Palaios

Exceptionally detailed soft tissues have been identified within the fossilized feces of a large Cretaceous tyrannosaurid. Microscopic cord-like structures in the coprolitic ground mass are visible in thin section and with scanning electron microscopy. The morphology, organization, and context of these structures indicate that they are the fossilized remains of undigested muscle tissue. This unusual discovery indicates specific digestive and taphonomic conditions, including a relatively short gut-residence time, rapid lithification, and minimal diagenetic recrystallization. Rapid burial of the feces probably was facilitated by a flood event on the ancient coastal lowland plain on which the fecal mass was deposited.

Patterns of longevity and rate of appearance of taxa in the fossil record indicate a different evolutionary dynamic between land plants and marine invertebrates. Among marine invertebrates, rates of taxonomic turnover declined through the Phanerozoic, with increasingly extinction-resistant, long-lived, clades coming to dominate. Among terrestrial vascular plants, rates of turnover increased through the Phanerozoic, with short-lived, extinction-prone clades coming to dominate from the Devonian to the present. Terrestrial vertebrates appear to approximate the marine invertebrate pattern more closely than the plant record. We identify two features which individually or jointly may have influenced this distinction. First, land plants continuously invaded stressful environments during their evolution, while marine invertebrates and terrestrial vertebrates did not. Second, the relative structural simplicity and indeterminate mode of plant growth vs. the relative structural complexity and determinate mode of animal growth may have influenced the timing of major clade origin in the two groups.

Late Permian reefs of the Capitan complex, west Texas; the Magnesian Limestone, England; Chuenmuping reef, south China; and elsewhere contain anomalously large volumes of aragonite and calcite marine cements and seafloor crusts, as well as abundant microbial precipitates. These components strongly influenced reef growth and may have been responsible for the construction of rigid, open reefal frames in which bryozoans and sponges became encrusted and structurally reinforced. These microbial and inorganic reefs do not have modern marine counterparts; on the contrary, their textures and genesis are best understood through comparison with the older rock record, particularly that of the early Precambrian. Early Precambrian reefal facies are interpreted to have formed in a stratified ocean with anoxic deep waters enriched in carbonate alkalinity. Upwelling mixed deep and surface waters, resulting in massive seafloor precipitation of aragonite and calcite. -from Authors

Short stratigraphic sections in apparently monotonous strata pose several challenges to high-resolution (<l m) correlation. A lack of distinctive marker horizons can prevent obvious visual correlations between the sections. The stratigraphic shortness of the outcrops further reduces the likelihood of any given section having a recognizable marker horizon. The Upper Ordovician Kope Formation of the Cincinnati, Ohio, area exhibits both of these problems and correlation within the Kope has not been accomplished easily, to date. However, cross-correlation of meter-scale cycles in the Kope can be used to identify potential correlations of small outcrops to larger, well-described outcrops. If multiple correlations are equally plausible, large-scale faunal transitions among facies fossils can then be used to select the best correlation. In this pilot study, two sections separated by 9 km are correlated successfully using these methodologies, which show promise for the correlation of numerous outcrops in the Cincinnati area. In addition, the methods described here may be applied easily to other areas of limited outcrop in which the rocks are so complexly cyclic that they, likewise, appear to be monotonous.

The Devonian to Carboniferous sinters of the Drummond Basin, Australia, are among the oldest well established examples of fossil subaerial hot springs. Numerous subaerial and subaqueous spring deposits are known from the geological record as a result of the occurrence of economic mineral deposits in many of them. Some are reported to contain fossils, but very few have been studied by paleobiologists; they represent an untapped source of paleobiological information on the history of hydrothermal ecosystems. Such systems are of special interest, given the molecular biological evidence that thermophilic bacteria lie near the root of the tree of extant life. The Drummond Basin sinters are very closely comparable with modern examples in Yellowstone National Park and elsewhere. Thirteen microfacies are recognisable in the field, ranging from high temperature apparently abiotic geyserite through various forms of stromatolitic sinter probably of cyanobacterial origin to ambient temperature marsh deposits. Microfossils in the stromatolites are interpreted as cyanobacterial sheaths. Herbaceous lycopsids occur in the lower temperature deposits.

