Jurassic Morrison Formation Research Articles

Seis-ing up the Super-Morrison formation sauropods.

The Upper Jurassic Morrison Formation sauropods Diplodocus (formerly "Seismosaurus") hallorum and Supersaurus vivianae are quantifiably the largest dinosaurian taxa from the formation, as well as being among the largest dinosaurs in the world. Their extreme body size (in particular body length, c. 50+ m) has fascinated the paleontological community since their discoveries and has sparked an ongoing discussion on the trends and limits of Morrison Formation sauropod body size. Although not an undeviating proxy, often the largest and skeletally most mature specimens are among the rarest (as exemplified in Triceratops). While their body size has no phylogenetic bearing, the extreme size and potential eco and biological significance of these two sauropod taxa are frequently discussed. Whether these rare and titanically proportioned sauropod specimens are large-bodied, senescent or both is an often-repeating rhetoric. To definitively make maturational inferences about these taxa, we osteohistologically sampled the holotype of D. hallorum (NMMNH P-25079) and the second known specimen of S. vivianae (WDC DMJ-021). Our age-determinant and maturational assessments indicate that both specimens were skeletally mature at their respective age of death. Retrocalculation methods for D. hallorum NMMNH P-25079 produce a maximum age-at-death estimation of 60 years, whereas S. vivianae WDC DMJ-021 lived well past skeletal maturity-so much so that reliable retrocalculated ages cannot be accurately determined at this time. Additionally, the rarity of such large sauropods within the Morrison Formation might be more parsimoniously explained as relating to their maturity as opposed to representing aberrant taxa on the Morrison landscape.

A new dryolestid fossil from the Late Jurassic illuminates molar root structure of dryolestids

ABSTRACT We report a micro-CT study of a new specimen of Dryolestes priscus (Dryolestidae, Cladotheria) from the Upper Jurassic Morrison Formation of Wyoming. The fossil (KUVP 134101) is an incomplete lower jaw preserving eight intact teeth, including two premolars (p3, p4) and six molars (m1–6), along with double-alveoli for m7 and a single alveolus for m8. Its preserved teeth and molar alveoli are consistent with the dental formula in more complete jaws of this species (i4.c1.p4.m8–m9). Here we have newly characterized diagnostic features on the molar trigonid of Dryolestes priscus and other species of Dryolestidae. Our analysis places this specimen close to Dryolestes priscus in terms of phylogeny. CT scans and visualization made it possible to illustrate the distinctive root structures of lower molars of dryolestid species. The two-rooted lower molars of D. priscus show a hypertrophied anterior root that is spatulate and much broader and longer than the simpler posterior root of the same molar. The disparate shape and size of the two molar roots are a highly derived pattern of dryolestids, which is absent in closely related genera formerly assigned to “paurodontids.” It is also unique among all mammaliaforms with typically two-rooted molars. The hypertrophied anterior root corresponds to most of the molar crown, and likely provided better support for occlusal forces acting on the large trigonid of the molar.

Bite and tooth marks on sauropod dinosaurs from the Morrison Formation.

Tooth-marked bones provide important evidence for feeding choices made by extinct carnivorous animals. In the case of the dinosaurs, most bite traces are attributed to the large and robust osteophagous tyrannosaurs, but those of other large carnivores remain underreported. Here we report on an extensive survey of the literature and some fossil collections cataloging a large number of sauropod bones (68) from the Upper Jurassic Morrison Formation of the USA that bear bite traces that can be attributed to theropods. We find that such bites on large sauropods, although less common than in tyrannosaur-dominated faunas, are known in large numbers from the Morrison Formation, and that none of the observed traces showed evidence of healing. The presence of tooth wear in non-tyrannosaur theropods further shows that they were biting into bone, but it remains difficult to assign individual bite traces to theropod taxa in the presence of multiple credible candidate biters. The widespread occurrence of bite traces without evidence of perimortem bites or healed bite traces, and of theropod tooth wear in Morrison Formation taxa suggests preferential feeding by theropods on juvenile sauropods, and likely scavenging of large-sized sauropod carcasses.

