Tetrapod association and palaeoenvironment of the Los Colorados Formation (Argentina): a significant sample from Western Gondwana at the end of the Triassic

The Los Colorados Formation constitutes a continuous continental succession deposited in Western Argentina during the Late Triassic, a time period that is crucial to the record of the faunistic turnover at the Triassic-Jurassic boundary. Many authors have pointed out that its rich tetrapod fauna represents a unique transitional assemblage with elements typical of both Late Triassic and Early Jurassic. However, the possibility that the fauna represented a mixture of Triassic and Jurassic horizons was also proposed. Recently, stratigraphic control of the fossiliferous levels was developed in order to correlate the different localities of the extense Los Colorados outcrops, and a revision of the taxonomic status of most tetrapods recovered is currently undergoing. Preliminary results confirm previous assumptions about the transitional nature of the assemblage where typical Triassic taxa are associated with dinosaur groups known from Early Jurassic levels in other Gondwanan areas. The fossiliferous levels of the upper third of the sequence included several basal archosaurs (aetosaurs, rauisuchids, sphenosuchians), protosuchian crocodiles, dinosaurs (sauropodomorphs, tetanuran theropods), derived therapsids and primitive chelonians. New evidence about tetrapod ichnites of chirotheroid affinities is added to the fossiliferous association.     

The first 50Myr of dinosaur evolution: macroevolutionary pattern and morphological disparity

The evolutionary radiation of dinosaurs in the Late Triassic and Early Jurassic was a pivotal event in the Earth's history but is poorly understood, as previous studies have focused on vague driving mechanisms and have not untangled different macroevolutionary components (origination, diversity, abundance and disparity). We calculate the morphological disparity (morphospace occupation) of dinosaurs throughout the Late Triassic and Early Jurassic and present new measures of taxonomic diversity. Crurotarsan archosaurs, the primary dinosaur 'competitors', were significantly more disparate than dinosaurs throughout the Triassic, but underwent a devastating extinction at the Triassic-Jurassic boundary. However, dinosaur disparity showed only a slight non-significant increase after this event, arguing against the hypothesis of ecological release-driven morphospace expansion in the Early Jurassic. Instead, the main jump in dinosaur disparity occurred between the Carnian and Norian stages of the Triassic. Conversely, dinosaur diversity shows a steady increase over this time, and measures of diversification and faunal abundance indicate that the Early Jurassic was a key episode in dinosaur evolution. Thus, different aspects of the dinosaur radiation (diversity, disparity and abundance) were decoupled, and the overall macroevolutionary pattern of the first 50Myr of dinosaur evolution is more complex than often considered.     

A New Basal Sauropodomorph Dinosaur from the Lower Jurassic Navajo Sandstone of Southern Utah

Background

Basal sauropodomorphs, or ‘prosauropods,’ are a globally widespread paraphyletic assemblage of terrestrial herbivorous dinosaurs from the Late Triassic and Early Jurassic. In contrast to several other landmasses, the North American record of sauropodomorphs during this time interval remains sparse, limited to Early Jurassic occurrences of a single well-known taxon from eastern North America and several fragmentary specimens from western North America.

Methodology/Principal Findings

On the basis of a partial skeleton, we describe here a new basal sauropodomorph dinosaur from the Lower Jurassic Navajo Sandstone of southern Utah, Seitaad ruessi gen. et sp. nov. The partially articulated skeleton of Seitaad was likely buried post-mortem in the base of a collapsed dune foreset. The new taxon is characterized by a plate-like medial process of the scapula, a prominent proximal expansion of the deltopectoral crest of the humerus, a strongly inclined distal articular surface of the radius, and a proximally and laterally hypertrophied proximal metacarpal I.

Conclusions/Significance

Phylogenetic analysis recovers Seitaad as a derived basal sauropodomorph closely related to plateosaurid or massospondylid ‘prosauropods’ and its presence in western North America is not unexpected for a member of this highly cosmopolitan clade. This occurrence represents one of the most complete vertebrate body fossil specimens yet recovered from the Navajo Sandstone and one of the few basal sauropodomorph taxa currently known from North America.     

