James G. Schmitt

Nest Structure for Sauropods: Sedimentary Criteria for Recognition of Dinosaur Nesting Traces

Abstract Six egg-filled depressions discovered in the Upper Cretaceous Anacleto Formation (Campanian) of Patagonia, Argentina, and interpreted as dinosaur nests, provide the only known evidence of titanosaurid sauropod nest construction. These nest trace fossils show truncation of sedimentary structures as well as differences in texture between the host substrate and in-filling sediment. Titanosaurid sauropods excavated and laid eggs in open nests rather than burying clutches in sediment. In addition, this paper establishes criteria for definitive recognition of excavated nests in the stratigraphic record.

Influence of inherited taper on structural variability and conglomerate distribution, Cordilleran fold and thrust belt, western United States

North-south changes in thickness of the prethrust stratigraphy of the North American Cordillera within the United States controlled kinematic development of the late Mesozoic (Sevier) fold and thrust belt. Variation in stratigraphic thickness determined the initial slope (β o ) of the basal decollement beneath the thrust wedge. As predicted by critical-wedge theory, segments of the Sevier thrust belt with initially thick Precambrian and Paleozoic stratigraphic sections are structurally simple and are composed of few thrust sheets; segments with thin prethrust stratigraphy and comparably low values of β o are intensely imbricated. The difference in structural style resulted because wedge segments with higher values of β o required less internal deformation to build and maintain the critical taper required for thrust-wedge advance. Transverse structural zones, across which kinematic style differed, formed at the terminations of thrust-belt segments and controlled the locations of long-lived antecedent drainage systems that transported gravel to the foreland. Therefore, along-strike changes in prethrust stratigraphic thickness resulted in an observed disjunct distribution of conglomerate in synorogenic deposits of the thrust wedge and foreland basin.

Late Cretaceous avian eggs with embryos from Argentina

MARY H. SCHWEITZER1,2, FRANKIE D. JACKSON2, LUIS M. CHIAPPE3, JAMES G. SCHMITT2, JORGE O. CALVO4, and DAVID E. RUBILAR5 1Department of Microbiology, Montana State University, Bozeman, Montana 59717, schweitz@montana.edu; 2Department of Earth Sciences and Museum of the Rockies, Montana State University, Bozeman, Montana 59717; 3Natural History Museum of Los Angeles, Los Angeles, California 90007; 4Museo de Geologia y Paleontologia, Universidad Nacional del Comahue, Neuqu6n, Argentina; 5Instituto de Geociencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile, and Secci6n Paleontologia, Museo Nacional de Historia Natural, Casilla 787, Santiago, Chile

Patterns of silicification in Permian pelecypods and brachiopods from Wyoming

ABSTRACT Silicified pelecypods and brachiopods are common in the Ervay, Franson, and Grandeur Members of the Permian Park City Formation of central and western Wyoming. Study of hundreds of these fossils has resulted in recognition of five patterns of silicification. Pattern I is characterized by megaquartz crystals projecting inward from and perpendicular to the skeletal boundary, whereas fossils exemplifying Pattern II consist of concentric laminae of chalcedonic quartz. In Pattern III, an outer lamina of microcrystalline quartz grades abruptly inward to laminated chalcedonic quartz which in turn grades abruptly inward to megaquartz. Shells demonstrating Pattern IV involve megaquartz crystals aligned parallel to the skeletal boundary and lying adjacent to beekite discs. Pattern V consists of megaquartz crystals aligned parallel to the skeletal boundary. In brachiopods, these crystals typically exhibit relict skeletal microstruture. Pelecypod wall layers constructed originally of aragonite are characterized by Patterns I, II, III and IV. Pattern V characterizes all brachiopods and originally calcitic layers of pelecypods. Patterns I, II, III and IV are interpreted to result from filling of voids created by dissolution of skeletal carbonate. Pattern V results from concurrent carbonate dissolution and silica precipitation.

