Roger M.H. Smith

The skull roof tracks the brain during the evolution and development of reptiles including birds

Major transformations in brain size and proportions, such as the enlargement of the brain during the evolution of birds, are accompanied by profound modifications to the skull roof. However, the hypothesis of concerted evolution of shape between brain and skull roof over major phylogenetic transitions, and in particular of an ontogenetic relationship between specific regions of the brain and the skull roof, has never been formally tested. We performed 3D morphometric analyses to examine the deep history of brain and skull-roof morphology in Reptilia, focusing on changes during the well-documented transition from early reptiles through archosauromorphs, including nonavian dinosaurs, to birds. Non-avialan taxa cluster tightly together in morphospace, whereas Archaeopteryx and crown birds occupy a separate region. There is a one-to-one correspondence between the forebrain and frontal bone and the midbrain and parietal bone. Furthermore, the position of the forebrain–midbrain boundary correlates significantly with the position of the frontoparietal suture across the phylogenetic breadth of Reptilia and during the ontogeny of individual taxa. Conservation of position and identity in the skull roof is apparent, and there is no support for previous hypotheses that the avian parietal is a transformed postparietal. The correlation and apparent developmental link between regions of the brain and bony skull elements are likely to be ancestral to Tetrapoda and may be fundamental to all of Osteichthyes, coeval with the origin of the dermatocranium.Brain and skull development are intimately related across tetrapods. Here, the authors show a close relationship between brain and skull roof across evolutionary transitions and ontogenetic stages of reptiles.

New information on the braincase and inner ear of Euparkeria capensis Broom: implications for diapsid and archosaur evolution

Since its discovery, Euparkeria capensis has been a key taxon for understanding the early evolution of archosaurs. The braincase of Euparkeria was described based on a single specimen, but much uncertainty remained. For the first time, all available braincase material of Euparkeria is re-examined using micro-computed tomography scanning. Contrary to previous work, the parabasisphenoid does not form the posterior border of the fenestra ovalis in lateral view, but it does bear a dorsal projection that forms the anteroventral half of the fenestra. No bone pneumatization was found, but the lateral depression of the parabasisphenoid may have been pneumatic. We propose that the lateral depression likely corresponds to the anterior tympanic recess present in crown archosaurs. The presence of a laterosphenoid is confirmed for Euparkeria. It largely conforms to the crocodilian condition, but shows some features which make it more similar to the avemetatarsalian laterosphenoid. The cochlea of Euparkeria is elongated, forming a deep cochlear recess. In comparison with other basal archosauromorphs, the metotic foramen is much enlarged and regionalized into vagus and recessus scalae tympani areas, indicating an increase in its pressure-relief mechanism. The anterior semicircular canal is extended and corresponds to an enlarged floccular fossa. These aspects of the braincase morphology may be related to the development of a more upright posture and active lifestyle. They also indicate further adaptations of the hearing system of Euparkeria to terrestriality.

Fossorial Origin of the Turtle Shell

The turtle shell is a complex structure that currently serves a largely protective function in this iconically slow-moving group [1]. Developmental [2, 3] and fossil [4-7] data indicate that one of the first steps toward the shelled body plan was broadening of the ribs (approximately 50 my before the completed shell [5]). Broadened ribs alone provide little protection [8] and confer significant locomotory [9, 10] and respiratory [9, 11] costs. They increase thoracic rigidity [8], which decreases speed of locomotion due to shortened stride length [10], and they inhibit effective costal ventilation [9, 11]. New fossil material of the oldest hypothesized stem turtle, Eunotosaurus africanus [12] (260 mya) [13, 14] from the Karoo Basin of South Africa, indicates the initiation of rib broadening was an adaptive response to fossoriality. Similar to extant fossorial taxa [8], the broad ribs of Eunotosaurus provide an intrinsically stable base on which to operate a powerful forelimb digging mechanism. Numerous fossorial correlates [15-17] are expressed throughout Eunotosaurus skeleton. Most of these features are widely distributed along the turtle stem and into the crown clade, indicating the common ancestor of Eunotosaurus and modern turtles possessed a body plan significantly influenced by digging. The adaptations related to fossoriality likely facilitated movement of stem turtles into aquatic environments early in the groups evolutionary history, and this ecology may have played an important role in stem turtlesxa0surviving the Permian/Triassic extinction event.

