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.
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.
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 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.
Late Kimmeridgian and Tithonian ammonoid distributions when compared with Late Jurassic paleoclimate simulations show likely causal relationships with sea surface water temperature and upwelling, and possibly shed light on the temperature limitations of ammonoids. Results from modelled seasonal sea surface temperature, sea ice distribution, precipitation-evaporation, and wind-driven upwelling permit the evaluation and quantification of paleoenvironmental factors favourable as well as pernicious for ammonoid distribution. -from Authors
Large mammals, especially Hippopotamus amphibius, have created a distinctive set of traces in a spring-fed freshwater wetland on the margin of the saline-alkaline crater lake in the Ngorongoro Crater, Tanzania. It is comprised of a ∼30 m diameter zone of deep (<2m) bioturbation due to hippo wallowing, surrounded by dendritic to radial hippo trails 1–5 m wide and <0.5 m deep, infilled with organic-rich mud. These trails narrow and thin as they grade into trackways on the lake flat. Tracks and trackways of more terrestrial mammals, such as bovids and equids, are found in the lake flat muds surrounding the hippo-dominated area. Associations of sedimentary structures such as these are important indicators of paleoenvironmental conditions where they are preserved in the sedimentary record, due to the strong affinity of Hippopotamidae for freshwater environments.
Substrate properties, such as grain size, water content, shear strength, and content of organic mucus, influence the life activity of benthic organisms and their trace-fossil record. This study deals with actualistic experiments using small crustaceans (amphipods and isopods) moving mainly over plaster of Paris surfaces in various stages of hardening. Several consistencies, such as semi-fluid, very soft, soft, soft-stiff, stiff, and very stiff are distinguished. The morphology of surface lebensspuren shows a broad variety that directly depends on stiffness of the substrate and the capability of the organisms to cope with it. Semi-fluid substrates hinder the organisms in their motility - they move by plowing, whereby sediment flows back behind the animal and refills the furrow, leaving an indistinct line on the surface. With increasing stiffness, traces acquire additional morphological details such as levees and median furrows. On stiff and very stiff substrates, the crustaceans do not penetrate into the sediment, but move by jumping, with the consequence of producing jumping traces instead of furrows. Some lebensspuren obtained in the experiments, especially furrows, are similar to some trace fossils attributed to several non-arthropod animal groups such as annelids, bivalves, and gastropods. This study helps clarify the interpretation and taxonomy of trace fossils and in reconstruct of substrate properties during their formation.
A neoichnological resin cast of modern burrows from a mudflat at Willapa Bay, Washington, reveals much of the architecture of the infaunal community present locally within sediments of this mudflat. The cast structures comprise Psilonichnus-, Skolithos-, Trichichnus-, Chondrites(?)-, Planolites-, Palaeophycus-, and Arenicolites-like burrows. The range of behaviors represented by these structures cannot be classified into an archetypal (Seilacherian) ichnofacies. The resin cast contains many coeval, composite burrows that may reflect certain commensal relationships. Notable composite associations include: (1) siphon shafts of Mya arenaria that are connected to and descend from crab domiciles; (2) an example of a pair of infaunal Mya arenaria that share a single siphonate shaft; (3) abundant Heteromastus burrows that connect to crab and bivalve burrows; and, (4) Nereis burrows that are connected to crab domiciles. It is suggested that such contemporaneous relationships, if passed into the fossil record, might be misinterpreted as representing tiering or faunal succession. The reported observations provide a new perspective on the potential significance of interpenetrating trace fossils and complex ichnofabrics.
