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The Kaiparowits Formation: A Remarkable Record of Late Cretaceous Terrestrial Environments, Ecosystems, and Evolution in Western North America
... Dinosaur Provincial Park (DPP; the Park), a small geographic area (80 km 2 ) in southern Alberta, Canada (Fig. 1), yields a rich and uniquely diverse assemblage of Late Campanian vertebrates, including non-avian dinosaurs. The Park is famous for abundant articulated and associated fossil skeletons and bonebeds that contribute to our understanding of peak dinosaur diversity during the Late Campanian and patterns of latitudinal variation in dinosaurian megaherbivore taxa across the Western Interior Basin (WIB) of North America (Lehman 1997(Lehman , 2001Currie and Koppelhus 2005, and papers therein; Barrett et al. 2009;Sampson et al. 2010;Gates et al. 2012;Eberth 2015;Ramezani et al. 2022;Roberts et al. 2005Roberts et al. , 2013. ...
... high-resolution, reproducible geochronology for the Park and the BRG, as well as correlation to other Campanianage dinosaur sites with 40 Ar/ 39 Ar ages in the WIB (e.g., Goodwin and Deino 1989;Rogers et al. 1993;Ogg et al. 2004;Roberts et al. 2005Roberts et al. , 2013Foreman et al. 2008;Jinnah et al. 2009). ...
The 100 m thick stratigraphic section exposed at Dinosaur Provincial Park (DPP; southern Alberta) contains bentonites that have been used for more than 30 years to date DPP’s rocks and fossils using the K–Ar decay scheme. Limited reproducibility among different vintages of K–Ar and ⁴⁰Ar/³⁹Ar ages inhibited the development of a high-resolution chronostratigraphy. Here, we employ and further test a recently completed U–Pb geochronology and associated age-stratigraphy model to update temporal constraints on the Park’s bentonites, formational contacts, and other markers. In turn, we document rock accumulation rates and calibrate ages and durations of informal megaherbivore dinosaur assemblage zones and other biozones. Weighted mean ²⁰⁶ Pb/238 U ages from five bentonites range from 76.718 ± 0.020 to 74.289 ± 0.014 Ma (2σ internal uncertainties) through an interval of 88.75 m, indicating a duration of ∼2.43 Myr and an overall rock accumulation rate of 3.65 ± 0.04 cm/ka. An increase in rate above the Oldman–Dinosaur Park formational contact conforms to a regionally expressed pattern of increased accommodation at ∼76.3 Ma across Alberta and Montana. Palynological biozone data suggest a condensed section/hiatus in the uppermost portion of the Oldman Formation. Dinosaur assemblage zones exhibit durations of ∼700–600 kyr and are significantly shorter than those in the overlying Horseshoe Canyon Formation. A decreased rate in dinosaur assemblage turnovers in the last eight million years of the Mesozoic in western Canada may be explained by withdrawal of the Western Interior Seaway and the expansion of ecologically homogenous lowlands in its wake.
... 76.46-74.69 Ma [123]) and detrital zircon U-Pb radiometic geochronology (as late as 73.10 Ma ± 1.20 Ma [124]), with multiple ootaxa occurring at the same locality [55]. The oogenera we document from the Mussentuchit Member are represented from at least one locality between MAZ1 (99.49 ...
The first fossil eggshell from the Cenomanian-age Mussentuchit Member of the Cedar Mountain Formation was described over fifty years ago. In the half-century since, oodiversity of this rock unit has been limited to a single, taxonomically unstable ootaxon, currently formulated as Macroelongatoolithus carlylei. Recently, there has been a renewed effort to recover and describe the macrofauna of the Mussentuchit; however, these advances are limited to the body fossil record. Here, we examine the range of eggshells present in the Mussentuchit Member and assess the preserved biodiversity they represent. Gross morphological and microstructural inspection reveals a greater diversity of eggshells than previously described. We identify six ootaxa: three Elongatoolithidae oogenera (Macroelongatoolithus, Undulatoolithus, Continuoolithus), eggs laid by oviraptorosaur dinosaurs; two oospecies of Spheroolithus laid by ornithopod dinosaurs; and Mycomorphoolithus kohringi, laid by a crocodylomorph. The diversity of Elongatoolithidae in the Mussentuchit requires a co-occurrence of at least three putative oviraptorosaurs, the oldest such phenomenon in North America. The occurrence of the crocodylomorph oogenus Mycomorphoolithus is the first recognized occurrence outside of Europe, and the youngest yet documented. This new ooassemblage is more representative of the known paleobiodiversity of Cenomanian-age strata of Western North America and complements the body fossil record in improving our understanding of this crucial—yet poorly documented—timeslice within the broader evolution of the Cretaceous Western Interior Basin.
