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Large theropod tracks have previously been attributed to Tyrannosaurus rex. Most identifications however, have not been supported by either clear comparison with T. rex osteology or the stratigraphic position of the track. There is a conspicuous absence of tracks in the Upper Cretaceous (Maastrichtian) Hell Creek, Lance, Scollard, Frenchman, and Denver Formations (Lancian, North American Land Mammal Age), where T. rex body fossils have been found. A large tridactyl track is described here from the Hell Creek Formation of Carter County, Montana, United States. This find constitutes the first record of a large theropod track from the Hell Creek Formation, which could have potentially been made by T. rex or another large theropod, based on the track morphology and stratigraphic position.
Upper Jurassic ammonoid shell concentrations on pelagic carbonate platforms formed by the mixture of well-preserved and moldic shells provide a unique opportunity to evaluate the effects of average shell durability and productivity on variations in shell abundance preserved in the fossil record. High abundance of primary cement has significantly negative statistical effects on taphonomic alteration, reducing the proportion of ammonoid shells affected by Fe-staining and syndepositional dissolution. High proportions of internal borings indicate that shell concentrations were not rapidly buried. Significantly negative effects of taphonomic alteration on ammonoid shell-packing density and spatial variations in shell-bed thickness show that variations in ammonoid abundance are related to variations in production and destruction rates rather than to variations in sediment dilution. The close spatial association of dissolved aragonite shells and precipitated calcite in shell-rich deposits and the higher proportion of dissolved molds in shell-poor beds demonstrate the simultaneous action of dissolution and cementation in the semiconsolidated mixed layer. These relationships imply positive feedback between the high abundance of ammonoid shells and the low rate of shell destruction, with dissolved carbonate ions from high aragonite input reducing the rate of ammonoid dissolution and providing a local source for carbonate cement. Cementation has the strongest positive relationship with shell-packing density in rank correlations and generalized linear models. Proportions of ammonoid embryonic stages and early juveniles have smaller but significantly positive statistical effects on shell-packing density in simple regressions. We hypothesize that (1) ammonoid shell concentrations correspond to long-term peaks in ammonoid production, with aragonite dissolution buffering the pore-water chemistry, and (2) the increase in ammonoid production rates was related to intervals with average high fecundity coupled with high juvenile mortality.
Taphonomic features of 156 graphoglyptids and other trace fossils preserved as hypichnia of thin-bedded turbidites in Oligo-Miocene flysch of the northern Apennines (central Italy) were analyzed. Two biogenic taphonomic categories—deformation and elongation—were produced in hemipelagic mud by the behavior of endobenthic organisms. Deformation includes such features typical of bulldozing and burrowing as twisting, squeezing, tilting, thickening, and widening. Elongation is considered a primary biogenic character controlled directly by the tracemaker. Taphonomic features induced by such physical agents as currents and creep usually developed unidirectionally and include stretching, straightening, smoothing, bending, tapering, thickening, and thinning. These features, associated with hundreds of microgrooves (5–10 per 0.01 m2) interpreted as mud-current lineations, suggest that currents were active and produced deformational structures of fluting before, during, and after the biogenic activity. Preservation of such delicate structures recognizable at different levels is particularly noticeable when a thin layer of fine material settled by suspension, molding all structures and producing a cemented film. Deformational structures may be particularly well preserved in thin-bedded (3–6-cm-thick) and fine-grained calcarenitic turbidites as in diluted turbulent flow deposits that fringed the isolated Verghereto High. Activities of epi- and infaunal communities in this area are also exceptionally well preserved. Physical taphocharacters of graphoglyptids are interpreted in two ways: (1) as true tool marks produced in mud by a tractive water mass preceding sand deposition by turbidite flows, or (2) as structures inherited from preturbidite phases. Taphonomic analysis in deep-sea deposits, therefore, is a promising methodology to resolve the preservational state of trace fossils above and below the soles of turbidites.