Stratigraphy, sedimentology, and paleoclimatic proxies of the Upper Jurassic Morrison Formation of central Montana: Geology of the Intermountain West

Since the discovery of dinosaurs in the late 19th century, the Upper Jurassic Morrison Formation has received considerable scholarly attention. However, the formation in central Montana needed to be sufficiently investigated. Recent dinosaur excavations from exposed Morrison strata on the southern flank of Spindletop dome near the town of Grass Range, Montana, prompted a review of the formation’s geology, paleobotany, and paleoclimate. A review of historical stratigraphic measurements of the formation in Montana reveals a considerable variance in measured thicknesses. Stratigraphic measurements and regional log data indicate that the formation averages 71 m thick across central Montana. A new regional isopach map of the formation from well-log data illustrates a broad distributive fluvial system that migrated from the southwest toward the northeast. The formation is divided into two informal facies in the study area: lower and upper depositional facies. The lower depositional beds represent the mud-rich distal-most distributive fluvial facies that overlies the stranded muddy tidal flat of the Swift Formation. An increased sandstone:mudstone ratio and small isolated fluvial channel and crevasse splay beds indicate that the upper depositional beds represent the slow progression of the distributive fluvial system. However, a review of the regional field stratigraphy and well-log data did not provide a regional correlatable facies change to warrant subdividing the formation into members.The stratigraphic positions and climatic interpretations for lithologic, faunal, and floral paleoclimatic proxies are specified. The compilation of climate proxy data from central Montana demonstrates that the climate in this region was wetter than in southern parts of the Morrison foreland basin. The climatic proxies signify that the environmental conditions were variable during the Late Jurassic in central Montana, displaying changing temperatures with mesic and xeric intervals of unknown duration.

Ancient Basidiomycota in an extinct conifer-like tree, Xenoxylon utahense, and a brief survey of fungi in the Upper Jurassic Morrison Formation, USA

AbstractAlthough the well-known Upper Jurassic Morrison Formation has yielded abundant fossil plants for nearly a century, relatively little is known about fossil fungi and their ecological relationships to the Morrison flora. The first mention of fungal decay in fossil wood was briefly made over three decades ago, and since then, a few more reports of fungal decay associated with Morrison plants have been published. However, up to now, detailed data on the fossil fungi themselves have not been given from the Morrison Formation. Here we describe in detail well-preserved fossil mycelia in a silicified log of Xenoxylon utahense Xie et Gee, 2021 from the Upper Jurassic Morrison Formation at Miners Draw, Blue Mountain, near Vernal in northeastern Utah, USA. The fungal hyphae are variable in form, ranging from straight to slightly curved to highly coiled to tubular; they measure ~1.53 μm in diameter and possess clamp connections, septa, and occasional bifurcations. The occurrence of clamp connections typical of living Basidiomycota indicates a taxonomic affinity to this division of fungi. On the basis of the patterns of wood decay in the Xenoxylon log, the fossil fungi are interpreted here as pertaining to saprotrophic, white-rot wood fungi. These fossil mycelia represent a new record of ancient Basidiomycota from the Upper Jurassic Morrison Formation and provide further evidence for plant–fungus interactions in Jurassic terrestrial ecosystems.

New palaeopathology cases of Allosaurus fragilis (Dinosauria: Theropoda)

ABSTRACT Here, we describe a pathological sub-adult theropod dinosaur from the Late Jurassic Morrison Formation in Dana Quarry, Wyoming, U.S.A. It exhibits multiple pathologies, involving three dorsal vertebrae, the right scapula and coracoid, and apparent pathology of the left scapula. The fourth dorsal vertebra has four irregularly shaped growths on the vertebral arch. The eighth and ninth vertebrae are fused. The scapulocoracoid joint has an obvious deformity. The distal part of the right scapula is more rounded than normal. The left scapula has a depression on the lateral blade. These pathologies can be identified as follows: the fourth dorsal vertebra is possibly neoplastic or exhibiting compensatory hyperplasia; the eighth and ninth vertebral and the right scapular pathology are consistent with spondyloarthropathy. Diagnoses considered for the left scapula pathology are infections, trauma or unknown. The animal survived the pathologies, which are unlikely the cause of its death. This specimen represents the first known occurrence of spondyloarthropathy in a theropod.

