Val britton impossible boundaries in dating

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val britton impossible boundaries in dating

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These phylogenetic issues are important as they affect the placement of fossils relative to crown clades in the stem lineage to Gnathostomata. Jawed Vertebrates The phylogeny of Gnathostomata Figure 3the jawed vertebrates, has been revised substantially, especially with regard to extinct groups.

Living gnathostomes are grouped into Chondrichthyes cartilaginous fishes, including the sharks, rays, and ratfishes and Osteichthyes bony fishes and tetrapodsand are joined by two extinct groups: The status of placoderms as a clade is unclear, with many phylogenetic analyses suggesting that they are a grade of stem gnathostomes Brazeau, ; Davis et al. The acanthodians, long difficult to position phylogenetically, appear to include either an assortment of stem gnathostomes, stem chondrichthyans, and stem osteichthyans Brazeau, ; Davis et al.

Within Osteichthyes, the division into Actinopterygii ray-finned fishes and Sarcopterygii lobe-finned fishes and tetrapods has long been clear reviewed in Friedman and Brazeau, To provide basic orientation, we provide dates for the bases of crown Gnathostomata, Chondrichthyes, Osteichthyes, Sarcopterygii, and Rhipidistia below, extending then to Tetrapoda terrestrial osteichthyans. Ray-Finned Fishes The dates presented here reflect divergences between important ray-finned fish model systems Danio, Oryzias, Gasterosteus, Takifugu, Tetraodonas well as splits that define major divisions of ray-finned fish classification, species-rich clades, or both Figure 4Figure 5and Figure 6.

Because of the coarse resolution of this survey, the precise formulation of calibrations given here differs from those provided in divergence-time analyses of actinopterygians that rest on a dense sample of extant lineages e. Where relevant, we explicitly identify updates relating to the age assessments or phylogenetic interpretations of specific fossils.

At the broadest scale, there is a now a stable hypothesis of relationships among living ray-finned fishes based on both morphological and molecular datasets. Cladistia bichirs and ropefish include two extant genera, and form the sister group to all remaining Actinopterygii, collectively known as Actinopteri. The divergence that defines crown Actinopteri is that between Chondrostei, which includes sturgeons and paddlefishes, and Neopterygii, the clade containing Halecomorphi bowfinGinglymodi garsand Teleostei teleosts.

Most possible patterns of relationships have been proposed between these three neopterygian clades, but two in particular have enjoyed considerable exposure: The halecostome hypothesis was dominant throughout the last quarter of the 20th century, but molecular analyses and renewed anatomical investigation targeting neopterygian relationships now strongly support a monophyletic Holostei e.

Although the relationships among extant representatives of the major actinopterygian divisions is stable, there is less certainty concerning the placement of Paleozoic fossils, with respect to the deepest splits between living ray-finned fish groups.

In general, the Permo-Carboniferous record of actinopterygians represents perhaps the most poorly studied aspect of the vertebrate fossil record Hurley et al. There are several published hypotheses depicting the relationships of Paleozoic actinopterygians relative to extant ray-finned clades Patterson, ; Gardiner, ; Gardiner and Schaeffer, ; Lund et al.

These studies vary considerably in terms of taxonomic and morphological scope, and there are instances of major disagreement between them e. This has led to a corresponding diversity of calibration schemes for deep divergences within actinopterygians e. We therefore restrict our list of calibrations for the deepest splits within the ray-finned fish tree to those examples broadly agreed upon across competing phylogenetic solutions.

Tetrapods The phylogeny of Lissamphibia modern amphibians and their ancestors and the basal Tetrapoda Figure 7 is based on various sources.

val britton impossible boundaries in dating

Most authors regard Lissamphibia as monophyletic, based on the morphological characters of pedicellate teeth and cutaneous respiration Parsons and Williams, ; Benton, ; Ruta et al. Most molecular phylogenetic studies e. An exception is the recent work by Fong et al. This pairing of Caudata and Gymnophiona is unexpected, as morphological evidence supports a pairing of Caudata and Anura Parsons and Williams, ; Benton, ; Carroll, ; Anderson, The clade Tetrapoda was defined by Andersonp.

These meanings are debated Laurin and Anderson,without conclusion. Amniotes The dates for key amniote nodes presented below are based on a single phylogeny of the key amniote groups, based on nearly unanimous current understanding of the roots of the phylogeny of modern birds and mammals Figure 8. The encompassing clade is Amniota, which includes all tetrapods that lay an amniotic egg, characterized by its tough sometimes mineralized.

