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Session Schedule & Abstracts

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Wednesday 29th June, 2016

Paleontology 1

Room: Salon G   2:30 pm–4:00 pm

Moderator(s): Mallat J, Mann AAM
PAL1-1  2:30 pm  Puzzle of the earliest vertebrates: “Blessed are the meek”. Mallatt Jon *, School of Biological Sciences, Washington State University
Abstract: Why are vertebrates such complex animals, compared to almost all the invertebrates? Some paleobiologists say vertebrate complexity arose when the prevertebrates evolved from filter feeders into predators. However, the earliest fossil vertebrates such as the jawless Cambrian fishes Haikouichthys and Metaspriggina are unlikely to have been predators and more likely were filter feeders on food-rich remnants of “microbial mats” on the sea floor. The main predators of the Cambrian were arthropods, with their grasping, manipulative limbs. Both the arthropods and their vertebrate prey participated in the Cambrian’s “predatory arms race,” which led in both clades to rapidly improving distance senses, enlarging brains, increasing cognition, and more complex behaviors. It also led to the most basic, sensory, type of consciousness (having any kind of experience at all), although a better case can be made for consciousness in the first vertebrates than in the arthropods. But why did the limbless, jawless, unarmored, harmless, and rarer proto-vertebrates – so low on the food chain – exceed the arthropods and other invertebrates in brain complexity (many more neurons) and genomic complexity (two rounds of genomic duplication), with their larger genomic repertoire allowing more evolutionary innovations (such as the later evolution of the predatory, jawed vertebrates)? The proposed solution is two-fold. First, ancestral vertebrates were unusual filter feeders with access to highly concentrated and nutritious food particles in the patches of microbial mat. Second, they had superior locomotor motility based on their streamlined, myomere-propelled bodies, unhindered by a heavy or awkward exoskeleton of the arthropod type.

PAL1-2  2:45 pm  Osteology of the Cretaceous †Ctenothrissiformes and †Pattersonichthyiformes: clues to primitive structure in euryptergyian fishes. Delbarre DJ*, Department of Earth Sciences, University of Oxford; Friedman M, Department of Earth Sciences, University of Oxford
Abstract: Eurypterygian fishes contain over a third of extant vertebrate species and comprise three major lineages: the myctophiforms and acanthomorphs (collectively the ctenosquamates), and the aulopiforms. There are also a number of exclusively fossil eurypterygian groups, including †Ctenothrissiformes and †Pattersonichthyiformes. Relationships among the three major eurypterygian clades are clear, but there is no consensus as to how these extinct groups fit into this framework. We studied three fossil taxa: †Aulolepis, †Ctenothrissa (†Ctenothrissiformes) and †Pattersonichthys (†Pattersonichthyiformes), by re-examining fossils and visualizing internal anatomy using computed tomography (†Aulolepis). We developed a new morphological matrix, used in isolation and combined with a molecular dataset, in order to place the three fossil among living teleosts. Finally, we used topological hypothesis testing to evaluate contrasting systematic interpretations. We find strong support for a stem ctenosquamate placement for the two fossil groups, with two favoured topologies: 1) †ctenothrissiforms and †pattersonichthyiforms are a clade, 2) †ctenothrissiforms and †pattersonichthyiforms are a grade, with †Ctenothrissa closest to the ctenosquamate crown and †Aulolepis the most remote. We tentatively prefer the second hypothesis, which demands fewer independent derivations of morphological specializations. Our internal examination of these stem ctenosquamates suggests more complicated patterns of character evolution among eurypterygians than is suggested by living species alone. For example, we found that a key aulopiform synapomorphy, an enlarged uncinate process on the second epibranchial, is present in †Aulolepis suggesting that it is a eurpyterygian synapomorphy subsequently lost in derived ctenosquamates.

PAL1-3  3:00 pm  A reconsideration of the aïstopod Lethiscus stocki (Tetrapoda: Lepospondyi) via micro-Computed Tomography (microCT), with implications for tetrapod phylogeny. Anderson J. S.*, University of Calgary; Pardo J. D., University of Calgary; Ahlberg P. E., Uppsala University; Szostakiwskyj M., University of Calgary; Germain D., Museum National d'Histoire Naturelle
Abstract: To investigate larger trends in tetrapod phylogeny, we revisited microCT dataset of Lethiscus, the oldest lepospondyl. Digital dissection shows for the first time fine structure of the skull, revealing previously unrecognized primitive morphology despite its derived body plan, including a spiracular notch, narrow, anteriorly-restricted parasphenoid, palatal fang-pit pairs, Meckelian ossifications, and previously noted deep circular atlantoccipital articulation. The braincase is elongate and atop a dorsally projecting septum of the parasphenoid, similar to what is seen in stem tetrapods such as embolomeres. This morphology is corroborated by that of a second aïstopod, Coloraderpeton. Lower jaw morphology is completely preserved, facilitating comparison with stem tetrapods. Phylogenetic analysis of a newly expanded matrix of early tetrapods, including critical new microsaurian braincase data, demonstrates lepospondyl polyphyly, placing aïstopods onto the tetrapod stem, whereas recumbirostrans are displaced into amniotes. These results show that stem tetrapods were much more diverse in their body plans than previously thought. Furthermore, our study requires a change in commonly used calibration dates for molecular analyses, and emphasizes the importance of taxonomic and character sampling for early tetrapod evolutionary relationships.