A nearly complete skeleton of a new eusphenodontian from the Upper Jurassic Morrison Formation, Wyoming, USA, provides insight into the evolution and diversity of Rhynchocephalia (Reptilia: Lepidosauria)

We describe a new, small-bodied rhynchocephalian reptile, Opisthiamimus gregori gen. et sp. nov., from the Upper Jurassic Morrison Formation of Wyoming, USA. Whereas many fossil rhynchocephalians are based on isolated incomplete jaws, the holotype of O. gregori includes most of the skull and postcranium and therefore represents one of the most complete specimens of Rhynchocephalia known from North America. We used micro-computed tomography to examine its skeletal anatomy in detail and to develop a three-dimensional reconstruction of the skull. The skull of O. gregori is similar to that of several non-neosphenodontian rhynchocephalians such as Planocephalosaurus (e.g. large orbits) and Clevosaurus (e.g. parietal parasagittal crests) yet exhibits a suite of other features related to the proal shearing mechanism that becomes increasingly elaborated among more phylogenetically nested taxa such as Sphenodon (e.g. lateral palatine tooth row parallels maxillary tooth row along its entire length, pyramidal dentary teeth with mesial shearing crests). The postcranial skeleton of O. gregori exhibits characteristics typical of a terrestrial rhynchocephalian. Our phylogenetic analyses use a substantially updated data set of 118 characters and 46 taxa, and both maximum parsimony and Bayesian frameworks. Results place O. gregori inside Eusphenodontia but outside Neosphenodontia, and therefore in a key position for contributing to character polarity for more deeply nested clades such as Clevosauridae, Sphenodontidae and Pleurosauridae. We also erect Leptorhynchia taxon nov., composed primarily of aquatically adapted taxa (e.g. Pleurosaurus, Sapheosaurus), which is supported by both cranial and postcranial characters. Because O. gregori is not particularly closely related to the other named Morrison rhynchocephalians (e.g. Opisthias rarus), it increases both the alpha and beta taxonomic diversities within the formation. Similarly, major differences in body size and inferred diet of the Morrison taxa imply considerable concomitant palaeoecological diversity just prior to a major global decline in rhynchocephalian diversity around the close of the Jurassic. http://zoobank.org/urn:lsid:zoobank.org:pub:888E055B-8AC1-4BD0-A37C-8CB192F79673

"Dragons" on the landscape: Modeling the abundance of large carnivorous dinosaurs of the Upper Jurassic Morrison Formation (USA) and the Upper Cretaceous Dinosaur Park Formation (Canada).

Counts of the number of skeletal specimens of "adult" megaherbivores and large theropods from the Morrison and Dinosaur Park formations-if not biased by taphonomic artifacts-suggest that the big meat-eaters were more abundant, relative to the number of big plant-eaters, than one would expect on the basis of the relative abundance of large carnivores and herbivores in modern mammalian faunas. Models of megaherbivore population density (number of individuals per square kilometer) that attempt to take into account ecosystem productivity, the size structure of megaherbivore populations, and individual megaherbivore energy requirements, when combined with values of the large theropod/megaherbivore abundance ratio, suggest that large theropods may have been more abundant on the landscape than estimates extrapolated from the population density versus body mass relationship of mammalian carnivores. Models of the meat production of megaherbivore populations and the meat requirements of "adult" large theropods suggest that herbivore productivity would have been insufficient to support the associated number of individuals of "adult" large theropods, unless the herbivore production/biomass ratio was substantially higher, and/or the large theropod meat requirement markedly lower, than expectations based on modern mammals. Alternatively, or in addition to one or both of these other factors, large theropods likely included dinosaurs other than megaherbivores as significant components of their diet.

The lithostratigraphic Tidwell Member of the Morrison or Summerville Formations (Upper Jurassic)—who, what, where, when?