The broad relationships within Amniota have been established thanks to numerous cladistic analyses e. The meanings of Amniota, Reptilia, Anapsida, Diapsida, Archosauria, and many other major clade terms have been much discussed, and debates have focused on whether each term should be given either a node-based or stem-based definition, and whether each should refer to its more or less traditional extent or to the crown group.

We follow the most recent discussions of the topic e. However, Modesto and Andersonp. Reptilia is the clade that includes parareptiles, turtles, and diapsids, but excludes synapsids, and the definition allows for a variety of phylogenetic positions for Testudines, whether they should turn out to be diapsids or not. Using the crown term Archosauria makes the date of origin of the clade equivalent to the common ancestor of modern crocodilians and birds.

We can provide dates for crown-group Amniota, Reptilia, Diapsida, Archosauria, Crocodylia, Aves, Lepidosauria, and Squamata, but not for major intermediate clades such as Eureptilia, Lepidosauromorpha, Archosauromorpha, Crurotarsi, Avemetatarsalia, or Dinosauria, because, although each contains extant members, the other major components of each of these clades are extinct. Birds The five key avian nodes dated below are based on a single phylogeny that reflects nearly unanimous current understanding of the roots of the phylogeny of modern birds Figure 9.

The encompassing clade is Ornithurae, a broad clade that includes all modern birds as well as many fossil forms such as Hesperornis and Ichthyornis, and this has been given a stem-based definition by Gauthier However, the node in the cladogram equivalent to Ornithurae Figure 7 cannot be investigated by phylogenetic study of extant forms because it contains many extinct taxa at its base and along the stem to Neornithes.

Within Ornithurae, all modern birds are included within the clade Neornithes Figure 7which consists of the subclades Neognathae and Palaeognathae, a long-held view Huxley, ; Pycraft,based initially on morphological differences in the palate, and confirmed by cladistic and molecular phylogenetic studies Cracraft et al.

Within Neognathae are two major subclades, Galloanserae for waterfowl, landfowl, and their relatives and Neoaves all other modern flighted birdsbased on several independent molecular and morphological phylogenetic analyses e. Mammals There is now little ambiguity concerning the phylogenetic branching patterns among mammalian clades reviewed in Springer et al.

The interrelationships implied by most mammalian orders, families, tribes, and genera have been accurately recognized since the 19th century.

val britton impossible boundaries in dating

Distinction of the three extant major groups monotremes, marsupials, placentals has a similarly long history among zoologists Gregory, What has changed substantially during the last decade is the level of confidence in the inter-ordinal relationships among mammals.

In the early s, questions such as the relations of hyraxes were hotly debated Fischer and Tassy, ; an affinity of bats with primates was generally suspected Kaas and Preuss, ; but see Beard, ; the integrity of the "Insectivora" was skeptically regarded MacPhee and Novacek, ; and the monophyly of terrestrial Artiodactyla was widely commended Prothero, Serious questions also surrounded the possibility that marsupials and monotremes were related to the exclusion of placentals Janke et al.

Sincedebate on each of these topics has been replaced with consensus, to the point where most participants in what used to be disagreements now consider a much more limited set of possible resolutions compared to the astronomically huge number of possible trees Felsenstein, for the over extant species of mammals.

Several authors including one of us, RJA who previously interpreted datasets in favor of phylogenetic affinity between for example tenrecs and lipotyphlan insectivorans Asher,or of marsupials plus monotremes Janke et al. This work reflects what is today a genome-backed consensus supporting, among other nodes, therians to the exclusion of monotremes and four major clades within placental mammals: Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires Murphy et al.

The increase in consensus in mammalian phylogenetics is relative, and there are of course some persistent areas of ambiguity reviewed in Asher et al. Nevertheless, stability in the mammalian Tree of Life enables progress on other fronts, including a framework by which common ancestors of mammalian groups may be hypothesized to have existed in the geological past.

Here, we review the fossil record pertinent to those nodes, expanding upon the discussion in Benton et al. Following the methods and criteria summarized in Benton et al. An issue relevant to many of the mammalian nodes discussed here is the paleontological case that has occasionally been made for the presence of crown placentals deep in the Mesozoic.

