Biostratigraphic and Biogeographic Implications of New Middle to Late Triassic Fossil Vertebrates; Morondava Basin, Madagascar

Prepared for the 2000 meeting of the Western Association of Vertebrate Paleontologists, Museum of Northern Arizona, Flagstaff, AZ Burmeister, K. C., J. J. Flynn, J. M. Parrish, R. L. Whatley, and A. R. Wyss. 2000. Biostratigraphic and biogeographic implications of new middle to Late Triassic fossil vertebrates; Morondava Basin, Madagascar. Abstracts of the 2000 Meeting of the Western Association of Vertebrate Paleontologists, Museum of Northern Arizona, Flagstaff. BIOSTRATIGRAPHIC AND BIOGEOGRAPHIC IMPLICATIONS OF NEW LATE TRIASSIC FOSSIL VERTEBRATES; MORONDAVA BASIN, MADAGASCAR BURMEISTER, Kurtis C., Univ. Calif. Santa Barbara, CA, 93106; FLYNN, J. J., Field Museum, Chicago, IL 60605; PARRISH, M. J., N. Ill. Univ., DeKalb, IL 60115; WHATLEY, R. L., Univ. Calif. Santa Barbara, CA 93106; WYSS, A. R., Univ. Calif. Santa Barbara, CA 93106 Vertebrate fossils recently collected from the Isalo II, the middle of three subdivisions within the Isalo “Group” of western Madagascar, are helping to shape a biochronology for the island’s early to middle Mesozoic continental strata. In addition, these fossils extend the biostratigraphic and biogeographic ranges of several well known, geochronologically significant taxa. The previous paucity of biostratigraphic markers, as well as the lack of radioisotopically datable horizons, clouded correlations of the Isalo II to continental sedimentary rock sequences elsewhere in the world. These abundant new age-diagnostic fossils provide a greatly improved biostratigraphy for the basal and upper levels of the Isalo II. Diverse synapsids, some of the earliest known dinosaurs, and the rhynchosaur, Isalorhynchus, have been collected from basal levels of the Isalo II in the southern Morondava Basin, near Sakaraha. A preliminary cladistic analysis of Isalorhynchus suggests a placement between Middle Triassic rhynchosaurs from England and Tanzania, and Late Triassic forms from Scotland and South America. Previous age estimates for the basal Isalo II in the southern Morondava Basin ranged from Middle Triassic to Jurassic. This new evidence and the unique co-occurrence of several taxa suggest an early Carnian (earliest Late Triassic) age (Flynn et al., 1999, Science, 286: 763-765). Fragmentary phytosaur, aetosaur, dinosaur, ray, and semionotid fossils have been collected from a horizon directly below the Andafia Limestone in upper levels of the Isalo II, west of Malaimbandy in the northern Morondava Basin. Though previously considered of Early Jurassic age, the presence of phytosaurs and aetosaurs, in addition to the apparent absence of rhynchosaurs, suggests this horizon to instead be of Late Triassic (Norian) age. This discovery represents the first report of aetosaurs from Madagascar and, indeed of southeastern Gondwana..

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