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      Reappraisal of Europe’s most complete Early Cretaceous plesiosaurian: Brancasaurus brancai Wegner, 1914 from the “Wealden facies” of Germany

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          Abstract

          The holotype of Brancasaurus brancai is one of the most historically famous and anatomically complete Early Cretaceous plesiosaurian fossils. It derived from the Gerdemann & Co. brickworks clay pit near Gronau (Westfalen) in North Rhine-Westphalia, northwestern Germany. Stratigraphically this locality formed part of the classic European “Wealden facies,” but is now more formally attributed to the upper-most strata of the Bückeberg Group (upper Berriasian). Since its initial description in 1914, the type skeleton of B. brancai has suffered damage both during, and after WWII. Sadly, these mishaps have resulted in the loss of substantial information, in particular many structures of the cranium and limb girdles, which are today only evidenced from published text and/or illustrations. This non-confirmable data has, however, proven crucial for determining the relationships of B. brancai within Plesiosauria: either as an early long-necked elasmosaurid, or a member of the controversial Early Cretaceous leptocleidid radiation. To evaluate these competing hypotheses and compile an updated osteological compendium, we undertook a comprehensive examination of the holotype as it is now preserved, and also assessed other Bückeberg Group plesiosaurian fossils to establish a morphological hypodigm. Phylogenetic simulations using the most species-rich datasets of Early Cretaceous plesiosaurians incorporating revised scores for B. brancai, together with a second recently named Bückeberg Group plesiosaurian Gronausaurus wegneri ( Hampe, 2013), demonstrated that referral of these taxa to Leptocleididae was not unanimous, and that the topological stability of this clade is tenuous. In addition, the trait combinations manifested by B. brancai and G. wegneri were virtually identical. We therefore conclude that these monotypic individuals are ontogenetic morphs and G. wegneri is a junior synonym of B. brancai. Finally, anomalies detected in the diagnostic features for other “Wealden” plesiosaurians have prompted reconsiderations of interspecies homology versus intraspecific variability. We therefore propose that the still unresolved taxonomy of B. brancai should emphasize only those character states evident in the examinable fossil material, and specifically accommodate for growth-related modifications delimited via osteologically mature referred specimens.

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          Mesozoic marine tetrapod diversity: mass extinctions and temporal heterogeneity in geological megabiases affecting vertebrates.

          The fossil record is our only direct means for evaluating shifts in biodiversity through Earth's history. However, analyses of fossil marine invertebrates have demonstrated that geological megabiases profoundly influence fossil preservation and discovery, obscuring true diversity signals. Comparable studies of vertebrate palaeodiversity patterns remain in their infancy. A new species-level dataset of Mesozoic marine tetrapod occurrences was compared with a proxy for temporal variation in the volume and facies diversity of fossiliferous rock (number of marine fossiliferous formations: FMF). A strong correlation between taxic diversity and FMF is present during the Cretaceous. Weak or no correlation of Jurassic data suggests a qualitatively different sampling regime resulting from five apparent peaks in Triassic-Jurassic diversity. These correspond to a small number of European formations that have been the subject of intensive collecting, and represent 'Lagerstätten effects'. Consideration of sampling biases allows re-evaluation of proposed mass extinction events. Marine tetrapod diversity declined during the Carnian or Norian. However, the proposed end-Triassic extinction event cannot be recognized with confidence. Some evidence supports an extinction event near the Jurassic/Cretaceous boundary, but the proposed end-Cenomanian extinction is probably an artefact of poor sampling. Marine tetrapod diversity underwent a long-term decline prior to the Cretaceous-Palaeogene extinction.
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            The pachypleurosaurids (Reptilia: Nothosauria) from the middle triassic of Monte San Giorgio (Switzerland) with the description of a new species.

