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Large (up to 7 cm in diameter) and deep-penetrating (up to 30 cm) vertical burrows are described from the lower Cambrian Wood Canyon Formation of the Death Valley region, United States, and their paleoenvironmental and paleoecological implications are addressed. Trace fossils occur as dense populations that precede the earliest occurrences of Skolithos pipe rock in the region. These trace fossils are assigned to the ichnogenera Bergaueria, Conichnus, and Dolopichnus, and each represents the burrowing behavior of an anemone-like organism. These ichnotaxa occur within oolitic and sandy dolostones of a flood-tidal delta and lagoonal environment, respectively, and burrow fill is dominated by echinoderm ossicles. Ichnofabric indices of 4–5 and a tidally influenced position place these ichnotaxa within the same nearshore locus of bioturbation as Skolithos pipe rock; however, unlike Skolithos pipe rock, the tracemaker of these trace fossils can be identified. This study reveals that anemone-like animals were responsible for extensive modification of the marine substrate and this style of deep-penetrating bioturbation appears first in nearshore environments. This is in accord with the observation that many evolutionary novelties originate in nearshore environments, and specifically that deep-penetrating bioturbation originated nearshore before expanding offshore. This study also provides insight into the paleoecology of earliest Cambrian noncalcified cnidarians and their behavior and paleoenvironmental distribution during the Cambrian radiation.
Bivalves are the most common macrofauna present in marine sequences spanning the end-Triassic mass extinction and document the initial ecological response to the crisis. In the west-Tethyan Kössen Basin, marine bivalves occur within distinctive low diversity episodic shell beds at the time of initial crisis and δ13C minimum, and continue for 1 m (<∼20 kyr) upward into the peak extinction phase devoid of macrofauna. The paleoecology, shell mineralogy, and paleobiogeographic context of these well-preserved bivalves suggest they are part of eurytopic opportunistic paleocommunities flourishing in a time of crisis and are consistent with some, but not all, of the paleoenvironmental scenarios hypothesized in the context of synchronous volcanic activity. The best model-to-data fit is found for an ocean acidification scenario, which predicts an increased extinction risk for taxa with thick calcareous skeletons and aragonite mineralogy due to reduced CaCO3 saturation of seawater for a period of <20 kyr. Other kill mechanisms, such as climatic change and reduced salinity in nearshore marine settings, are less well supported but not fully incompatible with our data and might have acted in concert with ocean acidification.
Oxygen and carbon isotope ratios are reported from exceptionally well preserved, early Late Cretaceous (Cenomanian) aragonitic mollusks (ammonites, bivalves, gastropods) from the Moonkinu Formation exposed on the southern coast of Bathurst Island, northern Australia. Samples were examined by means of X-ray diffraction and electron microscopy to screen for diagenetic alteration. The oxygen isotope data derived from planispiral ammonites (largely Euomphaloceras and Acanthoceras), interpreted in terms of temperature, provide the warmest temperatures, ranging up to 34 °C, in accord with the subtropical paleolatitude and with other temperature estimates associated with the middle Cretaceous thermal maximum. Oxygen isotope data from benthic mollusks suggest shelf bottom-water temperatures of ∼21 °C. The oxygen isotope data derived from the straight-shelled baculitid heteromorphic ammonite Sciponoceras are consistent with these organisms having a nektobenthic mode of life, differing from previous views of Baculites which suggest habitation of the mid to upper parts of the water column. Isotopic analyses of shells of the inoceramid bivalve (Actinoceramus) in general show relatively positive carbon values and the most negative oxygen isotope values. Such values are more characteristic of surface-dwelling organisms, and approach those we have measured for planispiral ammonites. Isotopic data from other benthic organisms including the gastropod Latiala and other bivalves provide δ18O data that are consistent with a normally stratified water column. We view the isotopic values for Actinoceramus as anomalous relative to expected values for the watermass in which the animals lived, and ascribe them to taxon-specific disequilibrium or a vital effect as the cause of depleted oxygen isotopic values that are characteristic of inoceramids in general.
