Browsing by Subject "Cretaceous"
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Item 2-Dimensional Seismic Refraction Mapping Study of the Cretaceous-Paleogene Boundary Complex from the Brazos, Texas Section(2012-07-16) Gowan, Joshua SmithMany scientific studies have been conducted on the Cretaceous-Paleogene boundary (KTB) in the Gulf coast region and, in particular, the Brazos River section in Falls County, Texas. Despite this, there remains much to be learned about the KTB and its depositional environment. Study of the KTB has been multidisciplinary, primarily in the fields of sedimentology and paleontology. Some researchers in these disciplines have questioned the consensus view of the placement of the KTB and subsequent interpretation of the timing of depositional events and mass extinction events. Geophysical methods have potential to provide additional understanding of the physical properties of the KTB. To date, study of the KTB has relied on point data and borehole information to create cross sections of the complex. Seismic refraction surveys can provide spatially continuous information on susburface horizons located adjacent to the KTB. In this study, seismic first-arrival traveltimes are processed with a tomographic modeling program to map the top of the hummocky cross-bedded sandstone (HCS), which is a key indicator of the deposition environment at the time of KTB boundary complex placement. The survey area is located at Cottonmouth Creek, a tributary of the Brazos River. Three seismic lines were surveyed, one across Cottonmouth Creek, and two parallel to the creek on either side. The data from the two parallel lines were processed using the 2-D seismic refraction tomography algorithm of Zelt and Smith. The reconstructed depth to the HCS in the survey area is approximately 6 m, with layer seismic velocities of 364, 1800, and 2200 m/s, respectively. Seismic tomography successfully mapped the HCS layer and reveals approximately 1 m amplitude undulations vertically and undulations on the order of several m horizontally. These variations are consistent with exposed surfaces of the HCS in the creek bed. Seismic refraction has been utilized successfully herein to map a key buried indicator, namely the top of the HCS layer, associated with the KTB complex. A detailed 3-D seismic refraction survey at this site is recommended to generate a high-resolution 2-D terrain map of the top of the HCS layer.Item A dyrosaurid crocodilian from the cretaceous (Maastrichtian) Escondido formation of Coahuila, Mexico(2012-12) Shiller, Thomas A; Lehman, Thomas; Chatterjee, Sankar; Barrick, James E.Two fragmentary marine crocodylian specimens were discovered in Upper Cretaceous marine strata near the town of Sabinas in Coahuila, Mexico. Although exposures are poor in the vicinity of the collection sites, associated ammonites indicate that the specimens were recovered from either the Sauz Creek Member or lower part of the Cuevas Creek Member of the Escondido Formation, and are Maastrichtian in age. The two specimens share several unique features, including a reduced seventh dentary alveolus and flat mandibular rostrum that indicate they pertain to the extinct crocodylian family Dyrosauridae. The two are assigned to the same species, referred to here informally as the "Sabinas dyrosaur." The proportions of the mandible in the Sabinas dyrosaur are unusual. Its rostrum is narrow, but relatively short, and intermediate in form between typical longirostrine and brevirostrine dyrosaurs. The distal end of the mandibular rostrum is uniquely shaped with laterally expanded margins and enlarged, closely spaced, first and second alveoli. Dentary alveoli thirteen through fifteen are grouped together, with the fourteenth alveolus laterally offset and confluent with the thirteenth. The splenials participate in only a small part of the mandibular symphysis, and do not diverge laterally with the dentaries in the mandibular rami. The mandible lacks internal or external mandibular fenestra. These unique features, and unusual combination of character states, indicate that the Sabinas dyrosaur represents a previously unknown species. Based on criteria used to recognize marine reptile feeding guilds, the skull and tooth morphology indicate that the Sabinas dyrosaur had a durophagus diet, and may have preyed on the thin-shelled ammonites that are abundantly preserved in the same deposits. Restoration of the fragmentary skull