Historically, "placentals" have been argued to exist prior to the K-Pg boundary. McKenna and Bell reported the oldest lipotyphlan as Otlestes from the Cenomanian of Uzbekistan, but Archibald regarded it as a basal eutherian, lacking derived characters of Lipotyphla, or any other modern order.

More recently, Averianov and Archibald synonymized it with Bobolestes from the same local fauna and regarded it as a possible zalambdalestoid.

Somewhat younger is Paranyctoides from the Turonian of Asia and the Campanian of North America, and Batodon from the Maastrichtian of North America, both regarded as lipotyphlans by McKenna and Bell but not unequivocally reconstructed as such by other authors e.

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Based on recent phylogenetic study Asher et al. A tenuous link between ca. Some molecular clock studies suggest deep Cretaceous roots for crown placental orders Springer et al.

Notably, Dos Reis et al.

val britton impossible boundaries in dating

This difference in data availability was likely behind the relatively small error margins from Dos Reis et al. Thus, we consider it likely that at least some placental mammal groups originated during the Mesozoic, even though the paleontological case for identification of any crown placental order prior to the K-Pg boundary remains lacking.

Given the presence of stem lineages of crown placental clades close to the K-Pg boundary e. Moreover, we regard the conclusion of O'Leary et al. A therian node deeper than Euarchontoglires with some paleontological documentation is the divergence of Eutheria from other mammals in the late Jurassic, represented by Juramaia Luo et al.

This taxon is known from the Daxigou site of the Tiaojishan Formation, Liaoning Province, Northeastern China, and has been constrained by radiometric dates to derive from deposits of just over Ma in age Luo et al. Biostratigraphic comparisons suggest the Tiaojishan Formation exhibits an older fauna and flora compared to the Jehol biota of the Yixian Formation Hu et al.

The equivalent marine stage in the GTS Gradstein et al. It is worth commenting here on some core issues about the determination of soft maxima for mammalian nodes, points that also have a bearing on the debate just reviewed between apparently ancient molecular dates and much younger fossil dates.

Inference of a soft maximum amounts to making the case that, in opposition to the cliche, absence of evidence really is evidence of absence. For example, there are many groups of Jurassic and Cretaceous mammaliaforms, thanks to an abundance of geological facies that preserve relevant habitat, but these deposits lack crown placentals Wible et al. Hence, when we use records of near-relatives of clade X to argue that clade X does not occur at time Y, this is of course contingent on the understanding that they could have been there but are not based on the existence of facies preserving their near relatives, among which are at least some ecological analogues.

We therefore take this as the basis for assessing a probability a "soft maximum" that clade X had not yet evolved. In this paper we have deleted several mammalian clades dated by Benton et al. Both Nishihara et al. In contrast, Meredith et al. The clade comprised of sponges and eumetazoans all animals bar spongestheir last common ancestor and all of its descendants. There has been debate concerning the monophyly versus paraphyly of sponges and whether ctenophores are a sister clade to sponges plus Eumetazoa, or whether ctenophores are themselves eumetazoans Sperling et al.

A basal position for ctenophores has been revealed to be an artifact of poor model selection and conflict between variable rate loci Nosenko et al. Our calibration for crown Metazoa is independent of whether sponges are monophyletic or paraphyletic. Fossil Taxon and Specimens. Several hundred specimens are kept at this institution. Kimberella preserves several features that demonstrate it is a bilateral metazoan with an anterior-posterior axis Fedonkin and Waggoner, ; Fedonkin et al.

Specimens are often found associated with a distinct bipartite feeding trace emerging from one end of the body indicative of a feeding apparatus with two major denticles and a grazing behavior. There appears to be a ventral creeping sole surrounded by concentric units of tissue and a dorsal soft-bodied carapace. The morphology and feeding behavior has been accredited to a molluscan affinity. No coherent argument has been presented that calls into question the lophotrochozoan affinity of Kimberella see Discussion.

Specimens are also known from the Ediacara of Australia Glaessner and Wade, ; Wade,but the age of this unit is less well constrained. We select the date published in as our minimum hard constraint. A soft maximum constraint can be established on the approximately coeval Halverson et al. The absolute age of the Bitter Springs Formation is the better constrained, through correlation to a volcanic sequence in the upper Loves Creek Formation, allied to the Gairdner Dyke Swarm Hill et al.

The morphology and observed feeding behavior of Kimberella is demonstrative of an agile bilaterian metazoan moving by the means of a creeping sole.