In the modern oceans, the removal of dissolved silica from sea water is principally a biological process carried out by diatoms, with lesser contributions from radiolaria, silicoflagellates, and sponges. Because such silica in sediments is often redistributed locally during diagenesis to from nodular or bedded chert, stratigraphic changes in the facies distribution of early diagenetic chert provide important insights into the development of biological participation in the silica cycle. The abundance of chert in upper Proterozoic peritidal carbonates suggests that at this time silica was removed from seawater principally by abiological processes operating in part of the margins of the oceans. With the evolution of demosponges near the beginning of the Cambrian Period, subtidal biogenetic cherts became increasingly common, and with the Ordovician rise of radiolaria to ecological and biogeochemical prominence, sedimented skeletons became a principal sink for oceanic silica. Cherts of Silurian to Cretaceous age share many features of facies distribution and petrography but they differ from Cenozoic siliceous deposits. These differences are interpreted to reflect the mid-Cretaceous radiation of diatoms and their subsequent rise to domination of the silica cycle. Biogeochemical cycles provide an important framework for the paleobiological interpretation of the organisms that participate in them.

Tidal flat and lagoonal dolostones of the Neoproterozoic Draken Formation, Spitsbergen, exhibit excellent preservation of carbonate fabrics, including heavily calcified microfossils. The crust-forming cyanobacterium Polybessurus is preserved locally by carbonate precipitated on and within sheaths in mildly evaporitic upper intertidal to supratidal environments. In contrast, calcified filaments in columnar stromatolites reflect subtidal precipitation. Filament molds in dolomicrites independently document extremely early lithification. The presence of heavily calcified cyanobacteria in Draken and other Proterozoic carbonates constrains potential explanations for the widespread appearance of calcified microorganisms near the Proterozoic-Cambrian boundary. We propose that the rarity of Proterozoic examples principally reflects the abundance and wide distribution of carbonate crystals precipitated on the sea floor or in the water column. Cyanobacterial sheaths would have competed effectively as sites for carbonate nucleation and growth only where calcitic and/or aragonitic nuclei were absent. In this view, the Proterozoic-Cambrian expansion of calcified microfossils primarily reflects the emergence of skeletons as principal agents of carbonate deposition.

To better understand temporal variations in species diversity and composition, ecological attributes, and environmental influences for the Middle Cambrian Burgess Shale community, we studied 50,900 fossil specimens belonging to 158 genera (mostly monospecific and non-biomineralized) representing 17 major taxonomic groups and 17 ecological categories. Fossils were collected in situ from within 26 massive siliciclastic mudstone beds of the Greater Phyllopod Bed (Walcott Quarry — Fossil Ridge). Previous taphonomic studies have demonstrated that each bed represents a single obrution event capturing a predominantly benthic community represented by census- and time-averaged assemblages, preserved within habitat. The Greater Phyllopod Bed (GPB) corresponds to an estimated depositional interval of 10 to 100 KA and thus potentially preserves community patterns in ecological and short-term evolutionary time.

We are pleased that our paper has generated interest from Retallack and relish the opportunity to comment further on the intriguing timing of the end-Permian extinction. Perhaps not surprisingly, we do not agree with any of Retallack’s assertions and deal with them here in the order he presented them.

