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Prior to the advent of widespread bioturbation during Cambro–Ordovician times, microbial mats may have covered large expanses of the continental shelf. Evidence of matgrounds in shallow-marine settings is provided by abundant wrinkle structures in Lower Cambrian strata of the Great Basin, United States. Wrinkle structures from the Lower Cambrian Harkless Formation commonly co-occur with a distinctive assemblage of invertebrate fossils, providing evidence for the possibility of selective metazoan colonization of matground substrates. Molds of linguliform brachiopods are abundant on many wrinkle surfaces. The agglutinated problematicum, Volborthella tenuis, also is found on wrinkle surfaces and in laminations beneath wrinkle-structure surfaces. Bedding-parallel trace fossils, such as Planolites, Diplichnites, and Taphrhelminthopsis, commonly crosscut wrinkle structures, while vertically oriented trace fossils are absent.
Microbial mats containing layered microbial communities would have considerably compressed stratified redox zones beneath the sediment-water interface in marine-shelf settings. Sulfidic and anoxic conditions within and beneath microbial mats would have precluded habitation by many metazoans, while those that adapted to such conditions may have found matgrounds a unique, though temporally fleeting, ecological niche. The distinctive, low-diversity fossil assemblage found in association with the wrinkle structures in the Great Basin suggests that some early animals may have been adapted to hypoxic and sulfidic conditions found in matground substrates, while others may have been physiologically excluded from these environments.
Early Ordovician conodonts have been recovered from the upper Munising Formation and the lower part of the Au Train Formation at Pictured Rock National Lakeshore in the Upper Peninsula of Michigan. Two trilobites from the lower Au Train probably represent the Symphysurina Zone (S. woosteri Subzone) and confirm the Early Ordovician age of the Au Train. The presence of middle Late Cambrian (Franconian) trilobites in the underlying Munising Formation led to traditional assumption of a major hiatus between the Munising and the Au Train. An unconformity between these formations has been difficult to recognize in outcrops because upper Munising sandstones grade upward into weathered, reworked beach sands deposited during Ordovician transgression. The reworked sands traditionally have been assigned to the Munising, with the base of the Au Train placed at the lowest occurrence of carbonate rocks in the sequence. Conodonts from the uppermost Munising and lower Au Train are characteristic of the Rossodus manitouensis Zone (upper Skullrockian Stage, Millardan Series, Lower Ordovician). The conodont and trilobite data permit correlation of the rocks of this interval with sequence stratigraphic units in the Burnout Canyon Member of the House Limestone in the Ibex area of Utah and with equivalent strata in other parts of Laurentia.
Crinoids reached their highest generic richness and overall abundance during the Mississippian, which thus has been dubbed the Age of Crinoids. The causes are hypothesized to be from the coincidence of two factors. First, in the wake of the Late Devonian mass-extinction event, the five major crinoid groups recovered and radiated in the Early Mississippian. The advanced cladids continued to radiate from their origin in the Early Devonian and reached a peak in the Middle Mississippian (Visean) that was not exceeded again until the Middle Pennsylvanian (Moscovian). Second, the Late Devonian mass-extinction event destroyed the extensive coral-stromatoporoid platform-edge reefs that had restricted circulation on carbonate platforms and limited the abundance of crinoids, which are stenohaline. The resulting carbonate ramps during the Mississippian had improved circulation, producing stenohaline conditions that resulted in an abundance peak for crinoids, recorded by widespread regional encrinites on multiple continents. This increased habitat space was ideal for camerate crinoids and resulted in a new radiation of camerate crinoids. The simultaneous radiation of pinnulate cladids and the short resurgence of camerates were responsible for the biodiversity spike in the Mississippian. The Age of Crinoids ended with a major drop in sea level at the end of the Mississippian as massive glaciers formed on Gondwana and epicontinental seas were drained.
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.
There has been significant biological and environmental change in Caribbean coral reefs during the past 30 million years, including two periods of accelerated turnover of species in the zooxanthellate coral biota that may have been correlated with changes in regional sea-surface productivity during the Early Miocene and the Early Pleistocene. Skeletal extension rates measured on x-radiographs of 11 massive genera of fossil corals collected from Late Oligocene to Pleistocene units from across the Caribbean were analyzed to determine whether average coral growth responded to these regional environmental changes. The observed patterns were evaluated by comparisons with records of Recent coral growth rates taken from published literature. These analyses suggest that there is significant variation in average growth rate among corals living in the Recent Indo-west Pacific, eastern Pacific, and Caribbean, even when broad ranges of taxa and habitats are intermingled. When applied to fossils, a similar analysis suggests that rates of growth do not change overall through time. One exception is during the Late Miocene, when rates of growth were significantly lower than from other fossil units or for Recent colonies from the Caribbean. However, the Late Miocene colonies sampled for this study lived in relatively deep, turbid habitats, so the observed reduced growth rates may have resulted from local low availability of light. Similar facies were not sampled in other stratigraphic intervals, so there is no strong evidence for reduced regional average growth rates for Caribbean corals during the past 30 million years.