The Tidwell Member, a lithostratigraphic unit on the Colorado Plateau, has variously been referred to the Upper Jurassic Morrison Formation or the Upper Jurassic Summerville Formation. Its authorship has been ascribed to U.S. Geological Survey geologists Robert B. O'Sullivan or Fred Peterson. Because Peterson and O'Sullivan have different type sections, a resolution to authorship is needed. Both authors meet the minimum requirements for a new stratigraphic unit as given by the 1983 North American Stratigraphic Code under which Peterson and O'Sullivan operated. However, both authors also operated under the more restrictive Stratigraphic Nomenclature in Reports of the U.S. Geological Survey, which makes it clear that O'Sullivan used Tidwell Member informally, whereas Peterson made a formal proposal. Thus, Peterson is the rightful author and the type section is at Shadscale Mesa, Emery County, Utah. To resolve the issue of which formation the Tidwell Member belongs, its strata were examined across the northern Colorado Plateau in context with both the underlying marine Summerville Formation and the overlying terrestrial Salt Wash Member of the Morrison Formation. Meters-thick gypsum beds, one criterion for including the Tidwell Member in the Summerville, was found to be mostly restricted to the west side of the Colorado Plateau. Numerous low-angled anticlines at the top of the Summerville Formation are truncated beneath this gypsum, thus the gypsum beds cannot be part of the Summerville Formation. The unconformity marks the J-5 unconformity. Detailed analysis of the gypsum beds shows a complex origin indicative of smaller playa lakes rather than broad coastal sabkhas. The gypsum beds show an interfingering relationship with the overlying and lateral interbedded thin sandstone-siltstone-mudstone-limestone facies assemblage of the Tidwell Member. This relationship is interpreted as localized gypsum playa lakes that formed at the terminus of prograding fluvial fans from the Elko highlands to the west, with possible lesser contribution from the growing fluvial fan exiting from the Grand Canyon bight near the present-day Arizona-Nevada border. An isopachous map of the Tidwell Member supports this interpretation, and also indicates additional, less significant source areas to the southeast and east of the northern Colorado Plateau. A widespread tabular sandstone at the base of the Tidwell Member lateral to the gypsum facies known as Bed A, is interpreted to have originated mostly as a sand sheet analogous to the Selima Sand Sheet of the eastern Sahara or the sand sheet of the Gran Dieserto in Sonora, Mexico. The source of the sand is from the fluvial fans to the west. Similarities between the lithofacies of the contemporaneous Tidwell and Ralston Creek Members of the Morrison Formation indicates deposition under similar environmental conditions. This supports the inclusion of the Ralston Creek Member in the Morrison Formation as previously suggested, rather than as a separate formation.

Multitrophic plant–insect–fungal interactions across 150 million years: A giant Agathoxylon tree, ancient wood-boring beetles and fungi from the Morrison Formation of NE Utah, and the brood of an extant orchard mason bee

Although plant–insect interactions can be traced back to the Silurian, little is known from Upper Jurassic to mid-Cretaceous ecosystems. In the Upper Jurassic Morrison Formation, evidence is limited to insect body fossils and trace fossils in mostly wood. However, until now, multitrophic interactions have not been reported from the Morrison Formation. Here we describe a complex web of interrelationships between an ancient tree, fungi, and wood-boring insect from the Morrison Formation with an extant bee in NE Utah, USA, using microscopy, thin sectioning, and microCT for non-destructive internal imaging. The tree is Agathoxylon hoodii (Araucariaceae), one of the largest individuals in a monospecific forest, whose trunk was infected and decayed by white rot while the tree was alive during the Jurassic. Fungal formation of voids in the wood facilitated the boring of large-diameter galleries by an insect, likely a beetle. One gallery was repurposed by a modern orchard mason bee, Osmia lignaria, as a linear nest for her brood. The fossil wood specimen was serendipitously collected with two bee larvae plus food provisions in cocoons in the lowermost two chambers. Thus, this example is not only a simple interaction between fossil plants and insects, but transcends 150 million years between the ancient tree–insect–fungal relationship and a living insect.

A new goniopholidid from the Upper Jurassic Morrison Formation, USA: novel insight into aquatic adaptation toward modern crocodylians.

Goniopholididae is a group of basal neosuchian crocodyliforms closely related to Paralligatoridae and Eusuchia that lived during the Jurassic and Early Cretaceous. Goniopholidids have the long, flat snout and secondary palate of modern crocodylians, the acquisition of which is regarded as a key feature in the early evolution of crocodylian body plan and their aquatic adaptation. Here, we report a new species, Amphicotylus milesi, with the description from the best-preserved specimen to date of Goniopholididae from Wyoming, USA. Its posterior extension of the nasopharyngeal passage (pterygoid secondary palate) and the shortening and dorsal deflection of the ceratobranchial suggest that basal neosuchians could raise their gular valve to separate oral and pharyngeal cavities as in modern crocodylians. The anatomy of Amphicotylus milesi sheds light on the acquisition of this new respiratory system in the crocodyliform evolution and their early aquatic adaptation, leading to modern crocodylians.

