Cynodont therapsids diversified extensively after the Permo-Triassic mass extinction event, and gave rise to mammals in the Jurassic. We use an enlarged and revised dataset of discrete skeletal ...characters to build a new phylogeny for all main cynodont clades from the Late Permian to the Early Jurassic, and we analyse models of morphological diversification in the group. Basal taxa and epicynodonts are paraphyletic relative to eucynodonts, and the latter are divided into cynognathians and probainognathians, with tritylodonts and mammals forming sister groups. Disparity analyses reveal a heterogeneous distribution of cynodonts in a morphospace derived from cladistic characters. Pairwise morphological distances are weakly correlated with phylogenetic distances. Comparisons of disparity by groups and through time are non-significant, especially after the data are rarefied. A disparity peak occurs in the Early/Middle Triassic, after which period the mean disparity fluctuates little. Cynognathians were characterized by high evolutionary rates and high diversity early in their history, whereas probainognathian rates were low. Community structure may have been instrumental in imposing different rates on the two clades.
Metabolic heat production in archosaurs has played an important role in their evolutionary radiation during the Mesozoic, and their ancestral metabolic condition has long been a matter of debate in ...systematics and palaeontology. The study of fossil bone histology provides crucial information on bone growth rate, which has been used to indirectly investigate the evolution of thermometabolism in archosaurs. However, no quantitative estimation of metabolic rate has ever been performed on fossils using bone histological features. Moreover, to date, no inference model has included phylogenetic information in the form of predictive variables. Here we performed statistical predictive modeling using the new method of phylogenetic eigenvector maps on a set of bone histological features for a sample of extant and extinct vertebrates, to estimate metabolic rates of fossil archosauromorphs. This modeling procedure serves as a case study for eigenvectorbased predictive modeling in a phylogenetic context, as well as an investigation of the poorly known evolutionary patterns of metabolic rate in archosaurs. Our results show that Mesozoic theropod dinosaurs exhibit metabolic rates very close to those found in modern birds, that archosaurs share a higher ancestral metabolic rate than that of extant ectotherms, and that this derived high metabolic rate was acquired at a much more inclusive level of the phylogenetic tree, among non-archosaurian archosauromorphs. These results also highlight the difficulties of assigning a given heat production strategy (i.e., endothermy, ectothermy) to an estimated metabolic rate value, and confirm findings of previous studies that the definition of the endotherm/ectotherm dichotomy may be ambiguous.
Sauropod dinosaurs were dominant, bulk-browsing herbivores for 130 million years of the Mesozoic, attaining gigantic body masses in excess of 60 metric tons 1, 2. A columnar-limbed, quadrupedal ...posture enabled these giant body sizes 3, but the nature of the transition from bipedal sauropodomorph ancestors to derived quadrupeds remains contentious 4–6. We describe a gigantic, new sauropodomorph from the earliest Jurassic of South Africa weighing 12 metric tons and representing a phylogenetically independent origin of sauropod-like body size in a non-sauropod. Osteohistological evidence shows that this specimen was an adult of maximum size and approximately 14 years old at death. Ledumahadi mafube gen. et sp. nov. shows that gigantic body sizes were possible in early sauropodomorphs, which were habitual quadrupeds but lacked the derived, columnar limb postures of sauropods. We use data from this new taxon and a discriminant analysis of tetrapod limb measurements to study postural evolution in sauropodomorphs. Our results show that quadrupedality appeared by the mid-Late Triassic (Norian), well outside of Sauropoda. Secondary reversion to bipedality occurred in some lineages phylogenetically close to Sauropoda, indicating early experimentation in locomotory styles. Morphofunctional observations support the hypothesis that partially flexed (rather than columnar) limbs characterized Ledumahadi and other early-branching quadrupedal sauropodomorphs. Patterns of locomotory and body-size evolution show that quadrupedality allowed Triassic sauropodomorphs to achieve body sizes of at least 3.8 metric tons. Ledumahadi’s Early Jurassic age shows that maximum body mass in sauropodomorph dinosaurs was either unaffected or rapidly rebounded after the end-Triassic extinction event.
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•A new species of Early Jurassic South African sauropodomorph weighed 12 metric tons•Proportional limb robusticity is useful for inferring posture in extinct tetrapods•Many early-branching sauropodomorphs were quadrupeds with flexed limbs•Quadrupedality evolves at low body mass but facilitates larger body masses
McPhee et al. present a new, gigantic sauropodomorph dinosaur and a quantitative method for inferring postures in extinct tetrapods using proportional limb robusticity. Quadrupedal postures with flexed limbs potentially evolved several times in sauropodomorph dinosaurs before the evolution of derived, columnar-limbed postures of Sauropoda.