Some contention has been aired about the rigor of this assessment and a more general bilaterian affinity has been aired Butterfield,but this assessment was based on the assumption that the feeding apparatus of annelids could be plesiomorphic for the crown group, which is a demonstrably derived character of aciculate annelids not present in stem-group annelids Eibye-Jacobsen, We have considered a number of claims for older records of Metazoa, but these are rejected.

These claims include the intriguing hypothesis that Dickinsonia is a placozoan-grade organism Sperling and Vinther,but which does not require Dickinsonia to be a placozoan clade organism.

There has been some debate over their age, some claiming a younger, Carboniferous or Permian age Gaucher et al. Regardless, the preservation of the traces is not convincing and is probably associated with a textured surface indicative of a microbial mat capable of preserving such structures Droser and Gehling, Thus, we exclude these traces as evidence for the minimum constraint on crown-Metazoa, though our soft maximum constraint must encompass it.

Finally, Love et al. Thus, a minimum constraint on the age of crown Metazoa could be established on the age of the younger, Marinoan glaciation, which is However, Siegl and colleagues have shown that genes encoding the isopropyl steroid biomarker are present in the genome of a eubacterium, Poribacteria, which has an apparently symbiotic relationship with demosponges Siegl et al.

Siegl and colleagues have not demonstrated that Poribacteria synthesizes the critical isopropyl steroid biomarker and, regardless, it could still be marshaled in evidence for demosponges if Poribacteria has a demonstrable obligate relationship with demosponges, and that it extends to the Cryogenian. But these remain variables and, therefore, the evidence from Poribacteria clearly calls into question its veracity as evidence of the existence of demosponges and, therefore, crown-Metazoans in the Cryogenian.

The clade comprised of Bilateria and Cnidaria, their last common ancestor and all of its descendants.

val britton impossible boundaries in dating

In this we assume that ctenophores are crown Eumetazoa, but our calibration is not contingent upon this since there are no older uncontentious records of ctenophores. Specimens are often found associated with a distinct bipartite feeding trace emerging from one end of the body, indicative of a feeding apparatus with two major denticles and a grazing behavior. No coherent argument has been presented that calls into question the lophotrochozoan affinity of Kimberella.

A soft maximum constraint is based on the maximum age interpretation of the Lantian Biota Yuan et al. This together with the Doushantuo Biota Yuan et al.

There are a number of claims for older records of Eumetazoa that we have considered but rejected as credible evidence on which to establish a minimum constraint on the diversification of crown Metazoa.

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There has been some debate over their age, some claiming a younger Carboniferous or Permian age Gaucher et al. Regardless, the preservation of the traces does not meet expectations, that it should be associated with a textured surface indicative of a microbial mat capable of preserving such structures Droser and Gehling, Thus, we exclude these traces in evidence for the minimum constraint on crown-Eumetazoa, though our soft maximum constraint must encompass it.

The clade comprised of Anthozoa and Medusozoa, their last common ancestor and its descendants. The monophyly of crown Cnidaria is well established on the basis of anatomical and molecular evidence Daly et al.

Olivooides multisulcatus Geological Museum of Peking University: Olivooides is known from embryonic and post-embryonic stages of development, including a polyp theca, characteristic of scyphozoans, and a medusa stage Dong et al. Olivooides multisulcatus co-occurs with Anabarites trisulcatus, which is indicative of the middle of the Fortunian Stage of the Terreneuvian Series, the first of the Cambrian.

The top of the Fortunian Stage is dated at This, together with the Doushantuo Biota Yuan et al. The oldest possible record of a cnidarian is Sinocyclocyclicus guizhouensis from the Ediacaran Doushantuo Formation Xiao et al. Putative Ediacaran medusoid cnidarians Wade, ; Glaessner, have been reinterpreted as microbial communities Grazhdankin and Gerdes, or trace fossils Jensen et al. Frond-like Ediacarans have been interpreted as pennatulacean cnidarians, but this comparison is unconvincing Antcliffe and Brasier, The terminal Ediacaran skeletonizing organisms Cloudina, Namacalathus, Nemapoika, and Sinotubulites have been considered total-group cnidarians Wood,but this comparison has no material basis.

Liu and colleagues described c. Thus, Olivooides multisulcatus, known from embryonic and post-embryonic stages of development including a polyp theca and medusa stage, is the oldest phylogenetically secure crown-cnidarian. Its pentaradiate symmetry has led to speculative hypotheses of an affinity to echinoderms, however, O.

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