James I. Kirkland

A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah

Therizinosauroids are an enigmatic group of dinosaurs known mostly from the Cretaceous period of Asia, whose derived members are characterized by elongate necks, laterally expanded pelves, small, leaf-shaped teeth, edentulous rostra and mandibular symphyses that probably bore keratinized beaks. Although more than a dozen therizinosauroid taxa are known, their relationships within Dinosauria have remained controversial because of fragmentary remains and an unusual suite of characters. The recently discovered ‘feathered’ therizinosauroid Beipiaosaurus from the Early Cretaceous of China helped to clarify the theropod affinities of the group. However, Beipiaosaurus is also poorly represented. Here we describe a new, primitive therizinosauroid from an extensive paucispecific bonebed at the base of the Cedar Mountain Formation (Early Cretaceous) of east-central Utah. This new taxon represents the most complete and most basal therizinosauroid yet discovered. Phylogenetic analysis of coelurosaurian theropods incorporating this taxon places it at the base of the clade Therizinosauroiden, indicating that this species documents the earliest known stage in the poorly understood transition from carnivory to herbivory within Therizinosauroidea. The taxon provides the first documentation, to our knowledge, of therizinosauroids in North America during the Early Cretaceous.

New Basal Iguanodonts from the Cedar Mountain Formation of Utah and the Evolution of Thumb-Spiked Dinosaurs

Background Basal iguanodontian dinosaurs were extremely successful animals, found in great abundance and diversity almost worldwide during the Early Cretaceous. In contrast to Europe and Asia, the North American record of Early Cretaceous basal iguanodonts has until recently been limited largely to skulls and skeletons of Tenontosaurus tilletti. Methodology/Principal Findings Herein we describe two new basal iguanodonts from the Yellow Cat Member of the Cedar Mountain Formation of eastern Utah, each known from a partial skull and skeleton. Iguanacolossus fortis gen. et sp. nov. and Hippodraco scutodens gen. et sp. nov. are each diagnosed by a single autapomorphy and a unique combination of characters. Conclusions/Significance Iguanacolossus and Hippodraco add greatly to our knowledge of North American basal iguanodonts and prompt a new comprehensive phylogenetic analysis of basal iguanodont relationships. This analysis indicates that North American Early Cretaceous basal iguanodonts are more basal than their contemporaries in Europe and Asia.

Bird-Like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace

Background Fossil tracks made by non-avian theropod dinosaurs commonly reflect the habitual bipedal stance retained in living birds. Only rarely-captured behaviors, such as crouching, might create impressions made by the hands. Such tracks provide valuable information concerning the often poorly understood functional morphology of the early theropod forelimb. Methodology/Principal Findings Here we describe a well-preserved theropod trackway in a Lower Jurassic (∼198 million-year-old) lacustrine beach sandstone in the Whitmore Point Member of the Moenave Formation in southwestern Utah. The trackway consists of prints of typical morphology, intermittent tail drags and, unusually, traces made by the animal resting on the substrate in a posture very similar to modern birds. The resting trace includes symmetrical pes impressions and well-defined impressions made by both hands, the tail, and the ischial callosity. Conclusions/Significance The manus impressions corroborate that early theropods, like later birds, held their palms facing medially, in contrast to manus prints previously attributed to theropods that have forward-pointing digits. Both the symmetrical resting posture and the medially-facing palms therefore evolved by the Early Jurassic, much earlier in the theropod lineage than previously recognized, and may characterize all theropods.

First definitive therizinosaurid (Dinosauria; Theropoda) from North America

monospecific genera from Asia (Perle, 1979; Barsbold and Perle, 1980; Barsbold, 1983; Russell and Dong, 1994; Xu et al., 1999). These taxa are known from partial skeletons that include one skull (Clark et al., 1994). Isolated frontals and an ungual from the upper Campanian Dinosaur Park formation of Alberta, Canada (Currie, 1987, 1992) and a possible astragalus from the upper Maastrichtian Hell Creek Formation of the northern United States (Russell, 1984) are the only therizino saurids reported from North America, albeit these identifica tions have been questioned (Barsbold and Maryanska, 1990; Clark et al., 1994; Maryanska, 1997). The recovery of a significant portion of a large therizino saurid skeleton in the lower member of the Moreno Hill for

VELAFRONS COAHUILENSIS, A NEW LAMBEOSAURINE HADROSAURID (DINOSAURIA: ORNITHOPODA) FROM THE LATE CAMPANIAN CERRO DEL PUEBLO FORMATION, COAHUILA, MEXICO

Abstract A new lambeosaurine hadrosaurid, Velafrons coahuilensis, is described as the first lambeosaurine from the Cerro del Pueblo Formation of Coahuila, Mexico, and the first lambeosaurine genus to be named from North America in more than 70 years. Although the holotype specimen is a juvenile individual—as evidenced by its incomplete crest development and relative size compared to other North American lambeosaurines—ontogeny independent autapomorphies have been identified including quadrate with narrow quadratojugal notch and a postorbital with well developed, dorsally positioned squamosal process. Additionally, this taxon is unique in that the prefrontal is not dorsally deflected and anteroposteriorly expanded as in other lambeosaurine taxa of its size, but rather retains the frontal-prefrontal “clamp” present in smaller individuals of other taxa. Phylogenetic analysis places Velafrons in a polytomy with numerous other fan-crested lambeosaurines. The crest structure of Velafrons more closely resembles that of Corythosaurus and Hypacrosaurus because it possesses an anteriorly projecting nasal process over the dorsal premaxilla process. Biogeographically, Velafrons is one of three distinct hadrosaurids known from approximately 73.5 Ma—two lambeosaurines and one hadrosaurine—all restricted to the southern region of the Western Interior Basin of North America.

