Ecosystem instability in the Late Triassic and the early evolution of dinosaurs.

The Late Triassic Petrified Forest Member of the Chinle Formation (Photo from AASG)

Dinosaurs likely originated in the Middle Triassic and the first unequivocal dinosaur fossils are known from the late Carnian, but much about the geological and temporal backdrop of early dinosaur history remains poorly understood. A key question is why early dinosaurs were rare and species-poor at low paleolatitudes throughout the Late Triassic Period, for at least 30 million years after their origin.

The oldest well-dated identified dinosaurs are from the late Carnian (approx. 230 Ma) of the lower Ischigualasto Formation in northwestern Argentina. Similarly, the Santa Maria and Caturrita formations in southern Brazil preserve basal dinosauromorphs, basal saurischians, and early sauropodomorphs. In North America, the oldest dated occurrences of vertebrate assemblages with dinosaurs are from the Chinle Formation, but are less abundant and species rich compared to those from South America. The fact that those assemblages were at moderately high paleolatitudes during the Late Triassic, and the North American assemblages were near the paleoequator supports the hypotheses for a diachronous rise of dinosaurs across paleolatitudes (Irmis et al., 2011).

A reconstructed scene from the Late Triassic (Norian) of central Pangea. (Image from Brusatte, S. L. 2008, Dinosaurs, Quercus Publishing, London).

The Late Triassic is marked by a return to the “hothouse” condition of the Early Triassic, with two greenhouse crisis that may also have played a role in mass extinctions and long-term evolutionary trends (Retallack, 2013). The paleoclimate was a very arid with intense evaporation rate. Although there was at least one time of significant increase in rainfall known as the “Carnian Pluvial Event”, possibly related to the rifting of Pangea. Now, a multiproxy study  suggests  that fluctuating aridity in tropical and subtropical Pangea could explain why Triassic dinosaur faunas at low latitudes are restricted to small, slower growing carnivorous forms, whereas large-bodied herbivores, including sauropodomorph dinosaurs, are absent at low paleolatitudes during the Late Triassic “hothouse.” The palynomorphs recovered from sediments of the Chinle Formation indicate a major change from a seed fern-dominated (Alisporites) assemblage with accessory gymnosperms to one dominated by conifers and seed ferns in the lower portion of the Petrified Forest Member. In addition, the extensive charcoal record in the Petrified Forest Member provides evidence of paleo-environmental variability and aridity. 

 

References:

Jessica H. Whiteside, Sofie Lindström, Randall B. Irmis, Ian J. Glasspool, Morgan F. Schaller, Maria Dunlavey, Sterling J. Nesbitt, Nathan D. Smith, and Alan H. Turner. 2015. Extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years. PNAS: doi:10.1073/pnas.1505252112

Brusatte, S. L., Nesbitt, S. J., Irmis, R. B., Butler, R. J., Benton, M. J., and Norell, M. A. 2010. The origin and early radiation of dinosaurs. Earth-Science Reviews, 101, 68-100

Holz, M., Mesozoic paleogeography and paleoclimates – a discussion of the diverse greenhouse and hothouse conditions of an alien world, Journal of South American Earth Sciences (2015), doi: 10.1016/j.jsames.2015.01.001

Nesbitt,  S. J., Irmis,  R. B, Parker,  W. G. (2007) A critical re-evaluation of the Late Triassic di-nosaur taxa of North America. J Syst Palaeontology 5(2):209–243

Sellwood, B.W. & Valdes, P.J. 2006. Mesozoic climates: General circulation models and the rock Record. Sedimentary Geology 190:269–287.