... For example, Champsosaurus remains are comparatively rare within the penecontemporaneous inland setting of the TMF (Gilmore 1917, Varricchio 1995, Hutchinson et al. 2022. The genus is present in Maastrichtian coastal deposits of the Laramie Formation in Colorado (Carpenter 1979), but not in isolatitudinal deposits of the Kaiparowits Formation to the west in Utah (Lehman and Barnes 2010), which is believed to have occupied a more inland setting during the Campanian (Roberts et al. 2013, Ramezani et al. 2022. It is therefore tempting to suggest that the presence of C. lindoei in both coastal flood plains and more seasonal upland alluvial plain environments indicates that this taxon could withstand seasonal drought conditions to some extent. ...
Although the neochoristodere Champsosaurus is well documented in Campanian deposits of western North America, species-diagnostic remains from these strata are restricted to the Dinosaur Park Formation of Alberta, Canada. Here, we describe an exceptionally well-preserved specimen of Champsosaurus lindoei from the Two Medicine Formation of Montana, USA—one of the few occurrences of the genus within the formation, and the first confirmed occurrence of C. lindoei outside of Dinosaur Provincial Park and its vicinity. This specimen preserves previously unknown aspects of this species, including the pes, tail, and integument, allowing for the first detailed postcranial description in this taxon. The integumentary impressions preserved on the ventral surface are the largest described thus far for Champsosaurus, supporting previous suggestions that Champsosaurus scales increase in size ventrally. We also conducted the first species-level phylogeny for Champsosaurus, where we recovered a pectinate tree topology with Palaeocene species as the sister taxa to older Maastrichtian and Campanian species, and provide further evidence for at least three lineages of Champsosaurus surviving the Cretaceous-Palaeogene (K-Pg) extinction. Finally, the recovery of the specimen from the semi-arid upland deposits of the Two Medicine Formation suggests that Champsosaurus may have been better able to withstand drier environments than previously thought.
... 4418), within the central Kaiparowits Plateau of Grand Staircase-Escalante National Monument, southern Utah (Fig. 1). The locality is approximately 200-300 meters above the lower contact with the Wahweap Formation, located stratigraphically higher than Ash Bed KP-07 of Roberts et al. (2013), U-Pb dated to 76.394 ± 0.040 Ma, and below Ash Bed KBC-109, dated to 75.609 ± 0.015 Ma , thus placing the locality at approximately 76 Ma. The youngest certain occurrence of Denazinemys nodosa, the type locality in the De-Na-Zin Member of the Kirtland Formation, is capped by Ash J (Fassett and Steiner 1997), dated to 73.496 ± 0.039 Ma . ...
Denazinemys nodosa is a Late Cretaceous representative of the North American turtle clade Baenidae diagnosed, among others, by a shell surface texture consisting of raised welts. We provide a detailed description of a partial skeleton from the late Campanian Kaiparowits Formation of Utah, USA, including bone-by-bone analysis of its cranium based on images obtained using micro-computed tomography. A revised phylogenetic analysis confirms placement of Denazinemys nodosa close to Eubaena cephalica and Boremys spp. within the clade Eubaeninae. Comparison with a second skull from the Kaiparowits Formation previously assigned to Denazinemys nodosa questions its referral to this taxon. An assortment of specimens from the Early to Late Campanian of Mexico and the USA had previously been referred to Denazinemys nodosa based on shell surface texture alone, even though this characteristic is known to occur in other baenids. Our review of all available material concludes that Denazinemys nodosa is currently only known from the Late Campanian of New Mexico and Utah.
