Adam K. Huttenlocker

Provincialization of terrestrial faunas following the end-Permian mass extinction

In addition to their devastating effects on global biodiversity, mass extinctions have had a long-term influence on the history of life by eliminating dominant lineages that suppressed ecological change. Here, we test whether the end-Permian mass extinction (252.3 Ma) affected the distribution of tetrapod faunas within the southern hemisphere and apply quantitative methods to analyze four components of biogeographic structure: connectedness, clustering, range size, and endemism. For all four components, we detected increased provincialism between our Permian and Triassic datasets. In southern Pangea, a more homogeneous and broadly distributed fauna in the Late Permian (Wuchiapingian, ∼257 Ma) was replaced by a provincial and biogeographically fragmented fauna by Middle Triassic times (Anisian, ∼242 Ma). Importantly in the Triassic, lower latitude basins in Tanzania and Zambia included dinosaur predecessors and other archosaurs unknown elsewhere. The recognition of heterogeneous tetrapod communities in the Triassic implies that the end-Permian mass extinction afforded ecologically marginalized lineages the ecospace to diversify, and that biotic controls (i.e., evolutionary incumbency) were fundamentally reset. Archosaurs, which began diversifying in the Early Triassic, were likely beneficiaries of this ecological release and remained dominant for much of the later Mesozoic.

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

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.

A New Specimen of Promoschorhynchus (Therapsida: Therocephalia: Akidnognathidae) from the Lower Triassic of South Africa and its Implications for Theriodont Survivorship Across the Permo-Triassic Boundary

ABSTRACT The anatomy of a new subadult specimen of eutherocephalian therapsid, attributed to Promoschorhynchus cf. P. platyrhinus, is described from lowermost Triassic Lystrosaurus Assemblage Zone strata in the main Karoo Basin of South Africa. The specimen preserves information previously unknown in the genus, including details of the posterior region of the skull and intertemporal region, and a partial (though disarticulated) postcranial skeleton. A cladistic analysis of 32 therapsid taxa, including 24 Permo-Triassic therocephalian genera, and 121 craniodental and postcranial characters supports the specimens placement within the Permian akidnognathid genus Promoschorhynchus (making it the youngest documented occurrence of this taxon) within a monophyletic Therocephalia. Inclusion of new postcranial characters strengthens support of the therocephalian clade. The new record of Promoschorhynchus offers insights into the diversity of eutheriodonts across the Permo-Triassic boundary (PTB) in the Karoo Basin. In contrast to cynodonts, therocephalians exhibited decreased rates of cladogenesis across the PTB, with several Triassic lineages having roots in the Late Permian rather than representing earliest Triassic radiations.

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.

Reassessment of the morphology and paleobiology of the therocephalian Tetracynodon darti (Therapsida), and the phylogenetic relationships of Baurioidea

ABSTRACT Several new specimens of the Triassic therocephalian Tetracynodon darti have become available in recent years, allowing substantial corrections and expansions to previous descriptions. We here analyze T. darti in the context of therocephalian relationships and biology, using computed tomographic (CT) scanning to reveal details of the skull. Histological sections, as well as the degree of ossification of individual elements, both suggest that the available specimens are subadults (rather than rapidly growing neonates), and probably yearlings. A maxillary shelf bearing a long suture with the vomer similar to that of Lycideops is present, and T. darti also shares a nasal-lacrimal contact with that taxon. There is no evidence for the presence of maxilloturbinal bones. The postcranial skeleton is slender and similar to that of regisaurids and other small baurioids. The skull bones show clear indentations demarcating the major divisions of the brain, allowing one of the most detailed reconstructions of a non-mammalian therapsid brain published to date. The brain is strikingly plesiomorphic in many features, particularly in the retention of large dorsally positioned optic lobes of the tectum, but generally shows features that are intermediate between gorgonopsians and cynodonts, thus clarifying this stage of the evolution of the synapsid brain. A cladistic analysis of 33 therapsid taxa and 131 morphological characters supports the monophyly of Therocephalia, and there is evidence for a monophyletic Lycideopidae within Baurioidea. Lycideopidae contains species with long snouts, nasal-lacrimal contacts, and incipient secondary palates, including Lycideops, Choerosaurus, T. tenuis, and T. darti.

Body size and growth patterns in the therocephalian Moschorhinus kitchingi (Therapsida: Eutheriodontia) before and after the end-Permian extinction in South Africa

Abstract The continuous fossil record of therocephalian therapsids (Eutheriodontia) across the Permo-Triassic boundary and their differential survivorship of the end-Permian extinction offer an exceptional deep-time perspective on vertebrate life-history evolution during episodes of large-scale ecological perturbation. To examine potential impacts of the extinction on body size evolution (e.g., “Lilliput” effects) and growth patterns, we investigated cranial sizes and limb bone histology in the therocephalian Moschorhinus kitchingi both before and after the end-Permian extinction, facilitated by analysis of thin-sections of 23 limb bones from an ontogenetic sample of ten individuals across the Permo-Triassic boundary. In general, early subadult Moschorhinus displayed propodial cortices with extensive woven- and parallel-fibered bone (PFB) with dense radial and reticular vascularization and a moderately thickened bone wall with few growth marks. The outer cortex of propodials and epipodials showed a transition to PFB and lamellar bone with longitudinally oriented canals in individuals interpreted as late subadults or adults (>80% largest size). Most elements displayed several (3+) growth marks, though growth marks were more faithfully recorded in the epipodials of Permian individuals. Pearson product-moment correlation tests were performed to examine the relationship between size and robusticity on growth proxies (% cortical vascularity, mean primary osteon diameter), but variation in histomorphology could not be explained by size alone. Variation in body size may be affected by differences in juvenile growth rate and duration, which are highly variable in environmentally stressed extant reptile species. Geologic stage was a more consistent predictor of cortical vascularity. We suggest that Permian and Triassic Moschorhinus exhibited differential rates of early skeletal growth, corroborating the hypothesis that increased environmental variability in the earliest Triassic was associated with rapid growth to a minimum body size requirement and, consequently, shortened developmental times.

