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Siliceous spicules, typically ≤3 cm high and ≤5 mm in diameter, are common just above water level alongside hot-spring pools and channels in the Waiotapu geothermal area of New Zealand, which contain waters of 50–80°C and pH of 1–3. These structures are formed of irregular sequences of silicified Cyanidium mats, silicified Alicyclobacillus mats, and homogeneous opal-A laminae. Allochthonous microorganisms (e.g., diatoms) form a minor part of the silicified biota. Lamina boundaries are discontinuities that reflect pauses in opal-A precipitation. Barite, gypsum, and kaolinite crystals were precipitated on the spicule surfaces during these periods.
Opal-A precipitation on subaerial spicules occurs when SiO2-rich fluid is delivered by (1) splashing water and spray, (2) temporary inundation by surging spring water, wind-driven waves, or local changes in the water table following periods of heavy rain, (3) steam condensate, and/or (4) capillary action. These abiogenic processes work in conjunction with biogenic processes controlled by microbial mats, which become temporarily established on spicule surfaces when conditions permit. These mats mediate spicule construction as they become silicified, providing substrates for opal-A cement precipitation. Nevertheless, stromatolite morphology cannot be attributed to a particular taxon because different localities have different microorganisms involved with spicule growth.
This paper explores the taphonomic effect that differences in body size and habitat breadth have on the end-Rancholabrean fossil record of carnivoran species. The preservation and recovery potential of carnivoran fossils in cave and noncave deposits are also compared. Identifying the nature and assessing the effect of taphonomic biases are among the preliminary steps in deciding the kind of evolutionary and paleoecological questions that can be addressed with the available fossil information. Few studies have examined taphonomic biases that affect the fossil record of mammals, and most of these have focused on herbivores.
Results suggest that differences in habitat breadth of extant carnivorans are not associated with differences in frequencies of their fossil remains. There is a significant, positive relationship between body size and frequency of identified fossil specimens in noncave deposits. Outside of caves, the frequency of identified fossil specimens of extant, small carnivorans is significantly less than that of large ones. Results indicate also that the species richness of extant, small carnivorans is underrepresented relative to that of larger carnivorans, especially in the end-Rancholabrean fossil record derived from noncave localities. No significant differences in the frequency of fossil remains of species that differed in body size were found within caves.
Ecological studies have revealed that the functional roles of dominant species in modern communities are often more important than overall diversity in governing community composition and functioning. Despite this recognition that abundance and diversity data are both required for a complete understanding of ecological processes, many paleoecological studies focus on presence-absence data, possibly because of concerns regarding the taphonomic fidelity of time-averaged fossil accumulations. However, the abundance of organisms in shell beds has been shown to provide a fairly accurate record of the living community, suggesting that the benefits of relative-abundance data should be reconsidered. Recognition of ecologically dominant species in local fossil assemblages should be based on counts of relative abundance and assessment of ecological role. Ecological dominance at larger spatial or temporal scales can be quantified using the mean rank order of a clade and the proportion of assemblages where the clade is present, providing unbiased, quantitative values for measuring the ecological importance of a clade. Their utility has been tested with three case studies encompassing a range of geographic and taxonomic scales, using a database of 1221 Ordovician–Paleogene quantitative fossil collections. The dominance metrics for rhynchonelliform brachiopods, bivalves, and gastropods broadly parallel anecdotal trends, even including some more detailed patterns documented by regional studies. An examination of substrate preferences for brachiopod and bivalve orders confirms the abundance of infaunal bivalves in siliciclastics and epifaunal bivalves in carbonates, but it also reveals intriguing patterns regarding substrate preferences among rhynchonelliform brachiopod orders. The final case study analyzed changes in dominance between early Mesozoic fossil assemblages from Tethys and Panthalassa, documenting significant geographic differences in the ecological importance of rhynchonelliform brachiopods and bivalves. These large-scale dominance patterns often approximately matched those inferred from diversity trends; however, there are also times when dominance was decoupled from diversity, indicating that further investigation of ecological dominance will provide additional insights into ecological influences on the Phanerozoic history of life.