... Therefore, it is suggested that the early juvenile individuals died right before the end of the (2015). Radiometric dates for Utah are taken from Roberts et al. (2013), except that the date for the Wahweap Formation is a recalibrated value reported by Freedman Fowler and Horner (2015) for a date originally published by Jinnah et al. (2009). Vertical bars indicate stratigraphic ranges for Gryposaurus species in Alberta, and diamond symbols (l) indicate approximate positions of non-Albertan Gryposaurus species. ...
A monodominant Gryposaurus sp. bonebed in the lower unit of the Campanian Oldman Formation of southern Alberta is the oldest hadrosauroid bonebed documented in the province and the first described from the formation. The sedimentology of the locality and the taphonomy of the hadrosaurid material indicates that the bonebed represents an assemblage of juvenile-sized individuals that were probably transported only a short distance from where they died to where they were finally deposited and preserved in a fine-grained mudstone within an overbank sequence. Histological examination of six limb elements confirms that all individuals are juveniles, with two age classes (<1 and <2 years of age at the time of death) that likely died in the same event. Bone microstructure data indicate that Gryposaurus experienced rapid growth over the 2-year life spans documented, equivalent to other Late Cretaceous hadrosaurids in North America. The parautochthonous nature of the bonebed, and the lack of small neonate (newborn) material and almost complete lack of large adult material, suggests that the bonebed represents a segregated group of juveniles. This group of immature individuals may have been an autonomous unit that had separated itself from a larger social grouping, possibly in an effort to increase their survivability.
Three catastrophic landslides make up the recently discovered Oligocene-Miocene Marysvale Gravity Slide Complex (MGSC), those being the Markagunt (MGS), Sevier (SGS), and Black Mountains (BGS) gravity slides, in south-western Utah. These collapses are hypothesized to have been driven by the igneous inflation of the Marysvale Volcanic Field (MVF). Here I assess the pre-volcanic strata that form the floor and environs of the MVF to constrain the regional paleogeography and sediment sourcing prior to volcanism. Detrital zircon U-Pb analyses (LA-ICPMS) were conducted on sandstones and the matrix of conglomerate beds of the Eocene Claron Formation (STATS) and the overlying Oligocene Conglomerate at Boat Mesa (STATS) and show a shift in provenance over time. The detrital zircon age spectra of the pink member (lower) of the Claron Formation is dominated by Mesoproterozoic zircons that I interpret as being recycled from Neoproterozoic Proterozoic-Mesozoic strata exposed in the Sevier thrust belt, which was proximal and to the west of the Claron Basin. The overlying white member of the Claron Formation also is dominated by Mesoproterozoic zircons but shows a larger presence of Mesozoic grains. All Claron pink samples are statistically indistinguishable. The Conglomerate at Boat Mesa is dominated by Jurassic grains that were sourced from the Cordilleran arc in the Sevier hinterland, indicating the loss of prominence of the Sevier fold and thrust belt at this latitude by Eocene time. The Conglomerate at Boat Mesa is overlain by the volcanogenic Brian Head Formation, which was sourced by the Indian Peak Caldera complex, several hundred kilometers to the west. Thus, these data indicate that the Laramide compression in this region had abated by the end of Claron deposition which is revealed by a shift in sediment provenance from proximal sources in the Sevier foreland to distal sources in the Sevier hinterland and the Sierran magmatic arc.