The St. Mary River Formation (Maastrichtian) consists of anastomosed fluvial deposits containing several hundred track-bearing beds. Paleontologic and sedimentologic analyses of these beds indicate that large herbivores, ornithopods, inhabited a seasonal wetland dominated by marshes and lakes. Shallow tracks in fine-grained sediments, formed as the sediments dewatered to the point of stiffness, display the highest resolution of detail. The preservation potential of tracks in anastomosed fluvial deposits is large because of the abundance of soft substrates to record the tracks and the occurrence of annual flooding to rapidly bury the footprints. Comparison of the St. Mary River Formation to other anastomosed fluvial deposits as old as the Early Jurassic confirms that tracks are common in this type of deposit. The variation in preservation of track types and depth of penetration raises the possibility that ornithopods employed a survival strategy involving seasonal wetlands. The wetlands provided an abundant food source and at the same time the combination of a soft substrate and flooded conditions would have effectively countered the superior speed and agility of large carnivores. The relatively common occurrence of ornithopod eggshells from anastomosed fluvial deposits suggests that the abundant food supply accompanying the wet season also made the wetlands an ideal location to rear young. These data can be used to refine the interpretations of depositional environment derived from the sediments by allowing estimates to be made regarding the early post-depositional conditions of the sediments. 95 refs., 7 figs., 1 tab.
The recognition of fossilized charcoal has revealed a long history of wildfire, although the earliest (pre-Late Devonian) records remain conjectural. A variety of approaches (experimental and natural charring, comparative anatomy of a range of plant tissues following combustion, and preliminary reflectance studies) demonstrates that smoldering surface fires already occurred ∼405 million years ago (Lochkovian; Early Devonian) in a vegetation of short stature composed mainly of small plants with smooth stems and terminal sporangia. In addition, the textures recorded in pyrite permineralizations are anomalous when compared with those of Lower Devonian and later examples, and indicative of the involvement of different taphonomic processes. From comparison with experimentally pyritized charcoal, they further suggest that the plants were burned before fossilization. The small millimeter-sized fossils (mesofossils) with remarkable, uncompressed cellular preservation indicate the importance of charcoalification in the determination of affinities and functioning of early land plants, and hence the reconstruction of ancient ecosystems.
The Great Bank of Guizhou (GBG) is an isolated Late Permian to Late Triassic carbonate platform in the Nanpanjiang Basin of Guizhou Province, southwest China. A faulted syncline exposes a cross section of the platform margin, including a well-preserved Anisian (earliest Middle Triassic) reef complex approximately 1 km wide and 800 meters thick. Geochronologic constraints from associated basin-margin strata indicate that reef development initiated late in the Early Triassic, making it the oldest-known platform-margin reef complex of the Mesozoic Era. The reef framework consists primarily of microspar-filled tubes ~100 µm wide and up to a few cm long that are embedded in irregular to branching, mm-scale masses of micrite, traditionally assigned to the problematic genus Tubiphytes . Based on preserved sporangia, the Nanpanjiang structures are interpreted as microbially induced micritic precipitates that formed in association with an otherwise uncalcified alga. A low-diversity metazoan and algal community also occurs within the reef complex, but these organisms did not contribute significantly to the reef framework or to the accretion of the reef complex. Rather, reef development is interpreted to have resulted largely from the stabilization of platform-margin sediments by algae and associated microbial mats. Only gradually, through the Middle and Late Triassic, did framework-building metazoans evolve to occupy and then construct reefs on the margins of carbonate platforms. Earth and Planetary Sciences Organismic and Evolutionary Biology
Large-bodied, semiaquatic herbivorous mammals have been a recur- ring component of most continental ecosystems throughout the Ce- nozoic. Identification of these species in the fossil record has largely been based on the morphological similarities with present-day hip- popotamids, leading to the designation of this pairing of body type and ecological niche as the hippo ecomorph. These morphological characters, however, may not always be diagnostic of aquatic habits. Here, enamel 13C and 18O values from living hippopotamuses were examined to define an isotopic signature unique to the hippo eco- morph. Although 13C values do not support unique foraging habits for this ecomorph, living and fossil hippopotamids typically have low mean 18O values relative to associated ungulates that fit a linear regression (18Ohippopotamids 0.96 0.09 ·18Ofauna 1.67 2.97; r2 0.886, p 0.001). Modeling of oxygen fluxes in large mammals suggests that high water-turnover rates or increased water loss through feces and urine may explain this relationship. This relation- ship was then used to assess the aquatic adaptation of four purported hippo ecomorphs from the fossil record: Coryphodon (early Eocene), Moeritherium and Bothriogenys (early Oligocene), and Teleoceras (middle-late Miocene). Only fossil specimens of Moeritherium, Both- riogenys, and large species of Coryphodon had 18 O values expected for hippo ecomorphs; 18O values for Teleoceras and a small species of Coryphodon were not significantly different from those of the as- sociated fauna. These results show that the mean 18O value of fossil
