The origin and early evolution of dinosaurs

Abstract

The oldest unequivocal records of Dinosauria were unearthed from Late Triassic rocks (approximately 230 Ma) accumulated over extensional rift basins in southwestern Pangea. The better known of these are Herrerasaurus ischigualastensis, Pisanosaurus mertii, Eoraptor lunensis, and Panphagia protos from the Ischigualasto Formation, Argentina, and Staurikosaurus pricei and Saturnalia tupiniquim from the Santa Maria Formation, Brazil. No uncontroversial dinosaur body fossils are known from older strata, but the Middle Triassic origin of the lineage may be inferred from both the footprint record and its sister-group relation to Ladinian basal dinosauromorphs. These include the typical Marasuchus lilloensis, more basal forms such as Lagerpeton and Dromomeron, as well as silesaurids: a possibly monophyletic group composed of Mid-Late Triassic forms that may represent immediate sister taxa to dinosaurs. The first phylogenetic definition to fit the current understanding of Dinosauria as a node-based taxon solely composed of mutually exclusive Saurischia and Ornithischia was given as "all descendants of the most recent common ancestor of birds and Triceratops". Recent cladistic analyses of early dinosaurs agree that Pisanosaurus mertii is a basal ornithischian; that Herrerasaurus ischigualastensis and Staurikosaurus pricei belong in a monophyletic Herrerasauridae; that herrerasaurids, Eoraptor lunensis, and Guaibasaurus candelariensis are saurischians; that Saurischia includes two main groups, Sauropodomorpha and Theropoda; and that Saturnalia tupiniquim is a basal member of the sauropodomorph lineage. On the contrary, several aspects of basal dinosaur phylogeny remain controversial, including the position of herrerasaurids, E. lunensis, and G. candelariensis as basal theropods or basal saurischians, and the affinity and/or validity of more fragmentary taxa such as Agnosphitys cromhallensis, Alwalkeria maleriensis, Chindesaurus bryansmalli, Saltopus elginensis, and Spondylosoma absconditum. The identification of dinosaur apomorphies is jeopardized by the incompleteness of skeletal remains attributed to most basal dinosauromorphs, the skulls and forelimbs of which are particularly poorly known. Nonetheless, Dinosauria can be diagnosed by a suite of derived traits, most of which are related to the anatomy of the pelvic girdle and limb. Some of these are connected to the acquisition of a fully erect bipedal gait, which has been traditionally suggested to represent a key adaptation that allowed, or even promoted, dinosaur radiation during Late Triassic times. Yet, contrary to the classical "competitive" models, dinosaurs did not gradually replace other terrestrial tetrapods over the Late Triassic. In fact, the radiation of the group comprises at least three landmark moments, separated by controversial (Carnian-Norian, Triassic-Jurassic) extinction events. These are mainly characterized by early diversification in Carnian times, a Norian increase in diversity and (especially) abundance, and the occupation of new niches from the ...

The origin and early radiation of dinosaurs

“…Taken together, these data indicate that the formation spans the Carnian-Norian boundary. Therefore, the oldest dinosaurs from the Ischigualasto Formation are late Carnian in age, not early Carnian as previously reported (e.g., Rogers et al, 1993;Martinez and Alcober, 2009), and some Ischigualasto dinosaurs, notably Pisanosaurus, may be Norian in age (Irmis et al, 2007b;Langer et al, 2010).…”

“…Most of these taxa resemble small predatory dinosaurs in their overall anatomy (e.g., Lagerpeton and Marasuchus: Sereno and Arcucci, 1994a,b), whereas recently discovered taxa such as Silesaurus (Dzik, 2003), Sacisaurus (Ferigolo and Langer, 2007), and Asilisaurus were quadrupedal herbivores or omnivores whose teeth superficially resemble those of ornithischian dinosaurs. Phylogenetic analyses indicate that these herbivorous taxa form their own distinct clade, Silesauridae, which is the immediate sister taxon (closest relative) of Dinosauria (Irmis et al, 2007a;Brusatte et al, 2008aBrusatte et al, , 2010bNesbitt et al, 2009bNesbitt et al, , 2010Langer et al, 2010). The recent discovery of Late Triassic representatives of these groups (e.g., Dzik, 2003 andIrmis et al, 2007a) demonstrates that they co-existed with dinosaurs for at least 15 million years.…”

“…As these two manuscripts were written independently and at the same time, we do not discuss the conclusions of Langer et al (2010) here, but note that the two papers largely complement each other in the discussion of early dinosaur evolution.…”

The precise temporal calibration of dinosaur origins

“…These events frame the evolutionary history of nonmarine tetrapod communities during the Triassic, resulting in a long-recognized threefold division: (i) lineages that survived the end-Permian mass extinction; (ii) a wide variety of new Triassic lineages that did not survive the end-Triassic mass extinction; and (iii) the first representatives of lineages that dominated later Mesozoic and Cenozoic ecosystems (2,3). Among the third group is arguably the most contentious of Mesozoic macroevolutionary events: the origin and rise of dinosaurs (4)(5)(6)(7)(8).…”

“…Although dinosaurs have often been cited as a classic case of an evolutionary radiation, many disparate hypotheses have been proposed for their origin and subsequent rapid rise to global dominance (4)(5)(6)(7)(8). One of the major difficulties with testing these hypotheses has been the lack of precise biostratigraphically independent age constraints for early dinosaur-bearing assemblages, which would provide a firm temporal basis for comparing origin scenarios across time and space (9).…”