Registered users receive a variety of benefits including the ability to customize email alerts, create favorite journals list, and save searches.
Please note that a BioOne web account does not automatically grant access to full-text content. An institutional or society member subscription is required to view non-Open Access content.
Contact firstname.lastname@example.org with any questions.
The Kirkpatrick Basalt (Jurassic) of South Victoria Land and the Central Transantarctic Mountains, Antarctica, includes sedimentary interbeds representing shallow lakes and ephemeral ponds (some with microbial mat accumulations), deep permanent lakes, and lake-margin areas, especially vegetated wetlands. Fossil assemblages in these sedimentary interbeds are dominated by spinicaudatans (conchostracans), but ostracodes, insect nymphs, actinopterygian fish, and plants are locally abundant. Similar biotas in contrasting contemporaneous deposits allow the taphonomy of these organisms to be compared across lacustrine depositional settings. Spinicaudatan carapaces and fish remains are preserved primarily in calcium phosphate, whereas ostracode carapaces are preserved in calcium carbonate, reflecting the original skeletal composition of the animals. Where microbial mats are present, silica replacement of spinicaudatan carapaces occurs more extensively than in other deposits; microbial processes may have enhanced silicification. This study is the first well-documented example of microbial mat influence on preservation in high-latitude lacustrine systems.
Microbialite masses within the lowest Triassic strata along the southern periphery of the tropical Yangtze Platform were produced in a post-Permian microbial regime. These microbialites (the Hindeodus parvus Zone), which are represented by thrombolites, occur only where terrigenous sediment influx was rare, even in shallow-marine settings. The lower parts of the thrombolites lie upon a distinctly unconformable Permian-Triassic boundary and exhibit a thin-bedded to thick-bedded planar structure. In contrast, the upper parts of the thrombolites contain domed macrostructures that interact in complex ways with skeletal grainstones and packstones. Irregular frameworks of thrombolite bodies differ in degree of lateral and vertical accumulation and in the amalgamation of mesoclots of microbial origin; they exhibit marked variations in texture. A transgressive episode occurred in the earliest Triassic following the mass extinctions, and this included the initiation of microbial regimes that usually formed planar thrombolite masses in lower-energy, deep subtidal environments. The varied textures and structures of thrombolites during deposition may reflect a combination of sea-level fluctuations, physicochemical ocean conditions, microbial activity, skeletal-sediment influx, and other factors. These earliest Triassic, uniquely microbial regimes collapsed in stepwise fashion and were succeeded by the Isarcicella staeschei and I. isarcica zones, which contain a predominance of mudstones, suggesting a marked sea-level transgression. Space-specific and time-specific, the earliest Triassic microbialites record short-term, high-resolution paleoenvironmental fluctuations immediately after the end-Permian extinctions.
Large-diameter (2–10 cm) crustacean burrows preserved in the latest Cretaceous and early Paleocene estuarine barforms of southern Wyoming's Ferris Formation exhibit simple, nonbranching architecture, elliptical cross sections, J- and U-shaped shaft morphologies, and walking-leg impressions along the burrow walls. The burrows are classified as Psilonichnus isp. based on their morphology and environmental occurrence. In the Ferris Formation, Psilonichnus is found within multistory channel deposits and is associated with lithofacies that indicate strong tidal influence. Thalassinoides is more abundant in single-story channels with shallower flow depths, and Camborygma is found in alluvial deposits of the overlying Paleocene Hanna Formation. The presence of Psilonichnus in estuarine barforms of the Ferris Formation supports reconstructions of the Western Interior Sea showing a much wider distribution during the Cretaceous–Paleogene transition than has normally been accepted.
Downslope fossil contamination is the result of erosion and subsequent redeposition of fossil material onto lower stratigraphic horizons. This produces time-averaged and potentially anomalous faunal records. Here, we describe vertebrate concentrations in Bighorn Basin (Wyoming) conglomerates that are early Wasatchian (earliest Eocene) in age (Wa-1) and rest erosionally upon Wa-0 strata deposited during the Paleocene–Eocene thermal maximum (PETM). The Wa-1 conglomerates were deposited during river channel migration and sheetflooding onto abandoned parts of an avulsion belt immediately after the PETM. Dark-colored Wa-1 fossil teeth eroding from the conglomerates are now mixed in places with the lighter-colored teeth of Wa-0 mammals. The spatial distribution of the conglomerate fossils and a vertical model of downslope contamination, based on species proportions from overlying stratigraphic intervals, are used to calculate an expected contribution of fossil contaminants to an assemblage. Results of this model are applied to address the principal weakness of the hypothesis that transient decreases in mammalian body size during the PETM were an evolutionary dwarfing response to climate change. Rare occurrences of large taxa with congeners of small size would refute the argument for evolutionary dwarfing, but our results indicate that such rare occurrences can be explained by downslope contamination alone. We conclude more generally that alluvial architecture is important for understanding the potential for downslope fossil contamination and that complications imposed by this type of contamination can be assessed quantitatively.