suggests that the Sabinas dyrosaur had a total body length of about 5 to 6 meters, and was among the largest dyrosaurs known. Although dyrosaurid crocodylians were abundant and diverse in Africa during Late Cretaceous and Paleogene time, the Sabinas dyrosaur represents only the second species thus far known from North America. Its occurrence in Mexico is compatible with an hypothesized trans-Atlantic dispersal event of dyrosaurs from Africa to North America during Late Cretaceous time.Item Barrier island associated washover fan and flood tidal delta systems: A geomorphologic analysis and proposed classification scheme for modern washover fans and examination of a flood tidal delta complex in the Cretaceous upper McMurray Formation, Alberta, Canada(2013-12) Hudock, Jessica Wager; Wood, Lesli J.; Flaig, Peter PaulA detailed study of modern washover fan (fan) morphologies will clarify common fan geometries, lead to a better model for a “typical” fan, identify the preservation potential and probable geometries of fan facies in subsurface datasets and outcrops, and ultimately improve hydrocarbon recovery in barrier island reservoir systems. This study uses satellite imagery to conduct a spatial analysis of 118 modern fans to quantify geomorphologic attributes of fans. A new classification scheme for fans is proposed that refines the current fan model, dividing fans into channelized and non-channelized fans. Channelized fans are subdivided according to the location of primary deposition: barrier depo-center or lagoonal depo-center. Non-channelized fans are subdivided based on morphology: dissipative, lobate, or apron-sourced. Quantitative cross-plots of morphologic relationships are analyzed to define trends in fan morphologies. The most common type of fan encountered in our study is a non-channelized, line-sourced, lobate washover fan with an area of less than 1 km2 that is fully contained on a barrier and exhibits no subaqueous deposition in back-barrier waters. The Lower Cretaceous McMurray Formation is the primary reservoir of the Athabasca Oil Sands in Alberta, Canada. The upper McMurray is commonly interpreted as deposits of embayed coastal systems. Our location is in an under-studied area located 80 km northwest of Fort McMurray. Lateral and vertical facies changes, sedimentary structures, key surfaces, trace fossils, and bitumen saturation were documented in eight cores located along a 20 km transect situated paleo-landward of a Devonian paleo-high acting as a bedrock-barrier. Our data indicate that a flood tidal delta complex prograded landward into a back-barrier embayment through the stable, bedrock-controlled inlet. This system overlies middle McMurray fluvial sands and Devonian basement and was transgressed by marine waters prior to deposition of the overlying Wabiskaw Formation. Flood tidal delta sandbodies are bitumen saturated and therefore make good reservoirs; however, heavily bioturbated tidal flats can act as a barrier to flow where they encase flood tidal deltas, as encased sands were devoid of bitumen. This complex coastal paleogeography produced back-barrier deposits that contain a slightly more diverse, marine trace fossil assemblage than might otherwise be expected.Item Characteristics of the Taylor Marl of Travis County, Texas(1928) Burford, Selwyn Oliver; Not availableItem Intermediate- to Deep-Water Circulation Changes on Short and Long Time Scales(2012-07-16) Murphy, Daniel PatrickOceanic circulation remains one of the poorly understood elements of the global climate system, despite its importance to planetary heat redistribution and carbon cycling. The nature of deep-water formation and circulation in ancient oceans are even more poorly constrained. In order to understand climate dynamics of past and future climates we must have a better understanding of the role of deep-ocean circulation. In this dissertation I investigated changes in intermediate- to deep-water circulation in three different ocean basins during two different geologic eras. The first study focused on the late Pleistocene (~25 ? 60 ka) California margin to investigate the role of intermediate water circulation in abrupt climate fluctuations. The other two studies investigated deep-water circulation during the Late Cretaceous (~70 ? 