The Early Triassic non‐mammaliaform epicynodont Galesaurus planiceps formed an important part of ecosystems following the Permo‐Triassic Mass Extinction, the greatest mass extinction in Phanerozoic ...history. Here, we re‐examine the postcranial skeleton of Galesaurus and present data which sheds light on the biology, ecology and possible survival strategies of this species. We find evidence for two distinct morphotypes, a gracile and a robust morph, which we interpret as stages in an ontogenetic series. The primary differences between the morphs manifest in the girdles, with further subtle differences in the fore and hind limbs. Our study also reveals postcranial differences between Galesaurus and the contemporaneous taxon Thrinaxodon liorhinus, allowing these taxa to be distinguished in the absence of cranial material. We also report the first evidence of intraspecific variation in the presence and distribution of disc‐like phalanges in a non‐mammaliaform cynodont. An analysis of the osteohistology of Galesaurus reveals rapid growth to skeletal maturity within one year, thereafter transitioning to slow intermittent growth. This growth pattern is similar to that of Thrinaxodon, which also grew rapidly and continuously to skeletal, and possibly reproductive, maturity within its first year of life. Features such as a strong, reinforced pelvis, elongated ilium, thick, robust forelimbs and stout unguals indicate that Galesaurus was capable of actively excavating burrows. The combination of rapid maturation and fossoriality may have aided its survival in the harsh, unpredictable post‐extinction Early Triassic environment.
Lystrosaurus is iconic for surviving the Permo-Triassic Mass Extinction and becoming the most abundant terrestrial vertebrate during the Early Triassic. Previous reports of skeletal remains of the ...Triassic species being found in fossilized burrows hint at a possible reason for its success, but unequivocal evidence showing that Lystrosaurus individuals were the burrow makers was lacking. I present here the first articulated skeleton of Lystrosaurus in a fossilized burrow from the Lower Triassic of the South African Karoo Basin, along with taphonomic evidence indicating that this individual was the burrow maker. The species is identified as L. curvatus, the only Lystrosaurus species recovered from above and below the inferred Permo-Triassic extinction horizon. It provides the first evidence of burrowing in a Permian species of Lystrosaurus, suggesting that this behavior was more prevalent than previously thought. Based on its size, the specimen is inferred to be a juvenile, showing that Lystrosaurus was capable of excavating burrows at young ontogenetic stages. The abundance of Lystrosaurus body fossils and similar-sized burrows from Lower Triassic strata suggests that burrowing played a pivotal role in the success of this genus in harsh, unpredictable postextinction conditions. Given the abundance of these burrows throughout the Lower Triassic Lystrosaurus Assemblage Zone, Lystrosaurus may have acted as an ecosystem engineer and refuge provider for other species, which may help to explain the high species diversity in the lowermost Triassic in the Karoo Basin.
Studies of the effects of mass extinctions on ancient ecosystems have focused on changes in taxic diversity, morphological disparity, abundance, behaviour and resource availability as key ...determinants of group survival. Crucially, the contribution of life history traits to survival during terrestrial mass extinctions has not been investigated, despite the critical role of such traits for population viability. We use bone microstructure and body size data to investigate the palaeoecological implications of changes in life history strategies in the therapsid forerunners of mammals before and after the Permo-Triassic Mass Extinction (PTME), the most catastrophic crisis in Phanerozoic history. Our results are consistent with truncated development, shortened life expectancies, elevated mortality rates and higher extinction risks amongst post-extinction species. Various simulations of ecological dynamics indicate that an earlier onset of reproduction leading to shortened generation times could explain the persistence of therapsids in the unpredictable, resource-limited Early Triassic environments, and help explain observed body size distributions of some disaster taxa (e.g., Lystrosaurus). Our study accounts for differential survival in mammal ancestors after the PTME and provides a methodological framework for quantifying survival strategies in other vertebrates during major biotic crises.
