J. Michael Parrish

Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly

ABSTRACT The Crocodylotarsi are a group erected by Benton and Clark (1988) for archosaurs that share the “crocodile-normal” ankle structure. In this study, the phylogeny of the Crocodylotarsi was re-examined based on study of most relevant fossil material of the early non–crocodyliform members of the clade. Relationships among the major archosaurian taxa (Ornithodira, Ornithosuchidae, and Crocodylotarsi) and their proximal outgroups (Proterochampsidae, Euparkeria) were also considered. The monophyly of the Archosauria, Crurotarsi, and Crocodylotarsi is supported by the current analysis. The Parasuchia are the most plesiomorphic clade within the Crocodylotarsi. The Suchia comprise the crocodylotarsans excluding the phytosaurs. The taxon Prestosuchidae is placed near the base of this clade, as are Lewisuchus and Turfanosuchus dabanensis. The new taxon Rauisuchiformes includes the last common ancestor of Aetosauria and Crocodylia and its descendants. Rauisuchia (new combination) includes Rauisuchidae, Gracil...

The Pangaean megamonsoon - evidence from the Upper Triassic Chinle Formation, Colorado Plateau

The Upper Triassic Chinle Formation was deposited at an exceptional time in Earths paleogeographic and paleoclimatic history. During the Triassic, the supercontinent Pangaea was at its greatest size, in terms of both aggregated continental crust and exposed land area. Moreover, the exposed land was divided symmetrically about the paleoequator between the northern and southern hemispheres. These conditions were ideal for maximizing monsoonal circulation, as predicted from paleoclimate models. The Chinle was deposited between about 5?to 15?N paleolatitude in the western equatorial region of Pangaea, a key area for documenting the effects of the monsoonal climate. This study summarizes sedimentologic and paleontologic data from the Chinle Formation on the Colorado Plateau and integrates that data with paleoclimatic models. The evidence for abundant moisture and seasonality attest to the reversal of equatorial flow and support the hypothesis that the Triassic Pangaean climate was dominated by monsoonal circulation. The Chinle Formation contains continental lithofacies deposited in fluvial channels, crevasse splays, lakes, bogs, marshes, and lacustrine deltas that reflect abundant precipitation and shallow water tables. Paleosols and ichnofossils indicate that water tables and lake levels fluctuated episodically. Interbedded lacustrine carbonates and marginal-lacustrine siltstones and mudstones indicate longer-term but regular, episodic fluctuations in lake level. Fine-scale laminations in lacustrine carbonates suggest a seasonal influx of clastic sediment, and thus precipitation, to the basin. Uppermost Chinle strata consist of lacustrine and marginal-lacustrine mudstones interbedded with minor eolian sand sheets and eolian dunes; thus, the later Triassic reflects continued precipitation, but was marked by more pronounced and extended dry seasons.

Late Jurassic Climates, Vegetation, and Dinosaur Distributions

The Jurassic and Cretaceous are considered to have been warmer than today on the basis of various climate data and model studies. Here, we use the available global record of climate‐sensitive sediments, plants, and dinosaurs to infer broadscale geographic patterns for the Late Jurassic. These provide a context for our more detailed accounts of the Morrison and Tendaguru Formations in North America and East Africa. At the global scale, evaporites predominated in low latitudes and coals in mid‐ to high latitudes. We ascribe these variations to a transition from drier to wetter conditions between the equator and poles. Plant diversity was lowest in equatorial regions, increasing to a maximum in midlatitudes and then decreasing toward the poles. Most dinosaur remains are known from low‐latitude to marginally midlatitude regions where plant fossils are generally sparse and evaporites common. Conversely, few dinosaur remains are known from mid‐ to high latitudes, which have higher floral diversities and abundant coals. Hence, there is an obvious geographic mismatch between known dinosaur distributions and their primary food source. This may be due to taphonomic bias, indicating that most dinosaur discoveries provide only a small window on the diversity and lifestyles of this group. On the basis of our global‐ and local‐scale studies, we suggest that dinosaur preservation was favored in environments toward the drier end of the climate spectrum, where savannas rather than forests predominated. A holistic approach, incorporating climate and vegetation as well as geography, is required to better understand patterns of dinosaur ecology and evolution.

The laterosphenoid bone of early archosauriforms

ABSTRACT The laterosphenoid is an ossification of the pila antotica neurocranial cartilage in the anterior sidewall of the braincase of crocodylians and birds. Contrary to published reports, the bone is present in the basal archosauriforms Proterosuchus fergusi, Euparkeria capensis, and Erythrosuchidae, taxa that diverged prior to the origin of the archosaurian crown group. Its presence is thus a synapomorphy of Archosauriformes rather than of Archosauria. The bone appears to have been induced by shifting jaw muscle origins from the skull roof to the anterolateral wall of the braincase. Additional innovations in the archosauriform feeding apparatus, such as a mandibular fenestra, appear to reflect changes in jaw muscles that may, in turn, facilitate predaceous habits. We review the historical development of the names of bones in the anterior sidewall of the amniote braincase and argue for the use of the term laterosphenoid, rather than either pleurosphenoid or orbitosphenoid, for this bone in all Archosau...