100 Ma) greenhouse interval, to determine if deep waters formed in the southern Indian or Atlantic basins. The above studies employed neodymium isotopes preserved in biogenic apatite (fish teeth and bones) and foraminiferal calcite to reconstruct the provenance of intermediate- to deep-water masses. Here I present data from two sites located at intermediate depths on the late Pleistocene California margin as well as seven Deep Sea Drilling Project and Ocean Drilling Program Cretaceous aged sites; four in the South Atlantic Ocean, and three in the Indian Ocean. The new Pleistocene data rule out changes in the source of intermediate waters to the California margin, thus the recorded changes in seafloor oxygenation were caused by changes in sea surface productivity. In the Cretaceous, the spread of deep waters formed in the high-latitude South Atlantic was hindered by tectonic barriers until the mid Campanian when the subduction of Rio Grande Rise allowed for the continuous flow of deep waters from the Southern Ocean into the North Atlantic. The deep Cretaceous Indian Ocean was filled with deep waters formed in the high-latitude Indian Ocean, until being replaced with waters sourced in the Pacific from the late Cenomanian to early Campanian before a return to southern Indian-sourced waters for the remainder of the study interval.Item Mid-Cretaceous Palynoflora from Central Mid-Pacific Ocean(2012-10-19) Hsiung, Shih-YiAlbian (late Early Cretaceous) pollen and spores were used to reconstruct the floral history of Allison Guyot in the Albian period, to better understand pollen and spore distributions on mid-oceanic islands, to investigate whether Allison Guyot supported land plants in the Albian, and to test previous hypotheses about the development of the guyot. Albian spores found in Allison Guyot sediments from ODP Leg 143 Site 865 include: Laevigatosporites ovatus, Cyathidites minor, Cicatricosisporites sp., Baculatisporites comaumensis, Ceratosporites equalis, Gleicheniidites senonicus, Leptolepidites verrucatus, Retitriletes circolumenus, Lycopodiacidites dettmannae, Osmundacidites wellmanii, Cicatricosisporites hughesii, Impardecispora excavate, and others. Albian pollen from these samples include Callialasporites dampieri, Ephedra, and others occur in Albian samples. The high abundance of terrestrial palynomorphs in these samples suggests that Allison Guyot was exposed in the Albian and supported land plants. The high frequency of spores (more than 90 percent) reflects a flora dominated by ferns.Item Middle-Hauterivian to Lower-Campanian sequence stratigraphy and stable isotope geochemistry of the Comanche platform, south Texas(2011-05) Phelps, Ryan Matthew, 1982-; Kerans, C. (Charles), 1954-; Loucks, Robert G.; Janson, Xavier; Scott, Robert W.; Fisher, William L.; Quinn, Terrence M.Carbonate platforms contain a wealth of information regarding the changing biota, sea level, ocean-chemistry, and climate of the Cretaceous Period. The Comanche platform of the northern Gulf of Mexico represents a vast, long-lived carbonate system that extended from west Texas through the Florida panhandle. In central and south Texas, excellent outcrops and an extensive suite of subsurface data provide an opportunity to document the evolution of this system, from the shoreline to the shelf-margin and slope. This study examines the changing facies, platform morphologies, and shelf-margin architectures of the mixed carbonate-siliciclastic, middle-Hauterivian to lower-Campanian interval. Stratigraphic results are integrated with stable-isotope geochemistry to document the detrimental effects of oceanic anoxic events on the carbonate platform. Seven second-order, transgressive-regressive supersequences of 3-14 Myr duration are defined in south Texas using sequence stratigraphic analysis of shelf-interior facies successions. Second-order supersequences are subdivided into several third-order depositional sequences of 1-3 Myr duration. In these sequences, facies proportions and stratal geometries of the shelf-interior are found to be the result of changing platform morphology and temporal evolution from distally-steepened ramp to rimmed-shelf depositional profiles. Shelf-margin trajectories, stratigraphic architectures, and facies proportions are a function of long-term accommodation trends expressed in second-order supersequences. These characteristics are modified by lateral variability in the underlying structural/tectonic setting and localized syndepositional faulting. The stratigraphic equivalents of oceanic anoxic events 1a, 1b, 1d, 2, and 3 are documented in the Cretaceous section of