Age constraints on the dispersal of dinosaurs in the Late Triassic from magnetochronology of the Los Colorados Formation (Argentina).

A measured magnetozone sequence defined by 24 sampling sites with normal polarity and 28 sites with reverse polarity characteristic magnetizations was established for the heretofore poorly age-constrained Los Colorados Formation and its dinosaur-bearing vertebrate fauna in the Ischigualasto-Villa Union continental rift basin of Argentina. The polarity pattern in this ∼600-m-thick red-bed section can be correlated to Chrons E7r to E15n of the Newark astrochronological polarity time scale. This represents a time interval from 227 to 213 Ma, indicating that the Los Colorados Formation is predominantly Norian in age, ending more than 11 My before the onset of the Jurassic. The magnetochronology confirms that the underlying Ischigualasto Formation and its vertebrate assemblages including some of the earliest known dinosaurs are of Carnian age. The oldest dated occurrences of vertebrate assemblages with dinosaurs in North America (Chinle Formation) are younger (Norian), and thus the rise of dinosaurs was diachronous across the Americas. Paleogeography of the Ischigualasto and Los Colorados Formations indicates prolonged residence in the austral temperate humid belt where a provincial vertebrate fauna with early dinosaurs may have incubated. Faunal dispersal across the Pangean supercontinent in the development of more cosmopolitan vertebrate assemblages later in the Norian may have been in response to reduced contrasts between climate zones and lowered barriers resulting from decreasing atmospheric pCO2 levels.

Dispersal of Late Triassic clam shrimps across Pangea linking northwestern Gondwana and central Pangea rift basins.

Clam shrimps are a group of freshwater crustaceans who prospered during the Late Triassic. They were abundant in lacustrine sedimentary records of continental basins distributed throughout Pangea during this time. However, they show significant taxonomic differences between the clamp shrimp faunas from the rift basins of central Pangea and the southern Gondwanan basins. In this contribution, we show new fossil clam shrimp assemblages from the lacustrine sedimentary successions of the Eastern Cordillera of Colombia (the Bocas and Montebel formations), providing information on the Late Triassic species that inhabited the northwestern Gondwana basins. This study demonstrates that the basins of northwestern Gondwana shared Norian clamp shrimp species with rift basins of central Pangea and differed in their faunas with the basins of the southern portion of Gondwana. In addition, the Late Triassic clam shrimps paleobiogeographic distribution reflects the dispersal of this fauna throughout fluvial-lacustrine environments established in the rift valleys along the central Pangea. Therefore, the rift valleys produced during the early fragmentation of central Pangea could have acted as corridors for dispersion. Simultaneously, rift valleys also provided paleobiogeographic barriers that isolated the central Pangea clam shrimp faunas from southern Gondwana.

A new sphenodontian (Lepidosauria: Rhynchocephalia) from the Late Triassic of Argentina and the early origin of the herbivore opisthodontians.

Sphenodontians were a successful group of rhynchocephalian reptiles that dominated the fossil record of Lepidosauria during the Triassic and Jurassic. Although evidence of extinction is seen at the end of the Laurasian Early Cretaceous, they appeared to remain numerically abundant in South America until the end of the period. Most of the known Late Cretaceous record in South America is composed of opisthodontians, the herbivorous branch of Sphenodontia, whose oldest members were until recently reported to be from the Kimmeridgian-Tithonian (Late Jurassic). Here, we report a new sphenodontian, Sphenotitan leyesi gen. et sp. nov., collected from the Upper Triassic Quebrada del Barro Formation of northwestern Argentina. Phylogenetic analysis identifies Sphenotitan as a basal member of Opisthodontia, extending the known record of opisthodontians and the origin of herbivory in this group by 50 Myr.

A new Late Triasssic phytogeographical scenario in westernmost Gondwana.

Floral provincialism within the Southern Hemisphere during the Late Triassic (230 Ma) is characterized by the Ipswich and Onslow provinces, recognized originally in eastern Gondwana. However, new palynological assemblages from the Ischigualasto Formation, northwestern Argentina (231-225 Ma), change the phytogeographic interpretation for the Carnian-Norian in the westernmost Gondwana, which was previously considered part of the southern floral Ipswich province. Here we show the presence of diagnostic Euramerican species within assemblages dominated by Gondwanan taxa that allows us to refer the palynofloras to the Onslow province. Our new data extend the Onslow floral belt, previously recognized from the western edge of Tethys to Timor, to the western margin of South America. This has implications for palaeophytogeography, palaeoclimate reconstructions and the palaeoecology of a Triassic ecosystem, which has yielded significant vertebrate remains and is regarded important in the early evolution of groups such as the Dinosauria.

Large-diameter burrows of the Triassic Ischigualasto Basin, NW Argentina: paleoecological and paleoenvironmental implications.

Large-diameter ichnofossils comprising three morphotypes have been identified in the Upper Triassic Ischigualasto and Los Colorados formations of northwestern Argentina. These burrows add to the global record of the early appearance of fossorial behavior during early Mesozoic time. Morphotypes 1 and 2 are characterized by a network of tunnels and shafts that can be assigned to tetrapod burrows given similarities with previously described forms. However, differences in diameter, overall morphology, and stratigraphic occurrence allow their independent classification. Morphotype 3 forms a complex network of straight branches that intersect at oblique angles. Their calcareous composition and surface morphology indicate these structures have a composite biogenic origin likely developed due to combined plant/animal interactions. The association of Morphotypes 1 and 2 with fluvial overbank lithologies deposited under an extremely seasonal arid climate confirms interpretations that the early appearance of burrowing behavior was employed by vertebrates in response to both temperature and moisture-stress associated with seasonally or perpetually dry Pangean paleoclimates. Comparisons of burrow morphology and biomechanical attributes of the abundant paleovertebrate fauna preserved in both formations permit interpretations regarding the possible burrow architects for Morphotypes 1 and 2. In the case of the Morphotype 1, the burrow constructor could be one of the small carnivorous cynodonts, Ecteninion or Probelesodon. Assigning an architect for Morphotype 2 is more problematic due to mismatches between the observed burrow morphology and the size of the known Los Colorados vertebrates.

A basal dinosaur from the dawn of the dinosaur era in southwestern Pangaea.

Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with (40)Ar/(39)Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement.