Karen J. Houck

North America's largest dinosaur trackway site: Implications for Morrison Formation paleoecology

Little-known exposures of the Upper Jurassic Morrison Formation from the Purgatoire Valley of southeastern Colorado have yielded the worlds largest continuously mapped assemblage of dinosaur trackways. Body fossils include plant, invertebrate, and vertebrate remains indicative of predominantly fresh-water conditions. The tracks occur in a lacustrine sequence characterized by (1) shallow-water shales; (2) micritic shoreface limestones with ooids, intraclasts, ripple marks, mud cracks; and (3) minor quartzose sandstone with salt-crystal casts. Analysis of these sedimentary facies suggests that in southeastern Colorado, lakes were larger and longer lived than in other Morrison paleoenvironments. Trackway orientations and footprint-depth contours pinpoint the location of the paleoshoreline at successive levels in the section. Detailed mapping of 1,300 footprints in bed 2 has revealed more than 100 trackways which testify to the activity of both quadrupedal and bipedal dinosaurs. The respective ratio based on trackway counts is ∼40/60. The quadrupedal tracks are attributed to sauropods, and represent the first ever discovered in North America. They also exhibit the first known manus claw impressions and are in need of formal description. More than 90% of the bipedal tridactyl prints lack claw impressions and are tentatively therefore referred to the Ornithopoda, cf. Gypsichnites possibly a Camptosaurus . The small proportion with distinct claw impressions may represent Allosaurus . Resulting estimates of species diversity (∼5) and predator-prey ratio (1:30) are in general agreement with the estimates of other authors based on skeletal remains, and they suggest that the Purgatoire River tracks may accurately reflect the composition of the dinosaur fauna. Distinctive groupings of parallel, non-overlapping trackways suggest gregarious behavior among sauropods and tridactylous forms.

Theropod courtship: large scale physical evidence of display arenas and avian-like scrape ceremony behaviour by Cretaceous dinosaurs

Relationships between non-avian theropod dinosaurs and extant and fossil birds are a major focus of current paleobiological research. Despite extensive phylogenetic and morphological support, behavioural evidence is mostly ambiguous and does not usually fossilize. Thus, inferences that dinosaurs, especially theropods displayed behaviour analogous to modern birds are intriguing but speculative. Here we present extensive and geographically widespread physical evidence of substrate scraping behavior by large theropods considered as compelling evidence of “display arenas” or leks, and consistent with “nest scrape display” behaviour among many extant ground-nesting birds. Large scrapes, up to 2 m in diameter, occur abundantly at several Cretaceous sites in Colorado. They constitute a previously unknown category of large dinosaurian trace fossil, inferred to fill gaps in our understanding of early phases in the breeding cycle of theropods. The trace makers were probably lekking species that were seasonally active at large display arena sites. Such scrapes indicate stereotypical avian behaviour hitherto unknown among Cretaceous theropods, and most likely associated with terrirorial activity in the breeding season. The scrapes most probably occur near nesting colonies, as yet unknown or no longer preserved in the immediate study areas. Thus, they provide clues to paleoenvironments where such nesting sites occurred.

Middle Pennsylvanian, late Atokan-early Desmoinesian echinoderms from an intermontane basin; the central Colorado Trough

Middle Pennsylvanian, Atokan and Desmoinesian, crinoids and echinoids are described from the Belden and Minturn Formations of the Central Colorado Trough. The echinoderms from the Minturn Formation were living in an active intermontane trough setting. They are found in two environments, on the flanks of phylloid algal reefs and in patches on abandoned delta lobes on the gently sloping substrate. Faunas are dominated by cromyocrinids and are similar to stable platform Terrigineous Facies Belt faunas of the Midcontinent. New species described are Dicromyocrinus beldenesis, Neoprotencrinus rockensis, and Sciadiocrinus wipsorum, Goleocrinus, and the columnal genera Blothronagma, Cyclocaudex, and Platyplateium are reported for the first time from the Minturn Formation. The columnal taxa Blothronagma cinctutum and Floricyclus angustimargo are believed to be cromyocrinids. B. cinctutum may be synonymous with Synarmocrinus molasensis, and Floricyclus angustimargo may be synonymous with Ulocrinus rockymontanus.

Dinosaur tracks in intrusive igneous rock

Well preserved brontosaur footprints are replicated as impressions at the top of a dacitic igneous sill that intruded the track‐rich Cretaceous Jindong Formation, Kyeongsang basin South Korea. Although an unusual occurrence, the example shows the potential of such intrusions for enhancing preservation of trace fossils in some cases

A Survey of Tetrapod Tracksites Preserved in Pyroclastic Sediments, with Special Reference to Footprints of Hominids, Other Mammals and Birds

Pyroclastic sediments provide an unusual and favorable medium for the recording, burial, and preservation of tetrapod tracks and other traces. Twenty-two tracksites were reviewed for the purpose of determining how these deposits contribute to track formation, burial, and preservation. These include Jurassic sites in Argentina and Mexico, Cretaceous sites in Korea, Miocene sites in Mexico and the United States, Pliocene sites in Tanzania, Pleistocene sites in Mexico, Korea, and Italy, and Holocene sites in Mexico, Turkey, Italy, Japan, New Zealand, Nicaragua, and the United States. Twelve of the sites contain hominid tracks. Tracksites occur most commonly in reworked tephra on the shorelines of rivers, lakes, and seas. They also occur on pyroclastic falls, flows, and surges, and on lahars. Most tracksites are in volcanic arcs, especially around the Pacific Rim. A few occur in continental rifts or near intraplate volcanoes. Most older tracksites (Jurassic, Cretaceous, and Miocene) occur in silicic tephras such as rhyolite and dacite. The younger tracksites are mostly associated with basaltic tephra, though other compositions are also represented. Volcanic eruptions contribute to formation of substrates suitable for recording tracks by producing abundant fine-grained pyroclasts that interact physically and chemically with water to become cohesive. Hiatuses between eruptions provide time for tracks to accumulate, and in some cases, to lithify rapidly. Both physical processes (drying and compaction) and chemical processes (mineral precipitation) appear to be involved in early lithification. Eruptions also contribute to burial of tracks through rapid sedimentation and aggradation that typically follow a pyroclastic eruption. Multiple, closely spaced track horizons are common in pyroclastic sequences. Most tracksites are buried by either fallout tephra or reworked, waterlain deposits. Others are buried by lahars, pyroclastic flows and surges, or windblown ash. Upon exhumation pyroclastic deposits contribute to the formation of bedding planes through clay drapes, friable ash layers, and biotite-rich layers. Precipitation of authigenic minerals contributes to erosional resistance within beds. Bedding plane formation and erosional resistance facilitate discovery and study of tracks.

Systematics and occurrences of Edestus (Chondrichthyes) worldwide and new occurrences from Colorado and Texas

The species of Edestus Leidy (Chondrichthyes, Edestidae) are divided into two groups: those close to Edestus minor Newberry and those close to Edestus heinrichi Newberry and Worthen. The occurrences of Edestus worldwide (North America, Britain and Russia) are reviewed. In North America, Edestus occurs in the Illinois Basin and in the western United States, including South Dakota, Colorado and Texas, but not in marine beds of the same age in the Appalachian Basin. The absence of Edestus in the Appalachian Basin supports the idea that Edestus required access to the open sea, even if its remains are sometimes found in shallow or marginal marine deposits. The chronostratigraphic range of Edestus is from the middle Atokan through the Desmoinesian (Middle Pennsylvanian). All Atokan and lower Desmoinesian occurrences are from the E. minor group. The ranges of the E. minor group and the E. heinrichi group overlap in the upper Desmoinesian. We report two teeth of E. minor from the latest Atokan, in the Minturn Formation at McCoy, Eagle County Colorado. This is the first report of the genus from the southern Rocky Mountain region. We report some teeth of Edestus from Texas for the first time.