Late Triassic (Late Norian) gastropods from the Wallowa Terrane (Idaho,USA)

A diverse Late Triassic (Late Norian) gastropod fauna is described from the Mission Creek Limestone of the Wallowa terrane (Idaho, USA). Sample standardization by rarefaction analysis indicates that the fauna is even more diverse than the Late Triassic gastropod fauna from the Pucara Formation (Peru) which represents the most diverse gastropod fauna from South America. The gastropod fauna consists of 66 species; several genera are reported for the first time from North America. A high percentage of the species are highly ornamented and several have distinct siphonal canals. This suggests that the appearance of truly Mesozoic elements among the gastropods began before the Mesozoic Marine Revolution in other clades. The fauna is dominated by high-spired strongly ornamented procerithiids, a group more characteristic for the Jurassic. Comparison of the present fauna and the Iranian Nayband Formation gastropod fauna show that the procerithiids underwent a first global radiation in the Late Triassic. The high number of new species in this fauna suggests that sampling of Late Triassic gastropod faunas is still incomplete and hinders palaeobiogeographic considerations. Previous suggesions that gastropod faunas from the Wallowa and Wrangellia terranes resemble each other and are distinct from those of Alexander, Chulitna, and Farewell terranes are basically corroborated. The gastropod fauna of the Mission Creek Limestone differs considerably from that of the western and central Tethys but shares several taxa with the Late Triassic gastropod fauna of the Pucara Formation in Peru. Thus, the Hispanic corridor was probably not present in the Norian but opened only in the Early Jurassic. The subfamily Andangulariinae is introduced and placed in the Zygopleuridae. The generaSpiniomphalus, Nodoconus, Gudrunella, Blodgettella, Idahospira, andSiphonilda and the subgenusCryptaulax (Wallowax) are introduced. 27 species are erected. A lectotype is designated forCryptaulax rhabdocolpoides Haas, 1953.      

Lower limits of ornithischian dinosaur body size inferred from a new Upper Jurassic heterodontosaurid from North America

The extremes of dinosaur body size have long fascinated scientists. The smallest (<1 m length) known dinosaurs are carnivorous saurischian theropods, and similarly diminutive herbivorous or omnivorous ornithischians (the other major group of dinosaurs) are unknown. We report a new ornithischian dinosaur, Fruitadens haagarorum, from the Late Jurassic of western North America that rivals the smallest theropods in size. The largest specimens of Fruitadens represent young adults in their fifth year of development and are estimated at just 65–75 cm in total body length and 0.5–0.75 kg body mass. They are thus the smallest known ornithischians. Fruitadens is a late-surviving member of the basal dinosaur clade Heterodontosauridae, and is the first member of this clade to be described from North America. The craniodental anatomy and diminutive body size of Fruitadens suggest that this taxon was an ecological generalist with an omnivorous diet, thus providing new insights into morphological and palaeoecological diversity within Dinosauria. Late-surviving (Late Jurassic and Early Cretaceous) heterodontosaurids are smaller and less ecologically specialized than Early (Late Triassic and Early Jurassic) heterodontosaurids, and this ecological generalization may account in part for the remarkable 100-million-year-long longevity of the clade.     

An analysis of dinosaurian biogeography: evidence for the existence of vicariance and dispersal patterns caused by geological events

As the supercontinent Pangaea fragmented during the Mesozoic era, dinosaur faunas were divided into isolated populations living on separate continents. It has been predicted, therefore, that dinosaur distributions should display a branching ('vicariance') pattern that corresponds with the sequence and timing of continental break-up. Several recent studies, however, minimize the importance of plate tectonics and instead suggest that dispersal and regional extinction were the main controls on dinosaur biogeography. Here, in order to test the vicariance hypothesis, we apply a cladistic biogeographical method to a large dataset on dinosaur relationships and distributions. We also introduce a methodological refinement termed 'time-slicing', which is shown to be a key step in the detection of ancient biogeographical patterns. These analyses reveal biogeographical patterns that closely correlate with palaeogeography. The results provide the first statistically robust evidence that, from Middle Jurassic to mid-Cretaceous times, tectonic events had a major role in determining where and when particular dinosaur groups flourished. The fact that evolutionary trees for extinct organisms preserve such distribution patterns opens up a new and fruitful direction for palaeobiogeographical research.     

