No description available.

The Triassic Period (~252–201.5 Ma) saw a transformative radiation and reorganization of continental tetrapod diversity following the end-Permian Extinction, including an assemblage of diverse forms that do not survive the end-Triassic (herein termed the 'endemic Triassic fauna', =ETF), as well as the earliest fossil representatives of all major modern tetrapod groups (herein termed the 'Living [Triassic to Recent] Fauna', =LTF; i.e. Salientia, Caudata, Gymnophiona, Mammaliaformes, Squamata, Rhynchocephalia, Testudinata, Crocodylomorpha, and Dinosauria). With few exceptions, only the LTF assemblage survives the end-Triassic Extinction (~201.5 Ma), highlighting the Late Triassic (~227–201.5 Ma) record as essential for understanding this pivotal transition and the evolutionary and ecological origins of post-Triassic non-marine tetrapod faunas, including those of present day. Micro-microvertebrate bonebeds are arguably the best proxy for tracking continental vertebrate biodiversity, however gaps in their Late Triassic record obscure patterns and drivers of evolutionary, ecological, and environmental change during the rise of LTF communities. In my dissertation, I use new data collected from Upper Triassic microvertebrate bonebeds from North America, and particularly the Thunderstorm Ridge site (PFV 456) in Petrified Forest National Park, Arizona, U.S.A, to fill gaps in the evolutionary record of specific groups (e.g., lissamphibians and lepidosaurs), as well as the vertebrate paleocommunity record of Triassic equatorial Pangaea. My first chapter describes and analyzes an assemblage of gymnophionomorph (stem caecilian) bones from PFV 456 which represent the oldest-known caecilian fossils globally. As the oldest caecilian fossils, they provide new support for the dissorophoid temnospondyl affinities of caecilians and other living amphibians, evidence of a step-wise acquisition of caecilian anatomies associated with fossoriality, and evidence of an ancient pattern of equatorial biogeographic restriction in caecilians from the Triassic to the present day. My second chapter describes and analyzes an assemblage of lepidosauromorphs from the Late Triassic of Equatorial Pangaea, providing new insights into the step-wise evolution tooth and jaw morphologies near the divergence of living lepidosaur clades (Squamata and Rhynchocephalia), and showing evidence for the Triassic acquisition in stem squamates and non-squamate lepidosaurs of dental features conserved in living squamates. The third chapter uses apomorphy-based identifications to describe the vertebrate diversity of the Thunderstorm Ridge site (PFV 456), providing evidence for the most species rich continental vertebrate community yet-known from the Triassic, with 55 vertebrate taxa. Nearly all LTF clades are present, predating similar assemblages from the early Jurassic by over 20 million years, and indicating that the assembly of the first LTF communities by at least 220 million years ago, long before the Triassic-Jurassic Extinction event (~201.5). The presence of this exceptional diversity may be linked to the climatic and environmental settings of equatorial Pangaea during the Triassic. / Doctor of Philosophy / The Triassic Period, lasting about 50 million years from approximately 252 to 201.5 million years ago, was a period of transformation for life living on land. During the Triassic, we see the first fossil evidence for the evolution of the tetrapod (animals with a backbone and limbs) groups familiar to us from the present-day Earth, including frogs, salamanders, caecilians, mammals, lizards, the tuatara, turtles, crocodilians, and dinosaurs. Understanding the early evolution of these groups is limited by gaps in the Triassic fossil record, particularly for groups with small-bodies and delicate skeletons like frogs, salamanders, caecilians, and lizards. The poor Triassic records of these groups also limits understandings of when and where tetrapod communities resembling those of the present-day first assembled, and whether events like the Triassic-Jurassic Extinction event (~201.5 million years ago) shaped the organization of these communities. To fill these gaps, I have focused on collecting data from microvertebrate bonebeds, layers of rock that preserve the small, delicate bones of small-bodied vertebrates that are typically rare elsewhere. New microvertebrate data collected from Late Triassic rocks in North America, and particularly the 220 million year old Thunderstorm Ridge site (PFV 456) in Petrified Forest National Park, Arizona, U.S.A., provide evidence for exceptionally diverse tetrapod communities, opening a window onto the early evolution of living tetrapods and their ecological interactions. In my first chapter I describe and analyze the bones of a stem caecilian amphibian from the Thunderstorm Ridge site. These are the oldest caecilian fossils on Earth, and they provide new evidence for the evolutionary relationships, ecologies, and biogeography of these enigmatic living amphibians. In my second chapter I describe and analyze the jaws and teeth of early lizards and their close relatives from North American Late Triassic microvertebrate sites, showing that the tooth and jaw morphologies of living lizards like geckos and skinks first evolved in their Triassic relatives. In my third chapter, I describe and identify the 55 vertebrate taxa recovered from the Thunderstorm Ridge site, showing that it is the most diverse tetrapod community known from the Triassic. The diversity of early members of living tetrapod groups at Thunderstorm Ridge suggests that tetrapod communities resembling those of the present-day first assembled in the Triassic, at least 20 million years prior to the Triassic-Jurassic Extinction.

