Comparative osteohistology of hyperelongate neural spines in basal synapsids (Vertebrata, Amniota): Growth and mechanical considerations

Basal synapsids comprise one of the best represented groups of early tetrapods and demonstrate a classic example of a macroevolutionary transition. The basal synapsid fossil record is of significant interest to paleontolists because it presents an excellent opportunity for studying the evolution of vertebrate morphology. In the present study, neural spine osteohistology is further examined across a broad range of basal synapsid genera with hyperlongate neural spines. Due to the limited availability of specimens and the comparative approach required for destructive analysis, the present study is divided into two major comparative investigations: analysis of hyperlongate neural spine in (1) Spenacodontidae and (2) Edaphosauridae

Bone microstructure and the evolution of growth patterns in Permo-Triassic therocephalians (Amniota, Therapsida) of South Africa

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 Carboniferous synapsid with caniniform teeth and a reappraisal of mandibular size-shape heterodonty in the origin of mammals.

Heterodonty is a hallmark of early mammal evolution that originated among the non-mammalian therapsids by the Middle Permian. Nonetheless, the early evolution of heterodonty in basal synapsids is poorly understood, especially in the mandibular dentition. Here, we describe a new synapsid, Shashajaia bermani gen. et sp. nov., based on a well-preserved dentary and jaw fragments from the Carboniferous–Permian Halgaito Formation of southern Utah. Shashajaia shares with some sphenacodontids enlarged (canine-like) anterior dentary teeth, a dorsoventrally deep symphysis and low-crowned, subthecodont postcanines having festooned plicidentine. A phylogenetic analysis of 20 taxa and 154 characters places Shashajaia near the evolutionary divergence of Sphenacodontidae and Therapsida (Sphenacodontoidea). To investigate the ecomorphological context of Palaeozoic sphenacodontoid dentitions, we performed a principal component analysis based on two-dimensional geometric morphometrics of the mandibular dentition in 65 synapsids. Results emphasize the increasing terrestrialization of predator–prey interactions as a driver of synapsid heterodonty; enhanced raptorial biting (puncture/gripping) aided prey capture, but this behaviour was probably an evolutionary antecedent to more complex processing (shearing/tearing) of larger herbivore prey by the late Early to Middle Permian. The record of Shashajaia supports the notion that the predatory feeding ecology of sphenacodontoids emerged in palaeotropical western Pangea by late Carboniferous times

Report on ICDP Deep Dust workshops: probing continental climate of the late Paleozoic icehouse–greenhouse transition and beyond

Chamberlin and Salisbury's assessment of the Permian a century ago captured the essence of the period: it is an interval of extremes yet one sufficiently recent to have affected a biosphere with near-modern complexity. The events of the Permian - the orogenic episodes, massive biospheric turnovers, both icehouse and greenhouse antitheses, and Mars-analog lithofacies - boggle the imagination and present us with great opportunities to explore Earth system behavior. The ICDP-funded workshops dubbed "Deep Dust," held in Oklahoma (USA) in March 2019 (67 participants from nine countries) and Paris (France) in January 2020 (33 participants from eight countries), focused on clarifying the scientific drivers and key sites for coring continuous sections of Permian continental (loess, lacustrine, and associated) strata that preserve high-resolution records. Combined, the two workshops hosted a total of 91 participants representing 14 countries, with broad expertise. Discussions at Deep Dust 1.0 (USA) focused on the primary research questions of paleoclimate, paleoenvironments, and paleoecology of icehouse collapse and the run-up to the Great Dying and both the modern and Permian deep microbial biosphere. Auxiliary science topics included tectonics, induced seismicity, geothermal energy, and planetary science. Deep Dust 1.0 also addressed site selection as well as scientific approaches, logistical challenges, and broader impacts and included a mid-workshop field trip to view the Permian of Oklahoma. Deep Dust 2.0 focused specifically on honing the European target. The Anadarko Basin (Oklahoma) and Paris Basin (France) represent the most promising initial targets to capture complete or near-complete stratigraphic coverage through continental successions that serve as reference points for western and eastern equatorial Pangaea. © 2020 Copernicus GmbH. All rights reserved

