The beginning of the ‘Age of Dinosaurs’: a brief overview of terrestrial biotic changes during the Triassic Nicholas C. Fraser 1 and Hans-Dieter Sues 2 1 National Museums Scotland, Chambers Street, Edinburgh EH1 1JF, UK 2 Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, MRC 121, PO Box 37012, Washington, DC 20013–7012, USA ABSTRACT: The first appearance of dinosaurs during the early Late Triassic coincided with marked faunal changes in terrestrial ecosystems. Most of the major groups of extant tetrapods (or their proximate sister-taxa), including mammaliaforms, crocodyliform archosaurs, lepidosaurs and turtles, also first appeared in the fossil record during the Late Triassic. On the other hand, a number of Palaeozoic ‘holdovers’, such as procolophonid parareptiles, dicynodont therapsids and many groups of temnospondyls, vanished near or at the end of the Triassic. The tempo and mode of this faunal turnover have long been debated, but there has been growing acceptance of a rather sudden event, although the precise dating of such an event remains controversial. However, new discoveries have cast doubt on this assumption. The persistence of non-dinosaurian dinosauromorphs alongside dinosaurs well into Norian times hints at a more protracted turnover. New data on Triassic insect assemblages indicate that turnover among insects may also have been more protracted and possibly not co-incident with the faunal changes among tetrapods. Future work directed toward improved absolute age assessments for major faunal assemblages will be critical for a better understanding of the transition from therapsid-dominated to dinosaur-dominated communities during the early Mesozoic. KEY WORDS: extinctions, insects, Pangaea The Triassic Period represents a major turning point in the history of life on land. Bracketed by the largest mass extinction of all time near or at the end of the preceding Permian Period, and by another major extinction event near or at its end, the Triassic can be viewed as a time of extraordinary change and innovation in vertebrate evolution (Sues & Fraser 2010). However, it goes much further than that – the Triassic can justifiably be considered the ‘Dawn of the Modern World’. At the beginning of this period, there are few if any terrestrial tetrapods with close links to the principal extant groups. Yet, by the beginning of the Jurassic Period, there are lepidosaurs, turtles, modern amphibians and close relatives of present-day mammals and crocodylians (Sues et al. 1994). Sphenodontian lepidosaurs were already widely distributed across Pangaea during the Late Triassic (Bonaparte & Sues 2006). Admittedly, many of these forms still differ considerably from the extant representatives of these clades. For example, the earliest croco- dyliforms were fully terrestrial, highly cursorial carnivores rather than amphibious predators. But other forms, such as turtles, would already have been readily recognisable as members of their respective clades. Alberti (1834) first coined the term ‘Trias’ for a tripartite succession of sedimentary rock units in southern Germany (in ascending order): Buntsandstein, Muschelkalk and Keuper. He noted that similar deposits were widely distributed across Europe and already suspected their presence in India and North America. Alberti’s threefold lithostratigraphic succes- sion corresponds roughly to the current standard division of the Triassic into Lower, Middle and Upper Triassic series. Following on this tripartite division, Romer (1966) grouped Triassic terrestrial vertebrate assemblages into three major successive ‘faunas’. His Early Triassic Fauna ‘A’ was still dominated by therapsids, but archosauriform and basal archo- saurian reptiles were already present, along with other less common faunal elements. Romer’s Middle Triassic Fauna ‘B’ was dominated by gomphodont cynodonts and rhynchosaurs, and there was a major diversification of archosaurs, including dinosaurian precursors. Finally, dinosaurs, along with various groups of non-dinosaurian archosaurs, dominated Romer’s Late Triassic Fauna ‘C’, but therapsids were only a relatively minor faunal component. Although admittedly oversimplified, these divisions illustrate the principal change that took place among terrestrial tetrapod communities during the Triassic – the replacement of therapsids as the dominant terrestrial tetrapods by archosaurian reptiles. In the introduction to an edited volume, Padian (1986) noted that Triassic vertebrate assemblages comprised three main and successive components: ‘holdovers’, ‘indigenous’ taxa that appear to be entirely restricted to this period, and ‘modern’ elements. Recent work suggests that insects also started to show a shift towards modern forms some time during the Triassic. For example, some Late Triassic beetles, dipterans and thysan- opterans (thrips) are already remarkably similar to extant representatives of these groups (Fraser & Grimaldi 2003; Grimaldi & Engel 2005; Blagoderov et al. 2007). Similar to Romer’s threefold division of Triassic tetrapod communi- ties, Shcherbakov (2008) recently divided Triassic insect assemblages into three groups. He identified a low-diversity group of assemblages from the Early Triassic, which com- prised taxa related to those from the Late Permian, as well as a few endemic forms. Shcherbakov argued that the peak Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 101, 1–12, 2011 (for 2010)  2011 The Royal Society of Edinburgh. doi:10.1017/S1755691011020019