            The largest and most diverse collection of Pachypleurosauridae (Nothosauria, Reptilia) comes from Monte San Giorgio, Switzerland. Several hundred complete skeletons were collected from four distinct horizons of bituminous limestones and shales of Anisian-Ladinian boundary to early Ladinian age (Middle Triassic). Serpianosaurus mirigiolensis comes from the oldest strata, the Grenzbitumenzone Beds. The three younger strata, all in the Lower Meride Limestone, yield three species of Neusticosaurus. Neusticosaurus pusillus comes from the Cava Inferiore horizon, Neusticosaurus peyeri, new species, from the Cava Superiore horizon, and Neusticosaurus edwardsii, new combination, from the Alla Cascina horizon. Neusticosaurus pusillus is biostratigraphically important because it is one of the rare species reported from both the Germanic and the Alpine Triassic. Neusticosaurus pusillus and N. peyeri are small and very similar in their anatomy. Neusticosaurus species are easiest separated by their number of presacral vertebrae. Ornamentation of the bone surface is distinctive for all four pachypleurosaurids. Soft parts are rarely preserved, except for one partial squamation. The biological age of Neusticosaurus individuals can be determined by skeletochronology (aging by bone annuli). Small species of Neusticosaurus were sexually mature after three to four years and lived for six to nine years. Taphonomic analysis of the small species indicates attritional mortality and suggests weak bottom currents in the Monte San Giorgio basin during early Ladinian times. Morphometric comparison of all four pachypleurosaurids indicates that the changing vertebral numbers between species are largely due to a change in number of segments. All Monte San Giorgio pachypleurosaurids are sexually dimorphic in forelimb development. Sex x has poorly differentiated and relatively short humeri whereas sex y has well differentiated and relatively long humeri. The sexes are of about the same size and represented in roughly equal numbers. Identification of gender was not possible. Good growth series, especially of Neusticosaurus peyeri, from embryo to large adult permitted qualitative and quantitative study of ontogeny. The skull grows with negative allometry; the humerus grows isometrically or with positive allometry, depending on sex and species; the femur grows isometrically. The adult size range in N. peyeri is considerably larger than in modern reptiles. The Monte San Giorgio pachypleurosaurids are a monophyletic group. The phylogeny of this group is congruent with the stratigraphic distribution of its members.
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              Faunal turnover of marine tetrapods during the Jurassic-Cretaceous transition.

              Marine and terrestrial animals show a mosaic of lineage extinctions and diversifications during the Jurassic-Cretaceous transition. However, despite its potential importance in shaping animal evolution, few palaeontological studies have focussed on this interval and the possible climate and biotic drivers of its faunal turnover. In consequence evolutionary patterns in most groups are poorly understood. We use a new, large morphological dataset to examine patterns of lineage diversity and disparity (variety of form) in the marine tetrapod clade Plesiosauria, and compare these patterns with those of other organisms. Although seven plesiosaurian lineages have been hypothesised as crossing the Jurassic-Cretaceous boundary, our most parsimonious topology suggests the number was only three. The robust recovery of a novel group including most Cretaceous plesiosauroids (Xenopsaria, new clade) is instrumental in this result. Substantial plesiosaurian turnover occurred during the Jurassic-Cretaceous boundary interval, including the loss of substantial pliosaurid, and cryptoclidid diversity and disparity, followed by the radiation of Xenopsaria during the Early Cretaceous. Possible physical drivers of this turnover include climatic fluctuations that influenced oceanic productivity and diversity: Late Jurassic climates were characterised by widespread global monsoonal conditions and increased nutrient flux into the opening Atlantic-Tethys, resulting in eutrophication and a highly productive, but taxonomically depauperate, plankton. Latest Jurassic and Early Cretaceous climates were more arid, resulting in oligotrophic ocean conditions and high taxonomic diversity of radiolarians, calcareous nannoplankton and possibly ammonoids. However, the observation of discordant extinction patterns in other marine tetrapod groups such as ichthyosaurs and marine crocodylomorphs suggests that clade-specific factors may have been more important than overarching extrinsic drivers of faunal turnover during the Jurassic-Cretaceous boundary interval.
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                Author and article information

                Contributors
                Journal
                PeerJ
                PeerJ
                peerj
                peerj
                PeerJ
                PeerJ Inc. (San Francisco, USA )
                2167-8359
                22 December 2016
                2016
                : 4
                Affiliations
                [1 ]Naturkundemuseum Bielefeld, Abteilung Geowissenschaften , Bielefeld, Germany
                [2 ]Engelskirchen, Germany
                [3 ]Hamburg, Germany
                [4 ]Museum of Evolution, Uppsala Universitet , Uppsala, Sweden
                Article
                2813
                10.7717/peerj.2813
                5183163
                ©2016 Sachs et al.

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ) and either DOI or URL of the article must be cited.

                Funding
                The authors received no funding for this work.
                Categories
                Evolutionary Studies
                Paleontology
                Taxonomy

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