Abstract
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).
Kurzfassung
Die 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).
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Zeigler, K.E., Lucas, S.G. & Heckert, A.B. Variation in the Late Triassic Canjilon quarry (Upper Chinle Group, New Mexico) phytosaur skulls: a case for sexual dimorphism. Paläontol Z 77, 341–351 (2003). https://doi.org/10.1007/BF03006946
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DOI: https://doi.org/10.1007/BF03006946