Kate E. Zeigler

Magnetostratigraphy of the Upper Triassic Chinle Group of New Mexico: Implications for regional and global correlations among Upper Triassic sequences

A magnetic polarity zonation for the Upper Triassic Chinle Group in the Chama Basin, north-central New Mexico (United States), supplemented by polarity data from eastern and west-central New Mexico (Mesa Redonda and Zuni Mountains, respectively), provides the most complete and continuous magnetic polarity chronology for the Late Triassic of the American Southwest yet available. Most of the Chinle Group sequence is composed of hematitic mudrocks that typically carry a well-defined, well-grouped magnetization (residing in both pigmentary and detrital hematite), with laboratory unblocking temperatures as high as 680 °C. Demagnetization experiments isolate magnetizations of south- or north-seeking declination and shallow inclination, which are interpreted as early acquired, Late Triassic magnetizations. Our proposed polarity correlations, coupled with biostratigraphic observations and recent U-Pb age determinations on detrital zircon–bearing strata in the Chinle Group in western New Mexico, West Texas, and Arizona, indicate that deposition of Chinle strata likely spanned a much shorter time span than previously considered. If this interpretation is correct, the Chinle Group can be correlated with only part of the Newark Supergroup or the Upper Triassic Tethyan sections. On a local scale, lower Chinle strata in the Chama Basin are significantly older than the Bluewater Creek Formation in western New Mexico, and the base of the Poleo Formation represents a disconformity of >13 m.y. duration. Magnetic polarity chronologies from upper Chinle strata in New Mexico and Utah suggest that strata considered to be part of the Rock Point Formation in north-central New Mexico are not time equivalent to type Rock Point strata in Utah or to the Redonda Formation of eastern New Mexico.

Variation in the Late Triassic Canjilon quarry (Upper Chinle Group, New Mexico) phytosaur skulls: a case for sexual dimorphism

The Canjilon quarry, located in north-central New Mexico near Ghost Ranch, contains a death assemblage of phytosaurs located stratigraphically high within the Petrified Forest Formation of the Chinle Group (Revueltian = early-mid Norian). The site has yielded numerous fossils ofPseudopalatus-grade phytosaurs, including at least 10 skulls collected from the locality byCharles Camp in 1928 and 1933 and another collected more recently byAlex Downs, curator of paleontology at Ghost Ranch. A re-examination of these skulls reveals two morphotypes that differ only in the relative lengths and relative robustness of their premaxillae. In these two morphotypes, the premaxillae define the shape and length of the rostral crest, the dimensions of which are independent of skull size. In one morphotype, the premaxillae are long, thin bones that lead to an abrupt, volcano-like narial crest. In the second morphotype, the premaxillae are of approximately the same length, but expand dorsoventrally halfway along their lengths, creating a longer and more robust crest. The most probable explanation of these two variants in rostral crest morphology in a Single, catastrophic death assemblage is thatPseudopalatus-grade phytosaurs are sexually dimorphic. Thus, the larger, more robust crest of the first morphotype is probably a display feature, most likely of the male animal. The more gracile snout änderest characterize the female morph. In the sample of phytosaurs examined, there are three individuals of the more robust (male) morphotype, five individuals of the gracile (female) morphotype, and three individuals (2 adult, 1 juvenile) that cannot be assigned to either morphotype because the skulls are too damaged to make an aecurate assessment. This is the first clear evidence of sexual dimorphism in phytosaurs, and has important implications for phytosaur species-level taxonomy, as well as for understanding aspects of their paleobiology (e.g., population dynamics).KurzfassungDie Canjilon-Fundstelle, die im nördlichen New Mexico in der Nähe von Ghost Ranch liegt, hat eine Taphozönose von Phytosauriern geliefert, die sich stratigraphisch hoch in der Petrified Forest-Formation befindet (Revueltium = frühes bis mittleres Nor). Es handelt sich um zahlreiche Funde von Phytosauriern auf der Evolutionshöhe von Pseudopalatus, darunter 11 Schädel. Von diesen wurden 10 von Charles Camp in den Jahren 1928 and 1933 geborgen und ein weiterer in jüngerer Zeit von Alex Downs vom paläontologischen Museum Ghost Ranch. Eine Neubearbeitung dieser Schädel zeigt, dass zwei Morphotypen vorhanden sind, die sich nur in der relativen Länge und relativen Robustheit des Prämaxillare unterscheiden. In beiden Morphotypen bestimmen die Prämaxillaria die Form und Länge des Rostralkammes, dessen Proportionen unabhängig von der Schädelgröße sind. Bei dem einen Morphotypus sind die Prämaxillaria lange, dünne Knochen, die in einem vulkanartigen Nasenkamm führen. Bei dem zweiten Morphotyp haben die Prämaxillaria zwar die gleiche Länge, aber nehmen auf der halben Länge in dorsoventraler Richtung an Größe zu, so dass ein längerer und robusterer Kamm entsteht. Die wahrscheinlichste Erklärung für diese beiden Varianten in einer durch eine Katastrophe verursachten Taphozönose ist, dass diese Phytosaurier sexuell dimorph waren. So ist der größere und robustere Kamm wohl eine Display struktur, vermutlich des männlichen Tieres. Die grazilere Schnauze und Kamm kennzeichnen das Weibchen. Im Material befinden sich 3 Individuen des robusteren Morphotyps (Männchen), 5 Individuen des grazileren Typs (Weibchen) und 3 Individuen (2 Adulttiere, 1 Juveniles), die zu keinem der Morphotypen gestellt werden können, weil die Schädel für eine sichere Ansprache zu stark beschädigt sind. Der Befund ist der erste eindeutige Beleg für einen Sexualdimorphismus bei Phytosauriern. Er hat bedeutende Implikationen für die Taxonomie der Phytosaurier auf dem Artniveau sowie für das Verständnis ihrer Paläobiologie (z.B. Populationsdynamik).