No description available.

No description available.

No description available.

Vertebrate fossils have been discovered at several locations in the Bighorn Basin (Wyoming). The Virginia Museum of Natural History's (VMNH) digsite is located in the eastern part of the Bighorn Basin, in the Coyote Basin. Many scientists have worked within these basins trying to describe the stratigraphy. One question specifically asked is where the boundary between the Morrison Fm. (Jurassic) and the Cloverly Fm. (Cretaceous) lies. This new study attempted to show if the current method (Kvale, 1986) of determining the boundary is appropriate. The stratigraphy of the area was examined using Kvale, 1986, Ostrom, 1970, and Moberly, 1960's work in order to see which model was more robust. The fossils in the VMNH digsite were used to supplement the stratigraphic data in determining the age of specific beds. All of Ostrom's units were identified throughout the study area. There is some doubt as to whether the units would be acceptable outside of the Coyote Basin because of laterally discontinuity. Nevertheless, his description of units is satisfactory for the study area, and is more appropriate than other methods. The geologic age of the dinosaurs uncovered in the VMNH quarry is in agreement with the age determined stratigraphically. The VMNH site is below Ostrom's Unit II, which would place it in the Late Jurassic. The determination of the Jurassic/Cretaceous stratigraphic boundary has not been resolved. However, since the Pryor Conglomerate member of the Cloverly Fm. can be identified throughout this area, it is proposed as the Morrison Fm./Cloverly Fm. boundary. / Master of Science

Gastrulation, literally “formation of the gut” is ultimately an inadequate term to describe one of the most dynamic periods during vertebrate developmental biology. During gastrulation coordinated cell movements reshape the non-descript blastula into the structured gastrula and simultaneously specify the three germ layers: endoderm, mesoderm and ectoderm. The morphogenetic movements of gastrulation are highly conserved between species, but the links between their genetic and biomechanical regulation are poorly understood. The zebrafish embryo – externally hatched, optically clear and amenable to genetic manipulation – is an ideal vertebrate model in which to study both morphogenetic movements and their genetic control. This thesis provides a detailed analysis of the zebrafish Mix-type homeobox transcription factor, Mtx2, both in terms of its role in gastrulation and the molecular mechanisms regulated by Mtx2. This approach involved detailed examination of the Mtx2 loss-of-function phenotype and, subsequently, use of this phenotype as the basis for a microarray screen to identify and investigate Mtx2-dependent genes. One specific Mtx2-dependent gene, katanin-like 1 was investigated further by loss-of-function studies. Prior to this study the mtx2 gene was identified by homology, within its homeodomain, to other Mix-family transcription factors, but both its function and transcriptional targets remained unknown. Using a morpholino knockdown approach, this thesis demonstrates that Mtx2 is essential for vegetal movement (epiboly), but not specification, of the embryonic germ layers and extra-embryonic tissues during zebrafish gastrulation. The recruitment of filamentous actin (F-actin) to a punctate band at the blastoderm margin, was previously shown to be responsible for progression of epiboly. However, formation of this structure is demonstrated to be Mtx2-dependent. Microarray expression profiling of the Mtx2 loss-of-function phenotype was performed to screen for novel genes with roles in gastrulation. This approach identified Mtx2-dependent genes with roles in cytoskeletal dynamics, cell-cell adhesion and endocytosis and vesicular trafficking – processes known to be involved in morphogenetic movements. Many Mtx2-dependent genes are co-expressed with mtx2 in the extra-embryonic yolk syncytial layer (YSL), the teleost functional equivalent of mammalian visceral endoderm. The subset of Mtx2-dependent genes co-expressed with mtx2 and that contain Mtx2-binding sites within their 2kb proximal promoter represent the genes with the greatest likelihood of being direct Mtx2 transcriptional targets. A novel homologue of the microtubule severing protein Katanin, known as katanin-like 1 (katnal1) met all these conditions. Morpholino knockdown of Katnal1 demonstrates that like Mtx2, Katnal1 is essential for gastrulation in zebrafish. A cloned Katnal1mCherry fusion construct was observed to associate with microtubules, and demonstrated bi-directional trafficking around transfected mammalian cells. Analysis of the microtubule network in wild-type and morpholino injected zebrafish embryos demonstrated that remodelling of the extensive microtubule network found in the YSL and yolk cytoplasmic layer (YCL) is Katnal1-dependent. Nuclear division within the YSL and F-actin recruitment to the blastoderm margin are also Katnal1-dependent. This thesis therefore demonstrates, for the first time directly, the multiple, specific roles played by the microtubule network of the YSL/YCL. Katnal1 is highly conserved from Drosophila to mammals and is dynamically expressed during mouse gastrulation. The Mtx2 binding motif in the katnal1 2kb proximal promoter can be bound by both Mtx2 and its putative mouse homologue Mixl1. This suggests that katnal1 may also be a direct target of Mtx2. At the technical level, these results demonstrate the validity of screening for novel developmentally important genes using a zebrafish microarray-based approach, the potential of such an approach to, ab initio, identify a candidate list of transcription factor targets and confirm the utility of the zebrafish as a developmental model. At the biological level, this work collectively suggests that Mtx2 is a central regulator of the morphogenetic movement of epiboly and that Katnal1-dependent microtubule remodelling drives multiple aspects of gastrulation, potentially from Drosophila through to humans.