The first Late Cretaceous dinosaur tracksite recorded from Arizona, preserving over 100 footprints, also preserves sinuous grooves here interpreted to be impressions left by the tail of a large vertebrate as it dragged through sediment now assigned to the Toreva Formation. Although no footprints are preserved that can be referred unambiguously to the individual or individuals that produced the tail-drag marks, the dynamic nature of the surface upon which the tracks were made, likely a river or stream shoreline, could have easily resulted in their disturbance or obliteration. As best as can be determined on the basis of a depauperate marine fauna from the lower part of the unit, and on the basis of a substantial hiatus that separates the lower and upper parts, the site is probably middle Coniacian in age. These tracks and drag marks add significantly to the exceptionally rare record of terrestrial vertebrates known from the Toreva Formation.
Filamentous and possible coccoid bacteria are preserved in calcite vein-fills cutting Devonian lacustrine limestones in the Tynet Burn fish bed near Fochabers, N.E. Scotland. Filamentous bacteria grew into open space off the walls of fractures and are concentrated as clumps on some detrital grains. The filaments were coated by iron hydroxide and are now represented by hollow canals 0.3-0.4 mu m diameter enclosed in a sheath of acicular hematite crystals each 2 mu m long by 0.2 mu m broad. Possible coccoid bacteria are represented,by hematite spheres 0.5 to 2 mu m diameter. The bacteria grew in cracks formed by deformation of the sediments after initial compaction, and formation of calcareous concretions. Fossil fish are preserved in red and purple colors and the surrounding matrix displays a halo of oxidation which is considered to have been bacterially mediated. Bacterial invasion of sediments took place during lowstands of the Middle Devonian Orcadian Lake when the sediment was in the vadose zone. A clear association is seen between the bacteria-bearing veins and reddening of the succession. Organismic and Evolutionary Biology
Live microbial mats and bacterial were collected from modern tidal flats. Over 50 experiments were set up in the laboratory in which live, naturally dead, and sterilized (autoclaved) dead filamentous cyanobacteria were inoculated with bacterial cultures. The experimental data demonstrated that precipitation of calcium carbonate only occurred on cyanobacterial filaments in the presence of live bacteria. Furthermore, dead cyanobacteria were coated with calcium carbonate much more quickly and to a greater extent than live cyanobacteria. The micritic, clotted fabric, exhibited by many ancient stromatolites as well as the mesoclotted fabric of thrombolites are inferred to be the result of bacterially induced precipitation of calcium carbonate. -from Authors
Bone fossilization is generally thought to be predominantly slow and geochemically controlled, whereas soft-tissue preservation is rapid and microbially enhanced. Microbial destruction of bone has been well researched, but potential preservational influences of microbes on bone are relatively unstudied. Building on previous work, this study examined evidence for microbially induced mineralization of bone buried in a simulated terrestrial setting using an actualistic, experimental approach designed to allow both biotic and abiotic precipitation to occur. Four trials were conducted in 2002 by burying bleached fresh bone cubes in river sand through which calcium carbonate saturated water was percolated. A "natural" trial used unmodified river sediment with natural bacterial populations. Two trials were run with bleach or sodium azide to reduce or eliminate bacterial populations. A fourth trial used washed, bleached sediment with reintroduced bacteria. After one week, the natural trial showed signs of mineral precipitation that cemented sand grains to the bone cubes. Bones from both the natural and washed trials were completely covered by adherent sediment after six weeks. After twelve weeks, cancellous bone in the nonsterile trials was permineralized while antiseptic trials and compact bone showed no indications of permineralization. Nonsterile trials also exhibited partially calcified fungal hyphae and possible lithified bacteria. This study provides additional experimental evidence that microbes are important agents of fossilization for bone as well as soft tissue. It also supports the inference that microbially induced mineral precipitation can play a significant role in bone fossilization by enhancing bone survival through early permineralization until apatite recrystallization occurs.
