Such skeletons are one of two variants. They are incomplete, often markedly so, but the preserved parts are highly articulated. Alternatively, they are complete, or nearly so, but articulation varies markedly between parts of the body. A characteristic feature is the absence of skeletal elements that, on the basis of their larger size and/or greater density, would be predicted to be present. Here we erect a model, termed “stick ‘n’ peel”, that explains how these distinctive patterns originate. The model emphasizes the role of decay products, especially fluids released from the carcass while resting on the sediment surface. These fluids permeate the sediment below and around the carcass. As a result, skeletal elements on the downward facing side of the carcass become adhered to the sediment surface, and are less likely to be remobilized as a result of current activity than others. The pattern of articulation and, especially, completeness is thus not what would be predicted on the basis of the size, shape and density of the skeletal elements. The effects of stick ‘n’ peel are difficult to predict a priori. Stick ‘n’ peel has been identified in vertebrate fossils in lacustrine and marine settings and is likely to be a common feature of the taphonomic history of many vertebrate assemblages. Specimens becoming adhered to the substrate may also explain the preservation in situ of the multi-element skeletons of invertebrates such as echinoderms, and integumentary structures such as hair and feathers in exceptionally preserved fossils