Cretaceous to early Tertiary paleogeography in the hinterland of the Sevier thrust belt, east-central Nevada

The Cretaceous to early Tertiary hinterland of the Sevier thrust belt in east-central Nevada underwent two periods of sedimentary basin development. The older (Early Cretaceous) episode is characterized by basins that formed coeval with deformation within a belt of contractional structures. Basins were characterized by periods of through-flowing fluvial drainage. The younger (Late Cretaceous to Eocene) episode of basin development was characterized by internally drained alluvial basins that subsequently expanded into more regional lacustrine basins. Basal alluvial detritus indicates that basin inception occurred as a result of rapid development of topographic relief. Syn-sedimentary tectonism was characterized by horizontal extension due to attainment of equilibrium topography in response to crustal thickening.

Provenance and Dispersal of Tectogenic Sediments in Thin-Skinned, Thrusted Terrains

The style of deformation in thin-skinned, thrusted terrains provides a setting for complex provenance and dispersal relations between source and sediment. Whereas it is commonly assumed that clastic wedges are shed by active thrusts and therefore can be used to date thrust motion, passive sources, formed when inactive terrain is transported over a ramp, may also generate tectogenic sediments. Furthermore, these sediments may be dispersed either synthetically or antithetically to tectonic transport. In some cases the clast composition of these deposits displays an inverted stratigraphy, reflecting unroofing in the source, while in other instances, blended clast compositions may form because of the simultaneous erosion of a thick stratigraphic section, complex geology in the source, or depositional amalgamation during sediment accumulation. Finally, during the evolution of a thrusted terrain, tectogenic sediments are tectonically transported, continually reworked, and cannibalized.

Influence of Late Cretaceous magmatism on the Sevier orogenic wedge, western Montana

Late Cretaceous large-volume pluton emplacement and accompanying volcanism within the evolving western Montana thrust wedge may have played important roles in determining the geometric and kinematic development of the thrust wedge, thereby influencing patterns of sediment dispersal and subsidence in the adjacent foreland basin. Intrusion of the Upper Cretaceous Boulder batholith and coeval eruption of the Elkhorn Mountains Volcanics were focused mainly at the trailing margin of the Lombard-Eldorado allochthon. The resultant thick (16–17 km) igneous culmination evolved over a relatively short interval (80–70 Ma), thickening the orogenic wedge to the point of supercritical taper and facilitating continued motion along the Lombard-Eldorado thrust system, thrust imbrication at the wedge toe, and forelandward translation of the Helena salient. Delivery of eroded volcaniclastic detritus from the thickened thrust wedge and accelerated basin subsidence due to thrust loading resulted in accumulation of a thick (4–5 km) sequence of Campanian-Maastrichtian volcaniclastic strata in the foreland basin (Livingston Group). The structural and sedimentological effects of this structural-magmatic culmination are similar to those of basement-cored culminations elsewhere in the Sevier orogen and to the Neogene central Andean orogenic wedge.

Comparative Taphonomy, Taphofacies, and Bonebeds of the Mio-Pliocene Purisima Formation, Central California: Strong Physical Control on Marine Vertebrate Preservation in Shallow Marine Settings

Background Taphonomic study of marine vertebrate remains has traditionally focused on single skeletons, lagerstätten, or bonebed genesis with few attempts to document environmental gradients in preservation. As such, establishment of a concrete taphonomic model for shallow marine vertebrate assemblages is lacking. The Neogene Purisima Formation of Northern California, a richly fossiliferous unit recording nearshore to offshore depositional settings, offers a unique opportunity to examine preservational trends across these settings. Methodology/Principal Findings Lithofacies analysis was conducted to place vertebrate fossils within a hydrodynamic and depositional environmental context. Taphonomic data including abrasion, fragmentation, phosphatization, articulation, polish, and biogenic bone modification were recorded for over 1000 vertebrate fossils of sharks, bony fish, birds, pinnipeds, odontocetes, mysticetes, sirenians, and land mammals. These data were used to compare both preservation of multiple taxa within a single lithofacies and preservation of individual taxa across lithofacies to document environmental gradients in preservation. Differential preservation between taxa indicates strong preservational bias within the Purisima Formation. Varying levels of abrasion, fragmentation, phosphatization, and articulation are strongly correlative with physical processes of sediment transport and sedimentation rate. Preservational characteristics were used to delineate four taphofacies corresponding to inner, middle, and outer shelf settings, and bonebeds. Application of sequence stratigraphic methods shows that bonebeds mark major stratigraphic discontinuities, while packages of rock between discontinuities consistently exhibit onshore-offshore changes in taphofacies. Conclusions/Significance Changes in vertebrate preservation and bonebed character between lithofacies closely correspond to onshore-offshore changes in depositional setting, indicating that the dominant control of preservation is exerted by physical processes. The strong physical control on marine vertebrate preservation and preservational bias within the Purisima Formation has implications for paleoecologic and paleobiologic studies of marine vertebrates. Evidence of preservational bias among marine vertebrates suggests that careful consideration of taphonomic overprint must be undertaken before meaningful paleoecologic interpretations of shallow marine vertebrates is attempted.