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.

Biostratinomy and paleoecology of a Cretaceous brackish lagoon

Shell beds in a Cretaceous lagoonal fill resulted from a combination of low sediment input, high organic productivity, and reworking by storm processes. The skeletal concentrations occur in a shelly shale unit 2-3 m thick in the upper sandstone member (Late Cenomanian) of the Dakota Formation of northeastern Arizona. The shell beds are parautochthonous relics of a former brackish-water community dominated by the infaunal suspension-feeding bivalve Caryocorbula ovisana. Analysis of 74 samples and more than 12,000 specimens allows subdivision of this association into four subsets characterized by the bivalves Flemingostrea prudentia/Anomia ponticulana, Brachidontes filisculptus, and corbulid sp. A, and the gastropod Voysa sp. Biostratinomic, sedimentologic, and paleoecologic data allow reconstruction of a stratified brackish-water lagoon system. Saline, low-oxygen waters below storm wave base occurred at least locally in deeper parts of the lagoon system and were characterized by a monospecific assemblage of lucinid bivalves. The low-oxygen water was overlain by brachyhaline waters that were below fairweather wave base, but were within the reach of storms. This habitat supported a moderately diverse fauna of benthic molluscs. The lagoon was capped by water of strongly reduced salinity (mesohaline regime) and contained a very low diversity fauna. Progradation of the low-energy lagoonal shore resulted in the establishment of nonmarine environments in some parts of the lagoon. This sequence is overlain by reworked barrier-bar sands deposited during the ensuing transgression

A new basal hadrosauroid (Dinosauria: Ornithopoda) from the Turonian of New Mexico

ABSTRACT MSM P4166, a specimen from the Moreno Hill Formation (middle Turonian) of New Mexico, is described as the holotype of a new genus and species of hadrosauroid dinosaur. Jeyawati rugoculus, gen. et sp. nov., is diagnosed by a rugose texture that covers the entire lateral surface of the postorbital and the presence of a large neurovascular foramen at the base of the jugal process of the postorbital, as well as a unique combination of characters. A preliminary phylogenetic analysis reveals that Jeyawati is a basal hadrosauroid more derived than Probactrosaurus, Eolambia, and Protohadros, but more basal than Shuangmiaosaurus, Bactrosaurus, and Telmatosaurus. Assessment of ontogenetic criteria indicates that MSM P4166 represents a subadult or adult individual. Even with the recognition of Jeyawati, Late Cretaceous hadrosauroid biogeography remains somewhat ambiguous because of the lack of material from the late Turonian—early Santonian in western North America.

Evidence of reworked Cretaceous fossils and their bearing on the existence of Tertiary dinosaurs

The Paleocene Shotgun fauna of Wyoming includes marine sharks as well as mammals. It has been suggested that the sharks were introduced from the Cannonball Sea. It is more likely that these sharks were reworked from a Cretaceous rock sequence that included both marine and terrestrial deposits as there is a mixture of marine and freshwater taxa. These taxa have not been recorded elsewhere after the Cretaceous and are not known from the Cannonball Formation. Early Eocene localities at Raven Ridge, Utah, similarly contain teeth of Cretaceous marine and freshwater fish, dinosaurs, and Eocene mammals. The Cretaceous teeth are well preserved, variably abraded, and serve to cast doubts on criteria recently used to claim that dinosaur teeth recovered from the Paleocene of Montana are not reworked. Another Eocene locality in the San Juan Basin has produced an Eocene mammalian fauna with diverse Cretaceous marine sharks. Neither the nature of preservation nor the degree of abrasion could be used to distinguish reworked from contemporaneous material. The mixed environments represented by the fish taxa and recognition of the extensive pre-Tertiary extinction of both marine and freshwater fish were employed to recognize reworked specimens.

Osteology of the basal hadrosauroid Eolambia caroljonesa (Dinosauria: Ornithopoda) from the Cedar Mountain Formation of Utah.

Background Eolambia caroljonesa is known from copious remains from the lower Cenomanian Mussentuchit Member of the Cedar Mountain Formation in eastern Utah; however, the taxon has been only briefly described. Thus, we present herein a complete osteological description of Eolambia. Methodology/Principal Findings The description of Eolambia presented here is based upon the holotype partial skeleton (CEUM 9758), paratype partial skull (CEUM 5212), and abundant disarticulated elements from two bonebeds that contain juvenile individuals. These remains allow the skeletal anatomy of Eolambia to be documented almost fully and a revised diagnosis to be proposed. Conclusions/Significance The description provided here facilitates comparisons between Eolambia and other iguanodontians and allows Eolambia to be coded for additional characters in phylogenetic analyses. The close affinity between Eolambia and Probactrosaurus gobiensis from the Early Cretaceous of China supports previous hypotheses of faunal interchange between Asia and North America in the early Late Cretaceous.