Global historical biogeography of hadrosaurid dinosaurs

Hadrosaurids were the most derived ornithopods and amongst the most diverse herbivore dinosaurs during the Late Cretaceous of Europe, Asia, and the two Americas. Here, their biogeographical history is reconstructed using dispersal‐vicariance analysis (DIVA). The results showed that Hadrosauridae originated in North America and soon after dispersed to Asia no later than the Late Santonian. The most recent common ancestor of Saurolophidae (= Saurolophinae + Lambeosaurinae) is inferred to have been widespread in North America and Asia. The split between saurolophines and lambeosaurines occurred in response to vicariance no later than the Late Santonian: the former clade originated in North America, whereas the latter did so in Asia. Saurolophine biogeographical history included a minimum of five dispersal events followed by vicariance. Four of these dispersals were inferred to have occurred from North America to Asia during the Campanian and Early Maastrichtian, whereas a fifth event represented a southward dispersal from North to South America no later than the Late Campanian. The historical biogeography of lambeosaurines was characterized by an early evolution in Asia, with a Campanian dispersal to the European archipelago followed by vicariance. Reconstruction of the ancestral areas for the deepest nodes uniting the more derived lambeosaurines clades (‘hypacrosaurs’, ‘corythosaurs’, and ‘parasaurolophs’) is ambiguous. The split between North American and Asian clades of ‘hypacrosaurs’ and ‘parasaurolophs’ occurred in response to vicariance during the Campanian. The evolutionary history of North American ‘hypacrosaurs’ and ‘parasaurolophs’ was characterized by duplication events. The latter also characterized the Late Campanian ‘corythosaurs’, which remained restricted to North America. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 159 , 503–525.     

The earliest fossil evidence of bone boring by terrestrial invertebrates,examples from China and South Africa

We report the oldest fossil evidence of osteophagia by terrestrial invertebrates on both the Asian and African continents. Bones attributable to the Middle Jurassic dinosaur Chuanjiesaurus (Dinosauria: Sauropoda) were found with post-mortem insect modification in the Chuanjie Formation, Yunnan Province, China. The morphology of the borings closely matches the ichnogenus Cubiculum. Based on the lack of bioglyphs observed in Cubiculum ornatus, a new ichnospecies is proposed here. The new trace fossil, Cubiculum inornatus isp. nov., is interpreted to have been constructed for pupation by an unknown taxon of insect. Additionally, we report even older borings from Early Jurassic dinosaur bones of the Elliott Formation in the Karoo Basin, which represent the second oldest occurrence of insect traces in bone from continental settings. Both trace fossils sites have palaeogeographic implications for the origins and dispersal of osteophagia amongst terrestrial invertebrates during the Mesozoic. These discoveries push back the antiquity of pupation in animal bones by more than 100 million years to the Middle Jurassic, indicating that this behaviour, and osteophagy more generally, originated early in the Mesozoic, roughly comparable with the origination of insect pupation in woody substrates (Late Triassic).     

Diversity variation and tempo-spatial distributions of the Dipteridaceae ferns in the Mesozoic of China

The extant family Dipteridaceae is a remarkable leptosporangiate fern because it includes only one genus with a restricted distribution to tropical regions. The fossil record of this family has been widely reported from the Mesozoic strata in Eurasia, America, Australia, and Greenland. In China, numerous fossils of the Dipteridaceae have been documented, in total, about 74 species of 6 genera. Geographically, they are distributed both in the Southern and Northern Floristic Provinces, and were particularly well developed in the Southern Floristic Province during the Late Triassic and the Early Jurassic intervals. Fossil diversity of Dipteridaceae varies in the different episodes of the Mesozoic in China. It is shown that Dipteridaceae has undergone a diversity development process and a distinct turnover during the Mesozoic. They appear to have diversified in the warm and humid Late Triassic–Early Jurassic, but declined sharply as aridity developed in the Middle Jurassic, and became extinct at the end of the Early Cretaceous. The diversity variation and tempo-spatial distribution pattern is suggested to be linked with paleoclimatic variations during the Mesozoic.     

