Strata In Utah Research Articles

Use of landmark and outline morphometrics to investigate thecal form variation in crushed gogiid echinoderms

Gogia is a primitive genus of pelmatozoan echinoderm the investigation of whose systematics, taxonomy, and palaeoecology has been invigorated recently by the recovery and description of articulated specimens and related genera of blastozoan species from Cambrian strata in Utah and China. To date investigation of gogiid systematics has been undertaken almost exclusively using qualitative methods to assess morphological variation owing, at least in part, to the crushed character of articulated fossils. This practice reflects the widespread belief among palaeontologists that quantitative approaches to morphological analysis can only be employed successfully on well-preserved specimens. Accordingly, no geometric morphometric analysis of gogiid morphology has been published to date. In order to investigate whether morphometric approaches to gogiid character analysis might be used to recover biological signals the thecae of 27 gogiid specimens collected from a single bedding plane were subjected to geometric analysis using landmarks, boundary outline equi-spaced semilandmarks, boundary outline sliding semilandmarks, and combined landmark+outline sampling protocols. Results indicate that, despite the crushed and deformed character of these specimens, robust and unquestionable biological signals can be extracted from the patterns of variation presented by these specimens by all the aforementioned methods. Based on these morphometric results it is concluded that (1) the sample exhibits several complex patterns and shape variation some of which may be inconsistent with the assumption that all specimens included in the sample should be assigned to the same species, (2) the primary pattern of thecal shape change appears to be highly correlated with an aspect of specimen size (thecal length) suggesting that various life-cycle stages are present within the sample, (3) assessments of right-left symmetry can be used to distinguish biological from post-mortem deformational aspects of shape variation, (4) the sliding semilandmark procedure failed to improve semilandmark correspondences across the sample and, indeed, appears to have degraded rather than improved the assessment of the biological shape variation, and (5) morphometric sampling strategies that combine the information recorded by landmarks and equi-spaced boundary outline semilandmarks appear to recover the greatest proportion of biologically interesting shape variation.

Uppermost Devonian (Famennian) to Lower Mississippian events of the western U.S.: Stratigraphy, sedimentology, chemostratigraphy, and detrital zircon geochronology

Upper Devonian to Lower Mississippian strata in Utah and Montana record global events through this important interval in Earth history. Late Famennian strata of the Beirdneau, Leatham, and Pilot Shale formations in Utah, and Three Forks and Sappington formations in Montana, record widespread deposition of generally fine-grained siliciclastic and carbonate strata. Integration of sedimentology, physical stratigraphy, chemostratigraphy, and published biostratigraphy allows for the recognition of important disconformities and regional stratigraphic patterns. These enable the reconstruction of uppermost Devonian to lowermost Mississippian depositional, tectonic, and eustatic history of the region. Carbon isotopic data allows for stratigraphic evaluation of the presence and absence of global bioevents of the Late Devonian, including the Annulata, Dasberg, and Hangenberg events, some of which are clearly recorded in hinterland deposits to the east in Colorado. While the Devonian–Mississippian boundary is also missing in our sections, a significant positive shift in δ13Ccarb in Lower Mississippian strata in Utah and Montana represents one of the largest positive δ13Ccarb isotope excursions of the Phanerozoic, linked to drawdown of atmospheric CO2 and glaciation in the Kinderhookian. Detrital zircon spectra from latest Devonian to Early Mississippian strata of Utah and Colorado include populations representing derivation from the Mazatzal and Yavapai provinces, Middle Proterozoic anorogenic granite bodies, and a small influx of Grenville and presumed Appalachian–Caledonian grains. Minor Paleoproterozoic and Late Archean peaks in Utah are likely multiple generation grains originally derived from the Peace River Arch of northwestern Canada and recycled in Ordovician rocks of Nevada. These patterns in detrital zircon geochronological data reflect, in part, changes in sediment dispersal patterns due to tectonic and eustastic variability within the Antler foreland basin during the Devonian–Mississippian boundary interval. This variability also led to irregular spatial patterns of unconformity development, as well as complicated physical stratigraphic and chemostratigraphic architecture.