“Are most species simply passengers in ecosystems that are run basically by a few dominants?” (Worm and Duffy, 2003, p. 631)
This study focuses on two main questions: (1) what types of shell damage occur in the death assemblage of upper Chesapeake Bay benthic mollusks; and (2) how does shell damage differ according to intrinsic factors such as life habit, shell mineralogy, and shell organic content. Extrinsic and intrinsic factors, ranging from the environment to shell composition, interact to influence the quality of fossil preservation. Our understanding of how extrinsic factors affect shell-damage profiles has improved dramatically with the development of taphofacies models, but the role that intrinsic factors play is still poorly understood. Molluscan death-assemblage material was obtained via box coring, identified, and assigned taphonomic damage states. The most common forms of shell damage were disarticulation, fine-scale surface alteration (FSA), periostracum loss, edge modification, and fragmentation. Four patterns were documented consistently across habitat types when shell damage was examined according to life habit and shell composition. Infaunal specimens exhibit significantly more severe damage due to internal FSA than epifaunal specimens. Calcitic specimens experience higher levels of external encrustation than noncalcitic specimens. Specimens with high levels of shell organics experience significantly more fragmentation and edge modification than specimens with low levels of shell organic content. The direction and degree to which other damage variables differ among intrinsic categories vary according to the variable assessed or the habitat type sampled. In the northern sites (i.e., deeper water), all the variables that recorded differential damage according to shell-organic content recorded greater damage in specimens with high shell organic content, suggesting that the latter may be experiencing selective removal from the death assemblage.
The degree to which the original community composition of the Middle Cambrian Burgess Shale was altered through transport and decay and how taphonomic conditions varied through time and across taxa is poorly understood. To address these issues, variation in fossil preservation was analyzed through a vertical succession of 26 bed assemblages, each representing a single obrution event, within the 7-m-thick Greater Phyllopod Bed of the Walcott Quarry. More than 50,000 specimens belonging to 158 genera—mostly benthic, monospecific and nonbiomineralized—were included in this analysis. The decay gradient of the polychaete Burgessochaeta setigera was used as a taphonomic threshold to estimate how far decay had proceeded in each bed assemblage.
Qualitative comparisons of the degree of preservation of 15 species, representing an array of different body plans, demonstrate that all bed assemblages contain a mix of articulated and in situ dissociated or completely dissociated organisms interpreted respectively as census- and time-averaged assemblages. Furthermore: (1) most organisms studied were preserved within their habitat and only slightly disturbed during burial; (2) most decay processes took place prior to burial and resulted in disarticulation of organisms at the time of burial; (3) the degree of disarticulation was variable within individuals of the same population and between populations; and (4) early mineralization of tissues across all body plans occurred soon after burial.
Canonical correspondence analysis summarizes the apparent variations in the amount of preburial decay, or time averaging, across species, individuals, and bed assemblages. The effect of time averaging, however, must have been limited because rarefaction curves reveal no link between decay and species richness. This suggests that decay is not an important community controlling factor. Overall, our data suggest that transport was trivial and the traditional distinction between a pre- and postslide environment is unnecessary. It is likely that all specimens present at the time of burial would have been preserved independent of their original tissue composition and degree of preburial decay.
The presence of extensive sheets of Morania confluens, a putative benthic cyanobacterium, in most bed assemblages suggests that it: (1) provided a stable substrate and food source for a number of benthic metazoans, and (2) played a possible role in the preservation of nonbiomineralized animals, acting as a barrier in maintaining local anoxic pore-water conditions.
The taphonomy of assemblages of disarticulated remains of marine vertebrates is not well studied. Examination of a Middle Pennsylvanian chondrichthyan assemblage from Kohl's Ranch, Naco Formation, central Arizona, contributes to knowledge of such assemblages and reveals a complex taphonomic history. This vertebrate assemblage is restricted to two thin horizons associated with a concentration of disarticulated and tightly packed invertebrates. The vertebrate specimens are associated with phosphatic internal molds of molluscs and bryozoans. Most vertebrate specimens show abrasion. Several lines of evidence suggest that the specimens were abraded in a nearshore wave environment and subsequently transported offshore by a storm surge, where they were incorporated into an environmentally condensed assemblage. In particular, the presence of abrasion, a concentrated skeletal assemblage, significant amounts of siliciclastic sand, presence of intraclasts, phosphatic molds, and a basal lithologic discontinuity support this hypothesis; similar characteristics are found in many bone beds in the fossil record.
To test whether wave-dominated nearshore marine environments are capable of abrading vertebrate teeth, modern elasmobranch teeth (Odontaspis and Myliobatis) were placed in an experimentally simulated abrasive environment of fine siliciclastic sand. Results indicate that progressive degradation of specimens by abrasion and cracking occurs as the duration of abrasion increases. Abrasion occurs along the edges of both the crown and root, and cracking proliferates across the surface of the crown. These data confirm that wave-dominated marine environments can progressively abrade vertebrate teeth, and are consistent with the taphonomic hypothesis presented for the Kohl's Ranch vertebrate assemblage.