Mammalian Petrosals from the Upper Jurassic Morrison Formation (Utah, USA) Reveal Non-canonical Evolution of Middle and Inner Ear Characters

Mammalian Petrosals from the Upper Jurassic Morrison Formation (Utah, USA) Reveal Non-canonical Evolution of Middle and Inner Ear Characters

Coarse-grained meandering distributive fluvial system of the basal Cedar Mountain Formation, U.S.A.

ABSTRACT The analysis of downstream changes in ancient fluvial systems can better inform depositional models for foreland-basin systems. Herein we analyze the basal deposits of the Early Cretaceous Cedar Mountain Formation of Utah to better understand the variety of fluvial deposits present and to develop a depositional model for the Sevier foreland basin. We also evaluate the long-held interpretation of a braided origin for these deposits and document numerous examples of point-bar deposition in highly sinuous meandering rivers by analysis of large (20 to 60 km2) plan-view exposures. These plan-view exposures allow comparisons between planform and cross-sectional geometries. The study utilizes outcrop data, virtual outcrop models, and satellite imagery to develop a facies model and analyze the architecture of channel bodies in the Buckhorn Conglomerate and Poison Strip Sandstone of the Cedar Mountain Formation. We document downstream (west to east) decreases in lateral channel migration, sinuosity, channel amalgamation, grain size, and percent of fluvial channel facies (conglomerate and sandstone). Fluvial channel deposits occur arranged into larger stratal bodies: multistory–multilateral channel bodies that are dominantly composed of clast-supported conglomerate in the west to a mix of multistory, multilateral, and isolated channel bodies composed of matrix-supported conglomerate in the east. The median width of highly sinuous point bars is similar across the field area (344 m to 477 m), but the inclusion of narrower (median = 174 m), low-sinuosity bar elements in the east indicates an overall reduction in lateral channel migration and sinuosity downstream. Net-to-gross values range from 100% in much of the western outcrops to as low as 38% in the east. Paleocurrent analysis reveals a transverse (west to east) paleoflow for the study interval that merges with axial (south–north) paleoflow near the Utah–Colorado state line. We estimate 104 m3/s-scale discharge and 106 kilometer-scale drainage area for axial rivers based on paleohydraulic analysis which represents a significant part of the Early Cretaceous continental-scale drainage. The observed downstream trends in lateral channel migration, sinuosity, channel amalgamation, grain size, and net-to-gross for the basal Cedar Mountain Formation are consistent with expected trends for sinuous single-thread distributive fluvial systems and are similar to observed trends in the Jurassic Morrison Formation. Medial (Buckhorn Conglomerate) to distal (Poison Strip Sandstone) zones are preserved and span the forebulge to backbulge depozones of a foreland-basin system. Postulated deposits of the proximal distributive fluvial system have been removed during erosion of the foredeep depozone. The easternmost Poison Strip Sandstone and coeval Burro Canyon Formation represent deposits of an axial system at which western-sourced distributive fluvial systems end. Distributive fluvial systems dominate modern foreland basins, and this study suggests that they may constitute a significant proportion of ancient successions.

Distinctive quadrangular seed-bearing structures of gnetalean affinity from the Late Jurassic Morrison Formation of Utah, USA

A new kind of seed-bearing structure is described based on three-dimensional casts and partially permineralized small cones from the Upper Jurassic Brushy Basin Member of the Morrison Formation, in the Henry Mountains of Utah. Cones of Dayvaultia tetragona gen. et sp. nov. are obovate in lateral view, 10.0–11.0 mm long, square in cross-section and 5.1–8.0 mm wide, with a thick wall composed of four tightly adhering bracts that open apically to expose the tips of six or eight elongate, four-lobed seeds. Micro-CT scanning reveals that the seeds are borne on a cup-shaped receptacle in a regular opposite and decussate manner. This regular arrangement, as well as similarities of the seeds to several kinds of Early Cretaceous chlamydospermous seeds, including those of Lobospermum and Battenispermum, suggests a relationship to extant and Cretaceous members of Gnetales. The sedimentary context in which the cones occur, combined with their local abundance, suggests that Dayvaultia was common on intermittently inundated well-drained floodplains during Morrison times, enhancing insight into the vegetation that supported the diverse vertebrate faunas for which the Morrison Formation is well known. http://zoobank.org/urn:lsid:zoobank.org:pub:0ECD4B37-E6B6-4050-B45A-28D713321EB8