Report on ICDP Deep Dust workshops: probing continental climate of the late Paleozoic icehouse–greenhouse transition and beyond

Chamberlin and Salisbury's assessment of the Permian a century ago captured the essence of the period: it is an interval of extremes yet one sufficiently recent to have affected a biosphere with near-modern complexity. The events of the Permian - the orogenic episodes, massive biospheric turnovers, both icehouse and greenhouse antitheses, and Mars-analog lithofacies - boggle the imagination and present us with great opportunities to explore Earth system behavior. The ICDP-funded workshops dubbed "Deep Dust," held in Oklahoma (USA) in March 2019 (67 participants from nine countries) and Paris (France) in January 2020 (33 participants from eight countries), focused on clarifying the scientific drivers and key sites for coring continuous sections of Permian continental (loess, lacustrine, and associated) strata that preserve high-resolution records. Combined, the two workshops hosted a total of 91 participants representing 14 countries, with broad expertise. Discussions at Deep Dust 1.0 (USA) focused on the primary research questions of paleoclimate, paleoenvironments, and paleoecology of icehouse collapse and the run-up to the Great Dying and both the modern and Permian deep microbial biosphere. Auxiliary science topics included tectonics, induced seismicity, geothermal energy, and planetary science. Deep Dust 1.0 also addressed site selection as well as scientific approaches, logistical challenges, and broader impacts and included a mid-workshop field trip to view the Permian of Oklahoma. Deep Dust 2.0 focused specifically on honing the European target. The Anadarko Basin (Oklahoma) and Paris Basin (France) represent the most promising initial targets to capture complete or near-complete stratigraphic coverage through continental successions that serve as reference points for western and eastern equatorial Pangaea.This research has been supported by the ICDP (DeepDust2019 grant).Ye

The Paleobiology of South African Therocephalian Therapsids (Amniota, Synapsida) and the Effects of the End-Permian Extinction on Size, Growth, and Bone Microstructure

Thesis (Ph.D.)--University of Washington, 2013Despite their relative diversity in terrestrial Permian and Triassic rocks, the fossil record of therocephalian therapsids (Eutheriodontia) and their utility for understanding evolutionary patterns in the therapsid forerunners of mammals remains poorly understood. In this study, I investigate the extent to which body size reductions and shifts in growth patterns in Triassic therocephalians were influenced by the end-Permian mass extinction (ca. 252.3 Ma) (rather than the culmination of longer-term phylogenetic trends traceable to their Permian predecessors). Specifically, I examine whether body size reductions observed in earliest Triassic therocephalians (`Lilliput phenomena') were the product of within-lineage size reductions, differential extinctions, or rapid diversifications of new small-bodied clades. To address this question, I first review the global diversity and taxonomic composition of therocephalians from the Middle Permian through early-Middle Triassic (Chapter 1). I then present a paleobiological investigation of the Permo-Triassic therocephalian Moschorhinus as a case study on within-lineage patterns of growth and body size evolution during the end-Permian mass extinction (Chapter 2). Finally, I examine clade-wide (among-taxon) size patterns by evaluating the stratigraphic and phylogenetic components of body size evolution (Chapter 3) and the underlying influences of bone histology and growth (Chapter 4). To examine within-lineage patterns, I studied cranial sizes and limb bone histology in Permian and Triassic specimens of Moschorhinus, the largest therapsid predator found both before and after the end-Permian mass extinction. Triassic specimens were found to have significantly decreased basal skull lengths compared to Permian specimens. Histological analysis indicated that variations in body size were associated with differences in subadult growth rate and duration (traits that are highly variable in environmentally stressed extant reptile species). Small Triassic individuals tended to display limb bones with fewer growth marks and more richly vascularized bone tissues than similarly sized Permian individuals, with an abundance of radially-oriented vascular canals, corroborating the hypothesis that conditions of the earliest Triassic favored rapid growth to a minimum body size requirement in Moschorhinus and, consequently, shortened developmental times. Broader-scale `Lilliput-type' patterns were examined in a large sample of therocephalians and compared with that of their sister clade, Cynodontia, in both geologic and phylogenetic contexts. Using a museum collections-based approach, I evaluated temporal and phylogenetic distributions of body size in Permo-Triassic eutheriodonts by time series analysis, rank order correlations, and phylogenetic model fitting. Results supported significant size reductions in earliest Triassic eutheriodonts, but suggested a pattern that was underscored largely by Brownian processes and constructive selectivity (a more general tendency to evolve smaller body sizes as in background intervals). Geologically brief size reductions were likely accomplished by the ecological removal of large-bodied species without rapid originations of new small-bodied clades or shifts from long-term evolutionary patterns. Finally, a survey of growth patterns and histomorphology in limb bones of Karoo therocephalians indicated that long-term changes in bone tissue vascularization (and thus growth) correlated with evolutionary changes in body size (e.g., smaller-bodied descendants tended to have less vascularized bone tissues than their larger-bodied ancestors). Results support a synergistic model of size reductions for Triassic therocephalians, influenced both by within-lineage heterochronic shifts in survivor taxa (e.g., Moschorhinus) and cladistically inferred survival of small-bodied taxa with 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 slowed, prolonged shell secretion in marine benthos. Subjecting new histologic data to phylogenetic comparative methods, as in these therocephalians, will improve our understanding of the generality of growth and size shifts in Lilliput faunas and interplay between macroevolution and extinction during this and other major geologic transitions

