Claudia A. Marsicano

The precise temporal calibration of dinosaur origins.

Significance Many hypotheses have been put forth to explain the origin and early radiation of dinosaurs, but poor age constraints for constituent fossil assemblages make these scenarios difficult to test. Using precise radioisotopic ages, we demonstrate that the temporal gap between assemblages containing only dinosaur precursors and those with the first dinosaurs was 5–10 million years shorter than previously thought. Thus, these data suggest that the origin of dinosaurs was a relatively rapid evolutionary event. Combined with our synthesis of paleoecological data, we demonstrate there was little compositional difference between the dinosaur precursor assemblages and the earliest dinosaur assemblages, and thus, the initial appearance of dinosaurs was not associated with a fundamental shift in ecosystem composition, as classically stated. Dinosaurs have been major components of ecosystems for over 200 million years. Although different macroevolutionary scenarios exist to explain the Triassic origin and subsequent rise to dominance of dinosaurs and their closest relatives (dinosauromorphs), all lack critical support from a precise biostratigraphically independent temporal framework. The absence of robust geochronologic age control for comparing alternative scenarios makes it impossible to determine if observed faunal differences vary across time, space, or a combination of both. To better constrain the origin of dinosaurs, we produced radioisotopic ages for the Argentinian Chañares Formation, which preserves a quintessential assemblage of dinosaurian precursors (early dinosauromorphs) just before the first dinosaurs. Our new high-precision chemical abrasion thermal ionization mass spectrometry (CA-TIMS) U–Pb zircon ages reveal that the assemblage is early Carnian (early Late Triassic), 5- to 10-Ma younger than previously thought. Combined with other geochronologic data from the same basin, we constrain the rate of dinosaur origins, demonstrating their relatively rapid origin in a less than 5-Ma interval, thus halving the temporal gap between assemblages containing only dinosaur precursors and those with early dinosaurs. After their origin, dinosaurs only gradually dominated mid- to high-latitude terrestrial ecosystems millions of years later, closer to the Triassic–Jurassic boundary.

A phylogeny of the Brachyopoidea (Temnospondyli, Stereospondyli)

Abstract In the thirty years since the last comprehensive review of the Brachyopidae many new brachyopid genera have been described and several different phylogenies proposed. This paper provides revised diagnoses of the Brachyopidae, their sister taxon, the Chigutisauridae, and the Brachyopoidea, erects a new higher taxon, Brachyopomorpha to include stem brachyopoids, and reviews the status of all material referred at one time to these taxa. In it we present revised illustrations of the first described member of the Brachyopidae and clarify parts of the morphology of it and other members of the Brachyopoidea including especially the mandible and postcranial skeleton. A new genus and species, Vigilius wellesi, is proposed for the skull of Hadrokkosaurus bradyi as the holotype of H. bradyi, a mandible, cannot be positively associated with the skull. A phylogenetic analysis including most taxa and all putative outgroups confirms the Chigutisauridae as sister group to the Brachyopidae, forming the Brachyopoidea. Bothriceps australis becomes a sister taxon to the Brachyopoidea. Xenobrachyops allos and Keratobrachyops australis are the most plesiomorphic members of the Brachyopidae and Chigutisauridae respectively.

Osteology of the Middle Triassic archosaur Lewisuchus admixtus Romer (Chanares Formation, Argentina), its inclusivity, and relationships amongst early dinosauromorphs

Lewisuchus admixtus is an enigmatic early dinosauriform from the Chañares Formation, Ladinian of Argentina, which has been recently considered a member of Silesauridae. Yet, it differs markedly from Late Triassic silesaurids in dental and vertebral anatomy. Indeed, a detailed redescription of its holotype allowed the identification of several features of the skeleton previously unrecognized amongst silesaurids. These include pterygoid teeth, a dorsomedial posttemporal opening on the otoccipital, foramina associated with cranial nerves X–XII on the caudal region of the prootic–otoccipital, and postaxial neck/trunk vertebrae with craniocaudally expanded neural spines. The presence of a single row of presacral scutes was also confirmed. Some elements previously referred to, or found associated with, the holotype, including a lower jaw, pedal elements and an astragalus, more probably correspond to proterochampsid remains. The anatomical information available for the holotype of L. admixtus was rescored into a new phylogenetic dataset for dinosauromorphs, mostly based on previous works. Lewisuchus admixtus and Pseudolagosuchus major are treated as distinct OTUs because their preserved skeletons mostly lack overlapping parts. The parsimony analysis supports the basal position of L. admixtus within dinosauriforms, prior to the silesaurid–dinosaur split, rather than at the base of Silesauridae. This suggests that a higher number of early dinosauriform clades branched in the Middle and Late Triassic than previously suggested.

