Time-averaged Assemblages Research Articles

Environmental and taphonomic controls on the shell beds and fauna of the middle Miocene Chipola Formation of Florida, USA

Thick (2–10's m) and laterally extensive shell beds are common in the stratigraphic record, however, relatively little is known about their genesis since no single cause can explain the formation of these shell deposits. The goal of this study is to use lithologic, faunal, and taphonomic analyses to elucidate how the thick and laterally extensive shell beds of the middle Miocene Chipola Formation of Florida formed and became one of the most species-rich Cenozoic faunas in the western Atlantic Province. This study examined Chipola Formation outcrops at Alum Bluff, Cooter Bluff, and Farley Creek. Lithologic and trace fossil analyses indicate that the Alum Bluff shell bed represents an upper to middle shoreface setting, whereas the Farley Creek and Cooter Bluff shell beds represent lower shoreface and/or proximal offshore-transitional environments. Faunal data further support that Farley Creek and Cooter Bluff were deeper environments relative to Alum Bluff. Environmental preferences of most of the fauna indicate that all localities had normal, open-marine salinities with seagrass and unvegetated substrates. Samples are classified as within-habitat time-averaged assemblages, which were spatially averaged by various physical and ecological factors. Spatial averaging mixed the mosaic of communities that are represented by different types of substrate-controlled habitats within, but not across, significant variation in water depth. Finally, shell beds were accumulated by winnowing and/or high-energy events, such as storms, which produced the relatively thick, laterally extensive shell beds at each locality. Understanding the high diversity of the Chipola Formation is complicated and is likely best explained by multiple controls. Variation in alpha diversity among the localities is linked with differences in water depth as well as time and/or spatial averaging. The elevated diversity at each site was likely influenced by spatial averaging of habitats within a single depth zone, within-habitat time averaging, and poleward expansion of tropical taxa into the northern Gulf of Mexico due to environmental conditions associated with the middle Miocene Climatic Optimum. Similar poleward shifts of tropical taxa as those recorded in the Chipola Formation could hypothetically occur in the future as global temperatures and sea level rise. Future studies need to examine the biological changes associated with the middle Miocene Climatic Optimum to better understand the future impacts of ocean-climate change on marine communities.

TAPHONOMIC INDICATORS OF DEAD OCEAN QUAHOG (ARCTICA ISLANDICA) SHELL AGE IN THE DEATH ASSEMBLAGE OF THE MID-ATLANTIC BIGHT CONTINENTAL SHELF

ABSTRACT Taphonomic indicators are often used to assess time-since-death of skeletal remains. These indicators frequently have limited accuracy, resulting in the reliance of other methodologies to age remains. Arctica islandica, commonly known as the ocean quahog, is a relatively widespread bivalve in the North Atlantic, with an extended lifespan that often exceeds two hundred years; hence, their shells are often studied to evaluate climate change over time. This report evaluates taphonomic age using 117 A. islandica shells collected from the Mid-Atlantic Bight offshore of the Delmarva Peninsula with radiocarbon dates extending from 60–4,400 cal years BP. These shells had varying degrees of taphonomic alteration produced by discoloration and degradation of periostracum. To determine if a relationship exists between taphonomic condition and time-since-death, radiocarbon ages were compared with the amount of remaining periostracum and type of discoloration. Old shells (individuals that died long ago) were discolored orange with no periostracum while younger shells (individuals that died more recently) had their original color, with some periostracum. Both the disappearance of periostracum and appearance of discoloration followed a logistic process, with 50% of shells devoid of periostracum and 50% discolored in about 1,000 years. The logistic form of long-term taphonomic processes degrading shell condition is first reported here, as are the longest time series for taphonomic processes in death assemblages within the Holocene record. This relationship can be utilized for triage when deciding what shells to age from time-averaged assemblages, permitting more efficient application of expensive methods of aging such as radiocarbon dating.