Oxygen isotopes suggest elevated thermometabolism within multiple permo-triassic therapsid clades

The only true living endothermic vertebrates are birds and mammals, which produce and regulate their internal temperature quite independently from their surroundings. For mammal ancestors, anatomical clues suggest that endothermy originated during the Permian or Triassic. Here we investigate the origin of mammalian thermoregulation by analysing apatite stable oxygen isotope compositions (δ18Op) of some of their Permo-Triassic therapsid relatives. Comparing of the δ18Op values of therapsid bone and tooth apatites to those of co-existing non-therapsid tetrapods, demonstrates different body temperatures and thermoregulatory strategies. It is proposed that cynodonts and dicynodonts independently acquired constant elevated thermometabolism, respectively within the Eucynodontia and Lystrosauridae + Kannemeyeriiformes clades. We conclude that mammalian endothermy originated in the Epicynodontia during the middle-late Permian. Major global climatic and environmental fluctuations were the most likely selective pressures on the success of such elevated thermometabolism. DOI: http://dx.doi.org/10.7554/eLife.28589.001

New whaitsioids (Therapsida: Therocephalia) from the Teekloof Formation of South Africa and therocephalian diversity during the end-Guadalupian extinction

Two new species of therocephalian therapsids are described from the upper Permian Teekloof Formation of the Karoo Basin, South Africa. They include two specimens of a whaitsiid, Microwhaitsia mendrezi gen. et sp. nov., and a single, small whaitsioid Ophidostoma tatarinovi gen. et sp. nov., which preserves a combination of primitive and apomorphic features. A phylogenetic analysis of 56 therapsid taxa and 136 craniodental and postcranial characters places the new taxa within the monophyletic sister group of baurioids—Whaitsioidea—with Microwhaitsia as a basal whaitsiid and Ophidostoma as an aberrant whaitsioid just outside the hofmeyriid+whaitsiid subclade. The new records support that whaitsioids were diverse during the early-late Permian (Wuchiapingian) and that the dichotomy between whaitsiid-line and baurioid-line eutherocephalians was established early on. The oldest Gondwanan whaitsiid Microwhaitsia and additional records from the lower strata of the Teekloof Formation suggest that whaitsioids had diversified by the early Wuchiapingian and no later than Pristerognathus Assemblage Zone times. Prior extinction estimates based on species counts are reflected in an analysis of origination/extinction rates, which imply increasing faunal turnover from Guadalupian to Lopingian (late Permian) times. The new records support a growing body of evidence that some key Lopingian synapsid clades originated near or prior to the Guadalupian-Lopingian boundary ca. 260–259 million years ago, but only radiated following the end-Guadalupian extinction of dinocephalians and basal therocephalian predators (long-fuse model). Ongoing collecting in older portions of the Teekloof Formation (e.g., Pristerognathus Assemblage Zone) will shed further light on early eutherocephalians during this murky but critical time in their evolutionary diversification.

An early geikiid dicynodont from the Tropidostoma Assemblage Zone (late Permian) of South Africa