Microfacies analysis and point Counts of thin sections from 608 hand samples were used to track changes in the abundance and diversity of fossil grains through the extended recovery interval following end-Permian mass extinction on the Great Bank of Guizhou (GBG)-an isolated Late Permian to Late Triassic carbonate platform in south China. Exposure of a two-dimensional cross-section of the platform permits the comparison of faunal patterns along an environmental gradient front shallow to deep water The diverse Late Permian biota was dominated by calcareous sponges, crinoids, articulate brachiopods, foraminifera, and calcareous algae. In contrast, Early Triassic communities were dominated by mollusks, with increasing abundance of crinoids beginning in the Spathian. Increase in the diversity and abundance of fossils 071 the GBG was confined to a brief interval near the Spathian-Anisian boundary and concentrated along the platform margin. Later Middle Triassic diversification, the return of calcareous algae and calcareous sponges, and the appearance of scleractinian corals did not substantially alter the mollusk-crinoid- Tubiphytes assemblage before the end of the Middle Triassic. The low abundance of skeletal grains in Lower Triassic strata implies: (1) similarities in the relative contributions of micrite, microbialites, and oolites to Neoproterozoic carbonates result, at least in part, from the temporary removal of skeletal sinks for calcium carbonate; and (2) animals with hard skeletons remained at low abundance from the time of the end-Permian extinction through much of the Early Triassic. Earth and Planetary Sciences Organismic and Evolutionary Biology
A recently discovered dinosaur tracksite from the Upper Jurassic Morrison Formation, Bighorn Basin, Wyoming, contains abundant sauropod tracks that exhibit varying degrees of preservation. Most of these tracks appear as indistinct bulges on the bottoms of sandstone beds, but several are well preserved and show foot-pad and skin impressions. Three track morphotypes are recognized: a sauropod pes print, a Brontopodus-like manus print, and a diplodocid manus print. The Brontopodus-like manus print most likely represents the footprint of a brachiosaur. This morphotype also contains evidence of phalangeal nodes—the first reported for a sauropod manus. The diplodocid manus print is unique because it contains impressions of a substantial ungual on digit I and a heel pad. A partial sauropod track cast also contains an impression of interlocking, polygonal scales. This is only the second known North American sauropod footprint that contains skin impressions.
The spectrum of preservational quality of the tracks and associated trace fossils is used to infer the relative moisture content of the original substrate. Moisture content of the original substrate is estimated to have been moist to borderline saturated. Observations of the tracks at the study areas also are used to establish a list of features that can be used to distinguish deep vertebrate tracks from load casts resulting from gravity-induced soft-sediment deformation.
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.
Publicación ISI Email : email@example.com A consensus on the biostratigraphic age and depositional environment of the Navidad, Ranquil, and Lacui formations exposed along the tectonic margin of central Chile has been elusive due to conflicting evidence. This study resolves this dilemma and gains further insight regarding the history of the Chilean coast. Problematic interpretations stem primarily from the remarkable similarity between the molluscan fauna of these units with those well documented for the late Oligocene to early Miocene of Peru. Planktic foraminifers, however, indicate that the Chilean sections accumulated in the late Miocene to early Pliocene interval following a regional hiatus that extends into the Eocene. The prevalence of mixed-depth bathyal assemblages of benthic foraminifers and ostracodes, the majority of which include lower-bathyal (> 2000 m) indicators, reveals that downslope displacement was a primary mode of deposition in the basins. Although the molluscan assemblages are dominated by shallow marine taxa, most include species that range into or are restricted to deeper waters. Sedimentary features connote rapid subsidence and deep-water deposition of gravity flows. Although older Tertiary and Cretaceous planktic foraminifers in several assemblages indicate reworking of older units, lack of data on pre-Tortonian faunas of this region precludes recognition of other age-discordant components that could constitute a significant portion of the recovered fauna. The findings of this study revise the prevailing conception of the region's geologic history that considered these units to be early to middle Miocene shelf deposits and indicate that infilling and uplift have characterized the nearshore basins since the late Pliocene.