Abnormal, multilayered titanosaur (Dinosauria: Sauropoda) eggs from in situ clutches at the Auca Mahuevo locality, Neuquen Province, Argentina

Abstract Abnormal, multilayered eggshells are frequently reported in fossil specimens. However, previous reports of this pathological condition pertain to taxonomically unidentified fossil eggs or eggshell fragments rather than in situ clutches. A survey of 393 in situ clutches, referable to titanosaur sauropod dinosaurs, from Upper Cretaceous rocks in northwestern Patagonia, Argentina, revealed that six of these clutches contain both normal eggs and abnormal, multilayered eggs within the same clutch. Excavation of one clutch exposed 30 eggs, distributed in three levels, including 27 normal eggs and 3 multilayered eggs. The three abnormal eggs occupied the highest level within the clutch and represent the last eggs laid by the female sauropod. The innermost eggshell layer in multilayered eggs from all six clutches exhibits typical megaloolithid calcite structure. The structure of the overlying, abnormal shell layer(s), however, varies among the clutches and within a single egg. Three types of abnormal eggshell morphology are documented. All previous studies that link abnormal eggshells to dinosaur physiology and the terminal Cretaceous extinction event are incomplete in terms of taphonomy, taxonomic identification, and lack rigorous statistical methods.

Development and exhumation of a Neogene sedimentary basin during extension, east-central Nevada

An analysis of the Miocene Horse Camp basin in the central Basin and Range province demonstrates that individual components of an extensional basin system may undergo initial subsidence and subsequent uplift and exhumation during extension. The Horse Camp basin developed during Miocene time above a west-dipping detachment fault and between two east-striking accommodation faults. Growth strata indicate that the west-dipping detachment was active throughout basin filling. Lateral facies variations in alluvial-fan, fan-delta, and lacustrine deposits indicate that the east-striking accommodation faults along the northern and southern basin margins bounded primary sediment source areas. During late Miocene time, the hanging-wall block of the detachment system was broken by the west-dipping Railroad Valley normal fault, thereby inducing development of an extensional basin, Railroad Valley, in the newly formed hanging wall and uplift and exhumation of the older Horse Camp basin in the footwall. The total amount of exhumation during Neogene extension is calculated for a single range block, the northern Grant Range. Early exhumation is recorded by the detrital compositions of middle-late Miocene strata in the southern part of the Horse Camp basin. Provenance modeling indicates that approximately 1600 m of Mississippian to Silurian section was exhumed in the range during middle-late Miocene Horse Camp deposition. From late Miocene to Holocene time, both the northern Grant Range and Horse Camp basin were exhumed in the footwall of the Railroad Valley fault. A comparison between the currently exposed bedrock geology and calculated late Miocene exposure levels from the provenance model indicates that 1000–2000 m of Cambrian–Ordovician section must have been exposed in the range after late Miocene deposition in the Horse Camp basin. Modern alluvial fans in eastern Railroad Valley have drainage networks that contain exposures of the fill of the Horse Camp basin, indicating that older basin fill is being recycled into the modern extensional basin. Extensional basin systems having similar basinward shifts of the major fault system are particularly susceptible to uplift, exhumation, and erosional recycling, not unlike the structurally dismembered margins of foreland basin systems.