David E. Fastovsky

High-precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs

The Triassic successions of the Colorado Plateau preserve an important record of vertebrate evolution and climate change, but correlations to a global Triassic framework are hampered by a lack of geochronological control. Tuffaceous sandstones and siltstones were collected from the Upper Triassic Chinle Formation exposed in the Petrified Forest National Park, Arizona, USA, within a refined stratigraphic context of 31 detailed measured sections. U-Pb analyses by the isotope dilution–thermal ionization mass spectrometry (ID-TIMS) method constrain maximum depositional ages for nine tuffaceous beds and provide new insights into the depositional history of the Chinle fluvial system. The base of the Blue Mesa Member of the Chinle Formation is placed at ca. 225 Ma, and the top of the Petrified Forest Member is placed at 208 Ma or younger, bracketing an ∼280-m-thick section that spans nearly the entire Norian Stage of the Late Triassic. Estimated sediment accumulation rates throughout the section reflect extensive hiatuses and/or sediment removal by channel erosion. The new geochronology for the Chinle Formation underscores the potential pitfalls of correlation of fluvial units based solely on lithostratigraphic criteria. A mid-Norian age (ca. 219–213 Ma) for the distinctive Sonsela conglomeratic sandstone bed constrains the Adamanian-Revueltian land vertebrate faunachron boundary. Our new data permit a significant time overlap between the lower Chinle sequence and the dinosauromorph-rich Ischigualasto Formation of northwestern Argentina. Near-contemporaneity of the trans-American deposits and their faunal similarities imply that early dinosaur evolution occurred rapidly across the Americas.

The Extinction of the Dinosaurs in North America

Rightly or wrongly, dinosaurs are poster children for the CretaceousTertiary (K-T) extinction. The rate and cause of their extinction, however, has been contentious, at least in part because of their rarity. Nonetheless, significant data have accumulated to indicate that the dinosaur extinction, in North America at least, was geologically instantaneous. The evidence comes from field studies in geologically disparate settings involving the reconstruction of dinosaur stratigraphic ranges as well as community structure in the Late Cretaceous, and from quantitative studies of the post-Cretaceous evolution of mammals. The hypothesis of extinction by asteroid impact is concordant with what is known of the rate of the dinosaur extinction, as well as the patterns of selective vertebrate survivorship across the K-T boundary. The precise nature of the kill mechanism(s), however, remains under discussion.

Major extinctions of land-dwelling vertebrates at the Cretaceous-Tertiary boundary, eastern Montana

A large database recording species of terrestrial vertebrates present in formations above and below the Cretaceous-Tertiary (K-T) boundary in eastern Montana was assembled by J. D. Archibald and L. J. Bryant. Division of the species in this database into freshwater and land-dwelling vertebrate assemblages reveals that the K-T vertebrate extinction was concentrated in land- dwelling forms. In data corrected for the effects of rare taxa, 90% of the species in the freshwater assemblage survived into the Tertiary, but only 12% of the land-dwelling forms survived. The pattern of differential extinction of terrestrial vertebrates in eastern Montana may be in large part the result of the dependence of land-based communities on primary productivity. This is in contrast to the riverine communities, which may derive much of their organic carbon from detritus. The pattern of extinction and survival is compatible with the hypothesis of an asteroid impact after which there was a temporary cessation of primary, photosynthetic productivity.

Paleoenvironments of vertebrate-bearing strata during the Cretaceous-Paleogene transition, eastern Montana and western North Dakota

Exposures of the Hell Creek and Tullock formations in eastern Montana and the Ludlow Formation in western North Dakota allow detailed reconstruction of the paleoenvironments associated with the Cretaceous-Paleogene (K-P) faunal transition in the Western Interior of North America. Paleoenvironments were reconstructed from five facies: 1) a cross-stratified sandstone facies interpreted as afluvial channel environment; 2) a siltstone facies interpreted as flood-plain environments; 3) a facies of epsilon cross-stratification inferred to represent point-bar deposition; 4) a facies of organic accumulation, interpreted to reflect peat deposition; and 5) a variegated siltstonefacies, interpreted as the remnant of extensive ponded deposits. Facies associations demonstrate an ancient meandering fluvial environment in which gleization in unstable, poorly drainedfloodplains modified incipient soils. Sedimentologic and pedologic features indicate that concurrent with the faunal transition that occurred in the region, the amount of standing water increased dramatically, changing the earliest Paleogene soils and landscape. Depositional environment imposes taphonomic constraints on interpretations of K-P faunas and floras. Contrary to recent reports, fossil assemblages in Hell Creek channel deposits are reworked; bone and sediment clasts of the channel fills have been subject to traction transport. Age estimations based on supposedly unreworked fossils in channel deposits are thus unreliable. The chronostratigraphic resolution of the sediments under study is to date simply not comparable to the resolution required by researchers of the K-P boundary.

Shape of Mesozoic dinosaur richness

The richness of Mesozoic Dinosauria is examined through the use of a new global database. Mesozoic dinosaurs show a steadily increasing rate of diversification, in part attributable to the development of new innovations driving an increasing variety of behavioral strategies. The data do not suggest that dinosaurs were decreasing in richness leading to extinction during the last ∼10 m.y. of the Cretaceous. Refinement of the dating of dinosaur fossils, rather than the collection of more dinosaurs, is the best way to resolve globally the rate of the Cretaceous-Tertiary dinosaur extinction.