The Late Cretaceous of western North America supported diverse dinosaur assemblages, though understanding patterns of dinosaur diversity, evolution, and extinction has been historically limited by unequal geographic and temporal sampling. In particular, the existence and extent of faunal endemism along the eastern coastal plain of Laramidia continues to generate debate, and finer scale regional patterns remain elusive. Here, we report a new centrosaurine ceratopsid, Lokiceratops rangiformis , from the lower portion of the McClelland Ferry Member of the Judith River Formation in the Kennedy Coulee region along the Canada-USA border. Dinosaurs from the same small geographic region, and from nearby, stratigraphically equivalent horizons of the lower Oldman Formation in Canada, reveal unprecedented ceratopsid richness, with four sympatric centrosaurine taxa and one chasmosaurine taxon. Phylogenetic results show that Lokiceratops , together with Albertaceratops and Medusaceratops , was part of a clade restricted to a small portion of northern Laramidia approximately 78 million years ago. This group, Albertaceratopsini, was one of multiple centrosaurine clades to undergo geographically restricted radiations, with Nasutuceratopsini restricted to the south and Centrosaurini and Pachyrostra restricted to the north. High regional endemism in centrosaurs is associated with, and may have been driven by, high speciation rates and diversity, with competition between dinosaurs limiting their geographic range. High speciation rates may in turn have been driven in part by sexual selection or latitudinally uneven climatic and floral gradients. The high endemism seen in centrosaurines and other dinosaurs implies that dinosaur diversity is underestimated and contrasts with the large geographic ranges seen in most extant mammalian megafauna.
Over the past thirty years, exploration of the terrestrial Mesozoic section in Utah has resulted in a more than fivefold increase in the known species of dinosaurs. A highly resolved temporal and sequence stratigraphic framework for these strata is facilitating the utility of these newly discovered dinosaur assemblages in geologic, evolutionary, paleoecologic, and paleogeographic research. Local subsidence due to salt tectonics in the northern Paradox Basin is responsible for this region of eastern Utah preserving basal Cretaceous dinosaur faunas, known nowhere else in North America, that document paleobiogeographic connections across the proto-North Atlantic with Europe. The more medial Cretaceous strata west of the San Rafael Swell, in central Utah, preserve a unique dinosaur assemblage on an isolated North America. These strata also record the first immigration of Asian dinosaurs into North America and the last occurrences of a number of endemic North American dinosaur lineages. Through the Late Cretaceous, extensive, fossiliferous floodplain deposits are exposed in the high plateaus of southern Utah within the Grand Canyon Bight on the western side of the Late Cretaceous Western Interior Seaway. Research on microvertebrate sites has resulted in a diverse record of vertebrate life substage by substage through most of the Upper Cretaceous sequence. Particularly, rich dinosaur-bearing beds through the Campanian have resulted in the discovery of many new dinosaur species distinct from the coeval dinosaur-bearing beds farther north along the western coast of the Western Interior Seaway in Montana and Alberta. The further development of these numerous rich dinosaur assemblages will provide the basis for considerable research in the future.
Snakes and lizards (Squamata) represent a third of terrestrial vertebrates and exhibit spectacular innovations in locomotion, feeding, and sensory processing. However, the evolutionary drivers of this radiation remain poorly known. We infer potential causes and ultimate consequences of squamate macroevolution by combining individual-based natural history observations (>60,000 animals) with a comprehensive time-calibrated phylogeny that we anchored with genomic data (5400 loci) from 1018 species. Due to shifts in the dynamics of speciation and phenotypic evolution, snakes have transformed the trophic structure of animal communities through the recurrent origin and diversification of specialized predatory strategies. Squamate biodiversity reflects a legacy of singular events that occurred during the early history of snakes and reveals the impact of historical contingency on vertebrate biodiversity.