The taphonomy of assemblages of disarticulated remains of marine vertebrates is not well studied. Examination of a Middle Pennsylva- nian chondrichthyan assemblage from Kohl's Ranch, Naco Forma- tion, central Arizona, contributes to knowledge of such assemblages and reveals a complex taphonomic history. This vertebrate assem- blage is restricted to two thin horizons associated with a concentra- tion of disarticulated and tightly packed invertebrates. The verte- brate 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 off- shore by a storm surge, where they were incorporated into an envi- ronmentally condensed assemblage. In particular, the presence of abrasion, a concentrated skeletal assemblage, significant amounts of siliciclastic sand, presence of intraclasts, phosphatic molds, and a bas- al lithologic discontinuity support this hypothesis; similar character- istics 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 simu- lated 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.

Years of over-fishing combined with increased nutrient pollution have had a catastrophic effect on the ecology of the Chesapeake Bay. The Holocene record of bay mollusks may provide a baseline for ecological restoration, but the effects of taphonomic bias on these assemblages first must be assessed. In this study, a five-dead comparison was carried out on four sites distributed in the main channel of the upper bay. Molluscan death-assemblage data were obtained from replicate box-core samples from which whole specimens and fragments were sorted, identified, and counted. Data on live communities at the same sites, sampled over the past twenty years, were provided by the Chesapeake Bay Program, making it possible to examine the degree to which death assemblages reflect long-term changes in the live community. Traditional live-dead metrics document a strong agreement between live-community and death-assemblage estimates of species composition, richness, and abundance - 77% of the species in the live community are found in the death assemblage, and 99% of the individuals of species found in the death assemblage are found in the live community. Correlations between live and dead estimates of species richness are not statistically significant, although they do improve with longer-term sampling of the live community, Rank abundance of taxa in the death assemblage is correlated strongly and significantly with live rank abundance regardless of the duration of live sampling. These results suggest that Holocene molluscan assemblages may provide useful estimates of richness and abundance for Chesapeake Bay restoration.

Although microvertebrate accumulations are commonly used for paleoecological reconstructions, taphonomic processes affecting the final taxonomic composition of an accumulation are often ignored. This research explores the effects of abiotic taphonomic processes on the taxonomic composition of terrestrial microvertebrate accumulations by comparing a floodplain and a channel lag deposit from the Maastrichtian Hell Creek Formation in eastern Montana. Distribution of skeletal elements with specific physical attributes and relative abundance of taxa correlate with the hydraulic indicators (i.e., grain size, sedimentary structures) of the depositional facies. Transport distances, hydraulic equivalencies of dominant skeletal elements, amount of hydraulic sorting and reworking, and degree of time averaging vary between deposits and significantly affect taxonomic distributions. Relative abundance data, in conjunction with chi-square test results and rank-order analysis, show that size, shape, abrasion, and taxonomic compositions vary significantly between assemblages. The fine-grained assemblage is dominated by tabular, low-density elements, such as cycloid scales and fish vertebrae. Dense, equidimensional elements, such as teeth and ganoid fish scales, dominate the sandstone assemblage. Rank-order analysis results demonstrate that relative abundance of hydraulically equivalent skeletal elements from morphologically similar organisms can be compared regardless of accumulation in nonisotaphonomic deposits. Statistical comparisons were made among osteichthyans using ganoid scales, caudates using vertebrae, ornithischians using teeth, and testudinates using shell fragments. Results show that portions of the assemblage analyzed using hydrodynamically equivalent elements are not significantly different, despite different depositional environments.

The Kimmeridgian shallow-water carbonates of the Swiss Jura Mountains display stratigraphic levels containing mass accumulations of nerineoidean gastropods. The macro- and microfacies of the rocks with these nerineoidean assemblages suggest that their occurrence was related to water depth, physical energy within the habitat, sedimentation rate, substrate stability, and food supply. Typical nerineoidean shells are high spired and exhibit unique spiral laminae (folds) on the internal walls. Mass accumulations consisting of large, heavy individuals are believed to represent a semisessile, suspensionfeeding community that utilized an external food source. During times of enhanced precipitation and run-off from the hinterland, terrestrial organic matter may have been supplied into the shallow sea, thus forming a source of food for the nerineoids. Alternatively, relative sea-level change during transgression may have led to largescale coastal erosion and flooding of the platform, also potentially leading to a mobilization of organic matter. Size and morphology of the high-spired shells seem to indicate a trend toward maximal external volume. Development of folds within the shell is interpreted as a modification to maintain a small internal volume. The faunal and floral composition of the strata indicates a suspension-rich, highenergy environment with a low-sedimentation rate and a stable substrate. This allowed mass colonization by epifaunal nerineoids, which, in turn, formed a frameworklike structure. The heavy shells acted as a grid, trapping the sediment in between. In this way, a reinforced carbonate accumulation formed.