south Texas. These oceanic anoxic events coincided with maximum flooding zones of supersequences and are linked to carbonate platform drowning events on four separate occasions. The occurrence of oceanic anoxic events is found to be a fundamental driver of carbonate platform morphology, faunal composition, and facies evolution in transgressive-regressive supersequences of the northern Gulf of Mexico.Item Occurrence and Stability of Glaciations in Geologic Time(2011-10-21) Zhuang, KelinEarth is characterized by episodes of glaciations and periods of minimal or no ice through geologic time. Using the linear energy balance model (EBM), nonlinear EBM with empirical ice sheet schemes, the general circulation model coupled with an ice sheet model, this study investigates the occurrence and stability of glaciations in geologic time. The simulations since the last glacial maximum (LGM) suggest that the summertime thawline of ice sheets conforms closely to the equatorward edge of the ice sheets and implies the relative stability toward deglaciation. CO2 levels are indispensable in controlling the initiation of ice sheet in the Cretaceous. At low CO2 levels, ice sheets exist in all periods no matter LGM or the last interglacial (LIG) orbital elements; however, at high CO2 levels ice sheets rarely exist. The simulations agree well with recent geological evidence of the hysteresis of glaciations in the Permo-Carboniferous. Gondwanaland reached its glacial maximum when CO2 level was roughly the same or slightly higher than the preindustrial value. With a further increase of CO2, deglaciation dominates and results in an ice free state. Again, if CO2 decreased to the present level, Gondwanaland would be glaciated once more and start a new cycle of glaciation and deglaciation. Simulations from five paleogeography maps in Gondwanaland with a suite of CO2 levels and different orbital elements reveal that paleogeography, CO2 levels and the Milankovitch cycles all contribute to the glaciations of Gondwanaland. This study shows that orbital elements alone are insufficient to account for the evolution of ice sheets. Net radiative forcing caused by greenhouse gases, such as CO2 and solar constant change are the primary drivers to glacial inception or demise. Continental geography, CO2 levels, solar constant change, and the Milankovitch cycles complicate the glacial history of Earth.Item Patch-reef and ramp interior facies architecture of the Early Albian Mural Limestone, southeastern Arizona(2010-12) Aisner, Rachel E.; Kerans, C. (Charles), 1954-; Loucks, Robert; Steel, Ronald J.The Mural Limestone, located in the Mule Mountains to the northeast and southeast of Bisbee, Arizona provides an exceptional outcrop analog for time-equivalent productive reservoirs in the Albian Glen Rose patch-reef play of the Maverick Basin. The Mural Limestone is exposed in a number of folds and east-dipping fault blocks in the Grassy Hill and Paul Spur localities in the Mule Mountains and represents a remnant of a south-facing distally-steepened carbonate ramp that prograded into the Chihuahua Trough in Albian time. This study documents the detailed facies architecture and sequence stratigraphic setting of a multicyclic patch-reef and its associated ramp interior facies at the Paul Spur and Grassy Hill localities, respectively. Small mud-dominated coral-algal buildups (~5 m thick) and tabular biostromes (up to 1.5 m thick) consisting of rudist floatstones are common in the bedded ramp interior carbonates at the Grassy Hill locality in the Mule Mountains 10 km landward of the Paul Spur reef. Buildups in this area are flanked by weakly-cyclic and well-bedded skeletal mud- and grain-dominated packstones. At the Paul Spur locality, Mural facies consist of a 10-35 m thick patch-reef with four distinct reef communities: microbial-Microsolena framestone, algal-Actinastrea boundstone, branching coral-skeletal framestone and caprinid-requienid floatstone. Measured reef dimensions show a distinct windward-leeward margin with reef frame facies extending ~70 m from the margin and extensive leeward rudstone debris and grainstone shoal facies extending a distance of 870 m. Reef and backreef shoal facies exhibit low preserved porosity but petrographic analysis of backreef grainstones shows that primary porosity and permeability was present. These extensive reservoir-prone shoals may be a suitable reservoir target similar to flank rudstones and grainstones