This study details occurrences of Teredolites ichnocoenoses from modern bay-margin settings in Willapa Bay, Wash- ington. In particular, the paper scrutinizes in situ log- grounds from two intertidal zones situated immediately seaward of low-lying, supratidal, forested marshlands that are fed by small streams. Vestiges of remnant marshes and streams are preserved on intertidal flats as in-situ roots, broken stumps, strewn logs, abundant organic detritus, and organic sandy mud. Xylic material and organic sedi- ment were deposited in the supratidal marshes: tide and wave processes truncated the swamps, exposing in situ tree- root networks and the lowermost supratidal sediments. Stream and swamp deposits overlie and incise older Pleis- tocene strata. Both units are overlain by discontinuous, modern intertidal deposits. The intertidally exposed stumps and logs support a di- verse community of animal and plant life. Boring organ- isms, encrusters, and refugium seekers are found on and within the xylic substrates. Some encrusting animals and all of the boring fauna produce traces that are comparable to ichnofossils reported by palichnologists. Wood-boring traces reported in this study are similar morphologically to the ichnogenera Caulostrepsis, Entobia, Meandropolydo- ra, Psilonichnus, Rogerella, Teredolites, Thalassinoides, and Trypanites. Most of these ichnogenera have not been re- ported from rock-record examples of the Teredolites ichno- facies. The stratigraphic and environmental significance of the reported (modern) locales is consistent with previous stud- ies that associate Teredolites ichnofacies with base-level rise in marginal-marine environments. At Willapa Bay, bored xylic media form a coeval surface with adjacent, bur- rowed firmgrounds as well as softgrounds.
Subsurface shell beds that are common on the eastern Alabama/western Florida Panhandle shelf provide paleoenvironmental and taphonomic data that demonstrate a dynamic depositional history with reactivation and amalgamation to the base of the Holocene marine transgressive package. In most of the ten shell beds we examined, shallow-marine mollusks, large soritid foraminifera, and cupularid bryozoans are common. Bioclasts occur in all preservation states, but a majority are pristine or only slightly altered. Normal grading, and concave-up, stacked, and random fabrics are common. In three of these shell beds, a relict estuarine component characterized by poorly-preserved Chione cancellata can be detected. Three other shell beds contain 1 of 2 estuarine molluscan assemblages. One assemblage is characteristic of fine-grained and the other of coarser-grained substrates. In all estuarine shell beds, bioclast preservation ranges from excellent to mixed, and bioclasts have random fabrics and locally are normally graded. Most molluscan remains are indigenous and assemblages range from within-habitat time-averaged to environmentally condensed. Shell beds are thick (up to about 75 cm) and overlie bay or shoreface ravinement surfaces, indicating that shell beds first accumulated as coarse transgressive deposits, and are composite concentrations modified by multiple events. Shell beds, however, are amalgamated. Only fabrics reflecting the final modifying events are preserved, and evidence of previous accumulation processes are obliterated. For marine shell beds especially, these final agents were, and possibly continue to be, high-energy events such as storms (winter cold fronts, hurricanes) or currents associated with Loop Current eddies. Episodically high sedimentation rates associated with these events may have acted as a buffer, preventing a long history of reworking and exposure (thus reducing shell alteration) typical of transgressive lags. Our results indicate that: 1) within-habitat time-averaged and environmentally condensed assemblages can be distinguished by combining taphonomic and environmental data, even when the ranges of environmentally disparate species overlap, 2) transgressive-lag deposits can be composed of well-preserved bioclasts, and 3) transgressive lags can be reworked and amalgamated but still be recognizable on the basis of their stratigraphic context.