Marine invertebrates are at risk of extinction if climate changes outpace their ability to adapt to thermal stress, and cold-adapted taxa may be especially vulnerable because of their specialized physiologies and because their high-latitude distributions permit only limited poleward migration. Here, we use a database of 1437 early and middle Permian eastern Australian fossil collections from the Paleobiology Database to test for latitudinal range shifts, extinctions, and faunal invasion among high-latitude marine invertebrates during climate changes in the late Paleozoic ice age. Latitudinal range shifts are not apparent, either because genera were unable to migrate or, more likely, because sampling noise or the scale of analyses prevent their recognition. Extinction rates were moderately elevated during the largest climate shifts, however, possibly suggesting that at least some taxa were unable to respond to the rate or magnitude of climate change. Although recognition of range shifts within Australia is difficult, warm-water brachiopods, bivalves, and ammonoids invaded the region during pronounced warming in the Artinskian, highlighting the importance of temperature on faunal distribution. That faunal invasion was coincident with substantial restructuring of local paleocommunities, but both likely resulted from the common cause of increasing temperature rather than having a causal relationship. Temperature warming would have stressed cold-adapted stenotherms, triggering changes in local dominance and allowing immigration of warm-water taxa. These local and regional shifts in dominance and distribution imply that physiological stresses from even gradual climate change can be sufficient to trigger biotic change.
The Lower Triassic Red Peak Formation of the Chugwater Group has long been considered to have an extremely poor paleontological record, although the cause for the apparent dearth of fossils has yet to be been determined. During the course of fieldwork in central Wyoming numerous vertebrate and invertebrate ichnogenera (n ≥ 11) were observed. Vertebrate tracks and trackways representative of dinosauromorph, archosaur, lepidosaur, and testudinate trackmakers were found (cf. Rotodactylus, Chirotherium barthii, Rhynchosauroides, and cf. Chelonipus respectively). An invertebrate ichnoassemblage composed of at least 6 ichnogenera consistent with the Scoyenia ichnofacies were also found (e.g., Diplichnites, Lockeia, Fuersichnus communis, Palaeophycus striatus, cf. Scoyenia, and cf. Scolicia). The majority of these tracks and traces were found in the upper platy facies (upper 10–20 m of the of the Red Peak Formation), which is thought to be no younger than upper Spathian in age. Sedimentary structures, architectural elements, and lateral stratigraphic relationships support the interpretation of floodplain, fluvial, and lacustrine deposition for the upper platy facies in central Wyoming. The Red Peak Formation vertebrate and invertebrate ichnoassemblages, along with their associated depositional environments, are consistent with a fluviolacustrine (continental) setting comparable to those described from Lower to Middle Triassic strata with a Pangean distribution, including the Moenkopi Formation in the southwestern United States. This ichnoassemblage provides the first opportunity to observe paleoecological diversity and associated paleoenvironments within the Lower Triassic of the Chugwater Group.
A major caveat of using gastropod drill holes to assess predator-prey interactions in modern and ancient ecosystems is identification of holes produced by predation versus other means. A recent method to confirm the predatory origin of holes uses Radulichnus-like microtraces left within the drill hole from the physical rasping of prey shell material, but the utility of these microtraces to assess biological information from the drilling predator has not been investigated. Generating artificial rasp marks using extracted radulae and producing drill holes by artificial means offered insights into the mechanical factors influencing microtrace morphology. Controlled laboratory feeding trials provided prey shells with drill holes of known origins, and corresponding gastropod predator radulae (Nucella lamellosa, Muricidae). These prey shell–predator radula pairs were analyzed under environmental scanning electron microscopy (ESEM) to comparatively assess spacing of radula intercusps and drilling microtraces. Artificial replication of microtraces using focused ion beam scanning electron microscope (FIB-SEM) micromanipulation demonstrates that microtraces provide reasonable approximations of radula dentition size and morphology. Depending on mechanical inconsistencies of the drilling process, however, microtrace expression may vary within and across individuals. Microtrace spacing parallels radula intercusp spacing (r = 0.50; p = 0.03), but no correlation was observed between intercusp spacing and predator size, which suggests that microtraces cannot serve as a proxy for predator size. With morphological conservation of the radula expressed in the microtraces, however, further investigation of microtraces may bolster the correct identification of predatory traces and afford insights into the identity of the drilling predator in the fossil record.