The phylogenetic relationships of the species commonly referred to as ‘hypsilophodontids’ remain one of the key questions in ornithischian dinosaur research, having profound implications for understanding the origin, evolution and taxonomical compositions of several more recently evolved neornithischian clades. Recent phylogenetic analyses have recovered two conflicting placements for these taxa: (1) primarily within Cerapoda (Ornithopoda + Marginocephalia), as a paraphyletic assemblage of early ornithopods; and (2) primarily outside of Cerapoda, within the clade Thescelosauridae. Here we assess three recent independent neornithischian phylogenetic studies that have recovered topologies congruent with one of these placements. We compare the compositions of these data matrices and test how each of them responds to manipulation of taxa and characters. The positions in which controversial clades are recovered is shown to be highly dependent on the sample of taxa analysed; however, taxon incompleteness or instability is not a contributing factor in altering topology. Character completeness and homoplasy is shown not to significantly alter tree topology either, although these factors can affect resolution. In one matrix investigated, femoral and dental characters are found to provide disproportionate support for the placement of key taxa outside of Cerapoda, and the exclusion of a small number of these characters results in ‘hypsilophodontids’ falling within Ornithopoda. In contrast, matrices that originally recovered ‘hypsilophodontids’ within Cerapoda are comparably more stable, with this array of taxa remaining in a consistent position throughout all analyses. There is still much work to be done to resolve these relationships, but our study provides several suggestions for future analyses with the aim of resolving areas of conflict within the neornithischian tree.
The Terlingua local fauna is a rich assemblage of predominantly terrestrial micro vertebrates from the Upper Cretaceous Aguja Formation of Trans-Pecos Texas. Marine invertebrates (which include elements of both Cretaceous Western Interior and Gulf Coast zoogeographic provinces) from conformably underlying strata suggest that the fauna is of late Campanian age, probably correlative with Judithian assemblages of the Western Interior. A Judithian “age” for the fauna is further supported by its mammal and theropod assemblages, and by the faunas of overlying deposits. The previously reported diversity of the Aguja Formation, which we summarize, is significantly enriched by this new fauna. The fauna also fills a major gap in the biogeography of Campanian terrestrial vertebrates.Notable occurrences in the Terlingua local fauna include the therian mammal Gallolestes, previously known only from Baja California, and a hitherto unrecorded type of primitive ‘tribothere.’ At least 4 marsupial and 6 multituberculate taxa are present, several of which represent new taxa. Squamates comprise at least 10 taxa, including xenosaurs, necrosaurs, glyptosaurines, scincids, teiids, and a snake, several of which represent new taxa. In addition, the fauna includes at least 7 dinosaurs, 1 pterosaur, 2 crocodylomorphs, 3 turtles, 3 lissamphibians, 3 actinopterygians, and 8 chondrichthyans. Wood, amber, leaves, seeds, pollen, molluscs, and dinoflagellates are also preserved. The fauna is not strictly comparable to others from the Western Interior. It includes taxa that are either endemic or otherwise known only from relatively low latitudes, indicating an appreciable degree of latitudinal differentiation among Campanian terrestrial faunas bordering the Western Interior seaway.
Many single-crystal Ar-40/Ar-39 ages and thermoremanent magnetization directions have resolved the problematic stratigraphic correlation of the laterally and vertically zoned rhyolite ash flow sheet of the Pahranagat Formation in the southern Great Basin. This outflow sheet was previously designated by four different stratigraphic names in different locations over its highly discontinuous exposure area of 33,000 km(2). We show that it is a single cooling unit emplaced at 22.639 +/- 0.009 Ma around its source, the Kawich caldera. The volume of the outflow sheet was about 1600 km(3) after compensation for 50% post volcanic east-west extension. A comparable volume of tuff likely accumulated inside the Kawich caldera. Modal and chemical compositions of bulk tuff and cognate pumice fragments, together with compositions of phenocrysts, show the preeruption magma body was zoned from high-silica rhyolite (two feldspars, quartz, biotite, and titanomagnetite) to underlying, silica-poor, more mafic rhyolite and trachydacite (plagioclase, minor biotite, titanomagnetite, amphibole, and clinopyroxene). Initial evacuation of the uppermost evolved zone produced proximal outflow hundreds of meters thick of relatively densely welded, pumice-poor, high-silica rhyolite tuff. As eruption progressed, tens of meters of more mafic ejecta were deposited in distal areas and locally near the caldera and consist of less welded, pumice-rich ash flow tuff derived by physical mixing of pyroclasts from all zones of the magma chamber. This mixing during eruption invalidates direct comparison of the composition of tuff and a particular part of the magma chamber. The Pahranagat ash flow sheet provides a rigorous test case for application of high-precision correlation tools because of the zonal emplacement of ejecta from the compositionally stratified magma chamber together with the subsequent tectonic dismemberment and erosion of the sheet that created widely scattered exposures.