On the earliest record of Cretaceous tyrannosauroids in western North America: implications for an Early Cretaceous Laurasian interchange event

The sudden appearance of Asian dinosaur clades within Lower Cretaceous strata of western North America has long been recognised as a biotic dispersion event related to initial establishment of a Beringian land bridge. To date, uncertainty exists regarding the timing of the Early Cretaceous Laurasian interchange event (EKLInE) and the pattern of associated biotic dispersal. Here, we report a tyrannosauroid premaxillary tooth (FMNH PR 2750) from the Cloverly Formation, Wyoming, USA, that pushes back the earliest Cretaceous record of the clade in North America. Although fragmentary, the tooth is consistent with mounting evidence for a pre-108 Ma initiation of EKLInE and earliest Albian emplacement of Beringia. Previous authors have considered the Aptian/Albian of western North America a depauperate dinosaur fauna, characterised by regional extinction and diversity decline. Documentation of Albian tyrannosauroids in the region indicates a more dynamic ecosystem than previously appreciated and marks an early start to faunal mixing between immigrant and endemic dinosaur clades. Finally, we find that the enamel microstructure of FMNH PR 2750 conforms to the morphotype of tyrannosaurids, yet exhibits poor columnar differentiation. This morphology bolsters prior interpretations on the phylogenetic utility of enamel microstructure and suggests a trend of increasing enamel complexity within Tyrannosauroidea.     

Evolution of ecospace occupancy by Mesozoic marine tetrapods

Ecology and morphology are different, and yet in comparative studies of fossil vertebrates the two are often conflated. The macroevolution of Mesozoic marine tetrapods has been explored in terms of morphological disparity, but less commonly using ecological‐functional categories. Here we use ecospace modelling to quantify ecological disparity across all Mesozoic marine tetrapods. We document the explosive radiation of marine tetrapod groups in the Triassic and their rapid attainment of high ecological disparity. Late Triassic extinctions led to a marked decline in ecological disparity, and the recovery of ecospace and ecological disparity was sluggish in the Early Jurassic. High levels of ecological disparity were again achieved by the Late Jurassic and maintained during the Cretaceous, when the ecospace became saturated by the Late Cretaceous. Sauropterygians, turtles and ichthyosauromorphs were the largest contributors to ecological disparity. Throughout the Mesozoic, we find that established groups remained ecologically conservative and did not explore occupied or vacant niches. Several parts of the ecospace remained vacant for long spans of time. Newly evolved, radiating taxa almost exclusively explored unoccupied ecospace, suggesting that abiotic releases are needed to empty niches and initiate diversification. In the balance of evolutionary drivers in Mesozoic marine tetrapods, abiotic factors were key to initiating diversification events, but biotic factors dominated the subsequent generation of ecological diversity.     

中生代鞘翅目扁甲总科化石的研究进展

扁甲总科隶属于鞘翅目多食亚目扁甲系,目前包含25个现生科和3个化石科,其中10个科仅分布于南半球,特别是澳大利亚、新西兰和南美。当前的分子系统发育学研究显示扁甲总科的起源时间为晚三叠世至早侏罗世,而冈瓦纳古陆与劳亚古陆的分离也发生在早侏罗世约1.8亿年前。因此对中生代扁甲总科化石的研究是解释扁甲总科如今分布格局的重要基础,也有助于研究扁甲总科的演化历史。本文简要回顾了现生扁甲总科的研究历史,并对其中生代的化石研究进行了概述,迄今在中生代地层发现有12科54种扁甲总科化石。     

Diversity variation and tempo-spatial distribution of Otozamites (Bennettitales) in the Mesozoic of China