Dinosaur origins: evidence from the footprint record

Triassic tracks and trackways assigned to dinosaur trackmakers or closest relatives have been mentioned from several Middle to the latest Triassic successions from both northern and southern Pangea. At present, the earliest gondwanan records are those from the Middle Triassic Los Rastros Formation in west-central Argentina. A reanalysis of Los Rastros ichnites at the Ischichuca area, including new material, has revealed the presence of a more diverse ichnofauna than previously suspected. The ichnocoenosis includes several tracks and trackways of bipeds with functionally tridactyl digitigrade pes, well developed claws, and a parasagittal posture of the hindlimbs. Previously, some large tridactyl footprints from the Ischichuca area were allied to theropod dinosaurs, although no synapomophies are preserved in the three-toed footprints that might discriminate among theropods, basal saurischians and basal ornithischian groups as their possible trackmakers. If the Ischichuca trackmakers are referred to a dinosaur taxon and/or to a close dinosaur sister-taxon, their presence in the Los Rastros levels suggests that derived dinosauriforms (including dinosaurs) had diverged and acquired their characteristic functionally tridactyl pes by at least the Middle Triassic, something that the body-fossil record has failed to document to date.

A Triassic tetrapod footprint assemblage from southern South America: palaeobiogeographical and evolutionary implications

A new Triassic tetrapod track assemblage recorded from levels of the Cuyana Basin (west-central Argentina) is described. The tracks came from multiple track-bearing horizons of the Upper Triassic (Carnian) Portezuelo Formation that indicate recurrent playa to mudflat conditions in a marginal lacustrine succession. This footprint assemblage is the most important one described up to now for the lower Mesozoic of South America, and forms a significant estimate of the fauna represented during that time in the region. Therefore, in order to understand the faunal significance of the assemblage, the track-makers were identified within a phylogenetic context. Although at a coarse taxonomic level, this analysis indicates the presence of non-mammalian therapsids (small cynodonts and large dicynodonts) and a quite diverse archosaur fauna that includes members of both crurotarsal archosaurs and dinosaurs (sauropodomorphs and theropods). Comparisons between the studied assemblage and other Early Mesozoic tetrapod track records from Gondwana suggest close relationships with those described from the Upper Triassic of the Karoo Basin in South Africa. In addition, the presence of putative basal sauropodomorph dinosaur tracks in the Portezuelo levels suggests that the stratigraphic range of the group extends back into the Carnian, farther than that documented by the known body-fossil record of the group in western Gondwana.

Dynamic Locomotor Capabilities Revealed by Early Dinosaur Trackmakers from Southern Africa

Background A new investigation of the sedimentology and ichnology of the Early Jurassic Moyeni tracksite in Lesotho, southern Africa has yielded new insights into the behavior and locomotor dynamics of early dinosaurs. Methodology/Principal Findings The tracksite is an ancient point bar preserving a heterogeneous substrate of varied consistency and inclination that includes a ripple-marked riverbed, a bar slope, and a stable algal-matted bar top surface. Several basal ornithischian dinosaurs and a single theropod dinosaur crossed its surface within days or perhaps weeks of one another, but responded to substrate heterogeneity differently. Whereas the theropod trackmaker accommodated sloping and slippery surfaces by gripping the substrate with its pedal claws, the basal ornithischian trackmakers adjusted to the terrain by changing between quadrupedal and bipedal stance, wide and narrow gauge limb support (abduction range = 31°), and plantigrade and digitigrade foot posture. Conclusions/Significance The locomotor adjustments coincide with changes in substrate consistency along the trackway and appear to reflect ‘real time’ responses to a complex terrain. It is proposed that these responses foreshadow important locomotor transformations characterizing the later evolution of the two main dinosaur lineages. Ornithischians, which shifted from bipedal to quadrupedal posture at least three times in their evolutionary history, are shown to have been capable of adopting both postures early in their evolutionary history. The substrate-gripping behavior demonstrated by the early theropod, in turn, is consistent with the hypothesized function of pedal claws in bird ancestors.

