102 research outputs found
Fullereneâlike structures of Cretaceous crinoids reveal topologically limited skeletal possibilities
There are few cases where numbers or types of possible phenotypes are known, although vast state spaces have been postulated. Rarely applied in this context, graph theory and topology enable enumeration of possible phenotypes and evolutionary transitions. Here, we generate polyhedral calyx graphs for the Late Cretaceous, stemless crinoids Marsupites testudinarius and Uintacrinus socialis (Uintacrinoidea Zittel) revealing structural similarities to carbon fullerene and fulleroid molecules (respectively). The U. socialis calyx incorporates numerous plates (e.g. graph vertices |V| â„ 197), which are small, light, lowâdensity and have four to eight sides. Therefore, the corresponding number of possible plate arrangements (number of polyhedral graphs) is large (â«1 Ă 1014). Graph vertices representing plates with sides >6 introduce negative Gaussian curvature (surface saddle points) and topological instability, increasing buckling risk. However, observed numbers of vertices for Uintacrinus do not allow more stable pentaradial configurations. In contrast, the Marsupites calyx dual graph has 17 faces that are pentagonal or hexagonal. Therefore, it is structurally identical to a carbon fullerene, specifically C30âD5h. Corresponding graph restrictions result in constraint to only three structural options (fullerene structures C30âC2v 1, C30âC2v 2 and C30âD5h). Further restriction to pentaradial symmetry allows only one possibility: the Marsupites phenotype. This robust, stable topology is consistent with adaptation to predation pressures of the Mesozoic marine revolution. Consequently, the most plausible evolutionary pathway between unitacrinoid phenotypes was a mixed heterochronic tradeâoff to fewer, larger calyx plates. Therefore, topological limitations radically constrained uintacrinoid skeletal possibilities but thereby aided evolution of a novel adaptive phenotype
Ammonite Faunal Dynamics Across Bio-Events During the Mid-and Late Cretaceous Along the Russian Pacific Coast
JagtâYazykova, E.A. 2012. Ammonite faunal dynamics across bioâevents during the mid â and Late Cretaceous along th
The genus Plesiophysa, with a redescription of P. ornata (Haas, 1938) (Gastropoda: Planorbidae)
Anatomical studies of sibling species within neotropical lymnaeids, snail intermediate hosts of fascioliasis
Upper Toarcian (Lower Jurassic) marine gastropods from the Cleveland Basin, England: systematics, palaeobiogeography and contribution to biotic recovery from the early Toarcian extinction event
Here we describe a new upper Toarcian (Lower Jurassic) marine gastropod fauna from rocks of the Cleveland Basin exposed on the North Yorkshire coast of England. The fossil assemblage consists of 16 species, of which three are new: Katosira ? bicarinata sp. nov., Turritelloidea stepheni sp. nov. and Striactaenonina elegans sp. nov. Four species are described in open nomenclature as Tricarilda ? sp., Jurilda sp., Cylindrobullina sp. and Cossmannina sp. The other species have previously been described: Coelodiscus minutus (SchĂŒbler in Zieten), Procerithium quadrilineatum (Römer), Pseudokatosira undulata (Benz in von Zieten), Palaeorissoina aff. acuminata (GrĂŒndel), Pietteia unicarinata (Hudleston), Globularia cf. canina (Hudleston), Striactaeonina cf. richterorum Schulbert & NĂŒtzel, Striactaenonina aff. tenuistriata (Hudleston) and Sulcoactaeon sedgvici (Phillips). Most of these species are the earliest records of their respective genera and show palaeobiogeographical connections with contemporary gastropod associations from other regions of Europe and South America. The taxonomic composition of the upper Toarcian Cleveland Basin gastropod assemblage differs substantially from the faunas of the upper Pliensbachian and lower Toarcian Tenuicostatum Zone, showing the strong effect of the early Toarcian mass extinction event on the marine gastropod communities in the basin. Only a few gastropod species are shared between the late Toarcian faunas and the much more diverse Aalenian gastropod faunas in the Cleveland Basin, suggesting that there was a facies control on gastropod occurrences at that time. This is also a potential explanation for the taxonomic differences between the late Toarcian gastropod faunas in the Cleveland Basin and those in France, and northern and southern Germany
Viaje a la America meridional : Brazil, Republica del Uruguay, Republica Argentina, La Patagonia, Republica del Chile, Republica de Bolivia, Republica de Peru realizado de 1826 a 1833
Histoire naturelle, g\ue9n\ue9rale et particuli\ue8re des c\ue9phalopodes ac\ue9tabuif\ue8res viants, et fossiles : comprenant: la description zoologique et anatomique de ces mollusques, des d\ue9tails sur leur organisation, leurs moeurs, leurs habitudes, et l\u27histoire des observations dont ils ont \ue9t\ue9 l\u27objet depuis les temps les plus recul\ue9s jusqu\u27\ue0 nos jours
t.1 [Text] (1835-1848
Histoire naturelle : generale et particuliere des cephalopodes acetabuliferes vivants et fossiles... /
t.2 (1835-1848
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