Dinosaur abundance was not declining in a “3 m gap” at the top of the Hell Creek Formation, Montana and North Dakota

An ∼3 m stratigraphic interval that entirely lacks dinosaur fossils or has very few fossils has been reported at the top of the Hell Creek Formation in the upper Great Plains of North America. The presence of the “3 m gap” in fossil distribution has been cited as evidence that dinosaurs had either become extinct or were on the verge of extinction prior to the bolide impact at the end of the Cretaceous Period. A survey in two areas of North Dakota and Montana reveals that dinosaur fossils in the upper 3 m of the Hell Creek Formation occur in numbers that are comparable to their abundance in other levels of the formation. Evidence for a gradual extinction is absent, and data are consistent with an abrupt extinction associated with an impact.

Foot posture in a primitive pterosaur

The nature of the hindlimb posture and gait of pterosaurs has been controversial, partly because most of the pterosaur skeletons that have been found were flattened in thin-bedded rocks, therefore obscuring three-dimensional anatomy. A major controversy concerns the extent to which pterosaurs move on the ground; they have been variously interpreted as ranging from sprawling, quadrupedal walkers to erect, bird-like bipedal cursors. Study of pelvis and femur material from the derived group Pterodactyloidea has resolved which movements are possible at the hip, but the lack of three-dimensional, articulated pterosaur feet has prevented examination of all of the movements that are possible within the foot. We have found a large, uncrushed, partial skeleton of a new species of the basal pterosaur Dimorphodon in thick-bedded deposits of Tamaulipas, Mexico; this material includes such a three-dimensional foot. The nature of this skeleton contradicts an important part of the cursorial interpretation, that is, that only the toes contacted the ground during terrestrial locomotion. The flattened metatarsal–phalangeal joint at the base of the first four toes of this specimen would not allow such a digitigrade posture without separating most of the joints. A flat-footed stance is consistent with presumed footprints of pterosaurs that show impressions of the entire sole of the foot.

New Oviraptorid Embryos from Bugin-Tsav, Nemegt Formation (Upper Cretaceous), Mongolia, with Insights into Their Habitat and Growth

Abstract Eggs containing well-preserved skeletons were collected from Bugin-tsav, an Upper Cretaceous locality in the Nemegt Formation, Ömnögov Aimag, Mongolia. These embryos, found in a weathered nest of eggs, are oviraptorid theropods. Eggshell morphotype is Elongatoolithidae, typical of theropods, including basal birds. Bone histology indicates that all embryos were probably close to hatching, based on the degree of ossification and in comparison with ossification patterns in living birds. Maturity of ossification indicates that oviraptorids hatched at a precocial stage of development. Two of the embryos are relatively large, while the other is 25% smaller, suggesting that size disparity of hatchlings in oviraptorids may be a consequence of asynchrony in egg laying.

Taphonomy and suggested structure of the dinosaurian assemblage of the Hell Creek Formation (Maastrichtian), eastern Montana and western North Dakota

This study quantifies the taphonomic context of fossil dinosaur elements in the Late Cretaceous Hell Creek Formation of Montana and North Dakota, USA. A previously published data base consisting of 649 individuals (counted at the family level) was used to establish statistically fundamental associations between the vertebrate faunal elements and the fluvial architectural elements in which they were found. In the Hell Creek, preservation is not equally distributed among the various fluvial architectural elements. Flood-plain and channel deposits preserve the preponderance of the Hell Creek dinosaur fauna. Articulated fossils most commonly occur within floodplain and point-bar deposits. Floodplain and related deposits, however, preserve the highest dinosaur faunal diversities. The dinosaur sample inferred to be most representative of the original dinosaurian assemblage structure, therefore, is obtained from floodplain and genetically related deposits. These yield eight families of dinosaurs represented in the following proportions: Ceratopsidae, 61%; Hadrosauridae, 23%; Ornithomimidae, 5%; Tyrannosauridae, 4%; Hypsilophodontidae, 3%; Dromaeosauridae, 2%; Pachycephalosauridae, 1%; and Troodontidae, 1%. Among these groups, dromaeosaurs and troodontids are represented only by teeth, a circumstance attributed at least in part to thin-walled bones whose potential for preservation in an active fluvial system is jeopardized. Ornithomimids constitute 5% of the total assemblage, which makes them the third most common dinosaur in this study. Their relatively high abundance may suggest a herbivorous dietary preference.

Paleoenvironments of Early Theropods, Chinle Formation (Late Triassic), Petrified Forest National Park, Arizona

Abstract Three localities in the Chinle Formation (Late Triassic), Petrified Forest National Park (PEFO), Arizona, provide insights into the paleoenvironments frequented by primitive North American theropods. At the Dinosaur Wash locality, an undetermined theropod is preserved in paleosols that indicate a transition from wet to dry conditions on the floodplain. Coelophysis bauri remains from the Dinosaur Hill locality are preserved in a filled channel scour. The paleosols at this locality contain carbonate nodules intimately associated with Fe and Mn oxides, indicative of alternating alkaline/acidic conditions around roots in response to a semi-arid climate with strong seasonal precipitation. At the Dinosaur Hollow locality, Chindesaurus bryansmalli is preserved in a setting similar to those encountered at Dinosaur Hill. The paleosols exhibit vertic features, such as pseudoanticlines, and are indicative of water-deficit periods during the year. The most complete theropod remains at PEFO are predominantly preserved in distinctive blue-colored paleosol horizons showing depletion in iron and aluminum, and exhibiting features such as localized Fe concentrations and mottling. These are interpreted as A-horizons of redoximorphic paleosols developed in wet areas of the floodplain where the degree of water saturation fluctuated. Preservational conditions of PEFO theropod localities indicate they are time-averaged attritional mortality assemblages representative of contemporaneous organisms. Comparison of these localities to the well-known Ghost Ranch, New Mexico, Coelophysis quarry reinforces the idea that the quarry represents unique preservational conditions.