Aeromagnetic anomalies encountered in three areas, two in the western United States and one in Central America, are shown to arise from magnetic sedimentary formations. The first area discussed is the northwest corner of Nebraska where the Miocene Arikaree formation, comprised of magnetic airfall and windblown tuffs, causes anomalies in areas of incised topography. The second area is located in south central Utah, where the Upper Cretaceous Kaiparowits sandstones contain detrital magnetite that causes large anomalies in tilted structures and over incised topography. The third area treated covers over half of southern Belize in Central America, including much of the offshore portion. Here, the Toledo formation of Paleocene-Eocene age contains a thick section of clastic detritus rich in lithic grains of volcanic rocks that produce magnetic highs over thrusted and folded anticlinal axes. These three examples of magnetic anomalies due to syngenetic magnetite in widely scattered areas and from different types of source materials bring into question the assumption of so-called 'diagenetic magnetite' (or other magnetic minerals) as a cause of magnetic anomalies in other petroleum basins. It is necessary in all cases to determine the magnetic source from surface or subsurface geology, as was done here, rather than making assumptions strictly from magnetic profiles or mathematical models.
Eutherian mammals are herein reported from the Wahweap and Kaiparowits formations, southern Utah, of probable Aquilan and Judithian age, respectively. Three species of the leptictid insectivoran Gypsonictops are tentatively recognized from the Kaiparowits Formation; none is identified, although the stratigraphically highest species bears some resemblance to G. clemensi from the lower Kirtland Shale of the San Juan Basin, New Mexico. Paranyctoides, the oldest undoubted North American eutherian and known elsewhere from faunas of Aquilan and Judithian age, is the stratigraphically lowest eutherian present in the local section and is represented in the Wahweap Formation by two unidentified species. Two other species of Paranyctoides, also unidentified and unnamed, are recognized from the overlying Kaiparowits Formation. Upper and lower premolars representing loci not previously known for Paranyctoides significantly augment knowledge of the genus. In known morphology, Paranyctoides contrasts with other known pre-Lancian eutherians and is structurally close to many Cenozoic placental groups, as has been previously suggested; a special relationship of the genus to nyctitheriid lipotyphlans is neither strongly supported nor contradicted by the new evidence. Avitotherium utahensis, of uncertain ordinal and familial affinities, is described as new from the Judithian Kaiparowits Formation. Although similar to Alostera and Paranyctoides in certain respects, Avitotherium lacks presumed autapomorphies seen in those genera, and in known morphology is suggestive of the latest Cretaceous and early Tertiary condylarth Protungulatum. Morphological diversity among known North American Cretaceous Eutheria suggests the possibility of considerable diversification on the continent prior to the Lancian.
The Wahweap Formation of southern Utah is probably early Campanian in age. Its mammalian fauna is generally similar to that of the Aquilan upper Milk River Formation, Alberta, but differs substantially from that and all other known Late Cretaceous mammalian local faunas. Wahweap taxa referable to the Marsupialia include a new, somewhat derived species of Protalphadon and an unidentified taxon. Two new genera and species of advanced therian mammals are described. The status of these taxa is uncertain, because in some respects (such as the “twinning” of lower molar hypoconulid with entoconid) they share advanced morphology with undoubted Cretaceous marsupials, while in others (such as the variable or complete absence of stylar cusp D on upper molars) they may be more primitive than known marsupials and, in fact, would be excluded from the group using most currently-accepted criteria. Although these two Wahweap species could represent members of advanced lineages in which characteristic marsupial specializations were lost, a simpler explanation is that they are primitive in these regards, and represent either primitive marsupials or marsupial sister-taxa, depending on how the group is defined.