Otozamites is a representative fossil leaf morphogenus of the extinct Bennettitales, with an extensive distribution during the Mesozoic, especially in China. Understanding the fossil diversity variation and distribution pattern of Otozamites in China will provide information on biodiversity of bennettitalean plants as well as for reconstruction of palaeogeography and palaeoclimate conditions during the Mesozoic. So far, 46 species of this genus have been described in China, excluding unspecified species. The results show that the fossils of Otozamites are extensively recorded in the Late Triassic, and then reach their maximum development in the Early Jurassic, followed by a reduction in diversity in the Middle and Late Jurassic, and finally become extinct at the end of Early Cretaceous. Geographically, they occur in both Northern and Southern Floristic Provinces in the Mesozoic of China, with a relatively higher abundance in the Southern Floristic Province. It implies that the diversity variation and distribution of Otozamites are closely related to the change of the palaeoclimatic conditions. The warm and humid climate prevailed in the Late Triassic and Early Jurassic in South China, propitious to the development of Otozamites. After the Middle Jurassic, dry and hot climate may have caused the lower diversity level and blocked the development of Otozamites; finally at the end of the Early Cretaceous, the frequent arid climate may be a major cause for the extinction of Otozamites.     

Impact of the Late Triassic mass extinction on functional diversity and composition of marine ecosystems

Mass extinctions have profoundly influenced the history of life, not only through the death of species but also through changes in ecosystem function and structure. Importantly, these events allow us the opportunity to study ecological dynamics under levels of environmental stress for which there are no recent analogues. Here, we examine the impact and selectivity of the Late Triassic mass extinction event on the functional diversity and functional composition of the global marine ecosystem, and test whether post‐extinction communities in the Early Jurassic represent a regime shift away from pre‐extinction communities in the Late Triassic. Our analyses show that, despite severe taxonomic losses, there is no unequivocal loss of global functional diversity associated with the extinction. Even though no functional groups were lost, the extinction event was, however, highly selective against some modes of life, in particular sessile suspension feeders. Although taxa with heavily calcified skeletons suffered higher extinction than other taxa, lightly calcified taxa also appear to have been selected against. The extinction appears to have invigorated the already ongoing faunal turnover associated with the Mesozoic Marine Revolution. The ecological effects of the Late Triassic mass extinction were preferentially felt in the tropical latitudes, especially amongst reefs, and it took until the Middle Jurassic for reef ecosystems to fully recover to pre‐extinction levels.     

The last dicynodont: an Australian Cretaceous relict

Some long-forgotten fossil evidence reveals that a dicynodont (mammal-like reptile of the infraorder Dicynodontia) inhabited Australia as recently as the Early Cretaceous, ca. 110 Myr after the supposed extinction of dicynodonts in the Late Triassic. This remarkably late occurrence more than doubles the known duration of dicynodont history (from ca. 63 Myr to ca. 170 Myr) and betrays the profound impact of geographical isolation on Australian terrestrial faunas through the Mesozoic. Australia's late-surviving dicynodont may be envisaged as a counterpart of the ceratopians (horned dinosaurs) in Cretaceous tetrapod faunas of Asia and North America.     

A palaeoequatorial ornithischian and new constraints on early dinosaur diversification

Current characterizations of early dinosaur evolution are incomplete: existing palaeobiological and phylogenetic scenarios are based on a fossil record dominated by saurischians and the implications of the early ornithischian record are often overlooked. Moreover, the timings of deep phylogenetic divergences within Dinosauria are poorly constrained owing to the absence of a rigorous chronostratigraphical framework for key Late Triassic–Early Jurassic localities. A new dinosaur from the earliest Jurassic of the Venezuelan Andes is the first basal ornithischian recovered from terrestrial deposits directly associated with a precise radioisotopic date and the first-named dinosaur from northern South America. It expands the early palaeogeographical range of Ornithischia to palaeoequatorial regions, an area sometimes thought to be devoid of early dinosaur taxa, and offers insights into early dinosaur growth rates, the evolution of sociality and the rapid tempo of the global dinosaur radiation following the end-Triassic mass extinction, helping to underscore the importance of the ornithischian record in broad-scale discussions of early dinosaur history.     

