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Session Schedule & Abstracts

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Wednesday 29th June, 2016

Paleontology 2

Room: Salon G   4:30 pm–6:00 pm

Moderator(s): Halliday TJD, Lloyd GT
PAL2-1  4:30 pm  A new important stage in the evolution of the turtle body plan. Sues H-D*, Smithsonian Institution; Schoch RR, Staatliches Museum für Naturkunde Stuttgart
Abstract: The phylogenetic relationships of turtles have been a particularly contentious issue in the study of vertebrate evolution. This is due to conflicting evidence from molecular and anatomical data but also a lack of transitional fossils from the critical time interval. The stem-turtle Odontochelys, from the early Late Triassic (~220 mya) of China, has a partially formed shell with an already fully developed plastron and other turtle-like features in its postcranial skeleton. Unlike Proganochelys, from the Late Triassic (214 mya) of Germany and Thailand, it retains marginal teeth and lacks a complete carapace. A large temporal gap separates Odontochelys from Eunotosaurus, from the late Middle Permian (~260 mya) of South Africa, which has been plausibly hypothesized as the oldest known stem-turtle. The recently discovered Pappochelys, from the late Middle Triassic (~240 mya) of Germany, represents a structural and chronological intermediate between Eunotosaurus and Odontochelys. It shares with the latter two taxa the possession of anteroposteriorly broad trunk ribs that are T-shaped in cross-section and bear sculpturing, elongate dorsal centra, and modified limb girdles. Unlike Odontochelys, Pappochelys has a cuirass of robust paired gastralia but no plastron. It provides new evidence for the hypothesis that the more posterior portion of the plastron formed through serial fusion of gastralia. Except for the modified trunk ribs Pappochelys has no carapace. The cranium of Pappochelys has upper and lower temporal fenestrae as well as a suborbital foramen, supporting diapsid affinities for turtles. The upper and lower jaws bear teeth. Phylogenetic analysis found Pantestudines as the sister-group of Sauropterygia and, in turn, this pair as the sister-taxon of Lepidosauriformes. However, statistical support for this result is not strong, at least in part due to our still inadequate knowledge of basal saurians.

PAL2-2  4:45 pm  Macroevolution of the crocodylomorphs. Stockdale MT*, University of Bristol
Abstract: Extant crocodilians might be considered something of a failure. Since the emergence of the Crocodylomorpha in the Late Triassic their diversity has declined to just 23 species, compared to over 10, 000 species in their sister clade, the birds. The extant crocodilians also show low morphological disparity, with all species being semi-aquatic ambush predators with a similar body plan. This low diversity and long fossil range has led to the crocodilians being described as 'living fossils', but is this justified? The Crocodylomorpha in the fossil record show great morphological diversity, including terrestrial, cursorial, fossorial and marine forms, insectivores, omnivores, herbivores and durophages. In this study we present a new super tree phylogeny of the Crocodylomorpha and evaluate the tempo and mode of crocodylomorph evolution using morphology. We find extreme conservatism of in the evolution of body size in the Crocodylomorpha and a number of its subclades. Rates of evolution are predominantly stable, but punctuated by environmental changes with rate-shifts in the crown-group associated with mass extinctions. Time-series modelling of Crocodylomorph diversity and morphological disparity compared with environmental variables finds evolution to be driven and constrained by temperature and the diversity of competing clades such as dinosaurs. We conclude that the Crocodylomorpha adhere strongly to the punctuated equilibrium and Court-Jester models of evolution. Therefore the limited diversity and disparity of the extant Crocodylomorpha is likely a reflection of their rather narrow range of ecological tolerances when compared to birds, and massively changed global environmental conditions that restrict the potential diversity of the group today.

PAL2-3  5:00 pm  Ornithischian dinosaur ‘cheeks’ are evolutionary epiphenomena of previously undescribed reorganization and enlargement of jaw musculature. Nabavizadeh A*, University of Chicago
Abstract: Numerous jaw muscle reconstructions in ornithischian dinosaurs have influenced over a century of jaw mechanics studies. These reconstructions, although loyal to extant phylogenetic bracketing (EPB), pose concerns of small adductor muscle bodies and caudally displaced insertions relative to mandibular proportions. Also, the presence of a buccal dental emargination bounded by a lateral dentary ridge (LDR) has led to reconstructions of novel buccinator-like ‘cheeks’, a trait requiring unlikely muscle differentiation and fiber reorientation. Here I present a new interpretation of jaw muscle anatomy in ornithischians with in-depth examination of osteological correlates and muscle scars in taxa spanning the entire clade, with reference to EPB. M. adductor mandibulae externus (mAME), a major jaw muscle group, has historically been reconstructed as inserting along the apex and caudal margin of the coronoid process. Although this is likely for deeper layers, the most superficial mAME layer is reconstructed here as a rostrolateral expansion of muscle along the coronoid process and its rostrally extending margin creating the LDR, a trait observed in some extant lepidosaurs with coronoid processes. Laterally flaring jugals in ornithischians, especially ceratopsids, create an opening allowing direct communication between the endocranial adductor chamber and the LDR. MAME exits this opening and fibers insert as a large muscle fan along the LDR extending farther rostrally than previously proposed. This new mAME reconstruction rejects a novel buccinator-like muscle and repurposes mAME as a major jaw adductor, cradling the jaw bilaterally, as well as a secondarily functional ‘cheek’ containing food within the oral cavity. Along with reorientation of pterygoideus musculature accommodating for orthal components, this new mAME reconstruction holds new and important implications for much stronger orthopalinal feeding strokes and long-axis dentary rotation in ornithischian jaw mechanics.