Diss. (sammanfattning) Stockholm : Karol. inst., 2006. / Härtill 6 uppsatser.

Processing of visual stimuli in the vertebrate retina is complex and diverse. The retinal output to the higher centres of the nervous system, mediated by ganglion cells, consists of several different channels. Neurons in these channels can have very distinct response properties, which originate in different retinal pathways. In this work, the retinal origins and possible functional implications of the segregation of visual pathways will be investigated with a detailed, biologically realistic computational model of the retina. This investigation will focus on the two main retino-cortical pathways in the mammalian retina, the parvocellular and magnocellular systems, which are crucial for conscious visual perception. These pathways differ in two important aspects. The parvocellular system has a high spatial, but low temporal resolution. Conversely, the magnocellular system has a high temporal fidelity, spatial sampling however is less dense than for parvocellular cells. Additionally, the responses of magnocellular ganglion cells can show pronounced nonlinearities, while the parvocellular system is essentially linear. The origin of magnocellular nonlinearities is unknown and will be investigated in the first part of this work. As their main source, the results suggest specific properties of the photoreceptor response and a specialised amacrine cell circuit in the inner retina. The results further show that their effect combines in a multiplicative way. The model is then used to examine the influence of nonlinearities on the responses of ganglion cells in the presence of involuntary fixational eye movements. Two different stimulus conditions will be considered: visual hyperacuity and motion induced illusions. In both cases, it is possible to directly compare properties of the ganglion cell population response with psychophysical data, which allows for an analysis of the influence of different components of the retinal circuitry. The simulation results suggest an important role for nonlinearities in the magnocellular stream for visual perception in both cases. First, it will be shown how nonlinearities, triggered by fixational eye movements, can strongly enhance the spatial precision of magnocellular ganglion cells. As a result, their performance in a hyperacuity task can be equal to or even surpass that of the parvocellular system. Second, the simulations imply that the origin of some of the illusory percepts elicited by fixational eye movements could be traced back to the nonlinear properties of magnocellular ganglion cells. As these activity patterns strongly differ from those in the parvocellular system, it appears that the magnocellular system can strongly dominate visual perception in certain conditions. Taken together, the results of this theoretical study suggest that retinal nonlinearities may be important for and strongly influence visual perception. The model makes several experimentally verifiable predictions to further test and quantify these findings. Furthermore, models investigating higher visual processing stages may benefit from this work, which could provide the basis to produce realistic afferent input.

During the Silurian, the Swedish island Gotland was positioned close to the equator and covered by a shallow sea called the Baltic Basin. The sedimentary rocks (predominantly carbonates) comprising most of the island today were initially formed in this warm sea, and the relatively complete succession of rocks often contains fossil fragments and scales from early vertebrates, including heterostracans, anaspids, thelodonts, osteostracans, acanthodians, and a stem-osteichthyan. Fossils of early vertebrates become increasingly more common in younger Silurian rocks, but are mostly represented by fragmentary remains and rarer occurrences of articulated jawless vertebrates (agnathans). However, the record of articulated specimens and jawed vertebrates (gnathostomes) are more numerous in rocks of the following Devonian Period. Isolated peaks of agnathan diversity during the Silurian and disarticulated remains of gnathostomes from this period hint at a cryptic evolutionary history. A micropaleontological approach with broader sampling may provide a better understanding of early vertebrate distribution patterns and hopefully give some insights into this history. The objective of this study was to build upon previous sampling on Gotland and to use established frameworks for disarticulated remains with the aim of making comparisons with similar studies performed in the East Baltic. However, difficulties locating the collections from these previous works necessitated a different focus. Undescribed museum collections and newly sampled material enabled some taxonomical revisions and greatly improved the understanding of vertebrate distribution in the youngest part of the Gotland sequence. It also indicated that this interval may represent the early stages of the diversification of gnathostomes that become increasingly dominant toward the end of the Silurian. Furthermore, the description of samples from partly coeval sections in Poland enabled some preliminary comparisons outside of Gotland, and presented a striking example of restricted environmental occurrences for a thelodont taxon. This is encouraging for future sampling and investigations on Gotland. Together with the establishment of a facies-framework comparable to that developed in the East Baltic and correlations to other areas, this may prove fruitful for an increased understanding of early vertebrate distribution and evolution during the Silurian.