Geospatial data collected with a Nikon Total Station from a dinosaur quarry in the upper part of the Morrison Formation in north-central Wyoming were plotted on ArcGIS ArcScene software. The resulting three-dimensional maps indicate two distinct sauropod bone assemblages with closely associated shed theropod teeth separated by a weakly developed paleosol. Consequently, previous hypotheses that all bone elements and theropod teeth in the quarry were chronologically connected are amended. Synthesis of geological and paleontological data provides evidence that a juvenile Camarasaurus was the center of feeding activity in a shallow-water, palustrine-lacustrine setting in the lower assemblage. The high ratio of juvenile to adult allosaurid teeth suggests one or two adults in the company of several juveniles during a scavenging event. A high incidence of theropod teeth in the upper assemblage suggests that another feeding event may have occurred, but data loss from initial traditional excavation techniques precludes a more detailed interpretation. Although the Upper Jurassic Morrison Formation in the western United States yields abundant sauropod and theropod remains, few sites documenting theropod-prey interactions have been reported. Evidence of theropod feeding activities has been difficult to establish in seemingly homogeneous continental deposits with traditional excavation techniques alone. Geographic Information Systems (GIS) is a valuable tool that allows paleontologists to establish chronostratigraphic constraints in complex continental assemblages, assess the degree of time averaging, and evaluate important geospatial patterns.
The record of fossil mammalian remains from Poggio Rosso, Upper Valdarno, represents one of the richest samples for deciphering paleobiological information on latest Pliocene land faunas of central Italy. The taphonomic imprinting of this bone accumulation formed by Pachycrocuta brevirostris is investigated to determine the time-averaging of the assemblage, the structure of the paleocommunity from which it originated, and the interactions among some of the species represented in it, as well as the paleobiology and paleoethology of the hyenas. The bones were accumulated in a short time (within a year) during an episode of severe drought. The harsh environmental conditions debilitated the fauna and exposed it to predators. Harsh conditions also forced clans of Pachycrocuta hyenas, which under normal circumstances would have been primarily scavengers, to turn into killers of prey that now were easier to subdue. Thus portions of killed prey were added to the scavenged carcass parts usually taken to the dens. Furthermore, the remains at Poggio Rosso seems to add weight to the hypothesis that middle latitude P. brevirostris might have had seasonal breeding and parental care of cubs.
Laboratory experiments on microbial decay were used to investigate the conditions required for pyritization of decaying twigs, as it provides an important source of data on the anatomy of fossil plants. Plane (Platanus acerifolia) was chosen as the experimental taxon, because this genus is preserved in pyrite in the Eocene London Clay. Experiments were designed to develop sulfate reduction under marine conditions, and each contained estuarine sediment with added iron oxide (1%) with a layer of pH-buffered artificial seawater medium above, which had a labile organic-matter source (yeast extract) and an inoculum of anaerobic, sulfate-reducing bacteria. Twigs (5) were pressed into the sediment and the systems incubated with a loose lid, in air at 15°C for up to 12 weeks. These conditions were varied to reflect those thought to promote pyrite formation in the natural environment (high concentrations of reactive iron and bioavailable organic matter, local concentration of decaying material, concurrent high concentrations of sulfide and iron, and oxidation of iron sulfides), plus variations in incubation time, anoxia, pH, and sulfate supply. Changes in the chemistry of the decay systems were monitored with oxygen and pH microelectrodes, and concentrations of sulfate, sulfide, ferrous iron, and sedimentary solid-phase sulfide pools were analyzed at the end of each experiment. All systems rapidly developed bacterial sulfate reduction, dissolved iron, and iron sulfides. In only 2 out of 18 reference systems were areas of some twigs pyritized, however, although this did occur rapidly (5.4 weeks). No twigs in the modified systems were pyritized despite up to a 240% increase in solid-phase iron sulfides, the presence of diffusion gradients of ferrous iron and sulfide, the focus of sulfate reduction on the twigs, and pyrite formation in the sediment. Neither slightly oxidizing nor completely anoxic conditions enhanced pyritization. These results suggest that conditions that promote formation of sedimentary pyrite differ considerably from those that facilitate pyritization of twigs. Pyritization can occur rapidly in conditions common in marine sediments with intense microbial activity, but the process is rather random and may be controlled by the nucleation of pyrite on decaying tissue rather than factors controlling pyrite formation.
The first report of a multitaxon aggregation from the Early Triassic Lystrosaurus Assemblage Zone is presented here. The aggregation includes two vertebrates—the suricate-sized cynodont Galesaurus planiceps and a gecko-like procolophonoid Owenetta kitchingorum—and a diplopod millipede. The high degree of articulation of the skeletal remains and the preservation of delicate bones in situ suggest that the animals were either rapidly buried or died in a place protected from weathering and agents of dispersal. Two hypotheses are possible to explain this multitaxon aggregation: trapping in a floodplain gully followed by immediate burial or shelter-sharing. The latter hypothesis is favored after the analysis of the fossil evidence recorded in the South African Karoo Basin.