Vertical changes in distribution, abundance, and ichnodiversity of ichnocoenoses in alluvial deposits of the Willwood Formation suggest significantly drier moisture regimes in the Bighorn Basin, Wyoming, during the Paleocene–Eocene Thermal Maximum (PETM), a transient period of global warming. The Willwood Formation at Polecat Bench contains an abundant assemblage of ichnofossils, including various types of rhizoliths and invertebrate trace fossils, such as Naktodemasis bowni, Camborygma litonomos, Edaphichnium lumbricatum, cf. Cylindricum isp., cf. Planolites isp., cf. Steinichnus, and cocoon traces. These comprise six distinct ichnocoenoses, which are categorized as dominantly terraphilic, hygrophilic, or hydrophilic based on the inferred moisture regimes of their most abundant ichnofossil morphotypes and associated pedogenic features, including other trace fossils and rhizoliths. The interpreted moisture regimes correlate well with the paleoenvironments of their host lithofacies, as inferred from sedimentology and paleopedology. Outside the PETM interval at Polecat Bench, abundant avulsion deposits and thin, compound paleosols containing hygrophilic and hydrophilic ichnocoenoses suggest frequent depositional events and predominantly poor to imperfect soil-drainage conditions. Within the PETM interval, thick, cumulative paleosol profiles with abundant terraphilic to hygrophilic ichnocoenoses suggest significantly improved drainage conditions. Lithofacies and ichnocoenoses above the PETM interval are not significantly different from those below the interval, indicating a return to pre-PETM moisture regimes. These conclusions support previous studies that suggest the Bighorn Basin experienced transient drying during this interval. This study demonstrates that ichnocoenoses and their ichnopedologic associations can be used to refine paleohydrologic and paleoclimatic generalizations inferred from paleoclimate models.
A distinctive, disturbed surface with numerous soft-sediment impressions occurs within a wet interdune interval of Jurassic Navajo Sandstone at the Coyote Buttes along the Arizona-Utah border. These high-density impressions are interpreted as footprints that comprise a dinosaur trampled surface. This surface displays an unusual combination of multiple overlapping track types and sizes, distinct to modified footprint features that include claws and toes and rare tail traces. The trampled surface covers ∼3000 m2 with an average density of ∼12 impressions/m2 in its main extent. Although modern water collection and biofilms typical of weathering potholes or pits are superimposed on this surface, the primary origin of the impression features are trace fossil structures formed prior to lithification. Four criteria distinguish the impressions as vertebrate in origin: (1) large—up to several tens of centimeters—repeating identifiable foot morphologies; (2) impression floors surrounded by soft-sediment marginal ridges; (3) impressions that are rarely flat and are typically oriented at an angle into the sediment (media) and indicate a clear direction of travel; and (4) multiple in situ ichnofossils on a moist interdune surface that resulted in soft-sediment deformation. At least three ichnogenera—cf. Eubrontes, cf. Anchisauripus, cf. Grallator— and the tracks attributed to a sauropodomorph appear as regular to asymmetric penetrations into the media with digitate features, commonly accompanied by soft-sediment marginal ridges of displaced sand preserved in the sandstone. The trampled surface provides paleoecologic and paleoclimatologic proxies that suggest a pluvial climate shift likely induced groundwater saturation of an eolian interdune that attracted dinosaurs to the area. The trampled surface provides valuable data for refining ecologic and climatic sensitivities recorded in Early Jurassic eolian deposits.
Scarcity of fossiliferous boundary sections makes the Triassic-Jurassic biotic crisis the most enigmatic of the five major Phanerozoic faunal turnovers. We report a bivalve-dominated level-bottom fauna from the Triassic-Jurassic transition in southern Tibet, which is unique for two reasons: (1) it documents the faunal turnover across the system boundary without major facies changes, and (2) it provides paleobiological data from the immediate postextinction interval. In the extinction pattern, selectivity against burrowing suspension feeders and taxa with completely aragonitic shells emerges, but no major clades or ecological groups disappeared. The earliest postextinction fauna differs in three significant ways from typical survival faunas of other mass extinction events: (1) it was surprisingly diverse, (2) it was heterogeneous (Simpson D = 0.1078) without dominance of disaster taxa or opportunists, and (3) there was a prevalence of highly specialized, morphologically complex forms reaching normal growth sizes. In spite of these unusual features, extinction of typical Triassic taxa occurred comparatively sharply within the underlying beds. These findings support scenarios of relatively short environmental disturbances triggered by the volcanic activity of the Central Atlantic Magmatic Province, which led to rapid extinction of taxa but did not impose ongoing environmental stress on the survival fauna. The nearly instantaneous recovery of level-bottom faunas is in sharp contrast to a prolonged reef eclipse, which probably indicates both higher extinction rates of reef-organisms and intrinsic limitations to the tempo of recovery due to their high level of co-evolution.