Cranial osteology of Longosuchus meadei and the phylogeny and distribution of the Aetosauria

ABSTRACT The aetosaur Longosuchus meadei is known from several specimens from the Upper Triassic (Carnian) of Texas. The structure of the single well preserved skull (TMM 31185-84B) was restudied in light of the appearance of new material of other archosaurs subsequent to Sawins original (1947) description. In particular, an ossified orbitosphenoid and several structures associated with the nasolacrimal duct system are described and discussed for the first time. Adaptations for herbivory in the Aetosauria include the ventral depression of the jaw joint, the edentulous beak, and the conical, unserrated teeth in later aetosaurs. The increased ossification of the braincase may serve to protect this region during food processing, and the nasolacrimal structures probably are associated with the lateral nasal sinus system widespread in archosaurs and particularly well developed in Longosuchus. The Aetosauria are a robust monophyletic group of Suchia that can be characterized by five synapo-morphies. Aetosaurus...

A new poposaurid from the Upper Triassic of Argentina

ABSTRACT The Poposauridae are a group of Middle to Late Triassic rauisuchian archosaurs that are the sister-group of the Crocodylomorpha. A new specimen from the Ischigualasto Formation (Carnian) of Argentina (PVSJ-85) is the first clear record of this group from Gondwana, and is designated the holotype of a new genus and species, Sillosuchus longicervix. The specimen consists of most of the vertebral column, both femora, pubes, and ischia, a partial right ilium, and a single paramedian plate. A distinctive feature shared with Chatterjeea, a poposaurid known from the Dockum Formation of Texas, is the presence of elongate cervical vertebrae with deep, dorsoventrally foreshortened excavations in the sides of the centra. The ilium of Sillosuchus has a prominent overhang that makes the upper surface of the acetabulum concave and partially encloses its dorsolateral edge. The pubes are elongate and narrow with a modest distal expansion forming a small foot. At least four and possibly as many as six sacral verte...

A NEW SPECIMEN OF AN EARLY CROCODYLOMORPH (CF. SPHENOSUCHUS SP.) FROM THE UPPER TRIASSIC CHINLE FORMATION OF PETRIFIED FOREST NATIONAL PARK, ARIZONA

ABSTRACT An associated skeleton of a crocodylomorph from the Painted Desert of Petrified Forest National Park is the second relatively complete specimen of a crocodylomorph described from the Upper Triassic Chinle Formation. The specimen lacks cranial and hind-limb elements commonly used for classification of crocodylomorphs, so character states of other features, notably armor, pelvic, tooth, and dentary morphology, are evaluated. Phylogenetic analysis of the Triassic/Jurassic non-crocodyliform crocodylomorphs (“sphenosuchians”) indicates that the new specimen is most similar to the Early Jurassic genus Sphenosuchus. Some of the features that ally the new specimen with Sphenosuchus may be artifacts of scaling, as the new specimen and the holotype of S. acutus are the two largest specimens of members of this group.

Phylogeny of the Erythrosuchidae (Reptilia: Archosauriformes)

ABSTRACT The Erythrosuchidae are a late Early to early Middle Triassic lineage that represent the first radiation of large terrestrial carnivores within the Archosauriformes. Erythrosuchid monophyly can be established by their possession of a suite of seven shared derived characters. At least five valid genera of erythrosuchids can be recognized. The most plesimorphic of these is Fugusuchus from China, followed by Garjainia from Russia and Erythrosuchus from South Africa. Vjushkovia and Shansisuchus share a later common ancestry. Vjushkovia is represented by two species, V. triplicostata from Russia and V. sinensis from China. Shansisuchus, also from China, is the only erythrosuchid with two fenestrae in the antorbital region. Erythrosuchids had a nearly worldwide distribution, although individual species appear to have restricted geographical ranges. The greatest diversity is in Russia and China but this probably reflects the fact that the most complete continental sequences spanning the stratigraphic ra...

Nonmarine extinction across the Cenomanian-Turonian boundary, southwestern Utah, with a comparison to the Cretaceous-Tertiary extinction event

There is a marked, possibly stepwise, extinction of marine taxa across the Cenomanian-Turonian boundary. Across the boundary in southwestern Utah, there is only minor species-level extinction of brackish-water taxa, and an actual increase in diversity of fully terrestrial organisms; significant family-level extinctions are restricted to aquatic taxa such as fishes and turtles. It is not possible in the nonmarine setting to determine if this is a gradual, stepwise, or instantaneous extinction, or to what degree it correlates to marine extinction events. Nonmarine faunas underwent no major change during the transgressive phase of the Greenhorn cycle, and the loss of aquatic taxa along with displacement (but not extinction) of brackish-water vertebrates and some marsupial mammals is first apparent in rocks deposited during regression in the Turonian. The loss of flood-plain habitat at maximum transgression may have caused the extinction of some of the aquatic taxa. The absence but not extinction of certain taxa on flood plains during the Greenhorn regression suggests that there may be some significant difference in transgressive and regressive flood plains. Drawdown increases the gradients of rivers and results in incision along coastal margins. This restricts the extent of brackish-water environments and may have had an impact on faunal compositions of riverine systems and contributed to extinction within aquatic communities. This pattern is quite different from that at the Cretaceous-Tertiary (K-T) boundary. Aquatic taxa underwent relatively minor losses at that boundary, whereas terrestrial organisms underwent major extinction. It appears that much of the Late Cretaceous aquatic community was restructured (mostly by exclusion of many taxa rather than extinction) and reduced in diversity during large-scale regression in the middle of the Maastrichtian before the end of the Cretaceous. This aquatic community was living in a rapidly expanding environment (overall regression of marine waters) at the K-T boundary. The extinction of terrestrial taxa at the boundary is unlike the pattern observed at the Cenomanian-Turonian boundary and suggests that some mechanism other than eustatic change played a significant role in the extinction.