Archosaur evolution during the Jurassic: a southern perspective

Revista de la Asociación Geológica Argentina 63 (4): 557 - 585 (2008) ARCHOSAUR EVOLUTION DURING THE JURASSIC: A SOUTHERN PERSPECTIVE Oliver W. M. RAUHUT and Adriana LOPEZ-ARBARELLO Bayerische Staatssammlung für Paläontologie und Geologie, München, Germany. E-mails: , ABSTRACT The fossil record of archosaurs - crocodylomorphs, pterosaurs and dinosaurs - from the Jurassic of the Southern Hemisphere is critically reviewed, and its evolutionary implications are evaluated. Although several important faunas and also isolated finds are known from Gondwana, the record in total is still very patchy, and any evolutionary scenario based on this record should be seen as tentative. Compared to the Northern Hemisphere, southern archosaurs are much more poorly known, which is especially true for terrestrial crocodiles and pterosaurs. Marine crocodiles are rather well represented in south-western South America, whereas the report of terrestrial archosaurs is currently best for Africa. However, in South America, important and especially promising archosaur faunas are known from the Callovian Cañadón Asfalto and the (?)Tithonian Cañadón Calcáreo formations of Chubut province, Argentina. Early and Middle Jurassic Gondwanan archosaurs demonstrate that the faunas of that period still had a generally Pangean distribution, whereas first indications of differential archosaur evolution in the Northern and Southern Hemispheres are evident in Late Jurassic Gondwanan faunas. Keywords: Crocodylomorphs, Pterosaurs, Dinosaurs, Gondwana, Fossil record RESUMEN: La evolución de los arcosaurios durante el Jurásico: una perspectiva desde el Sur. El presente trabajo incluye una revisión crí- tica del registro fósil de los arcosaurios - cocodrilos, pterosaurios y dinosaurios - del Jurásico del Hemisferio Sur. Se evalúan además, sus implicancias evolutivas. Aunque se conocen varias faunas importantes y hallazgos aislados, el registro, en su totalidad, es todavía incompleto y cualquier escenario evolutivo basado en este registro debería verse como tentativo. En comparación con la situación en el Hemisferio Norte, es mucho menos lo que sabemos sobre los arcosaurios del Hemisferio Sur, y esto es especialmente cierto para los cocodrilos terrestres y pterosaurios. Los cocodrilos marinos están bastante bien representados en el sudoeste de Sudamérica, en tanto que el registro de arcosaruios terrestres es actualmente mejor en África. No obstante, en Sudamérica, se conocen faunas importantes, y especialmente prometedoras, del Caloviano de la Formación Cañadón Asfalto y del Titoniano (?) de la Formación Cañadón Calcáreo, en la provincia de Chubut, Argentina. Las faunas de arcosaurios del Jurásico Temprano y Medio de Gondwana muestran una distribución pangeica, en tanto que los primeros indicios de diferenciación en la evolución de los arcosaurios de los Hemisferios Norte y Sur se evidencian en las faunas gondwánicas del Jurásico Tardío. Palabras clave: Cocodrilos, Pterosaurios, Dinosaurios, Gondwana, Registro fósil INTRODUCTION Archosaurs - crocodylomorphs, pterosaurs, dinosaurs, and several basal, mainly Triassic groups - are certainly one of the most prolific clades of Mesozoic terrestrial vertebrates, and their living representatives, modern crocodiles and birds, are important components of recent vertebrate faunas. All three derived groups (crocodylomorphs, pterosaur, dinosaurs) originated in the later stages of the Triassic, but experienced their first and most important radiations in the Jurassic. In crocodylomorphs, the Jurassic saw the origin and diversification of the Mesoeucrocodylia, the largest clade, which also includes the modern forms (Fig. 1, e.g. Benton and Clark 1988, Tykoski et al. 2002, Pol and Gasparini 2007). In pterosaurs, the early "rhamphorhynchoid" faunas reached their peak diversity during this period, and the pterydactyloids, to which almost all Cretaceous pterosaurs belong, originated and first diversified in the Upper Jurassic (Fig. 2, Kellner 2003, Unwin 2003, 2006). Dinosaurs started with only a few clades into the Jurassic, almost none of which survived this period. However, by the end of this period, basically all important lineages were established (Fig. 3, Sereno 1999a, Weishampel et al. 2004). The Jurassic thus witnessed important faunal turnovers in all major clades of archosaurs and is therefore of greatest importance for our understanding of the evolutionary history of these groups. It is still rather unclear what caused these important changes in Jurassic archosaur faunas. In the Jurassic the initial break-up of the supercontinent of Pangea took 557 558 O. W. M . R AU H U T A N D A . L O P E Z - A R BA R E L L O Jurassic archosaur evolution. In the following, the colloquial term "crocodile" is used to refer to crocodylomorphs in general. True members of the Crocodylia are so far unknown from the Jurassic and only appear in the Cretaceous. Institutional abbreviations: MACN: Museo Argentino de Ciencias Naturales "Bernardino Rivadavia", Buenos Aires, MB: Museum für Naturkunde, Berlin, MPEF: Museo Paleontológico Egidio Feruglio, Trelew, Argentina. JURASSIC ARCHOSAURS FROM THE SOUTHERN HEMISPHERE Figure 1: Phylogeny of crocodiles showing the origin and first diversification of the Mesoeucrocodylia during the Jurassic. Based on Pol and Gasparini (2007). Dotted lines indicate uncertain limits of the stratigraphic range of the bearing sediments of a certain taxon. place, during which the western Tethian and northern Atlantic oceans opened and finally separated Gondwana from the northern continents in the Late Jurassic (Scotese 1991, Smith et al. 1994, Scotese et al. 1999, Ford and Golonka 2003). Vicariance as a consequence of this separation has been proposed as an important aspect of dinosaurian radiations (Russell 1993, Sereno 1999a, b), and evidence for continent-wide vicariance was found in empirical analyses of dinosaurian biogeography (Upchurch et al. 2002). Furthermore, these changes in geography brought about other modifications, such as the end of the megamonsoonal climate that had dominated large parts of Pangea in the Triassic and early stages of the Jurassic (Parrish 1993, Scotese et al. 1999). Archosaur radiations and faunal turnovers certainly have to be seen in the context of these environmental changes (see Forster 1999), but even the timing and the modes of these evolutionary events are still largely unknown, owing to the rather poor Jurassic fossil record in many groups. Furthermore, our current ideas about Jurassic faunal changes are almost entirely based on the fossil record of the Northern Hemisphere, making it basically impossible to check possible effects of the separation of Gondwana on the evolutionary history of the different groups. In the current paper, we critically evaluate the Jurassic archosaur fossil record of the Southern Hemisphere and its importance for our current understanding of Jurassic terrestrial archosaurs from the Souther (...truncated)