The promise of taphonomy as a nomothetic discipline: taphonomic bias in two dinosaur-bearing faunas in North America1

The fossil record of dinosaurs is a rich, if biased, one with nearly complete skeletons, partial skeletons, and isolated parts found in diverse, well-studied faunal assemblages around the world. Among the recognized biases are the preferential preservation of large dinosaurs and the systematic underrepresentation of small dinosaurs. Such biases have been quantitatively described in the Upper Cretaceous (Campanian) Dinosaur Park Formation of Alberta, where large, nearly complete dinosaurs were found and described early in collecting history and small, very incomplete dinosaurs were found and described later. This pattern, apparently replicated in the Maastrichtian Hell Creek Formation of Montana, is so striking that it begs the question of whether this is a nomothetic principle for the preservation of dinosaur faunas elsewhere. We tested this hypothesis by analyzing the very well-studied dinosaur fauna of the Upper Jurassic (Kimmeridgian) Morrison Formation of the western United States. The Morrison Formation fails to show any correlation between body size and completeness, order of discovery, or order of description. Both large and small dinosaurs of the Morrison include highly complete as well as highly incomplete taxa, and both large and small dinosaurs were discovered and described early in collecting history as well as more recently. The differences in preservation between the Dinosaur Park Formation and the Morrison Formation are so striking that we posit a Dinosaur Park model of dinosaur fossil preservation and a Morrison model. Future study will show whether either or both represent durable nomothetic models for dinosaur fossil preservation.

Beveling the Colorado Plateau: Early Mesozoic Rift‐Related Flexure Explains Erosion and Anomalous Deposition in the Southern Cordilleran Foreland Basin

AbstractDeposition of the Late Jurassic Morrison Formation in a back‐bulge depozone and formation of the overlying sub‐Cretaceous unconformity above a forebulge mark the birth of the foreland basin system in the central U.S. Cordillera. In the southern U.S. Cordillera, the Morrison Formation is either anomalously thick or absent and the sub‐Cretaceous unconformity cuts out progressively older stratigraphy toward the south on the Colorado Plateau. Based on results of 2D and 3D flexural modeling, we suggest that flexural uplift of the northern rift flank of the Bisbee segment of the Borderland Rift Belt can explain these observations. Structural restoration of the sub‐Cretaceous unconformity indicates a minimum of 1.5 km of uplift and flexural models with an effective elastic thickness of 55 ± 5 km can reproduce the geometry of the unconformity and rift flank. This implies that effective elastic thickness has decreased between the Jurassic and the present, consistent with hypotheses for uplift and modification of the Colorado Plateau lithosphere during the Late Mesozoic to Cenozoic. Modeling results also predict the presence of a rift‐related flexural trough in the Four Corners region of the Colorado Plateau, which may explain above‐average thickness of the Morrison Formation. Constructive interference between a flexural back‐bulge depozone and a flexural rift‐flank trough may help explain anomalously high Late Jurassic subsidence.

Anatomy and systematics of the diplodocoid Amphicoelias altus supports high sauropod dinosaur diversity in the Upper Jurassic Morrison Formation of the USA.

Sauropod dinosaurs were an abundant and diverse component of the Upper Jurassic Morrison Formation of the USA, with 24 currently recognized species. However, some authors consider this high diversity to have been ecologically unviable and the validity of some species has been questioned, with suggestions that they represent growth series (ontogimorphs) of other species. Under this scenario, high sauropod diversity in the Late Jurassic of North America is greatly overestimated. One putative ontogimorph is the enigmatic diplodocoid Amphicoelias altus, which has been suggested to be synonymous with Diplodocus. Given that Amphicoelias was named first, it has priority and thus Diplodocus would become its junior synonym. Here, we provide a detailed re-description of A. altus in which we restrict it to the holotype individual and support its validity, based on three autapomorphies. Constraint analyses demonstrate that its phylogenetic position within Diplodocoidea is labile, but it seems unlikely that Amphicoelias is synonymous with Diplodocus. As such, our re-evaluation also leads us to retain Diplodocus as a distinct genus. There is no evidence to support the view that any of the currently recognized Morrison sauropod species are ontogimorphs. Available data indicate that sauropod anatomy did not dramatically alter once individuals approached maturity. Furthermore, subadult sauropod individuals are not prone to stemward slippage in phylogenetic analyses, casting doubt on the possibility that their taxonomic affinities are substantially misinterpreted. An anatomical feature can have both an ontogenetic and phylogenetic signature, but the former does not outweigh the latter when other characters overwhelmingly support the affinities of a taxon. Many Morrison Formation sauropods were spatio-temporally and/or ecologically separated from one another. Combined with the biases that cloud our reading of the fossil record, we contend that the number of sauropod dinosaur species in the Morrison Formation is currently likely to be underestimated, not overestimated.