Magnetostratigraphy of the Upper Triassic Chinle Group of New Mexico: Implications for regional and global correlations among Upper Triassic sequences

A magnetic polarity zonation for the Upper Triassic Chinle Group in the Chama Basin, north-central New Mexico (United States), supplemented by polarity data from eastern and west-central New Mexico (Mesa Redonda and Zuni Mountains, respectively), provides the most complete and continuous magnetic polarity chronology for the Late Triassic of the American Southwest yet available. Most of the Chinle Group sequence is composed of hematitic mudrocks that typically carry a well-defined, well-grouped magnetization (residing in both pigmentary and detrital hematite), with laboratory unblocking temperatures as high as 680 °C. Demagnetization experiments isolate magnetizations of south- or north-seeking declination and shallow inclination, which are interpreted as early acquired, Late Triassic magnetizations. Our proposed polarity correlations, coupled with biostratigraphic observations and recent U-Pb age determinations on detrital zircon–bearing strata in the Chinle Group in western New Mexico, West Texas, and Arizona, indicate that deposition of Chinle strata likely spanned a much shorter time span than previously considered. If this interpretation is correct, the Chinle Group can be correlated with only part of the Newark Supergroup or the Upper Triassic Tethyan sections. On a local scale, lower Chinle strata in the Chama Basin are significantly older than the Bluewater Creek Formation in western New Mexico, and the base of the Poleo Formation represents a disconformity of >13 m.y. duration. Magnetic polarity chronologies from upper Chinle strata in New Mexico and Utah suggest that strata considered to be part of the Rock Point Formation in north-central New Mexico are not time equivalent to type Rock Point strata in Utah or to the Redonda Formation of eastern New Mexico.

New phylogenetic insights into the Cambrian radiation of arachnomorph arthropods

The Cambrian fossil record of the Arachnomorpha is rich and diverse and includes trilobites, chelicerates, and many taxa known from various soft-bodied faunas including the Burgess Shale and the Chengjiang. Exceptionally well-preserved arthropod fossils are also known from Middle Cambrian strata in Utah. Recently, two new arachnomorphs (DicranocarisBriggs, Lieberman, Hendricks, Halgedahl, and Jarrard, 2008 andNettapezouraBriggs, Lieberman, Hendricks, Halgedahl, and Jarrard, 2008) were described from the Wheeler and Marjum formations of Utah. Cladistic analysis is undertaken to investigate arachnomorph relationships in light of these two new genera. The character matrix of Edgecombe and Ramsköld (1999) serves as the foundation for this study, augmented by new characters and taxa. The results of our cladistic analysis suggest that at least three distinct arachnomorph clades had diverged by the Middle Cambrian, and perhaps much earlier; the Utah genera can be referred to groups within one of these clades.

Charles D. Walcott: A Few Comments on Stratigraphy and Sedimentation

Throughout his career, Charles D.Walcott was a prodigious scientist.Although perhaps best known for his contributions to our understanding of the Cambrian System, including discovery of the Burgess Shale Biota,Walcott made other important contributions to stratigraphy and sedimentology that have had a lasting impact. Some contributions seem to have been quite deliberate, whereas others may have been less conscious. Examples of his deliberate scientific efforts are: measuring and describing more than 8250 m (25,000 ft) of strata in Utah and Arizona; interpreting the origin of limestone breccia and conglomerate within a limestone matrix; calculation of geologic time from rates of sedimentary deposition; and consideration of the Ozarkian and Canadian as time periods. An interesting contribution that may have been less obvious is the observation that limestones could form quickly in shallow marine water.

Undichna gosiutensis, isp. nov.: A new fish trace fossil from the Jurassic of Utah

Undichna gosiutensis isp. nov. is described from the Middle Jurassic (Callovian) Arapien Shale of central Utah. The new ichnospecies consists of three regularly sinuosidal intertwined grooves and is interpreted as a swimming trail produced by a fish with a caudal fin and a double anal fin (or an anal fin with an anterior spine) whose tips reached lower than the rest of the body. This is the first reported Mesozoic occurrence of Undichna in North America and the first evidence of fish in Jurassic marine strata of Utah. The occurrence of Undichna in marine strata confirms that the ichnogenus is not restricted to freshwater environments as was once thought. Undichna gosiutensis occurs in fine‐grained, laminated limestones associated with a small, generalistic, and shallow‐tier invertebrate ichnofauna. The overall trace fossil assemblage suggests that the boundary between dysoxic and oxygenated waters was located very close to or at the sediment‐water interface.

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