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We introduce new terminology and a new classification scheme for describing tetrapod tail traces, focusing on the interpretation of dinosaur tail traces. Our classification divides tail traces into (1) tail impressions—there is no evidence of forward motion; (2) protracted tail traces—they persist for at least one stride length; and (3) abbreviated tail traces—they persist for less than one stride length. Protracted tail traces are simple or compound, based on the amount of interruption of the tail trace, which we quantify by the percent interruption metric (PIM). Abbreviated tail traces are also simple or compound. Classifications are modified further by sinuosity, which we describe as low or high. The PIM approximates vertical tail motion, and sinuosity approximates lateral tail motion. Sediment variations, preservation, and lateral motion resulting from locomotion must be taken into consideration when interpreting tail traces. This new classification scheme is applied to a partial theropod trackway with associated tail trace from the Upper Jurassic Morrison Formation, Bighorn Basin, Wyoming, United States. The tail trace is protracted and simple with low sinuosity and a low PIM; we interpret this as the result of relatively low tail motion. We hypothesize that significant differences exist between ornithopod and theropod tail trace patterns. We also suggest that protracted tail traces associated with bipedal dinosaur trackways are not the result of the use of the tail as a stabilizing third leg; some may represent incidental contact of the sediment by the tail owing to backward rotation about the pelvis during deceleration.
Death assemblages from contemporary marginal marine settings carved into ancient shell deposits are composed of fossil shells exhumed by currents or tides and shells derived from living populations. A better understanding of the bias produced by such a mixing process is of interest for studies that use modern death assemblages as analogues of similar past habitats. In order to evaluate the magnitude of reworking and redeposition of fossil shells in modern environments, a taxonomic (composition, abundance, and richness) and taphonomic (taphofacies) study was carried out in the Mar Chiquita coastal lagoon, Argentina (37°40′S, 57°20′W). The nature and extent of reworking was explored along a gradient in tidal energy from the outer to the inner reaches of the coastal lagoon. Results indicate that modern death assemblages in the lagoon are composed mostly of fossil (late Holocene) reworked shells and that reworking varies along a gradient in tidal energy, being higher in the outer reaches of the coastal lagoon, where tidal action is more significant. Temporal mixing in the coastal lagoon appears to be associated with condensation (remanié) rather than with a subtle mixing of shells, as occurs in time-averaged deposits. This reworking process leads to an abundance of old shells in modern death assemblages, which has negative consequences for their utilization as modern analogues of past lagoons. Multidisciplinary studies involving various biological indicators need to take this type of bias into consideration in order to avoid erroneous inferences on the Quaternary evolution of coastal lagoons.
A substantial complication to using the oxygen isotope composition (δ18O) of vertebrate bioapatite in paleoclimate studies is the need to distinguish variation due to temporal changes in the δ18O of surface waters from that due to temperature-dependent fractionation during biomineralization. One solution is multiple-taxon comparisons using data from coexisting homeothermic and heterothermic animals. Fossil emydid turtles have been suggested to be potentially useful as functional homeotherms because (1) modern emydids employ behaviors, such as basking, to restrict skeletal growth to a narrow temperature range; (2) their aquatic habitat constrains the isotopic variability of dietary inputs; and (3) emydids have a dense fossil record. But because turtles lack teeth and therefore tooth enamel, sampling must focus on bone, which is potentially more susceptible to diagenetic alteration. This study examines the δ18O of carbonate (δ18Oc) and phosphate (δ18Op) in hydroxylapatite from co-occurring emydids and heterotherms (crocodilians and gars) from the Paleocene–Eocene of the Clarks Fork Basin, Wyoming. Previous isotopic studies of this area provide an extensive data set for comparison with the results of this study. Bone and enamel δ18Oc values measured here exhibit a greater range (16‰–32‰ Vienna Standard Mean Ocean Water) than previously observed, suggesting alteration, while the range of δ18Op values (9‰–15‰) is within that predicted by presumably unaltered mammalian tooth enamel δ18Oc. While high crystallinity indices (0.28–0.55) and a lack of covariation between δ18Oc and δ18Op suggest alteration of one or both of these constituents, a strong correlation between crocodilian enamel and bone δ18Op suggests bone phosphate may be reliable.1