Based on specimens previously identified as Tropidostoma, a new taxon of dicynodont (Bulbasaurus phylloxyron gen. et sp. nov.) from the Karoo Basin of South Africa is described. Bulbasaurus is a medium-sized dicynodont (maximum dorsal skull length 16.0 cm) restricted to the Tropidostoma Assemblage Zone (early Lopingian) of the Beaufort Group. Bulbasaurus can be distinguished from Tropidostoma by an array of characters including the presence of a tall, sharp premaxillary ridge, large, rugose, nearly-confluent nasal bosses, a nasofrontal ridge, massive tusks, robust pterygoids, prominently twisted subtemporal bar, and absence of a distinct postfrontal. Inclusion of Bulbasaurus in a phylogenetic analysis of anomodont therapsids recovers it as a member of Geikiidae, a clade of otherwise later Permian dicynodonts such as Aulacephalodon and Pelanomodon. Bulbasaurus exhibits many of the characters typical of adult Aulacephalodon, but at substantially smaller skull size (these characters are absent in comparably-sized Aulacephalodon juveniles), suggesting that the evolution of typical geikiid morphology preceded gigantism in the clade. Bulbasaurus is the earliest known geikiid and the only member of the group known from the Tropidostoma Assemblage Zone; discovery of this taxon shortens a perplexing ghost lineage and indicates that abundant clades from the later Permian of South Africa (e.g., Geikiidae, Dicynodontoidea) may have originated as rare components of earlier Karoo assemblage zones.

Taphonomy and Paleoenvironments of Middle Triassic Bone Accumulations in the Lifua Member of the Manda Beds, Songea Group (Ruhuhu Basin), Tanzania

ABSTRACT We report new data on the climate, paleoenvironments, and burial history of tetrapod fossils in the Middle Triassic Lifua Member of the Manda Beds (Songea Group) of southern Tanzania. Two bone-bearing intervals have been identified, both hosted by rubified floodplain mudrocks deposited alongside rivers that flowed from the Ruhuhu rift scarps into a series of subsiding basins under a warm, seasonally wet climate. The lower occurrence is a bonebed containing fossils of a large dicynodont (Dolichuranus), large cynodonts (Cynognathus), temnospondyls, small reptiles, and at least two archosauromorph reptiles. A chaotic melange of semiarticulated, disarticulated, and reworked bones associated with pedogenically mottled sandy siltstone is interpreted as having accumulated in a distal crevasse splay complex. The middle to upper Lifua bone accumulations are associated with floodplain pond and sheetwash deposits. Outcropping as isolated patches of strongly calcified rubified mudstones with lenses of reworked glaebule conglomerate, these accumulations contain partially articulated archosaur (Asilisaurus, Nundasuchus) and cynodont (Scalenodon) skeletons along with vertebrate coprolites and nonmarine bivalves (‘Unio’). Changes in floodplain facies, faunal assemblage, and taphonomic style between lower and middle to upper Lifua strata are similar to those recorded between the middle and upper Burgersdorp Formation (subzones B to C of the Cynognathus Assemblage Zone) of the main Karoo Basin of South Africa. We propose that an increase in mean annual temperature and rainfall in southwestern Gondwana during Early to Middle Triassic times resulted in vegetated, semipermanent water bodies in the floodplain depressions that supported a relatively diverse assemblage of herbivorous dicynodont, cynodont, and early archosaur populations.

Updated Geology and Vertebrate Paleontology of the Triassic Ntawere Formation of Northeastern Zambia, with Special Emphasis on the Archosauromorphs

ABSTRACT The two vertebrate fossil assemblages from the ?Middle Triassic Ntawere Formation have been known since the 1960s, but little new work has been done since the description of novel taxa in the 1960s and 1970s. Three recent field seasons have increased vertebrate diversity in the upper Ntawere assemblage and expanded biostratigraphic connections between the lower and upper Ntawere assemblages and assemblages in fossiliferous basins across southern Pangea. The upper Ntawere contains hybodontoid sharks, ptychoceratodontid lungfish, large- and small-bodied stereospondyl amphibians (Cherninia, ‘Stanocephalosaurus’ Batrachosuchus, a new taxon), stahleckeriid dicynodonts (Sangusaurus, Zambiasaurus), traversodontid and trirachodontid cynodonts (Luangwa, a new species, Cricodon), and at least four archosauromorphs, including a large loricatan pseudosuchian, a shuvosaurid poposauroid, and silesaurid dinosauriforms (Lutungutali), whereas the lower Ntawere contains the cynodonts Cynognathus and Diademodon and species of the dicynodont Kannemeyeria. The lower and upper Ntawere assemblages have been correlated with the middle and upper subzones of the Cynognathus Assemblage Zone of the Karoo Basin, South Africa, into a network of connections between assemblages in modern day Tanzania, Argentina, Brazil, Namibia, Antarctica, and India. Although lower Ntawere correlations are reinforced by the occurrence of Cynognathus, new observations from the upper Ntawere, in combination with field work in Tanzania, Namibia, and Brazil, have shifted the geographic focus of biostratigraphic connection away from the Karoo later in the Triassic. A recent radiometric date from Argentina from below the horizon correlated with both the Karoo and the lower Ntawere places these, and all higher assemblages, into the Carnian Stage of the Late Triassic.