Conservation Paleobiology and Taphonomy: Differential Preservation and Time-Averaging of Echinoids and Mollusks

Fossils from surficial death assemblages and shallow cores are an important archive used in the field of conservation paleobiology. Understanding the taphonomic filters and time-averaging that affect modern biomineralized taxa is crucial for using their fossil record as a source of geohistorical data. Through comparative analysis of the live-dead patterns of echinoids and mollusks, we aim to assess multiple hypotheses regarding differences between the two taxa in terms of preservation potential and fossil record resolution. In this pilot study, six sediment samples (five surface [0-10cm] and one subsurface [30-40cm]) were collected in a shallow, subtidal habitat off the coast of Cedar Key, Florida. These samples were sieved for mollusks and echinoids. Specimens were segregated by taxa and classified into live, whole dead, and fragments. Results were consistent among surface samples, which showed that, by weight, an average of 8% of all mollusk specimens and 55% of all echinoid specimens were live collected. Fragmentation rate by weight was notable higher for echinoids than for mollusks: 77% for mollusks and >99% for echinoids. The subsurface sample lacked complete echinoid tests, but complete mollusks shells, mollusk fragments and echinoid fragments remained common. These results support the hypothesis that echinoid tests degrade more quickly than mollusk shells, making them less likely to be preserved as part of time-averaged assemblages formed under low net-accumulation rates. Echinoid fragments do not seem to follow this pattern, and instead seem to preserve similarly to mollusks. The results suggest that geohistorical records provided by echinoids and mollusks may be fundamentally distinct from one another in terms of temporal resolution and completeness of the fossil record.

A downcore increase in time averaging is the null expectation from the transit of death assemblages through a mixed layer

AbstractUnderstanding how time averaging changes during burial is essential for using Holocene and Anthropocene cores to analyze ecosystem change, given the many ways in which time averaging affects biodiversity measures. Here, we use transition-rate matrices to explore how the extent of time averaging changes downcore as shells transit through a taphonomically complex mixed layer into permanently buried historical layers: this is a null model, without any temporal changes in rates of sedimentation or bioturbation, to contrast with downcore patterns that might be produced by human activity. Assuming stochastic burial and exhumation movements of shells between increments within the mixed layer and stochastic disintegration within increments, we find that almost all combinations of net sedimentation, mixing, and disintegration produce a downcore increase in time averaging (interquartile range [IQR] of shell ages), this trend is typically associated with a decrease in kurtosis and skewness and by a shift from right-skewed to symmetrical age distributions. A downcore increase in time averaging is thus the null expectation wherever bioturbation generates an internally structured mixed layer (i.e., a surface, well-mixed layer is underlain by an incompletely mixed layer): under these conditions, shells are mixed throughout the entire mixed layer at a slower rate than they are buried below it by sedimentation. This downcore trend created by mixing is further amplified by the downcore decline in disintegration rate. We find that transition-rate matrices accurately reproduce the downcore changes in IQR, skewness, and kurtosis observed in bivalve assemblages from the southern California shelf. The right-skewed shell age-frequency distributions typical of surface death assemblages—the focus of most actualistic research—might be fossilized under exceptional conditions of episodic anoxia or sudden burial. However, such right-skewed assemblages will typically not survive transit through the surface mixed layer into subsurface historical layers: they are geologically transient. The deep-time fossil record will be dominated instead by the more time-averaged assemblages with weakly skewed age distributions that form in the lower parts of the mixed layer.

INSIGHTS FROM FOSSIL FISH TAPHONOMY INTO THE DEPOSITIONAL ENVIRONMENT OF THE LOWER MILWAUKEE FORMATION (BERTHELET MEMBER, LATEST MIDDLE DEVONIAN)