The terrestrial crisis that reportedly parallels the P / Tr marine mass extinction is based mainly on Northern Hemisphere microfloral assemblages and Southern Hemisphere Gondwanan macrofloral collections. It is well established that taphonomic filters control the ultimate collectable fossil assemblage in any depositional regime. Recognition and comparison of isotaphonomic assemblages are critical before conclusions can be drawn about evolutionary trends over time. Such an approach has been taken in the investigation of pre-boundary, trans-boundary, and post-boundary plant-fossil assemblages in the Karoo Basin, South Africa. Fourteen stratigraphic sections were evaluated in the Balfour and Normandien formations (Lower Beaufort Group), Katberg Formation, and overlying Burgersdorp Formation (Upper Beaufort Group). These include previously published (e.g., Bulwer, Bethulie, Carlton Heights, Wapadsberg, Commando Drift) as well as newly discovered (e.g., Clouston Farm) localities, and span the Late Permian to Middle Triassic. Fossiliferous intervals were characterized with respect to their sedimentology and plant taphonomy, and bulk collections were made at several stratigraphic levels for future evaluation of floristic and plant-insect associational trends. The depositional regimes and plant taphonomic character of assemblages change through time. Much of the Lower Beaufort Group is characterized by parautochthonous assemblages within oxbow-lake channel fills. Below the P / Tr boundary, these are replaced by allochthonous assemblages, poorly preserved in lateral-accretion deposits and barforms of relatively shallow fluvial nature. Allochthonous assemblages within the same fluvial context continue across the boundary into the earliest Triassic (Palingkloof Member and Katberg Formation, and typify the Middle Triassic where scour-and-fill structures preserve plant debris. Based on the literature, parautochthonous assemblages reappear in the Upper Triassic Molteno Formation. Hence, the change in taphonomic regime to poorly preserved allochthonous assemblages (dispersed, fragmentary adpressions) at the critical interval on either side of the P / Tr extinction event, but not coincident with, requires extreme caution when interpreting global patterns from these data. Additionally, the presence of plant fossils in the Early Triassic provides evidence for a vegetated landscape during a time when sedimentation patterns are interpreted to be the result of a land-plant die-off.

The predominantly fluvial strata of the Late Permian Beaufort Group in the southwestern Karoo Basin contain numerous fossils of therapsid reptiles. This study investigates the taphonomy of these fossils and the sedimentology of the floodplain deposits in which they are most commonly preserved. -from Author

Floodplain paleosurfaces are depositional surfaces that were exposed on ancient floodplains and have been preserved in the rock record. Three types of floodplain paleosurfaces have been recognised in the Late Permian Beaufort Group strata of the southwestern Karoo Basin of South Africa. They occur in sedimentary sequences which represent point bar proximal floodplain, and distal floodplain facies. The distribution of paleosurfaces in the Beaufort strata was ultimately controlled by flash-floods which transported sand out of the large meandering river channels into the floodplains. Preservation of the vertebrate and invertebrate traces was enhanced by a silty-clay veneer that accumulated on the sand surfaces during waning flood. The surfaces were buried without significant modification in parts of the floodplain where water became ponded. These were most commonly in crevasse splay channels, on distal crevasse splay lobes prograding into axial floodbasin lakes and in swales on the downstream portion of point bars. Ichnotaxa that appear to be environmentally specific include arthropod trackways of Umfolozia type on proximal crevasse splays, algal matted textures on distal crevasse splays and large 'septate' traces of Beaconites type on point bar ridges. Trackways of synapsid reptiles are present on many of the paleosurfaces and do not appear to be environmentally specific. Cross-cutting relationships of the ichno-fossils and desiccation features are compared with those of modern flash-flood deposits to reconstruct some time constraints on the formation, exposure, and burial of these surfaces.