Comparative ecological dynamics of Permian-Triassic communities from the Karoo, Luangwa, and Ruhuhu basins of southern Africa

ABSTRACT The Permian-Triassic mass extinction (PTME) was one of the transformative events of the Phanerozoic, marked by extinction, post-Permian transformation of surviving ecosystems, and the formation of new communities. The South African Karoo Basin has served as the primary source of data on the terrestrial component of these events, but its global applicability remains poorly known. Here, we compare Permian-Triassic communities of the Karoo Basin with those from the Luangwa and Ruhuhu basins of Zambia and Tanzania, respectively, analyzing their functional structures and simulating dynamic responses to environmental perturbation. Results show that compositional similarities of late Permian communities among the basins underlie similarities in their dynamics and resistance to secondary extinction. The Karoo Basin ecosystem also displays evidence of a transformation to increased resistance during the late Permian. Although the Karoo Basin ecosystem was reduced significantly by the PTME, structural features of that resistance persisted into the Early Triassic, facilitated by a greater susceptibility to extinction of small-body-sized amniotes and large carnivorous amniotes. It was undone by the initial stages of postextinction restructuring. Continued evolution of the Triassic ecosystem led to a recovery of resistance, but in a community compositionally dissimilar from its Permian antecedents. Likewise, the upper part of the Lifua Member of the Manda Beds (Middle Triassic) of Tanzania was structurally distinct from the Karoo Basin communities but displayed similar dynamics. The recurrence and convergence of communities with different histories toward similar dynamics suggest that there are taxon-independent norms of community assembly and function operating on geological timescales.

Paleosols of the Permian-Triassic: proxies for rainfall, climate change and major changes in terrestrial tetrapod diversity

ABSTRACT Stable carbon isotope analysis of coexisting soil calcite and organic matter sampled from modern, California soil profiles representing 18 different U.S. Department of Agriculture (USDA) official soil series yields 51 paired calcite–organic matter δ13C values (Δ13Ccc-om values). These paired values correspond to atmospheric pCO2 estimates ranging from less than ~100 to 2200 ppmv using standard assumed soil pCO2 concentrations at temperatures spanning the typical range of modern soil calcite crystallization. Nevertheless, there is a strong negative correlation of Δ13Ccc-om values with mean annual precipitation (MAP) among these modern sites, offering a potential means to estimate paleo-MAP values from calcite and associated, occluded organic matter in paleosols. One hundred and twenty-five Δ13Ccc-om values from lower Permian–lower Middle Triassic paleosol profiles in the southwestern U.S.A., northwestern China, and Africa are presented as a proxy for paleorainfall over ~85° of paleolatitude. The results indicate substantial spatiotemporal variation in MAP and appear to reflect the development and strengthening of global megamonsoonal atmospheric circulation associated with assembly of the supercontinent Pangea. The data also indicate extremely low rainfall values in regions with endemic tetrapod fossil assemblages, suggesting that climate change was a principal driver of biogeography, biodiversity, and evolution of tetrapod faunas across continental landscapes during Permo-Triassic time.