PAL1-4  3:15 pm  Morphological innovations in the earliest post-Devonian tetrapods: adaptations towards increasing terrestriality? Clack J. A., University of Cambridge; Smithson T. R.*, University of Cambridge
Abstract: Although the earliest Carboniferous (early Mississippian) Tournaisian stage has been considered a depauperate interval for vertebrates and particularly tetrapods (known as “Romer’s Gap”), our recent work has unearthed a rich and diverse suite of Tournaisian fossils from the UK. These include many new tetrapod genera, spread across the phylogenetic and morphological divide between Devonian and Carboniferous forms. While some retain plesiomorphic characters, others demonstrate a number of innovations to morphology that are seen for the first time in the Tournaisian tetrapods. 1) Parasphenoids that cross the ventral cranial fissure and underplate and fuse to the braincase base, in one case also underlying the basipterygoid processes. This may relate to strengthening and consolidating the braincase as it is increasingly subjected to gravity and the forces of muscle contraction during head movements. 2) An exoccipital separate from the basioccipital in contrast to the conjoined ex- and basioccipital of more primitive tetrapods may relate to development of a more mobile occipital joint, also associated with more flexible head movement. 3) Deeply emarginated jugals may imply enlarged orbit and eye sizes compared to Devonian forms and suggest the increased importance of vision in terrestrial forms. 4) Humeri in which the foramen for the brachial artery and median nerve passes from the dorsal to the ventral side of entepicondyle. This is related to 5), humeri that show greater than 25 degree twist, related to increasing stride length, and 6) humeri in which the deltopectoral crest becomes more proximally placed and which show the development of a true shaft. 7) The earliest occurrence of a five-digit autopod. Some of these innovations, seen for the first time in small animals, may be directly linked to increasing terrestrialization among post-Devonian taxa.

PAL1-5  3:30 pm  A re-description of Amphibamus grandiceps (Temnospondyli: Dissorophoidea) from the Francis Creek shale, Mazon Creek, Illinois. . Mann A.*, Carleton University; Maddin H. C., Carleton University
Abstract: The Carboniferous temnospondyl Amphibamus grandiceps (Cope, 1865) is one of the oldest known members of Amphibamidae from the Middle Pennsylvanian aged (309 Mya) Mazon Creek deposits in Grundy County, Illinois, USA. Amphibamus was once considered a pivotal taxon in the debate over the origins of modern amphibians. It has since been usurped by the closely related amphibamids Doleserpeton and Gerobatrachus. Despite this, several features, including the presence of small pedicellate bicuspid teeth, an abbreviated skull table, reduced ribs, a pectoral girdle incorporating a small interclavicle and bar like clavicles, link Amphibamus to modern amphibians. Taxonomic revision of Amphibamus has been hindered by the loss of the original type specimen in a fire, and a lack of subsequent descriptive papers on significant specimens, including the near complete neotype YPM 799. In addition, many specimens from Mazon Creek have since been reassigned to Amphibamus, including Micrerpeton caudatum (Moodie, 1909), Miobatrachus romeri (Watson, 1940), Mazonerpeton longicaudatum (Moodie, 1912), as well as a variety of larval specimens. Many of these specimens vary in their morphology and their ontogenetic stages have not been assessed. Anatomical features including the shape of the skull and cranial elements, morphology of the terminal phalanges, length of the limbs, and number of caudal vertebrae (tail length) all remain unclear. Here we re-describe the neotype of Amphibamus and include a discussion of new anatomical and ontogenetic data from reassigned specimens. Exceptional preservation of soft tissues found in Mazon Creek nodules also allows for a new analysis of labile structures including the integument (scale impressions), and structures of the eye (scleral ossicles). A phylogenetic analysis including YPM 799 was performed for the first time, recovering it as the sister taxon to Amphibamus.

PAL1-6  3:45 pm  Evolution of genome size in recent and fossil salamanders. Stein Koen*, Earth System Science - AMGC, Vrije Universiteit Brussel; Royal Belgian Institute of Natural Sciences Directorate 'Earth and History of Life'; Skutchas Pavel, Saint Petersburg State University, Vertebrate Zoology Department, Biological Faculty; Schoch Rainer, Staatliches Museum für Naturkunde; Fröbisch Nadia, Museum für Naturkunde Berlin
Abstract: Modern salamanders possess giant genomes, and directly correlated with this, the by far the largest cells among tetrapods. While no entire genome of a salamander has been sequenced thus far, recent studies have shown that large introns and novel genes contribute to the enormous genome sizes. The extreme cells size impacts many aspects of salamander biology and has been suggested to be closely associated with the enormous plasticity in life history pathways and their high regenerative capacity that includes limbs, eyes, spinal cord and other complex tissues. In the current study, we sectioned long bones (N>18, mostly femora) of members of all modern salamander families with known genome sizes as well as long bones of members of several fossil amphibian lineages to investigate (1) if the correlation between genome size and cell size that has been established based on leucocytes holds with respect to osteocyte size and (2) when within the long evolutionary history of the clade the large genome sizes evolved. For outgroup reference, we sampled femora of a number of modern amniote and frog taxa with known genome sizes. Understanding the correlation between osteocyte lacuna size and genome size in modern taxa is vital for an investigation of the genome sizes in fossil taxa. In order to minimize errors caused by variation in location and orientation of sections, we aimed to sample homologous elements in the same location and the same plane of sectioning. Our results show that genome size and osteocyte size are tightly correlated in salamanders. Moreover, the large genomes of urodeles were already present in stem-group salamanders (Karauridae) and most likely evolved early in the evolutionary history of the salamander lineage, possibly as early as Paleozoic dissorophoid temnospondyls. This provides new insights into the deep time genomic evolution of urodeles and a novel dataset for understanding salamander origins and the evolution of central aspects of their biology, such as life history patterns, miniaturization, and developmental rates.

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