Bone microstructure has long been known as a powerful tool to investigate lifestyle-related biomechanical constraints, and many studies have focused on identifying such constraints in the limb bones ...of aquatic or arboreal mammals in recent years. The limb bone microstructure of fossorial mammals, however, has not been extensively described. Furthermore, so far, studies on this subject have always focused on the bone histology of small burrowers, such as subterranean rodents or true moles. Physiological constraints associated with digging, however, are known to be strongly influenced by body size, and larger burrowers are likely to exhibit a histological profile more conspicuously influenced by fossorial activity. Here, we describe for the first time the limb bone histology of the aardvark (
), the largest extant burrowing mammal. The general pattern is very similar for all six sampled limb bones (i.e., humerus, radius, ulna, femur, tibia, and fibula). Most of the cortex at midshaft is comprised of compacted coarse cancellous bone (CCCB), an endosteal tissue formed in the metaphyses through the compaction of bony trabeculae. Conversely, the periosteal bone is highly resorbed in all sections, and is reduced to a thin outer layer, suggesting a pattern of strong cortical drift. This pattern contrasts with that of most large mammals, in which cortical bone is of mostly periosteal origin, and CCCB, being a very compliant bone tissue type, is usually resorbed or remodeled during ontogeny. The link between histology and muscle attachment sites, as well as the influence of the semi-arid environment and ant-eating habits of the aardvark on its bone microstructure, are discussed. We hypothesize that the unusual histological profile of the aardvark is likely the outcome of physiological constraints due to both extensive digging behavior and strong metabolic restrictions. Adaptations to fossoriality are thus the result of a physiological compromise between limited food availability, an environment with high temperature variability, and the need for biomechanical resistance during digging. These results highlight the difficulties of deciphering all factors potentially involved in bone formation in fossorial mammals. Even though the formation and maintaining of CCCB through ontogeny in the aardvark cannot be unambiguously linked with its fossorial habits, a high amount of CCCB has been observed in the limb bones of other large burrowing mammals. The inclusion of such large burrowers in future histological studies is thus likely to improve our understanding of the functional link between bone growth and fossorial lifestyle in an evolutionary context.
Therocephalians were a speciose clade of nonmammalian therapsids whose ecological diversity and survivorship of the end-Permian mass extinction offer the potential to investigate the evolution of ...growth patterns across the clade and their underlying influences on post-extinction body size reductions, or 'Lilliput effects'. We present a phylogenetic survey of limb bone histology and growth patterns in therocephalians from the Middle Permian through Middle Triassic of the Karoo Basin, South Africa. Histologic sections were prepared from 80 limb bones representing 11 genera of therocephalians. Histologic indicators of skeletal growth, including cortical vascularity (%CV) and mean primary osteon diameters (POD), were evaluated in a phylogenetic framework and assessed for correlations with other biologically significant variables (e.g., size and robusticity). Changes in %CV and POD correlated strongly with evolutionary changes in body size (i.e., smaller-bodied descendants tended to have lower %CV than their larger-bodied ancestors across the tree). Bone wall thickness tended to be high in early therocephalians and lower in the gracile-limbed baurioids, but showed no general correlation with cross-sectional area or degree of vascularity (and, thus, growth). Clade-level patterns, however, deviated from previously studied within-lineage patterns. For example, Moschorhinus, one of few therapsid genera to have survived the extinction boundary, demonstrated higher %CV in the Triassic than in the Permian despite its smaller size in the extinction aftermath. Results support a synergistic model of size reductions for Triassic therocephalians, influenced both by within-lineage heterochronic shifts in survivor taxa (as reported in Moschorhinus and the dicynodont Lystrosaurus) and phylogenetically inferred survival of small-bodied taxa that had evolved short growth durations (e.g., baurioids). These findings mirror the multi-causal Lilliput patterns described in marine faunas, but contrast with skeletochronologic studies that suggest slow, prolonged shell secretion over several years in marine benthos. Applications of phylogenetic comparative methods to new histologic data will continue to improve our understanding of the evolutionary dynamics of growth and body size shifts during mass extinctions and recoveries.
A new species of the erythrosuchid archosauriform reptile Garjainia Ochev, 1958 is described on the basis of disarticulated but abundant and well-preserved cranial and postcranial material from the ...late Early Triassic (late Olenekian) Subzone A of the Cynognathus Assemblage Zone of the Burgersdorp Formation (Beaufort Group) of the Karoo Basin of South Africa. The new species, G. madiba, differs from its unique congener, G. prima from the late Olenekian of European Russia, most notably in having large bony bosses on the lateral surfaces of the jugals and postorbitals. The new species also has more teeth and a proportionately longer postacetabular process of the ilium than G. prima. Analysis of G. madiba bone histology reveals thick compact cortices comprised of highly vascularized, rapidly forming fibro-lamellar bone tissue, similar to Erythrosuchus africanus from Subzone B of the Cynognathus Assemblage Zone. The most notable differences between the two taxa are the predominance of a radiating vascular network and presence of annuli in the limb bones of G. madiba. These features indicate rapid growth rates, consistent with data for many other Triassic archosauriforms, but also a high degree of developmental plasticity as growth remained flexible. The diagnoses of Garjainia and of Erythrosuchidae are addressed and revised. Garjainia madiba is the geologically oldest erythrosuchid known from the Southern Hemisphere, and demonstrates that erythrosuchids achieved a cosmopolitan biogeographical distribution by the end of the Early Triassic, within five million years of the end-Permian mass extinction event. It provides new insights into the diversity of the Subzone A vertebrate assemblage, which partially fills a major gap between classic 'faunal' assemblages from the older Lystrosaurus Assemblage Zone (earliest Triassic) and the younger Subzone B of the Cynognathus Assemblage Zone (early Middle Triassic).
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