A more southerly occurrence of Xenoxylon in North America: X. utahense Xie et Gee sp. nov. from the Upper Jurassic Morrison Formation in Utah, USA, and its paleobiogeographic and paleoclimatic significance

Although Xenoxylon is a global genus of Mesozoic wood in the Northern Hemisphere, it is striking that the valid records in North America occur only in the northernmost parts of the continent. Decades ago, two species of Xenoxylon were described from the Upper Jurassic Morrison Formation in Utah, USA. However, a recent review of all North American reports of Xenoxylon excluded both species from the genus based on their anatomy, rendering all records of this genus south of Montana invalid in 2019. Here we describe a fossil log from the Morrison Formation near Vernal, northeastern Utah, USA, with true Xenoxylon anatomy with locally xenoxylean radial tracheid pitting and fenestriform crossfield pitting. Due to a novel combination of radial tracheid pitting, crossfield pitting and ray height, a new species, Xenoxylon utahense Xie et Gee sp. nov., is recognized here. Located at the southernmost occurrence of Xenoxylon in North America during the Mesozoic, this new species confirms that Xenoxylon trees were not only restricted to a circumpolar zone, but also grew in more southerly areas. In comparison with other fossil plants in the Morrison Formation of Utah, along with growth ring characteristics and climatic preferences of living conifers, the new fossil log with true growth rings with abundant, well-formed earlywood and extremely little latewood argues for a warm and humid climate with very mild seasonality for the Late Jurassic in this region, which is inconsistent with previous interpretations of Xenoxylon as a reliable indicator of cool climates.

Jurassic dinosaurs on the move: Gastrolith provenance and long‐distance migration

AbstractHere, we present the detrital zircon age spectra for five (n = 36, 68, 66, 41, 29) red quartzite gastroliths collected from the top of the Upper Jurassic Morrison Formation in the eastern Bighorn Basin, Wyoming, USA. The detrital zircon age spectra reveal geon 17 maximum depositional ages, and age peaks that are Yavapai (geon 17), Penokean (geon 18) and Archean (>geon 25). The colour, texture, composition and zircon age spectra of these exoliths are indistinguishable from those of geon 16 (i.e. Baraboo interval) quartzites present in the Laurentian midcontinent more than 1,000 km to the east. We interpret that these gastroliths were ingested by dinosaurs, most likely sauropods, in the Laurentian midcontinent and then transported in their digestive tracts to the site of deposition. These data support the hypothesis of long‐distance dinosaur migration, perhaps following low energy, continental‐scale drainage systems that flowed from the Appalachian highlands to the Morrison Formation depositional basin.

New social insect nests from the Upper Jurassic Morrison Formation of Utah

This paper reports a new assemblage of social insect ichnofossils from the Brushy Basin Member of the Upper Jurassic Morrison Formation near Green River, Utah. At least seven distinct nests are visible in the locality horizon, identifiable at the outcrop scale by loci of anastomosing, and orthogonally connected hor-izontal burrows and vertical shafts. A boulder-sized block from the in situ horizon has eroded and rolled downhill, revealing the ventral aspect of the nest, showing a view of the overall nest architecture. Burrow and shaft clusters are organized into mega-galleries which have branching arms and ovate, bulbous cham-bers. The organization of distinct trace morphologies is consistent with ethological complexity of the social insects. A small sample was collected and analyzed by serial sectioning and petrographic thin sectioning to observe small-scale morphological features. Centimeter-scale analysis shows chamber, gallery, and burrow walls have complex topography. Pebble-sized, hollow, ellipsoid features are distributed throughout the up-permost facies of the nest and have undergone complete silicification of their outer surfaces. The ellipsoids share similarity with pellet structures made of mud or carton produced by modern termites. This trace fossil assemblage suggests it is possible that termites had acquired subterranean nesting behavior, and mud or carton utilization in nest construction in seasonally arid habitats by the Late Jurassic.