The end-Permian mass extinction was the largest Phanerozoic biotic crisis that resulted in significant and permanent ecological change. In order to examine ecological aspects of the recovery, shell beds deposited in environments ranging from nearshore to storm wave base were examined from three stratigraphic intervals in the Lower Triassic of the western U.S. Shell beds of the first interval, the Griesbachian Dinwoody Formation, are low-diversity, monospecific beds of Claraia and Promyalina, commonly with the inarticulate brachiopod Lingula. Data from the Nammalian Sinbad Limestone (Moenkopi Formation) provide a small window into the second time interval, in which common low-diversity (bivalves and microgastropods) shell beds occur. Within the third interval, represented by the Spathian Virgin Limestone (Moenkopi Formation) and Thaynes Formation, the bivalves Promyalina and Permophorus are found in both monospecific and polytaxic beds. Crinoids are also commonly found as encrinites and as significant contributors to the matrix of these beds. Shell beds range in thickness from pavements to 10s of centimeters and show variable internal complexity. The persistence of monospecific shell beds throughout these three intervals is significant as support for long-term stress during the recovery interval that is not apparent from sedimentological data alone. Although these Early Triassic beds primarily are comprised of members of the Modern Evolutionary Fauna, they are more similar to beds from the Paleozoic in thickness and taphonomic characteristics.
Dinosaur track beds occur at several localities in the uppermost Cretaceous (Maastrichtian) North Horn Formation in the Wasatch Plateau, central Utah. The track bed localities, separated by up to 80 km, also contain dinosaur body fossils. At the type locality at North Horn Mountain in Emery County, more than 100 individual exposures and / or stratigraphic levels within a 1.2-km 2 study area exhibit tracks in vertical cross-sectional view. These biogenic structures are similar to others that have been interpreted elsewhere as deep dinosaur tracks. At the type section, track, beds vertically span at least 183 m from the base of the formation up to a few meters below the highest dinosaur eggshells, which are interpreted to occur immediately below the Cretaceous-Tertiary (K-T) boundary interval. Track occurence in the North Horn Formation demonstrates that large dinosaurs were present in central Utah until very shortly before the K-T boundary. The track structures feature deformation, overprinting, and slip striae that are interpreted to exhibit individual dinosaur behavior. Some striae exhibit repeating patterns that suggest tubercle configuration or scale patterns on dinosaur feet. Track groupings at any individual level suggest that many animals repeatedly congregated in topographic lows of the floodplain or near shifting anastomosing river systems, and track-size distribution largely suggests the presence of different sizes or age groups of herbivores. The conditions produced by anastomosed fluvial environments provided the depositional setting responsible for formation and preservation of most of the North Horn tracks.
Twenty-four ichnospecies of vertebrate trackway have been recorded from the Permian sediments of Scotland, most found in the Corncockle Sandstone Formation of Dumfries and Galloway and the Hopeman Sandstone Formation of Grampian. Studies on these trackways suggest that the trackmakers were slow, broad animals that moved with a distinctly sprawling or semi-erect gait but without dragging their tails or feet. Total body lengths vary from 116 mm to 830 mm. A study of manus and pes skeletons and body lengths of the various groups of Permian vertebrates suggests that the Cotylosauria, the Pelycosauri and the Therapsida were the main producers of the Scottish trackways. -from Author
Using abundance data, this study explores quantitative patterns from marine benthos, including implications for paleogeography, depositional environment, stratigraphic position, taxonomic groups (brachiopod or mollusc), substrate preferences, and ecological niches. Twenty-nine brachiopod- and bivalve-dominated fossil assemblages from the Pennsylvanian and Early Permian of North and South America, Thailand, and Australia were analyzed from carbonate-platform environments; specifically, Nevada, Kansas, Oklahoma, Texas, Utah, New Mexico, Venezuela, Kanchanaburi (Thailand), and Queensland (Australia). Samples were categorized by paleogeographic location, depositional environment, and age to help differentiate factors controlling the faunal patterns. Pooled from primary and summary literature resources, 336,321 specimens were identified to genus level and classified in terms of taxonomic membership, substrate preference, and ecological niche. Data were analyzed using detrended correspondence analysis (DCA) and multi-response permutation procedure cross-validated a-priori categories (e.g., paleogeography, depositional environment, stratigraphic position, and specimen ecology). Multivariate analyses indicate that the separation between genera and the orthogonal trends implies that paleoecological patterns within the studied late Paleozoic faunal associations were influenced strongly by the abundance of sessile versus mobile faunal components.