ABSTRACT The fossiliferous, argillaceous dolostone of the latest Middle Devonian (Givetian) Milwaukee Formation contains abundant disarticulated fish skeletal elements. This study refines depositional environment reconstruction of the Milwaukee Formation through taphonomic analysis of this fish assemblage. Robust skeletal elements of placoderm grinding teeth, fin spines, and armored plating dominate the assemblage. Specimens display variation in taphonomic attributes including color, luster, corrasion (chemical corrosion and/or physical abrasion), presence of pyrite and phosphate permineralization, and epibionts. Rare occurrences of teeth belonging to osteichthyan fish are also taphonomically variable. Associated invertebrates are diverse and include abundant cephalopod internal molds and pyritized brachiopods often randomly oriented in shell hashes. In sum, these observations suggest deposition on a marine shelf between normal and storm wave base. During low-energy background periods, fish plates exposed on the seafloor would undergo initial disarticulation, epibiont attachment, and color- and luster-altering corrasion and phosphatization. Buried skeletal elements were ‘protected' from epibiont attachment and corrasion, but occasional low oxygen conditions in the substrate led to early diagenetic pyrite permineralization. Periodic high-energy storm events were responsible for complete disarticulation as well as reworking and reexposure of buried skeletal elements (and burial of others), resulting in a time-averaged assemblage with a wide range of taphonomic attributes. Robust placoderm skeletal elements were best suited to survive fossilization in this setting and therefore the placoderm-dominated fossil assemblage appears to be an artifact of taphonomic processes and energy of the depositional environment.

A middle Miocene vertebrate assemblage from the Czech part of the Vienna Basin: Implications for the paleoenvironments of the Central Paratethys

Czujan's sandpit is an abandoned quarry in the Vienna Basin (Mikulov, Czech Republic) that has yielded an important middle Miocene vertebrate assemblage. Here we re-describe the site from the perspective of sedimentology, taphonomy, and paleoenvironments, and further review the biochronology of the fauna to clarify the age. The updated faunal list includes two testudines (one trionychid and one medium-sized testudinid), and 12 species of terrestrial mammals (three proboscideans, four perissodactyls, four artiodactyls, and one carnivoran), consistent with an early Astaracian (MN6) age. The position of the Wielician/Kosovian boundary just below the floor of Czujan's sandpit, and our new biostratigraphic data, further allow us to constrain the fossil assemblage to the latest MN6 (late Badenian, ⁓13.6 Ma) and resolves a longstanding controversy about the age of the site. The site exposes a coarsening-upward succession deposited in a braid delta environment, and comprises three facies association: from bottom to top, pelagic sediments (FA1); prodelta and delta slope sediments (FA2); and distributory channel infills of the delta front and delta plain (FA3), the latter containing all the studied terrestrial vertebrates. We propose two taphonomic explanations for the genesis of the vertebrate assemblage: (1) a time-averaged assemblage generated by riverine transport, or (2) a transported assemblage from a mass death site(s), with mass death episode(s) caused by seasonal droughts in the river catchment. Our new findings allow the more precise reconstruction of late Badenian terrestrial paleoenvironments in the northwest area of the Vienna Basin and adjacent Carpathian Foredeep Basin. This region comprised a mosaic of continental habitats dominated by woodlands but also including forest patches and more open environments.

Late Quaternary sporadic development of Desmophyllum dianthus deep-coral populations in the southern Labrador Sea with specific attention to their 14C- and 230Th-dating