of the Maverick Basin reefs. Three aggradational to retrogradational cycles of reef growth are evident at the Paul Spur locality. Retrogradational stacking is consistent with that of time-equivalent Lower Glen Rose patch-reefs in the Maverick Basin of Texas, which suggests a eustatic driver for stratigraphic architecture along the Bisbee/Comanche shelf. Backstepping of reef frame facies in Cycle 3 is interpreted to be time-equivalent to patch-reef development at the Grassy Hill locality.Item Pre-Cretaceous erosional surface of the Llano Uplift region, Central Texas(2011-08) Sobehrad, Susan Je; Long, Leon E., 1933-; Wilson, Clark R.Historical research reveals a repeating pattern of uplift, erosion, and deposition in the region of the Llano Uplift, central Texas. This report examines the topography of the pre-Cretaceous landscape. The data consist of points, in three dimensions, that are located on the erosional surface, as determined by three methods. Category I data lie upon the contact between Cretaceous strata and underlying Paleozoic sediments or Precambrian basement; Category II data are defined in the subsurface from well logs; and Category III data are topographic high points where the Cretaceous has eroded away, but the underlying unit has not eroded (an exhumed surface). Digital mapping procedures were used to create triangulated irregular networks, three dimensional scenery, and topographic profiles. The digitally reconstructed surface is compound, consisting of higher, older erosional surfaces, incised into by rejuvenated stream activity to create lower, younger surfaces. This valley/divide topography, which is regional in extent, could not have been visualized without modern GIS technology.Item Revision of the early Cretaceous flora from Hope Bay, Antarctica(1987-05) Gee, Carole T.; Delevoryas, Theodore, 1929-The Early Cretaceous (Berriasian) Hope Bay flora is one of the most diverse assemblages from the Mesozoic of Antarctica. Collected in 1902 by the Swedish Antarctic Expedition of 1901-1903 from Hope Bay at the northern tip of the Antarctic Peninsula, and described in 1913 by T. G. Halle, it has served as a classic reference collection for Jurassic and Cretaceous southern hemisphere paleobotanical studies. Because the systematics of the flora were outdated by the enormous advances in our understanding of fossil plants during the last 70 years, it was in serious need of revision. This revision has reduced the number of taxa from 61 to 42 species. Newly erected species are Otozamites rowleyi, Kachchhia schopfii, Ticoa jeffersonii, and Araucaria antarctica. New combinations are Todites grahamii and Thinnfeldia salicifolia. The genera Kachchhia, Ticoa, and Weltrichia are new occurrences at Hope Bay. Represented in the flora are members of the Hepatophyta, Arthrophyta, Pteridophyta, Pteridospermophyta, Cycadophyta, Cycadeoidophyta, and Coniferophyta. Not surprisingly, when compared with other Gondwana floras, the Hope Bay flora shows the greatest similarity with other Antarctic floras. There is also a close affinity with the floras of South America and New Zealand. Taxonomic similarity between these floras is best explained by paleogeographic proximity.Item Shelf-to-basin architecture and facies variability of a Cretaceous intrashelf basin in the northwest Gulf of Mexico(2015-12) Sitgreaves, Jeffrey Robert; Kerans, C. (Charles), 1954-; Loucks, Robert G; Fisher, William LThe geomorphic expression of intrashelf basin systems (ISBs) and their associated facies patterns is extremely subtle, with shelf-to-basin dip angles that can average 0.3° across the slope profile. This presents an issue to stratigraphers working to understand facies variability at the reservoir-scale because the changes in stratal geometries at the shelf-to-basin transition will occur beneath the resolution of conventional subsurface datasets. Exposures along the Pecos River Canyon provide a unique opportunity to observe the transition from grain-dominated facies of the ramp crest into planktonic foraminifera mudstones/wackestones of the intrashelf basin. For this study, 475 m of detailed sections were collected at five localities and integrated with a high-resolution 3D digital outcrop model (DOM) to document the relationship between vertical facies successions and stratal geometries of the intrashelf basin profile. The high-resolution DOM provides the ability to accurately interpret the subtle depositional dips of