The Kaiparowits Formation is the youngest Cretaceous unit in central-southern Utah. One 2,750-ft-thick section of the formation was measured, described, and sampled in the type locality, The Blues, Garfield County, Utah. Palynological documentation is based on the study of 15 samples distributed throughout the formation. Palynomorphs assignable to 80 species in 41 genera were described. One genus and 36 species are believed to be new. The Kaiparowits Formation is equivalent to the upper Lance or Hell Creek. Aquilapollenites spp., Azolla cretacea Stanley, and Proteacidites spp. proved to be of greatest significance for correlation and dating purposes. Comparisons of the entire flora also indicated that the Kaiparowits Formation is Late Cretaceous. The lower 2,200 ft of the Kaiparowits Formation was deposited as a delta in a rapidly subsiding basin. The sediments indicate a western provenance-probably central or western Nevada. The Blues was in the fluvial part of the delta with low, marshy or swampy topography. Uplands and semi-arid to arid areas also were present within the drainage basin. Sedimentation of the upper 550 ft of the formation was probably similar to that of the lower 2,200 ft, but is incompletely understood. Considerable volcanic activity in the region is indicated by the presence of large volumes of bentonitic material within the formation. No evidence was observed to indicate that any of the Kaiparowits Formation in this area was deposited under marine conditions. End_of_Article - Last_Page 729------------
Latest Cretaceous (Maastrichtian) terrestrial vertebrates from western North America occur in two faunal provinces. The Triceratops fauna is found in Canada, Montana, and Wyoming; and the Alamosaurus fauna occurs in Utah, New Mexico, and Texas. Although the two faunas are thought to have been contemporaneous, only the Northern Province contains intercalated volcanic units that have been isotopically dated. The first isotopic age from within the southern province is presented. A single outcrop of distal tuff within the Upper Cretaceous Javelina Formation in northern Big Bend National Park, Texas, contains monazite with a U-Pb age of 69.0 ± 0.9 Ma (2 sigma). The age is from a Pb/Pb vs. U/Pb isochron, an approach chosen to avoid the effects of Th-derived excess Pb. The age falls within the boundary interval between the poorly calibrated Edmontonian and Lancian North American Land Mammal Ages. The tuff bed occurs approximately in the middle of the fluvio-lacustrine Javelina Formation, about 90 m stratigraphically below the position of the Cretaceous-Tertiary boundary. This position is within the local range of the sauropod Alamosaurus, below two sites that have yielded remains of the pterosaur Quetzalcoatlus, and above a site with petrified logs of the dicotyledonous tree Javelinoxylon. The range zones of all three taxa span the full thickness of the Javelina Formation elsewhere in the Big Bend region. The Alamosaurus fauna is therefore Lancian to late Edmontonian in age.
Cretaceous basaltic pyroclastic strata have been discovered in a fault block on the southern edge of the Rosillos Mountains laccolith in the Big Bend area of Trans-Pecos Texas. The sequence comprises base-surge and pyroclastic-fall deposits inferred to have accumulated on the flanks of a small phreatomagmatic volcano. A diverse assemblage of freshwater turtles ( including Aspideretes), crocodile teeth, and dinosaur bones have been recovered from the uppermost part of the sequence. The fauna indicate a Late Cretaceous, probably Campanian age. An outward-dipping normal fault bounding the pyroclastic strata on the southeast juxtaposes sediments of the Upper Cretaceous Javelina Formation in the hanging wall with pyroclastic rocks in the footwall, indicating the pyroclastic strata must be Maastrichtian or older. U-Pb SHRIMP-RG analyses of zircons separated from a basaltic block thrown out of the volcano yield an inferred igneous crystallization age of Ma, consistent with the biostratigraphic and struc- 72.6 +/- 1.5 tural evidence. The pyroclastic strata in the fault block provide the first evidence for Late Cretaceous volcanism in the Trans-Pecos region. Previously, the onset of igneous activity in the area was thought to be no older than 64 Ma. We speculate that the basaltic pyroclastic rocks represent an extension of the Upper Cretaceous Balcones magmatic province into Trans-Pecos Texas.