About 70 fossil and live-collected specimens of Desmophyllum dianthus were collected from steep rocky surfaces and time-averaged assemblages at their base (“coral graveyards”) in the southern Labrador Sea at depths ranging from ~1700 m (Orphan Knoll) to ~2200 m (Flemish Cap). Aside from stable isotope and mineralogical analysis, U-series and 14C age determinations were performed on the samples. Fossil corals from the coral graveyards display two principal age clusters corresponding respectively to the Holocene and Marine Isotope Stage (MIS) 5c intervals. Three samples lay outside these clusters, at ~13.5 ka (Bølling-Allerød), ~ 64 ka (MIS 3) and at ~181 ka (MIS 7a). The clusters are thought to record intervals with both i) high food availability, either through pelagic primary production in the overlying water column or more effectively in relation with particulate and dissolved organic carbon transport via an active Western Boundary Undercurrent (WBUC), and ii) high carbonate micrite advection by this current from more open North Atlantic coccolithophorid production areas, maintaining an aragonite saturation horizon (now at 2300 m) slightly deeper than the coral habitat (~ 2200 m). Attempts at estimating ventilation ages from 230Th vs 14C age offsets in Holocene samples failed. This is due to uncertainties about the initial 14C-apparent age of coral skeletons and to their high initial excesses in 230Th. This initial excess relates mostly to the scavenging of 230Th produced by the dissolved marine U by the coral-feeding organic matter carried along the WBUC trajectory. In addition, discrete diagenetic U-fluxes can be documented in fossil samples. They are well illustrated in specimens from MIS 5c age, which depict variable excesses in 234U vs 238U, but similar 230Th/238U activity ratios, pointing to a recent but fractionating redistribution of uranium in the samples. These are also documented by the development of FeMn coatings of specimens exposed at the sea floor, due to the winnowing of embedding slope sediments, with the mid- to late Holocene intensified WBUC. Such diagenetic processes are to be notably taken into consideration for the dating of pre-Holocene samples. Thus, both the initial isotopic properties of corals from this WBUC-influenced, mid-continental slope, and subsequent diagenetic effects impact their calculated ages, and blur estimates of “ventilation ages” based on their 14C vs 230Th-ages offsets.

Environmental controls on shallow subtidal molluscan death assemblages on San Salvador Island, The Bahamas

Ecological studies that span multiple marine depositional environments frequently find that benthic invertebrate assemblages are correlated primarily with an onshore-offshore gradient. Variability within the same depositional environment is inferred to not result from water depth differences, but other parameters such as substrate consistency and shear stress. Modern studies focusing on assemblage variations within the same depositional environment can identify which variables affecting assemblage compositions are independent of water depth. This study examines the environmental controls on the composition of molluscan death assemblages from the shallow subtidal environments of San Salvador Island. Molluscan assemblages were tabulated from twenty-nine bulk samples collected from four locations around San Salvador Island representing seagrass meadows, open sandflats, and patch reef systems common in the shallow subtidal. In addition, quantitative environmental data for each sample was characterized, including seagrass density, water depth, temperature, pH, dissolved oxygen, and grain-size, as well as qualitative observations of wave energy and bioturbation. Cluster analysis and canonical correspondence analysis (CCA) were performed to identify environmental gradients associated with molluscan assemblage composition. Cluster analysis and CCA Axis 1 identify seagrass coverage and substrate consistency as the primary environmental variables describing molluscan death assemblage compositions. CCA Axis 2 identifies wave energy, bioturbation, and dissolved oxygen as the secondary environmental variables, though the small range of dissolved oxygen values (17–21 mg/L) collected using single, instantaneous measurements make it unclear whether the oxygen gradient is ecologically significant for our time-averaged assemblages. Our analysis demonstrates the variability of assemblages within a single depositional environment, as well as the utility of fine-resolution studies for identifying secondary environmental variables that control the composition of marine invertebrate assemblages.

Multi-storm events recorded on Panopea burrows (Pliocene, Spain): The importance of sequestered information inside burrows

The fossil record is essentially characterized by time-averaged assemblages as a consequence of the mixture of multiple generations of organisms into a single stratigraphic horizon. In order to investigate this issue a quantitative taphonomic analysis of fossil assemblages filling up Panopea burrows (ichnogenus Scalichnus) and those preserved in sediment without this trace fossil on of a Pliocene deposit of SW Spain were carried out. A combination of processes can be inferred from the analysis of the fossils infilling Scalichnus burrows. Passive concentration of shells within trace fossils were is interpreted to have been produced during event processes (most likely storm events) and active burrowing of bivalve trace-makers probably represented the background conditions.These shell concentrations inside Scalichnus burrows constitute a potential sedimentological implement to understand the multi-storm events recorded on the taphonomic signature of the biofabric that otherwise may be unnoticed. The studied examples can be used as proxies for sequencing a series of amalgamated storm events inside bivalve burrows, assessing the magnitude and frequency of erosional events, and analysing the positive taphonomic effect of these equilibrium trace fossils.

