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      Testing evolutionary constraint hypotheses with early Paleozoic gastropods

      Paleobiology
      Cambridge University Press (CUP)

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          Abstract

          The evolution of higher taxa among early Paleozoic gastropods is similar to that among early metazoans as a whole, as higher taxa diversified rapidly and early. There are two issues pertinent to this pattern. First, were greater morphologic changes concentrated in the early phases of evolution? Second, does the pattern better fit models of increasing phylogenetic constraints or increasing ecologic restrictions? This paper presents a phylogeny-based method designed to test whether amounts of morphologic evolution decreased over time. It also explores whether the data better fits models of increasing phylogenetic (i.e., developmental or genetic) constraint or increasing ecologic restriction. Two metrics of morphologic separation (i.e., the morphologic difference between sister-species) are used: (1) Euclidean distance in morphospace and (2) transition magnitude. The latter metric is calculated by a multivariate analysis of sister-species contrasts, which determines both types and magnitudes of morphologic transitions. The advantage of using transition magnitudes is that it balances the effects of transitions that either affect more morphometric characters or occur more frequently. Both metrics indicate that larger morphologic separations between sister-species were concentrated early in gastropod evolution. Among gastropods, gross shell morphology often reflects basic trophic strategy and function whereas basic internal anatomy does not. Transition magnitudes can be broken down into transitions associated with differences in basic trophic strategies and shell functional biology (“external”), and those associated with differences in basic internal anatomy (“internal”). Internal transition magnitudes show a highly significant decrease over time ( p < 10 –04) whereas external transition magnitudes show a much less significant decrease over time ( p < 0.10) and no significant decrease after the earliest Ordovician ( p ≅ 0.50). The results therefore suggest that increasing phylogenetic constraints played a greater role in the early evolution of gastropods than did increasing ecologic ones.

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          The operated Markov´s chains in economy (discrete chains of Markov with the income)

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            The Mesozoic marine revolution: evidence from snails, predators and grazers

            Tertiary and Recent marine gastropods include in their ranks a complement of mechanically sturdy forms unknown in earlier epochs. Open coiling, planispiral coiling, and umbilici detract from shell sturdiness, and were commoner among Paleozoic and Early Mesozoic gastropods than among younger forms. Strong external sculpture, narrow elongate apertures, and apertural dentition promote resistance to crushing predation and are primarily associated with post-Jurassic mesogastropods, neogastropods, and neritaceans. The ability to remodel the interior of the shell, developed primarily in gastropods with a non-nacreous shell structure, has contributed greatly to the acquisition of these antipredatory features.
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              Disparity as an evolutionary index: a comparison of Cambrian and Recent arthropods

              Disparity is a measure of the range or significance of morphology in a given sample of organisms, as opposed to diversity, which is expressed in terms of the number (and sometimes ranking) of taxa. At present there is no agreed definition of disparity, much less any consensus on how to measure it. Two possible categories of metric are considered here, one independent of any hypothesis of relationship (phenetics), the other constrained within an evolutionary framework (cladistics).
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                Author and article information

                Journal
                Paleobiology
                Paleobiology
                Cambridge University Press (CUP)
                0094-8373
                1938-5331
                1995
                February 08 2016
                1995
                : 21
                : 3
                : 248-272
                Article
                10.1017/S0094837300013294
                5e9c4ce3-b31c-46ee-be61-2a5f9c0a06ad
                © 1995

                https://www.cambridge.org/core/terms

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