the shelf-to-ISB profile that range from less than 0.1° to 0.7°. The development of the differential topography and facies changes associated with the formation of the Maverick ISB is attributed to differential sediment accumulation rates between active rudist-skeletal shoal formation versus deeper-water foraminiferal mudstones of the basin-center. Rudist bank deposition early in the Albian 6 Composite Sequence formed the positive topographic relief (1-3m) that led to the localization of rapid shallow-water sediment accumulation. After the development of subtle topographic expression, ensuing changes in relative sea-level promoted the development of ISB margins that were dominated by rudist faunal assemblages. The development of the ISB margin increasingly led to the differentiation between the grain-dominated facies along the margin and deposition of globigerinid mudstones in the basin-center. The extensive and largely undeformed exposures along the lower Pecos River Canyon and adjacent Amistad Reservoir provide clear evidence of the constructional differential-accumulation-driven formation for the Maverick ISB. Similar constructional models are likely for the East Texas and Fort Stockton ISBs on the Texas Comanche Shelf. Similar constructional progressions have been called on for the Bab intrashelf basin and the Natih-E Formation in the Cretaceous of the Middle East.Item Stratigraphy and Paleodepositional Interpretation of the Lower Nacatoch Formation (Maastrichtian), Robertson County, Texas(2014-02-03) Sims, III, Frank WayneThe lower Nacatoch Formation of the Maastrichtian Navarro Group is considered to be a secondary target for energy companies exploring oil and gas reservoirs. Previous investigators of the northern and western sections of the Nacatoch Formation interpreted this unit to represent a variety of coastal depositional environments. However, investigations have been limited in the southern extent of the Nacatoch Formation, which has created a gap in the data. Well logs, drill cuttings, and core descriptions were gathered to create cross-sections and maps used to interpret geometries, sediment distribution, grain sizes, fauna, and internal stratigraphy of the lower Nacatoch. In the study area on the northwestern corner of Robertson County, Texas, the lower Nacatoch interval is composed of laminated sand and shale of variable thickness and has bar forms trending northwest to southeast. It is interpreted to represent a starved, shallow shelf storm-dominated transgressive ridge/bar deposit. Stratigraphic heterogeneity of the lower Nacatoch is significant because of the effects it has on the vertical and horizontal permeability, as well as, completion methods needed for optimal extraction.Item Structural framework and seismic geomorphology of the Cretaceous beneath the Mad Dog Area, deep to ultradeep waters Gulf of Mexico(2013-05) Markez, Damian; Wood, Lesli J.Recent drilling of deep stratigraphy in subsalt offshore Gulf of Mexico has revealed the presence of thick, amalgamated, Cretaceous siliciclastic reservoirs with the potential to become valid exploration targets. Similar to the Lower Tertiary deepwater play, the significant down-dip distance (> 400 km) from the source deltaics, the data gap across the modern structurally complex salt-tectonics-dominated slope and the difficulties of imaging subsalt stratigraphy pose challenges for the construction of meaningful deepwater system models to aid in exploration and appraisal efforts. A 3D seismic dataset in the Mad Dog field at the basinward end of the modern allochthonous salt canopy and outboard of the Sigsbee Escarpment offers the opportunity to study the nature of the deep stratigraphy at central positions in the basin. The nature of the Cretaceous sedimentary system has been investigated through detailed structural and seismic geomorphologic mapping. An early syndepositional contractional event has been identified and temporally associated with Mesozoic emplacement of a deep salt sheet. These events are masked by the major Neogene-age phase of fold amplification that dominates the present-day subsalt structural framework. Ponded-basin deepwater sedimentation processes control early phases of deposition in the Cretaceous Mad Dog area and sediment-gravity flows are deposited as complexes of low sinuosity amalgamated channelized deposits in roughly-confined sediment pathways. Ponded fills show internal lateral accretion architectures