The Ingersoll shale, a thin (<1 m) clay-dominated lens within the Upper Cretaceous Eutaw Formation in eastern Alabama, contains a well-preserved, primarily continental biota that includes a diverse, carbonized, and variably pyritized flora, abundant amber with fossil inclusions, and common feathers. Geometry of the Ingersoll shale lens and its position between high-energy tidal deposits below and estuarine central bay deposits above indicate deposition in a shallow, narrow channel in the lower reaches of a bayhead delta in response to estuarine transgression. Tidal rhythmites and textural trends indicate that the channel filled very rapidly (55–80 cm per yr), under progressively waning energy regimes. Ichnofabrics, high organic carbon contents, and abundant pyrite indicate highly fluid and reducing sediments. Marine palynomorphs, sulfide contents, and δ34S values of pyrite sulfur indicate normal to near-normal marine salinities. Environmental factors and sedimentary processes contributing to preservation of the Ingersoll shale biota include (1) rapid deposition and burial (obrution); (2) reducing pore waters (stagnation), which limited bioturbation and scavenging, promoted pyritization of some fossils (diagenetic mineralization), and facilitated bacterial replacement of others (i.e., feathers); and (3) concentration of allochthonous or para-autochthonous amber clasts (preservation traps) by tidal currents. Lessons from the Ingersoll shale may help prospect for similar, isolated yet fossil-rich marginal marine deposits.
Raptors concentrate the remains of their small mammal prey in pellets rich in skeletal material. Stratified pellet deposits beneath long-term roost sites should, therefore, represent valuable archives of Holocene faunal change. Accurate paleoecological reconstruction from such deposits, however, requires a complete assessment of factors that may bias the ecological information that such records preserve. Three factors that could bias or obscure the community structure of a small mammal death assemblage relative to the living community include: (1) short-term transient dynamics of prey populations; (2) feeding activity of the raptors; and (3) extent of time averaging represented in individual stratigraphic horizons. Here I model (1) how much summed time is necessary for a raptor-derived small mammal death assemblage to capture a long-term (centennial to millennial) signal of relative abundance; and (2) the accuracy of the relative abundance information preserved in such death assemblages given short-term (decadal) cycling of small mammal prey populations. Results generated from an empirically parameterized model of prey dynamics assuming a multi-species type III functional response of raptors to fluctuations in density of two prey species suggest that the maximum extent of time averaging necessary to capture a stable relative abundance signal in a death assemblage is ∼140 years. This estimate is highly conservative, yet still remains fine enough to analyze phenomena operating at the centennial to millennial time scales critical for addressing long-term community response to habitat transitions through the Holocene. Results also suggest that the mismatch between relative abundance information in the living community and the death assemblage is generally low (<1%), except for a few specific parameter combinations that result in the population dynamics of the prey species being extremely similar to one another.
Within the Middle Jurassic Entrada Sandstone of south-central Utah, cylindrical burrows, 15–95 mm diameter, are abundant in large-scale, eolian cross-strata. Burrows are oriented at a high angle to stratification and commonly extend more than 30 cm below their surface termini. They are rarely inclined more steeply than 22°. Many are sinuous, and they sometimes branch (∼120°) at bends. Burrows terminate upward against flat-topped cones of structureless sandstone that are up to 15 cm deep and present at numerous, closely spaced stratigraphic horizons. Entrada eolian dune deposits also host abundant burrows likely produced by small insects. Both large and small burrows are most numerous in the uppermost parts of very thick (up to 35 m) compound sets of cross-strata generated by superimposed dunes migrating along the lee slopes of giant dune ridges. The size and morphology of the large burrows and the nature of their fills suggest that they were excavated by vertebrates, possibly insectivores, but the possibility that scorpions or spiders dug the burrows cannot be ruled out. In modern dunes, the top 20 cm of rain-moistened sand dries quickly, but underlying sediment can remain moist for long periods. Conical pits formed on the dry surface of Jurassic dunes at the tops of burrows that were primarily excavated in underlying moist sand. Cones composed of structureless sandstone are active fills produced when burrowers pushed moist sand to the surface, forming spoil piles. Most cylindrical portions of the burrows were also actively backfilled; remaining parts were passively filled when burrow walls collapsed. Cones at burrow tops now delineate thin (∼5–10-cm-thick) packages of cross-strata that record slow (seasonal?) dune migration. Rainfall on dune surfaces allowed scattered plants, insects, and insectivorous vertebrates to inhabit the Entrada sand sea. Burrows provided animals with refuge from the hot, desiccating surface conditions.