The dominance of an extant gregarious taxon in an attritional accumulation: Taphonomy and palaeoecological implications

A recurrent aspect of the fossil record is the observation of a disproportionate number of specimens or individuals of a single taxon in some deposits, which is stated as dominance. Commonly, the dominance is explained as the result of catastrophic or short-term mass death events or are proxies for palaeoecological inferences regarding gregariousness. However, taphonomic, stratigraphic and chronologic analyses of fossiliferous deposits have shown that this is not always true. To contribute to the study of dominance in fossil assemblages, we describe a probable Quaternary skeletal accumulation dominated by the extant gregarious rock-dwelling rodent Kerodon rupestris recovered from Sumidouro do Sansão, a 65 m deep pitfall cave in northeast Brazil, and discuss the palaeoecological implications of our findings. We provide taxonomic identification, taphonomic analyses, and chronological assessment. Besides K. rupestris (minimum number of individuals, MNI = 35), we recorded three taxonomic groups, that are the ground sloth Catonyx cuvieri (MNI = 1), the anteater Tamandua tetradactyla (MNI = 1), and the cougar Puma concolor (MNI = 1). The taphonomic analysis of the K. rupestris remains supports the idea that entrapment of individuals was the main process of bone accumulation and that the death of cave inhabitants followed by short transport to the main hall possibly occurred. Kerodon rupestris remains persisted on the surface of the cave deposit for different time spans and were exposed to fragmentation, weathering, invertebrate boring and encrustation inside the cave environment. Direct dating suggests differences in the ages obtained and intermittent deposition of the individuals inside the cave. It highlights the influence of taphonomic controls on gregarious taxa, such as K. rupestris, towards dominating attritional time-averaged assemblages. Furthermore, it demonstrates that such assemblages do not necessarily support inferences about single event mortality and gregariousness of the dominant taxon.

Sclerobionts on Patagonian Oysters from the Puerto Madryn Formation (Early—Late Miocene, Argentina)

Fil: Romero, Virginia. Universidad Nacional de Rio Negro. Instituto de Investigacion en Paleobiologia y Geologia. Rio Negro, Argentina

TAPHONOMIC CLOCK AND BATHYMETRIC DEPENDENCE OF CEPHALOPOD PRESERVATION IN BATHYAL, SEDIMENT-STARVED ENVIRONMENTS

The dependence of skeletal alteration on time spent in the taphonomic active zone (TAZ) can generate a taphonomic clock, which can be used to quantify scales of time averaging and rates of skeletal production and recycling in the fossil record. However, the strength of the taphonomic clock is variable in present-day shallow marine environments and it is unclear how this strength varies with depth. Here, we assess the strength of the taphonomic clock in Nautilus macromphalus dead shells that were collected in cool-water, sediment-starved, epi- and mesobathyal environments off New Caledonia and range in postmortem age from few decades to several millennia. We find that, first, differences in the onset and extent of alteration states in the epibathyal zone ( 1,000 years). The onset of dissolution and formation of clay-goethitic coating occurs earlier (∼ 200 years) in the mesobathyal zone (> 750 m) than in the epibathyal zone. Clay-goethitic rims and boring infills show signs of microbial binding, pelletization, and coccolith dissolution and can represent nascent stages of glauconitization. Second, shells several centuries old show differences between right and left flanks (1) in the degree of encrustation and sponge bioerosion in the epibathyal zone and (2) in the extent of clay-goethitic coating in the mesobathyal zone. The upper flanks are ultimately removed in both depth zones, leaving relict half-shells that are millennia old. Third, the depth dependence of alteration can reflect a bathymetric decline in disintegration rates by heterotrophic borers due to a reduced productivity in the mesobathyal zone and an increase in dissolution rates due to the proximity of the aragonite compensation depth. The between-flank asymmetry in preservation implies (1) horizontal position of shells close to the sediment-water interface for several decades or centuries without being overturned or subjected to reworking and (2) shell exposure to two distinct taphonomic regimes separated by few centimeters, with the upper flank located in the TAZ and the lower flank facing a less aggressive sediment zone. The stable exposure of shells in the taphonomic active zone and slow alteration rates in bathyal environments allow discriminating between within-habitat time-averaged assemblages on one hand and environmentally or stratigraphically condensed assemblages on the other hand.