Depositional cycles in fluvial successions are described here as chronostratigraphic packages of strata founded on a laterally extensive, scour-based, amalgamated channel-sand body, overlain by mudrocks, isolated channel fills, avulsion and splay complexes, and paleosols. Ten packages are described from the lower Horseshoe Canyon Formation (Campanian-Maastrichtian), one of a succession of clastic wedges filling the Alberta foreland basin in south-central Alberta. The structure of these packages is consistent with the fall-rise-fall cycle of base-level described in other studies, but the package-bounding scours and internal surfaces are discontinuous and difficult to trace in the mudrock-dominated strata. Terrestrial vertebrate fossils are preserved in relatively fossiliferous, facies-independent horizons 1 to 3 m thick that statistically correlate with the stratigraphic position of package scours and surfaces. Fossiliferous horizons formed as a result of attritional accumulation under an optimum, relatively low, regional deposition rate. Not only do these horizons aid in locating package surfaces, but they also provide insight to the interaction of the package-scale, base-level oscillation with the larger-scale fluctuation in accommodation associated with the formation of the clastic wedge. As such, fossiliferous horizons in the Horseshoe Canyon Formation make better boundary markers than do paleosols, splays, coal seams, or even the surfaces associated with package structure. Therefore, the vertebrate fossil record may supply a means of stratigraphically evaluating sections in other locations in which typical sedimentological and architectural cues for surfaces are absent.

Models of the ecological succession of encrusting organisms in the recent coralline-algal reefs from Atol das Rocas (South Atlantic, Brazil)are presented for a windward and a leeward transect. These models are constructed on probability calculations that use the relative frequencies of upward (overgrowing)transitions (succession)of observed pairings of reef- framework builders. These are taken from blocks excavated across a reef-front / reef-crest / reef-flat section in the windward transect, and a reef-crest / reef-flat section in the leeward transect. The main substrate available for encrustation is the dominant primary framework - building coralline alga Porolithon cf. pachydermum. The diversity (number)and complexity of competitive interactions among encrusters increase from reef-front and reef-crest to reef-flat in both transects. Vermetid gastropods and homotrematid and acervulinid foraminiferans are the most important secondary framework builders in the surveyed reefs. The proposed successional models present an account of the operation of competitive processes over time and space, providing further evidence of a non-hierarchical pattern of competition for space. Changes in reef-framework succession along the reef transects are controlled by extrinsic factors. However, intrinsic factors are likely to be the main control of the succession when windward and leeward reef-flat environments are compared. Finally, reef-framework successional models derived from transition-probability matrices can be used to investigate paleoenvironmental gradients (such as wave energy)in preserved frameworks from ancient reefs and to understand reef-community structure based on the succession of encrusting epibionts. Pages: 145-158