Biotic association and palaeoenvironmental reconstruction of the Loma del Pterodaustro' fossil site (Early Cretaceous, Argentina)

Abstract A sedimentological analysis of the basal section of the Early Cretaceous, lacustrine LagarcitoFormation at “Loma del Pterodaustro” (San Luis, Argentina) and a summary of its biological components are presented. Three sedimentological facies can be recognized in the basal sequence of the Lagarcito Formation. Fossil remains are particularly abundant in laminated claystones of a facies interpreted as deposits formed in offshore areas of the lake. The preservation of delicate structures allows recognition of these deposits as a Konservat Lagerstatte. Up to now, rocks at “Loma del Pterodaustro” have yielded plants, conchostracans, semionotid and pleuropholid fishes, pterodactyloid pterosaurs, and a variety of invertebrate traces. The chronology of the Lagarcito Formation is discussed and it is concluded that this unit is of Albian age. The palaeoenvironment of deposition of the basal sequence of the Lagarcito Formation at “Loma del Pterodaustro” is interpreted as a perennial, shallow lake developed within an alluvial plain, under semiarid climatic conditions.

Revision of the Brachyopidae (Temnospondyli) from the Triassic of the Sydney, Carnarvon and Tasmania Basins, Australia

Australian Triassic strata have produced more species of brachyopid Amphibia than are known elsewhere. Some were described last century and most before the widespread use of cladistics. There is thus a need for redescriptions which include identification of characters useful for character analysis. This paper re-examines the Australian brachyopids as a preliminary to a cladistic analysis of the Brachyopidae, clarifies some controversial aspects of their anatomy and taxonomy, and concludes that all except Trucheosaurus major are true brachyopids. Notobrachyops picketti is unlike other members of the taxon in the absence of a sensory canal system.

Paleoenvironments and taphonomy of a Triassic lacustrine system (Los Rastros Formation, central-western Argentina)

Abstract The Triassic Ischigualasto-Villa Unión Basin is an extensional basin located in central-western Argentina. It includes the Los Rastros Formation, a lacustrine-deltaic sequence comprising several coarsening-upward cycles of black shale, siltstone, and sandstone. We performed a taphonomic analysis of the floral and faunal fossils of the Los Rastros succession and have defined five plant taphofacies, four invertebrate taphofacies, and four vertebrate taphofacies. Our taphonomic model characterizes four subenvironments within the lacustrine-deltaic environment of Los Rastros Formation. These include 1) offshore lacustrine, 2) prodelta, 3) deltaic mouth bar, and 4) deltaic plain subenvironment. Our analysis of fossil assemblages allows us to reconstruct the structure of the original ecosystem. The lake margins were vegetated with small ginkgophytes, corystosperms, and sphenophytes. River margins were characterized by riparian thickets of sphenophytes, while the proximal floodplains supported closed woodlands of corystosperms, cycadophytes, and pteridophytes. More distal floodplains were covered with open conifer forests. The invertebrate fauna included insects (Blattoptera, Hemiptera, Coleoptera) associated with lakeshore vegetation and conchostracans that inhabited both the lake shoreline and smaller ponds in the floodplains. Fish and temnospondyl amphibians probably inhabited the delta plain and incoming fluvial systems. The activity of nonmammalian therapsids, crurotarsal archosaurs, and putative dinosaurs is recorded by trackway surfaces in the lake shoreline subenvironment.

New Permian fauna from tropical Gondwana

Terrestrial vertebrates are first known to colonize high-latitude regions during the middle Permian (Guadalupian) about 270 million years ago, following the Pennsylvanian Gondwanan continental glaciation. However, despite over 150 years of study in these areas, the biogeographic origins of these rich communities of land-dwelling vertebrates remain obscure. Here we report on a new early Permian continental tetrapod fauna from South America in tropical Western Gondwana that sheds new light on patterns of tetrapod distribution. Northeastern Brazil hosted an extensive lacustrine system inhabited by a unique community of temnospondyl amphibians and reptiles that considerably expand the known temporal and geographic ranges of key subgroups. Our findings demonstrate that tetrapod groups common in later Permian and Triassic temperate communities were already present in tropical Gondwana by the early Permian (Cisuralian). This new fauna constitutes a new biogeographic province with North American affinities and clearly demonstrates that tetrapod dispersal into Gondwana was already underway at the beginning of the Permian.