Active dispersal is differentially affected by inter‐ and intraspecific competition in closely related nematode species

De Meester, N., Derycke, S., Rigaux, A. and Moens, T. 2016. Active dispersal is differentially affected by inter- and intraspecific competition in closely related nematode species. – Oikos 124: 561–570. Figure 1: labels on y-axis are corrected Time-averaged assemblage compositions (relative abundances of the four cryptic species of Litoditis marina) in the current experiment (‘inoculation’ and ‘dispersal’ for resp. both plates of the dispersal plate) and the experiment of De Meester et al. (2011, no dispersal treatment) at two different salinities (15 and 25) (n = 24).

Shoal-water dynamics and coastal biozones in a sheltered-island setting: Upper Devonian Pillara Limestone (Western Australia)

Along the Canning Basin's Lennard Shelf in Western Australia, the 80-km-long Oscar Range is composed of folded Palaeoproterozoic quartzite and phyllite and surrounded by limestones of the Great Devonian Barrier Reef including reef complex, related back-reef and lagoonal deposits of the Frasnian Pillara Limestone. The range represents an exhumed cluster of palaeoislands. Near the east end of the Oscar Range, a palaeoislet is encircled by the Pillara Limestone showing outward dips that dramatically shallow to expose nearly horizontal bedding planes offshore. From shore and outward, the facies zones observed in the Pillara Limestone include unfossiliferous laminated sediments followed by biozones with abundant Amphipora and Stachyodes, and domal stromatoporoids. An additional outermost lagoonal facies with a diverse molluscan fauna preserved in fine limestone/dolostone is described in this study. High-spired Murchisonia in a time-averaged assemblage with other gastropods, bivalves, brachiopods and scaphopods dominate this zone. Uneven distribution of biozones is due to intermittent shoals controlled by the complex relief of basement rocks or recent erosion into underlying layers. The orientations of dendroid stromatoporoids and high-spired gastropods were analysed to appraise the dynamics of prevailing shoal-water settings on the inner, more sheltered side of the Oscar Range facing the Devonian mainland to the north. Oscillatory wave action is interpreted as the main agent of transport. Palaeocurrent data for the lighter dendroid stromatoporoids suggest that fair-weather prevailing winds originated from the SE. Pebble clasts, oncoids, bivalves and gastropods indicate episodes of wave agitation and stronger wind from a SE and southerly direction.

Inferring skeletal production from time-averaged assemblages: skeletal loss pulls the timing of production pulses towards the modern period

AbstractAge-frequency distributions of dead skeletal material on the landscape or seabed—information on the time that has elapsed since the death of individuals—provide decadal- to millennial-scale perspectives both on the history of production and on the processes that lead to skeletal disintegration and burial. So far, however, models quantifying the dynamics of skeletal loss have assumed that skeletal production is constant during time-averaged accumulation. Here, to improve inferences in conservation paleobiology and historical ecology, we evaluate the joint effects of temporally variable production and skeletal loss on postmortem age-frequency distributions (AFDs) to determine how to detect fluctuations in production over the recent past from AFDs. We show that, relative to the true timing of past production pulses, the modes of AFDs will be shifted to younger age cohorts, causing the true age of past pulses to be underestimated. This shift in the apparent timing of a past pulse in production will be stronger where loss rates are high and/or the rate of decline in production is slow; also, a single pulse coupled with a declining loss rate can, under some circumstances, generate a bimodal distribution. We apply these models to death assemblages of the bivalveNuculana taphriafrom the Southern California continental shelf, finding that: (1) an onshore-offshore gradient in time averaging is dominated by a gradient in the timing of production, reflecting the tracking of shallow-water habitats under a sea-level rise, rather than by a gradient in disintegration and sequestration rates, which remain constant with water depth; and (2) loss-corrected model-based estimates of the timing of past production are in good agreement with likely past changes in local production based on an independent sea-level curve.

Analysis of marrow cavity fillings as a tool to recognise diverse taphonomic histories of fossil reptile bones: Implications for the genesis of the Lower Muschelkalk marine bone-bearing bed (Middle Triassic, Żyglin, S Poland)

Abstract The bone-bearing bed from Żyglin (S Poland), which likely represents the oldest Lower Muschelkalk accumulation of reptile remains in the SE part of the Germanic Basin, has been investigated in order to determine its genesis. The basic methods were supported by petrographic analysis of the marrow cavity and large inter-trabecular pore space fillings of fossil bones, which was used to check its usefulness to identify the environments where the bones were initially deposited and to decipher taphonomic histories of the remains. The bone-bearing bed is a composite deposit, which consists of three distinct layers (from bottom to top): micritic limestone (mudstone, bioturbated autochthonous mud), crinoidal limestone (grainstone to packstone, calcirudite), and shell-rich limestone (packstone to wackstone, calcirudite). The crinoidal limestone layer, the main bone-bearing bed, is recognised as the proximal tempestite deposited in the mid-ramp zone. The petrographic analysis of the fillings reveals the prevalence of minute ostracod carapaces, accompanied by other grains, embedded together with micrite to microspar. Such compositions suggest that these sediments may have been inserted into the bone pore spaces in lagoons and tidal-flat ponds. Features of some bones record their early diagenetic burial and lithification before the final redeposition. The isopachous spar, blocky spar and weathered pyrite document changes in the chemical composition of fluids that flowed into the bone interiors. The burrow found in the marrow cavity of specimen IGUAM-ZOV-7 provides evidence that some remains were inhabited by minute or juvenile invertebrates. All recognised features indicate that both invertebrate and vertebrate bioclasts, included in the tempestite, were initially deposited in various settings of the carbonate ramp and in the end redeposited as a result of a heavy storm or a hurricane. The examined bone-bearing bed represents time-averaged assemblage, which originated due to hydraulic concentration of the vertebrate bioclasts. The petrographic analysis of the fillings is a valuable tool to identify complex taphonomic pathways of vertebrate bioclasts.

TAPHONOMY AND DEPOSITIONAL SETTING OF THE BURGESS SHALE TULIP BEDS, MOUNT STEPHEN, BRITISH COLUMBIA

ABSTRACT Burgess Shale–type deposits represent exceptional preservational windows for examining the biodiversity and ecological structure of some of the earliest metazoan communities that evolved during the Cambrian Explosion. While much attention has been paid to the original Burgess Shale locality, the Walcott Quarry on Fossil Ridge, temporal and regional variations of the depositional environment of the Burgess Shale biota as a whole are still poorly understood. Here we present the first comprehensive taphonomic and sedimentological study of the Tulip Beds on Mount Stephen (Campsite Cliff Shale Member, Burgess Shale Formation), based on a time-averaged assemblage of nearly 10,000 specimens. The taphonomic characteristics—size sorting, resistance to decay, and potential flow alignment—and mode of deposition of this assemblage are compared specifically to those of the nearby and stratigraphically younger Walcott Quarry assemblage. Like other Burgess Shale–type deposits, the Tulip Beds consist of millimet...

Large dinosaurian tracks from the Upper Cretaceous (Cenomanian–Turonian) portion of the Winton Formation, Lark Quarry, central-western Queensland, Australia: 3D photogrammetric analysis renders the ‘stampede trigger’ scenario unlikely