that grow sigmoid in nature as the migrating systems interact with the approaching minibasin margins making evident the structural control on sediment architecture. Later phases of deposition are characterized by slightly sinuous feeder channels with multiple lobe development at their terminus. Variable directions of sediment source pathways indicate a linear-sourced slope apron depositional model for these systems. In addition to the more structured morphologic elements, there were also pervasive mass-transport processes active, presumably triggered by Mesozoic halokinesis. Data in sparse deep wells in the GoM that penetrate the Cretaceous suggest that the Late Cretaceous deepwater depositional system was composed of coarse-grained high density gravity flows. The geometries seen in seismic beneath the Mad Dog area support the existence of such a basinwardly extensive deepwater fan systems developed during the Cretaceous, and the low sinuosity channel geometries and small length:width ratio and amalgamated nature of fan lobes suggest that these systems may have indeed been high-density in nature.Item Sutural form and shell morphology of Placenticeras and systematic descriptions of Late Cretaceous ammonites from the Big Bend region, Texas(2006-08) Waggoner, Karen J.; Lehman, Thomas; Barrick, James E.; Chatterjee, Sankar; Strauss, Richard E.; Mulligan, KevinTwo related investigations document the sutural form, shell morphology, and taxonomy of several Late Cretaceous ammonites. A morphometric study utilizes geographical information systems (GIS) and multivariate statistical tools to examine the taxonomy, phylogeny, and biostratigraphic utility of ten Placenticeras species common in North American Cretaceous strata. A second biostratigraphic study provides systematic descriptions of ammonites collected from Upper Cretaceous (Campanian) strata in the Big Bend and Sierra Vieja areas of West Texas, using the GIS and statistical methods employed in the first study to assist with identification of the Placenticeras specimens. A high degree of intraspecific variation and strong interspecific similarity among the ten Placenticeras species results in gradational shell morphology and sutural form to such an extent that confident discrimination is only possible for the extreme end members of the gradational series of species. Two distinct sutural lineages are recognized when GIS and morphometric analyses are based on sutural similarity, disregarding previous species assignment. The two separate sutural lineages persisted through most of Late Cretaceous time while shell morphology varied widely. This suggests that sutural form may be a more conservative character, whereas most aspects of shell morphology are more plastic. The Pen and Aguja formations in the Big Bend area and the Ojinaga and San Carlos formations in the Sierra Vieja area preserve ammonites representative of the Scaphites hippocrepis III zone, the Submortoniceras tequesquitense zone, the Menabites (Delawarella) delawarensis zone, and the Baculites maclearni zone. These Campanian biostratigraphic zones allow for correlation of Big Bend and Sierra Vieja strata with Gulf Coast and Western Interior strata. The two sutural form lineages documented in the morphometric study are also recognized in four species of Placenticeras represented in the Big Bend and Sierra Vieja collections. The importance of sutural form for examining the phylogeny of North American Placenticeras species suggests that future application of the methods utilized in this study to specimens representing various ontogenetic stages collected from well constrained stratigraphic successions may result in a complete revision of the genus Placenticeras.Item Sutural form and shell morphology of Placenticeras and systematic descriptions of Late Cretaceous ammonites from the Big Bend region, Texas(Texas Tech University, 2006-08) Waggoner, Karen J.; Lehman, Thomas; Barrick, James E.; Mulligan, Kevin; Strauss, Richard E.; Chatterjee, SankarTwo related investigations document the sutural form, shell morphology, and taxonomy of several Late Cretaceous ammonites. A morphometric study utilizes geographical information systems (GIS) and multivariate statistical tools to examine the taxonomy, phylogeny, and biostratigraphic utility of ten Placenticeras species common in North American Cretaceous strata. A second biostratigraphic study provides systematic descriptions of ammonites collected