The Rancho La Brea tar pits represent a collection of Pleistocene fossils from an unusual sedimentary environment. A taphonomic analysis of a single tar seep, Pit 91, reveals a complex history of deposition and diagenesis for specimens found there. Radiometric dating of 46 bones from Pit 91 documents at least two episodes of deposition, one from 45,000 to 35,000 yr and another, shorter interval from 26,500 to 23,000 yr. Interestingly, the law of superposition was not upheld consistently in this case study, as some younger bones were found at a greater depth than older bones, implying that taphonomic time averaging took place. Bones are distributed as disarticulated elements in two large concentrations that span both depositional episodes. In general, long bones are oriented horizontally, with little or no preference for cardinal orientation. Degree of weathering or abrasion is not correlated with depth. Bone-on-bone contact (pit wear), however, increases with depth, suggesting possible compaction of bones through time. These results, combined with the disarticulation common to nearly all recovered specimens, suggest a postentrapment journey for the bones unique to asphalt deposits.
Analysis of Middle Triassic data indicates that biogeography influences sample distributions, whereas depositional environment and stratigraphic position play secondary roles in governing sample patterns. During this time, taxa differed among biogeographic realms, while the general ecology remained the same: epifaunal benthos— pedunculate and epibyssate suspension feeders—dominate Middle Triassic samples much as they did in the Early Triassic. In contrast, Late Triassic data prove to be more complex in terms of ecology compared to Middle Triassic. Here, guild structure dictates the faunal patterns in addition to biogeographic realm and stratigraphic position, and an overall increase of infaunal life habits occurs—burrowing suspension and deposit feeders increase. Although diversity after mass extinction began to recover at the Early-Middle Triassic boundary, our results indicate that ecology remained stable through the Middle Triassic until the more modern life habits (e.g., infaunalization) increased in the Late Triassic. We conclude that the taxonomic and ecological differences among Late Triassic geographic regions recorded the initiation of a more mobile and infaunal life habit indicative of a modern lifestyle. Our results also indicate that this modernization did not necessarily unfold simultaneously and in coordinated fashion within regions and throughout time. Instead, details of guild expansion or stability may be region specific.
Microscopic morphologic variations of Epiphyton thalli in microbial buildups were examined in order to detail controlling factors on morphology and calcification processes, and their implications for identification of calcified microbes. Microbial carbonate of the Zhangxia Formation (Middle Cambrian), Shandong Province, China comprises thrombolites, stromatolites, and Epiphyton buildups, as well as consortia of microbial and metazoan communities. Epiphyton, a rigid framework of the buildups, is subdivided into four types based on characteristics of the branches. Type 1 consists of ∼75-μm-diameter dendritic rods of dense micrite that form bush-shaped and chambered thalli. Type 2 has ∼80-μm-thick branches characterized by transverse segments. Type 3 consists of thin, ∼50-μm diameter micritic branches that form round thalli. Type 4 is characterized by laterally arrayed, branching tubes that form fan-shaped thalli. All morphologic types have bipartite branched filaments as a basic growth pattern. Bush-shaped thalli are dominant in the inner part of the Epiphyton bioherms, while chambered thalli, which are solitary, connected, and tiered, are common in the outer part. Such physical energy conditions as currents or waves control the density, length, and sheath thickness of the branches and propagation pattern of the thalli. Chamber outlines of Epiphyton thalli resemble those of Renalcis, although internal structures of the branching rods and tubes in the chamber walls are distinct. Thalli become similar to that of Renalcis when calcification and diagenesis remove or obliterate the internal fabric of the chamber wall. Morphologic variations of Epiphyton thalli and subsequent textural changes can result in identification as different groups of calcified microbial taxa.