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The interpretation of fossil eggshells can be problematic because eggshells may be transported by hydraulic flow in floodplains, making it difficult to interpret the reproductive behavior and ecology of parent animals. A series of flume studies was conducted to establish analytical techniques for assessing eggshell hydraulic transport in the fossil record. We investigated preferred eggshell orientation after transport, the relationship of flow competence with eggshell height and volume, and the size of clastic sediment expected to be associated with transported eggshells. Goose, emu, and ostrich eggshell fragments were released in a flume with decelerating flow. The transport of each eggshell was observed five times on each of four substrates (coarse sand, sparse gravel, dense gravel, and polyvinyl chloride). At eggshell deposition, eggshell orientation and flow depths were recorded. Critical bed shear stress for eggshell deposition was estimated based on the flow depth at the point of eggshell deposition, tested relative to eggshell height and volume, and used to estimate the size of hydraulically equivalent particles. The probability of concave-down orientation after transport was > 85% regardless of eggshell type or substrate. The bed shear stress at eggshell deposition reflected the eggshell height and volume. The estimated size of hydraulically equivalent particles was coarse sand or larger. A high proportion of concave-down eggshells in a fossil assemblage may indicate transport. In addition, eggshells may be sorted according to their height and volume. Coarse sand or larger particles observed in a matrix of fossil eggshells may suggest eggshell transport.
Parental care is found in all extant archosaurs (crocodylians and birds) and parsimony suggests this behavior is homologous. There are known ‘parent atop eggs’ fossils of nonavian theropod dinosaurs (ancestors to birds), but no equivalent fossil for crocodylians has been reported yet within this context. Here we present a remarkable fossil of an adult crocodylian (Diplocynodon darwini) preserved in situ with eggs from the middle Eocene of Geiseltal, Germany, providing the first-reported evidence for the antiquity of parental care in the crocodylian lineage. The degree of articulation, the unusual curled posture of the adult, the position of the eggs, and the surrounding sediment indicate the adult may have died atop its nest after oviposition. Size relationships between the adult and eggs are consistent with values from modern related taxa and no other crocodylian was found within 12 m of the eggs. Thorough documentation of fossil vertebrate skeletons collected near the fossil crocodylian does not indicate flow influenced its curled posture. Despite being sexually mature, the adult crocodylian did not exhibit full fusion of the neurocentral sutures, an indicator of immaturity in nonavian archosaurs. Even in a paratropic environment, temperatures may temporarily drop below cold tolerance for warm-adapted crocodylians, possibly explaining the death of the adult and the young inside the eggs. Although still indicating egg attendance, the fossil may alternatively indicate the mother died from dystocia (egg-binding) during oviposition, which would be to our knowledge the first published record of this rare phenomenon in a fossil archosaur.
Paleontologists routinely study fossils using high-magnification and high-resolution backscattered electron (BSE) images acquired via scanning electron microscopy (SEM). In BSE imaging, contrast corresponds to differences in backscattering of primary electrons and BSE detection among points in the electron beam raster scan. In general, BSE images are known for compositional contrast corresponding to backscattering monotonically related to average atomic number. However, two other types of contrast are relevant to BSE-SEM of fossils: (1) topographic contrast corresponding to backscattering and BSE detection varying with specimen shape and (2) mass-thickness contrast corresponding to backscattering varying with the relative masses and thicknesses of materials in the uppermost few microns of a sample. Here, we demonstrate the significance of these contrast mechanisms for resolving three-dimensional and subsurficial features of fossils. First, we show—through study of mass-thickness contrast in BSE images of carbonaceous compressions from the Triassic Solite Quarry Lagerstätte (Virginia)—that some tissues (e.g., leaf and insect wing veins) are preserved as thicker carbonaceous films than others (e.g., leaf laminae and insect wing membranes), possibly reflecting taphonomic differences among anatomical tissues. Second, we show that the problematic phosphatic shelly fossil Sphenothallus (lower Cambrian, China) is covered by low-relief transverse ribs and made up of exteriorly sculptured and interiorly unsculptured carbon- and phosphorus-rich layers with microstructures. Taking advantage of both topographic and mass-thickness contrast mechanisms, these case studies demonstrate that BSE imaging elucidates morphological details that are not obvious in surficial light microscopy or secondary-electron SEM and are otherwise only evident via tomography.
Exceptionally preserved, phosphatized gastropod egg capsules from the uppermost Jurassic (upper Volgian) in Central Russia are reported. The egg capsules were attached to the inner side of the shell wall of empty body chambers of two ammonites. Due to phosphatization, the egg capsules retained their original morphology preserving both the lower attachment base and upper hemispherical cover. Comparison with recent and fossil gastropod egg capsules indicates that these were not produced by neritimorphs, the egg capsules of which are known from the Lower Jurassic and Upper Cretaceous. Since these fossil egg capsules share many similarities with those produced by some recent rissoids and muricid neogastropods, it is evident that they were produced by some representatives of Caenogastropoda. The abundant filamentous microbial structures, as well as micron-sized apatite globules, preserved inside the egg capsules suggest that phosphatization processes were mediated by microbial consortia under anaerobic and low-pH conditions.
The stratigraphic record of shallow-water shoreline paleoenvironments is characterized by significant facies heterogeneity and laterally discontinuous stratal geometries. In contrast, we investigate a uniquely extensive and microstratigraphically spatially uniform interval of upper Famennian (Upper Devonian) marginal marine strata in the Rocky Mountains. This transgressive deposit (< 5 m thick) rests on a depositional sequence boundary, and is composed of a thin, discontinuous basal transgressive sandstone bed, two thin fossiliferous shale beds, and several meters of oncolite-bearing carbonate wackestone. The lower shale is a consistently thin (~ 0.1 m) bed with a fossil fauna of spinicaudatans (clam shrimp or conchostracans), which are extant, bivalved, chitinous, benthic crustaceans that live in fresh to brackish water, in ephemeral ponds, estuaries, and other shoreline settings. Isotopic data for the oncolitic unit record deposition in an epicontinental seaway with restricted circulation and locally brackish conditions. Sulfur isotope data may also reflect short-term changes in redox conditions, consistent with our interpretation of temporary hypoxia during deposition of the spinicaudatan bed, based on its faunal assemblages. The remarkably large areal extent (~ 1600 km north to south, and ~ 1000 km east to west) of this uniformly thin interval with marginal marine fauna is one of the most unusual paleoecological events of the latest Devonian in Laurentia. It is considered an artifact of exceptional depositional processes including passive transgression (i.e., little or no wave or tidal ravinement) along with rapid opportunistic takeover of habitats during transgression. The radiation, possibly associated with temporary hypoxia, was aided by the spinicaudatan reproductive strategy of numerous offspring, high growth rate, and rapid reproduction.