from Upper Cretaceous (Campanian) strata in the Big Bend and Sierra Vieja areas of West Texas, using the GIS and statistical methods employed in the first study to assist with identification of the Placenticeras specimens. A high degree of intraspecific variation and strong interspecific similarity among the ten Placenticeras species results in gradational shell morphology and sutural form to such an extent that confident discrimination is only possible for the extreme end members of the gradational series of species. Two distinct sutural lineages are recognized when GIS and morphometric analyses are based on sutural similarity, disregarding previous species assignment. The two separate sutural lineages persisted through most of Late Cretaceous time while shell morphology varied widely. This suggests that sutural form may be a more conservative character, whereas most aspects of shell morphology are more plastic. The Pen and Aguja formations in the Big Bend area and the Ojinaga and San Carlos formations in the Sierra Vieja area preserve ammonites representative of the Scaphites hippocrepis III zone, the Submortoniceras tequesquitense zone, the Menabites (Delawarella) delawarensis zone, and the Baculites maclearni zone. These Campanian biostratigraphic zones allow for correlation of Big Bend and Sierra Vieja strata with Gulf Coast and Western Interior strata. The two sutural form lineages documented in the morphometric study are also recognized in four species of Placenticeras represented in the Big Bend and Sierra Vieja collections. The importance of sutural form for examining the phylogeny of North American Placenticeras species suggests that future application of the methods utilized in this study to specimens representing various ontogenetic stages collected from well constrained stratigraphic successions may result in a complete revision of the genus Placenticeras.Item Taxonomic revision of latest Cretaceous North american basal neonithischian taxa and a phylogenetic analysis of basal ornithischian relationships(2012-05) Boyd, Clint Aaroen; Clarke, Julia A.The systematic relationships of basal ornithischian dinosaurs remain contentious, especially the position of basal neornithischians (i.e., ‘hypsilophodontids’). Prior analyses of basal ornithischian relationships have been hampered by the fact that the hypodigm material of many basal neornithischian taxa is fragmentary, denying access to character data crucial to resolving their relationships. The recent discovery of several new basal neornithischian taxa and the referral of more complete specimens to known taxa provide important new data pertinent to resolving these relationships. The results of this study supplement those recent advances by improving our understanding of the anatomy and systematic relationships of basal neornithischian taxa from the Late Cretaceous of North America. These new insights are accomplished through a taxonomic revision of the Maastrichtian taxa Bugenasaura and Thescelosaurus, a detailed anatomical description of the cranial anatomy of Thescelosaurus neglectus based on the referral of a specimen that includes a nearly complete skull (NCSM 15728), and description of a new basal neornithischian taxon from the Kaiparowits Formation (Campanian) of Utah. All of these new data are compiled into a dataset composed of 255 characters for 65 terminal taxa (all species exemplars) focused on assessing basal ornithischian relationships. The recovered strict consensus topology is the most highly resolved, stratigraphically congruent phylogenetic hypothesis of basal ornithischian relationships yet proposed. This analysis places all basal neornithischians except Hypsilophodon foxii outside of Cerapoda, substantially reducing the taxonomic contents of Ornithopoda. A new clade containing fourteen basal neornithischian taxa is recovered as the sister taxon to Cerapoda and includes all North American basal neornithischians from the Cretaceous. The historical biogeography of Ornithischia is also reconstructed using a method that incorporates time calibrated branch lengths that represent the implied missing fossil record of each taxon. The results of this analysis support two dispersals of neornithischian taxa into South America during the Cretaceous: one consisting of basal iguanodontians dispersing from Australia (possibly via Antarctica) and a second consisting of basal neornithischians dispersing from Asia through North America.