The largest dinosaurian tracks at Lark Quarry, central-western Queensland, Australia, were re-examined using revised analytical protocols that incorporate three-dimensional (3D) structure. Comparisons were made with archival photographs, replica specimens (c. 1977) and the in situ tracks (2013) to account for changes to the track surface. Damage caused both during and after the excavation of the tracks was evident, and in cases where the archival photographs and 1970's replicas strongly differ from the in situ tracks, it is apparent that restoration has modified the original track morphology.Even after accounting for recent damage and alteration, several of the track morphologies obtained from new 3D evaluation models differ considerably from the track outlines that were published in the original description of the site. Compared with the new set of representations, some of the original outlines seem to represent simpler, stylized versions of the tracks. A number of the original outlines are >20% larger than the in situ tracks, while others appear to incorporate cracks as part of the margin of digit impressions. Overall, the best-preserved tracks show blunt digit impressions, reaffirming the idea that the trackmaker was a large ornithopod and supporting a reassignment to cf. Iguanodontipus. The new analysis also reveals the nature of the displacement rims associated with the tracks, and the overprinting of these rims by other ichnites—initially by tool marks (presumably caused by floating vegetation) and then by other dinosaurian tracks (assignable to Wintonopus latomorum). In the context of these observations, we see no evidence for an interaction between the cf. Iguanodontipus trackmaker and the smaller-bodied W. latomorum trackmakers, as neither can be inferred to have been present at the tracksite at, or even close to the same time. Similarly, there is no evidence to support the idea that the approach of the cf. Iguanodontipus trackmaker in some way triggered the movement of the W. latomorum trackmakers. Rather than a snap-shot of dinosaurian ‘stampede’, this study supports the idea that Lark Quarry most likely represents a complex time-averaged assemblage of multiple dinosaurian ichnites, preserved over an extended period of time (hours to days) and bracketed by discrete phases of trackmaker activity and fluctuations in water depth.

Cultural Transmission and Diversity in Time-Averaged Assemblages

Anthropologists have adopted methods from population genetics to study modes of cultural transmission in time-transgressive cultural data sets. However, it remains unclear to what extent methods originally developed to assess neutrality in genes sampled from a population at a single point in time are applicable to cultural variants sampled from assemblages. This report applies a suite of previously published methods to assemblages formed under unbiased cultural transmission. The results show that, even under the controlled conditions afforded by computer simulation, all but one method (the variants frequency approach) fail to identify unbiased cultural transmission in samples collected from moderately to severely time-averaged assemblages. While it is encouraging that the method that is best at identifying unbiased cultural transmission in simulated time-averaged data also is relatively robust to relative sample size, additional work is needed to determine whether even the variants frequency approach is powerful enough to identify “weakly” biased forms of cultural transmission in real-world assemblages.

Diversity and species abundance patterns of the Early Cambrian (Series 2, Stage 3) Chengjiang Biota from China

Lagerstättenfrom the Precambrian–Cambrian transition have traditionally been a relatively untapped resource for understanding the paleoecology of the “Cambrian explosion.” This quantitative paleoecological study is based on 10,238 fossil specimens belonging to 100 animal species, 11 phyla, and 15 ecological categories from the lower Cambrian (Series 2, Stage 3) Chengjiang biota (Mafang locality near Haikou, Yunnan Province, China). Fossils were systematically collected within a 2.5-meter-thick sequence divided into ten stratigraphic intervals. Each interval represents an induced time-averaged assemblage of various event (obrution) beds of unknown duration. Overall, the different fossil assemblages are taxonomically and ecologically similar, suggesting the presence of a single community type recurring throughout the Mafang section. The Mafang community is dominated by epibenthic vagile hunters or scavengers, sessile suspension feeders, and infaunal vagile hunters or scavengers represented primarily by arthropods, brachiopods, and priapulids, respectively. Most species have low abundance and low occurrence frequencies, whereas a few species are numerically abundant and occur frequently. Overall, in structure and ecology the Mafang community is comparable to the Middle Cambrian (Series 3, Stage 5) Burgess Shale biota (Walcott Quarry, Yoho National Park, British Columbia, Canada). This suggests that, despite variations in species identity within taxonomic and ecological groups, the structure and ecology of Cambrian Burgess Shale-type communities remained relatively stable until at least the Middle Cambrian (Series 3, Stage 5) in subtidal to relatively deep-water offshore settings in siliciclastic soft-substrate environments.