PAL2-4  5:15 pm  Distribution of purported cursorial adaptations in Mesozoic theropod dinosaurs through phylogeny, time, and space. Holtz TR*, University of Maryland
Abstract: Cursoriality is broadly defined as adaptations towards increased terrestrial locomotor performance at less energetic cost. One such clade which has been previously studied is Theropoda (the carnivorous dinosaurs and their descendants, including birds). Exclusive of the fully volant birds (the clade Ornithothoraces, containing crown-group birds Aves), terrestrial theropods were striding obligate bipeds with a parasagittal stance, despite a size range of <1 to over 8000 kg. But within this group osteological traits associated with cursoriality are variably developed. The present analysis examines these traits in the context of increased knowledge of theropod diversity (especially the early branches of lineages long known from the Late Cretaceous); of new comparative techniques to reconstruct the trait evolution, and new more phylogenetically-informed myology of the pelvic-hindlimb-caudal complex. A new species-level supertree of theropod dinosaurs and their outgroups is assembled. Traits affiliated with increased locomotory performance are mapped onto this tree. These include: proportional increases in the pre- and postacetabular portions of the ilium from the ancestral condition; elongation of the distal limb elements (tibia; metatarsus); alternative morphologies interlocking and reducing the mediolateral width of the metatarsus; size and position of the lesser and fourth trochanters; and size and distribution of the caudal transverse processes. These traits are not randomly distributed, but phylogenetically co-associated. Clades in which they are strongly expressed are the Jurassic "elaphrosaurs" and Cretaceous Noasauridae within Ceratosauria, sporadically expressed in Oviraptorosauria and Troodontidae within Maniraptora, and especially well-developed Tyrannosauridae, Ornithomimidae, and Parvicursorinae in basal Coelurosauria. The strongest examples are present only in the Late Cretaceous Asiamerican (Asia plus North America) landmass.

PAL2-5  5:30 pm  Measuring morphological diversity and tempo using discrete characters: advantages and disadvantages of including additional phylogenetic information. Lloyd G. T.*, Macquarie University
Abstract: Discrete characters offer a rich resource of morphological data, the ability to compare extremely disparate organisms, and a mature set of tools for dealing with missing data. In addition, they offer the opportunity to ask questions relating to both morphological diversity (often termed simply “disparity”) and tempo. Here I present two case studies exploring how the combination of discrete characters and phylogeny affects: 1) properties of an ordination (“phylomorphospace”), and 2) time series of evolutionary rate. The respective data sets are: 1) a coelurosaurian theropod dinosaur matrix of 152 taxa and 853 characters, and 2) a lungfish matrix of 86 taxa and 91 characters, and I used the recent R package Claddis for all analyses. For the former case I compared five different phylomorphospaces using scree and ordination plots, time series, and correlations. Overall, these suggested a pre-ordination ancestral state reconstruction that limits the amount of missing data estimated should be favoured as this minimises the amount of introduced phylogenetic signal. For the latter case I compared four different time series using the Wobble Index of Alroy, showing that use of a likelihood algorithm and a rudimentary correction for missing data creates dramatically smoother time series than a parsimony algorithm and no correction. In conclusion, the usage of phylogenetic data in analysing morphological diversity and tempo offers some clear benefits. However, results are highly contingent on algorithm choice and at present clear optimality criteria for comparative tests can be hard to identify.

PAL2-6  5:45 pm  Dynamics of morphological evolution in Cretaceous and Paleocene eutherian mammals. Halliday T.J.D.*, University College London; Goswami A., University College London
Abstract: The end-Cretaceous mass extinction marks a transition in mammal evolution from small Cretaceous taxa to more diverse faunas of larger taxa in the Cenozoic. Despite this, many studies have found no change in evolutionary parameters across the Cretaceous-Paleogene (K-Pg) boundary, although few of these studies have explicitly considered early Paleogene taxa, largely due to a lack of a resolved phylogeny. We conducted the largest phylogenetic analysis (177 taxa) of Paleocene placentals to date. We dated the resulting trees using a recently described stochastic method, predicting a latest Cretaceous origin of Placentalia, but earliest Paleocene interordinal diversification. We reconstructed ancestral states for 680 characters and binned character transitions, identifying significant increases in rate of evolution at the K-Pg boundary and at the origin of Placentalia. We binned morphologies of ancestral nodes as well as those of the tips, and estimated morphological disparity for Cretaceous and Paleogene time bins. Range-based disparity metrics were low in the Cretaceous, but increased at the K-Pg boundary. Variance-based metrics were stable during the Cretaceous, decreased from the Campanian to the Maastrichtian, before increasing only in the middle Paleocene. We reconstructed body size for all sampled taxa, testing several models of evolution. Our results support an increase in rate of body size evolution at the origin of Placentalia. Combined, these results suggest a three phase model of eutherian morphological evolution. First, extinction of most stem eutherians in the Campanian, alongside a diversification of near-crown eutherians and early Placentalia and an increase in rate of body size evolution. Second, further extinction of stem eutherians at the K-Pg boundary, rapid diversification of Placentalia and exploration of morphospace. Third, niche specialisation and morphospace clustering through the Paleogene. Together, our results support the diversification of Placentalia as an archetypal adaptive radiation.

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