The study of Upper Cretaceous and Lower Paleogene benthic foraminifera from the Bidart section (SW France) provides detailed data on the paleobathymetry as well as paleoenvironmental conditions across the Cretaceous/Paleogene (K/Pg) boundary. A quantitative analysis of benthic foraminiferal assemblages from the Upper Maastrichtian Abathomphalus mayaroensis Biozone and the Danian Guembelitria cretacea, Parvularugoglobigerina eugubina, and Parasubbotina pseudobulloides Biozones was performed. Benthic foraminifera indicate that the upper Maastrichtian and lower Danian sediments at Bidart were deposited in the upper–middle part of the slope.
Benthic foraminiferal assemblages indicate mesotrophic conditions during the late Maastrichtian and a strong decrease in the food supply to the sea floor coincident with the K/Pg boundary. This change in the trophic regime was related to the collapse of the food web triggered by the mass extinction of calcareous primary producers. Benthic assemblages in the lower Danian are strongly dominated by few species, and suggest that primary productivity was dominated by blooms of non-calcareous primary producers, creating a stressful environment for the benthic fauna. The faunal turnover, together with the geochemical evidence, is compatible with an asteroid impact scenario. Benthic foraminiferal assemblages suggest that primary productivity had not completely recovered more than 200 kyr after the K/Pg boundary event.
The diversity and abundance of bird and animal tracks preserved in Eocene strata of the Chuckanut Formation in Washington contrasts to the scarcity of body fossils. These ichnofossils were made by vertebrates that inhabited river margins, the only depositional environment favorable for track preservation. Three of the four localities described herein contain tracks from at least two different types of animals. Site SM-6 contains approximately 200 shallow circular plantigrade footprints, perhaps made by a type of archaic mammal of the Orders Pantodonta or Dinocerata. Site RU-1 yielded footprints from a small shorebird and tracks from an early equid or tapiroid. The same type of perissodactyl tracks were preserved at Site KC-1, along with a single webbed bird track, and trackways from a large heron-like bird and a turtle. Site SM-9.5 contained multiple bird tracks of a type not found at the other localities. The discovery of tracks only at Chuckanut Formation sites that expose large bedding planes indicates the importance of considering outcrop architecture during the search for vertebrate ichnofossils, and inspires the hope that similar fossils may eventually be found in correlative formations in the Pacific Northwest.
A transitional Scoyenia–Mermia ichnocoenose from the Saint John River, Fredericton, New Brunswick, Canada, is dominated by elements of the Mermia ichnofacies, with traces comparable to Curvolithus, Helminthopsis, Gordia, Spirophycus, and Lockeia. Environmental characteristics are, however, more typical of the Scoyenia ichnofacies, with an emersion event providing conditions favorable to viewing traces preserved in a sand-softground substrate. Observation of in situ trace-making behavior allowed traces to be attributed to their progenitors, which include unionid and sphaeriid bivalves. An omission assemblage of vertebrate tracks also was present, comprising gull, raven, and mink. Oichnus-like borings were observed in some unionid shells. The shallow-tier trace assemblage created in a high-energy river channel may be expected to have a poor preservational potential, with loss of trace definition observed at the water margin during emersion and subsequent deterioration by eolian sediment transport.