Vertebrate Paleontology of Nooitgedacht 68: A Lystrosaurus maccaigi-Rich Permo-Triassic Boundary Locality in South Africa

The farm Nooitgedacht 68 in the Bethulie District of the South African Karoo Basin contains strata that record a complete Permo-Triassic boundary sequence providing important new data regarding the end-Permian extinction event in South Africa. Exploratory collecting has yielded at least 14 vertebrate species, making this locality the second richest Permo-Triassic boundary site in South Africa. Furthermore, fossils include 50 specimens of the otherwise rare Late Permian dicynodont Lystrosaurus maccaigi. As a result, Nooitgedacht 68 is the richest L. maccaigi site known. The excellent preservation, high concentration of L. maccaigi, presence of relatively rare dicynodonts such as Dicynodontoides recurvidens and Dinanomodon gilli, and the large size of many of these Permian individuals makes Nooitgedacht 68 a particularly interesting site for studying the dynamics of the end-Permian extinction in South Africa.

Breeding Young as a Survival Strategy during Earth's Greatest Mass Extinction.

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.

Comparative anatomy and osteohistology of hyperelongate neural spines in the sphenacodontids Sphenacodon and Dimetrodon (Amniota: Synapsida)

Osteohistological investigations of hyperelongate vertebral spinous processes (neural spines) are presented to elucidate previously unknown aspects of dorsal sail form and function in two, closely related genera of “sail‐backed” synapsids: Sphenacodon and Dimetrodon. Although recent and classic surveys of bone histology in extinct vertebrates have sampled the genus Dimetrodon, new sectioning of Sphenacodon material allows a comparative analysis of these structures among Sphenacodontidae for the first time. Variability within the histological organization of the neural spine is assessed by examining multiple regions along its length, and implications for soft tissue correlates, growth and mechanics are considered here. Both genera exhibit extensive parallel‐fibered and fibrolamellar bone, in addition to lamellar bone. Several features vary along the length of the spine in each species. Muscle scars and extensive Sharpeys fibers are present at the base of the spine; no scars and fewer fibers are manifested ∼55–60 mm above the zygapophyses in mature individuals. The distal cortex of the spine does not exhibit greater vascularity than the proximal region in either genus. However, both genera manifest distinct vascular grooves of variable size along the distal periosteal surface, some of which become incorporated into the distal cortex. The observed histovariability appears to record the transition from the proximal (epaxial muscle embedded) to the distally protruding portion of the spine. These observations and independent pathological evidence support the existence of a short dorsal crest in Sphenacodon and possibly other basal sphenacodontids. Although the thermoregulatory capacity of such a crest remains uncertain, developmental and mechanical features are readily interpretable and are discussed with respect to the origins and early evolution of the dorsal sail in sphenacodontid synapsids. J.Morphol., 2010.

A new eutherocephalian (Therapsida, Therocephalia) from the upper Permian Madumabisa Mudstone Formation (Luangwa Basin) of Zambia

ABSTRACT A new therocephalian therapsid, Ichibengops munyamadziensis, gen. et sp. nov., is described on the basis of two partial skulls from the upper Permian (Wuchiapingian) upper Madumabisa Mudstone Formation of the Luangwa Basin, Zambia. The specimens offer insights into the diversity of therocephalians in a poorly sampled region, preserving unique maxillary structures, dental morphology that is intermediate between basal therocephalians and eutherocephalians, and a maxillovomerine bridge forming an incipient secondary palate. A phylogenetic analysis of 135 craniodental and postcranial characters from 56 therapsid taxa (including 49 therocephalians) recovered I. munyamadziensis as the sister taxon of the Russian Chthonosaurus, with both taxa resolving near the hofmeyriid C whaitsiid C baurioid clade (either as the sister group to this clade or nested near whaitsiids). Ichibengops shares with Chthonosaurus several features, including a ventral maxillary flange in which the upper postcanines are situated (also in Lycosuchus), anteroposteriorly short suborbital vacuities with strongly scalloped anterior borders, a furrowed or ridged surface texture on the palatal surface of the palatine, and a possible maxillovomerine bridge (although this latter structure is incompletely preserved in Chthonosaurus). The new taxon, along with its proposed relationship to Chthonosaurus, adds to a list of sister-group pairs of Wuchiapingian tetrapods in southern Gondwana and Laurasia, indicating that effective, though largely unknown, dispersal routes persisted in Pangea at least through early late Permian times.