Although onshore to offshore retreat of brachiopods, in terms of their community-level abundance, took place through the Mesozoic and Cenozoic, this study shows that comparable trends also occurred repeatedly on a short time scale and mainly were driven by variations in sediment and nutrient supply. In the Kössen Formation (Upper Triassic), brachiopods retreated to offshore habitats during nutrient-rich, siliciclastic regimes and expand to onshore habitats during nutrient-poor, carbonate regimes. Epifaunal bivalves occupied onshore and offshore habitats during both siliciclastic and carbonate regimes. Infaunal suspension-feeding bivalves expanded to offshore habitats during nutrient-rich, siliciclastic regimes and retreated from offshore habitats during nutrient-poor, carbonate regimes. Thus, the onshore to offshore retreat of brachiopods and the offshore expansion of infaunal bivalves repeatedly coincided with the switch from a nutrient-poor, carbonate regime to a nutrient-rich, siliciclastic regime. Because brachiopods and epifaunal bivalves were abundant in micrite-rich, soft-bottom habitats, the replacements between infaunal and epifaunal communities cannot be explained by variations in substrate consistency alone. Differences in guild structure between siliciclastic and carbonate regimes and onshore to offshore replacements indicate that distribution of bivalves and brachiopods is related to their differential response to low nutrient supply, turbidity, and, possibly, oxygen levels. Based on actualistic evidence, brachiopods are able to thrive in nutrient-poor conditions due to low metabolic demands and are less tolerant of high-turbidity conditions than bivalves. Epifaunal bivalves that co-occur with brachiopods in nutrient-poor habitats may have been characterized by higher clearance rates in contrast to infaunal bivalves with similar metabolic requirements. Although higher biogenic sediment disturbance or other biotic interactions could play a significant role in the retreat of brachiopods to offshore habitats, this study highlights the importance of varying nutrient supply and turbidity in governing onshore to offshore replacements on short time scales.

In a companion study to earlier work in the Indo-Pacific, taphonomic alteration in reef-coral death assemblages was assessed in four distinct reef habitats ranging from 2-30 m water depth in the Florida Keys reef tract. Physical and biological taphonomic attributes measured from coral specimens showed great variability with respect to reef environment. Physico-chemical degradation (abrasion and dissolution) was greatest in reef-crest and patch-reef environments. With the exception of encrusting foraminifera, coverage by epi- and endobionts was higher in deep-reef environments (20 m and 30 m). Variability in dissolution and abrasion is likely the result of the different energy regimes present in the reef habitats examined. Variability in biological attributes results from a combination of increased residence time of coral skeletons on substrates in deep-reef environments, higher overall coral skeletal densities of corals inhabiting deep reef environments, and increased nutrient availability in the deep reefs sampled. Clear gradients in the degree of taphonomic alteration of reef corals with reef habitat indicate the utility of corals as taphofacies indicators in ancient reef settings. In contrast to shallow-water reefs on the Great Barrier Reef, taphonomic alteration of corals in the Florida Keys was equitable across growth forms.

Different morphotypes of vertically embedded ammonoid shells are abundant in the 1-km-thick deposits of the Santa Marta Formation, Antarctica, that record the evolution of a deep-water delta system. Vertical shells deposited in water depths well below the theoretical limit imposed by hydrodynamic and hydrostatic analyses are preserved as isolated specimens, dense concentrations associated with abundant wood fragments (pod preservation), or as dense concentrations inside and around large ammonites (sheltered preservation). Taphonomic analysis indicates that (1) vertical shell orientation is primary; (2) postburial reworking was minimum or absent, as indicated by consistent shell orientation parallel to regional paleocurrents and complete preservation of fragile shells with phragmocones filled with drusy calcite; and (3) vertical orientation is not biased towards a preferred morphotype. Rapid sedimentation, including deposition from high- and low-density currents, tempestites, and weak bottom currents carrying a dense suspension of ammonoid shells and wood fragments, was one of the main factors controlling the vertical preservation below the limits imposed by theoretical hydrostatic analyses. Another important factor was the plugging of the siphuncular tube with clay particles during transportation. The clay plug was stiff enough to resist the ambient hydrostatic pressure, avoiding or delaying the waterlogging of the phragmocone.

A remarkable encounter between two Cretaceous mosasaurs of the same species and a sick or recently dead nautiloid (Argonautilus catarinae Sundberg) is recorded. It is possible that the mosasaur was training its young to attack shelled cephalopods. This is the first recorded attack by two mosasaurs on a nautiloid in the eastern Pacific Ocean, although ammonites preyed upon by mosasaurs are known from the west coast of North America.