Six populations of seeds of the aquatic monocotyledon Stratiotes (Hydrocharitaceae) from the Paleogene of England have been studied to assess morphological evolution through the Eocene–Oligocene transition. Morphometric methodologies (including eigenshape analysis) have been used to quantify evolution within the genus and compare results to previous qualitative studies. Previously hypothesized broad evolutionary trends of increasing size and more elongate shape are found to be mainly correct, but, in places, can be elucidated further. The results of this study indicate a single evolving lineage in the Paleogene of southern England with an increase in seed size and keel width in late Eocene specimens, followed by a reversal of this trend in the early Oligocene. Two Miocene populations from continental Europe are shown to be morphologically distinct from those of the English Paleogene. Changes in overall shape of the seed are shown to be controlled dominantly by the relative size of the keel structure, rather than the seed body. Comparisons show that the microevolutionary trend of Stratiotes across the Eocene–Oligocene transition differs from that of the charophyte, Harrisichara. This may suggest that factors other than climatic change, such as animal/plant interactions, played a role in evolution of Stratiotes seeds. Type and figured material of named Paleogene species was added passively to the dataset, and results suggest that taxonomic splitting may have led to previous evolutionary hypotheses of multiple clades, which is not supported by this study.
The experimentally determined transportation characteristics of Nautilus pompilius shells by bottom currents reveal that planispiral cephalopod shells can be transported, reoriented, and restrained. Shells of N. pompilius become reoriented when the water flow is about 0.20 m/s, regardless of shell size, and are transported by the flow of 0.25–0.37 m/s, with velocity proportional to shell size. The shells are restrained in a strongly preferred orientation (aperture downstream). The transportation characteristics of various ammonoid groups differ based on differences in shell shape (especially drag coefficient in water). The shells of most ammonoids (especially leiostraca, serpenticones, and cadicones) had reorientation and threshold velocities similar to N. pompilius, and therefore were highly sensitive to paleo-bottom currents. The character and strength of paleo-bottom currents (direction and flow velocity) can be reconstructed from the taphonomic attributes of ammonoids (e.g., three-dimensional orientation within matrix, shell morphology, and ornamentation).
Post-eruptive lacustrine sediments that infill a Middle Eocene kimberlitic diatreme near Lac de Gras (Northwest Territories, Canada) have spectacular preservation of diatom microfossils. A single undiagnosed species belonging to the genus Aulacoseira is abundant in this material and reveals, in addition to the full complement of siliceous structures comprising the frustule, an array of soft parts that hitherto have been unavailable for study in diatoms of this age. These features include the velar complex, which lines the interior of valves, layered internal membranes, lamellate plastid fragments with pyrenoid-like inclusions, and extracellular mucilagenous bodies. Both ultrastructural and cytoplasmic characters have pronounced affinities with extant congeneric taxa, with an especially close resemblance to the resting cells observed in living Aulacoseira. These observations suggest that modern cellular organization and associated ecological adaptations were present early in the history of nonmarine aulacoseiroid diatoms, implying that prolonged evolutionary stasis has characterized the ecologically important genus Aulacoseira.
Although insect size and robustness often have been hypothesized to be factors that lead to taphonomic bias in the insect fossil record, no studies have examined how these factors directly affect an insect's preservation potential. In this study, laboratory experiments were performed on modern Coleoptera (beetles) to examine the importance of insect morphology on preservation potential. A rotary tumbling barrel was used to determine how insect size and robustness would influence sinking and disarticulation rates. Although size and robustness were not correlated directly, beetles that were larger and more robust were more resistant to disarticulation than smaller, less-robust beetles. Waterlogged specimens gained increased flexibility in their exoskeletons, and were difficult to puncture. Sinking and disarticulation rates were correlated, although it took fewer days for beetles to sink than it took to begin disarticulating. A white-colored film was apparent on all specimens within a few days of their introduction to the tumbling barrel; however, major disarticulation did not occur until the specimens sank to the bottom. An examination of the fossil-beetle literature also suggests the importance of hardness in preservation potential. Although often considered fragile, given the right physical characteristics and environmental conditions, insects can be resistant to disarticulation and decay.