Results of rank correlations, Spearman's rho (<i>ρ</i>) and Kendall's tau (<i>τ</i>), for global age ranks.

<p>Results of rank correlations, Spearman's rho (<i>ρ</i>) and Kendall's tau (<i>τ</i>), for global age ranks.</p

Phylogenetic model fitting results.

<p>Model selection based on comparison of corrected AICs and Akaike weights (AW) for Brownian motion (BM), Brownian motion with trend (BM_T), Ornstein-Uhlenbeck (OU), and early burst (EB) models;</p><p>AICc, Akaike's Information Criterion corrected for small sample size; AW, Akaike weight;</p><p><i>β</i>, BM rate parameter; <i>μ</i>, step mean; <i>θ</i>, trait mean; <i>α</i>, constraint parameter; <i>r</i>, decay rate;</p><p>bold = best model; italics = non-negligible model (AW ≥1/8 best model).</p

Body Size Reductions in Nonmammalian Eutheriodont Therapsids (Synapsida) during the End-Permian Mass Extinction

<div><p>The extent to which mass extinctions influence body size evolution in major tetrapod clades is inadequately understood. For example, the ‘Lilliput effect,’ a common feature of mass extinctions, describes a temporary decrease in body sizes of survivor taxa in post-extinction faunas. However, its signature on existing patterns of body size evolution in tetrapods and the persistence of its impacts during post-extinction recoveries are virtually unknown, and rarely compared in both geologic and phylogenetic contexts. Here, I evaluate temporal and phylogenetic distributions of body size in Permo-Triassic therocephalian and cynodont therapsids (eutheriodonts) using a museum collections-based approach and time series model fitting on a regional stratigraphic sequence from the Karoo Basin, South Africa. I further employed rank order correlation tests on global age and clade rank data from an expanded phylogenetic dataset, and performed evolutionary model testing using Brownian (passive diffusion) models. Results support significant size reductions in the immediate aftermath of the end-Permian mass extinction (ca. 252.3 Ma) consistent with some definitions of Lilliput effects. However, this temporal succession reflects a pattern that was underscored largely by Brownian processes and constructive selectivity. Results also support two recent contentions about body size evolution and mass extinctions: 1) active, directional evolution in size traits is rare over macroevolutionary time scales and 2) geologically brief size reductions may be accomplished by the ecological removal of large-bodied species without rapid originations of new small-bodied clades or shifts from long-term evolutionary patterns.</p></div