Re-Examination of Some Large Early Mesozoic Tetrapod Footprints from the African Collection of Paul Ellenberger

The Late Triassic—Early Jurassic ichnofauna described mainly by Paul Ellenberger from southern Africa (Lesotho) is a valuable window on first phases of dinosaur diversification. Unfortunately, the present taxonomic status of several forms from that ichnofauna is unclear. The revision of this material has been frequently invoked and partially done without reaching many definitive results. Due to the enormous amount of data, a global revision seems at present impossible and must be accomplished in smaller steps. A small number of Ellenberger's ichnogenera including Tetrasauropus, Pseudotetrasauropus, Pentasauropus, Paratetrasauropus, Sauropodopus and Deuterosauropodopus, which different authors have ascribed to basal sauropodomorphs, are here revised in a consistent manner and their attribution to osteological clades is considered. Tetrasauropus and Pseudotetrasauropus are here validated as the only ichnotaxa related to sauropodomorphs. Pentasauropus is retained as valid, and a therapsid trackmaker is suggested. Paratetrasauropus and Sauropodopus are also validated and ascribed to non-dinosaurian trackmakers, and Deuterosauropodopus is synonymized with Sauropodopus.     

Biogeography and diversification of colletid bees (Hymenoptera: Colletidae): emerging patterns from the southern end of the world

Aim The evolutionary history of bees is presumed to extend back in time to the Early Cretaceous. Among all major clades of bees, Colletidae has been a prime example of an ancient group whose Gondwanan origin probably precedes the complete break‐up of Africa, Antarctica, Australia and South America, because modern lineages of this family occur primarily in southern continents. In this paper, we aim to study the temporal and spatial diversification of colletid bees to better understand the processes that have resulted in the present southern disjunctions. Location Southern continents. Methods We assembled a dataset comprising four nuclear genes of a broad sample of Colletidae. We used Bayesian inference analyses to estimate the phylogenetic tree topology and divergence times. Biogeographical relationships were investigated using event‐based analytical methods: a Bayesian approach to dispersal–vicariance analysis, a likelihood‐based dispersal–extinction–cladogenesis model and a Bayesian model. We also used lineage through time analyses to explore the tempo of radiations of Colletidae and their context in the biogeographical history of these bees. Results Initial diversification of Colletidae took place at the Late Cretaceous (≥ 70 Ma). Several (6–14) lineage exchanges between Australia and South America via Antarctica during the Late Cretaceous and Eocene epochs could explain the disjunctions observed between colletid lineages today. All biogeographical methods consistently indicated that there were multiple lineage exchanges between South America and Australia, and these approaches were valuable in exploring the degree of uncertainty inherent in the ancestral reconstructions. Biogeographical and dating results preclude an explanation of Scrapterinae in Africa as a result of vicariance, so one dispersal event is assumed to explain the disjunction in relation to Euryglossinae. The net diversification rate was found to be highest in the recent history of colletid evolution. Main conclusions The biogeography and macroevolutionary history of colletid bees can be explained by a combination of Cenozoic vicariance and palaeoclimatic changes during the Neogene. The austral connection and posterior break‐up of South America, Antarctica and Australia resulted in a pattern of disjunct sister lineages. Increased biome aridification coupled with floristic diversification in the southern continents during the Neogene may have contributed to the high rates of cladogenesis in these bees in the last 25–30 million years.     

Considerations on the age of the Tiouaren Formation (Iullemmeden Basin,Niger, Africa): Implications for Gondwanan Mesozoic terrestrial vertebrate faunas

Pre-Aptian mid-Mesozoic terrestrial vertebrates from the African continent are still very poorly known. In Niger, the Tiouaren Formation in the Iullemmeden Basin has yielded dinosaur and other vertebrate remains, and this unit has been dated as Early Cretaceous, most probably pre-Aptian, on the basis of its fish fauna and geological relations to other units in the basin. A review of the fish fauna and invertebrates from this formation does not provide any evidence for such an age, and the geological relations only help to constrain the upper limit for the age of the formation (Aptian). In contrast, the described dinosaur taxa are phylogenetically nested with late Middle Jurassic to Early Late Jurassic taxa from other localities, and thus indicate a pre-Kimmeridgian, probably late Middle Jurassic age for the Tiouaren Formation. Under the assumption of such an age, the dinosaur fauna of this formation provides new insights into dinosaur faunal provincialism during the latest Middle Jurassic. Northern Gondwanan faunas of that time seem to have been different from southern Gondwanan faunas, and show closer affinities to Eurasian faunas than to the latter. A possible explanation for this might be a climatically controlled geographic barrier due to pronounced arid conditions and thus desert environments in central Gondwana during this time.