Emberton K C (38 results)

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The shell convergence between Neohelix dentifera and Inflectarius ferrissi is an important key to understanding the unusual evolutionary pattern of polygyrid land snails in eastern North America. This group is characterized by conchologically distinct shell-static clades that have radiated ecologically to the extent that shell shape and habitat are not detectably correlated at the species level; furthermore, entire shell-static clades converge conchologically between anatomically distinct lineages (triodopsines and polygyrines). The triodopsine N. dentifera and the polygyrine I. ferrissi both represent unique, drastic shifts from their shell-static clades, and thus are possible founders of new shell-static clades. This study shows that, within a context of high intraspecific variance, N. dentifera and I. ferrissi exhibit ecological parallelism or convergence in addition to conchological convergence. In coarse-grained ecology, both are more associated with acidic, anionic, deep, low- density soils in talus in high-altitude, exposed non-oak-hickory forests than are their close relatives, even though the two are not very close to each other. In fine-grained ecology, N. dentifera converges on I. ferrissi by increased rock-association relative to outgroups, and I. ferrissi likewise converges on N. dentifera by decreased log association. Thus the shell shape unique to these two species appears to be adaptive, whereas their intraspecific ecological variances appear high enough to permit ecological radiation. 16 pp., 4 figs, gr. 8.…

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Nine species of Madagascan acavid land snails were compared in a phylogenetic context. The two most plesiomorphic, Clavator johnsoni and C. moreleti, differ from the others by their high-spired shells, short tentacles, short tails, long necks, and crawling mode of hitching the shell along the ground. In the seven more apomorphic species, the crawling mode is smooth, with the shell resting on the tail, and the relative lengths of tail and shell correlate significantly. Among these seven species, three pairs of closest relatives (Helicophanta petiti and H. uesicalis, H. farafanga and H. souverbiana, Ampelita decaryi and A. julii) show evidence of phylogenetic constraints on ranked shell size. Aestivation site (as tentatively inferred from rare data) does not correlate with shell shape or size: burrowers have H/D = 2.7 to 0.6 and D = 70 to 25 mm; arboreals have H/D = 0.8 to 0.5 and D = 70 to 30 mm; the species with both the highest spire and the smallest diameter (C. moreleti) is neither a burrower or an arboreal, but stays on the ground surface. Inferred aestivation sites are randomly distributed phylogenetically. Climate shows no correlation, except that the arborcals are only from humid to wet regimes. Uniform shell colouration occurs only in burrowers (C. johnsoni, H. petiti, H. vesicalis), but disruptive shell colouration occurs in all others, including burrowers (H. farafanga, A. decaryi), ground-surface aestivators (C. moreleti), arboreals (H. souverbiana, A. julii), and semi-arboreals (Ampelita subfunebris). Among all nine species, burrowers have significantly thicker shells (than their close relatives of similar size), wider bodies, and longer snouts than non-burrowers (H. souverbiana is exceptional in being arboreal despite its huge size and in having the broad foot and snout of a burrower). Thus, although there is some evidence for phylogenetic constraints, natural selection for aestivation and crawling behaviours seems to have dominated the evolution of external body morphology and of shell thickness (but not shell size and shape) in these snails. 13 pp., 3 figs, gr. 8.…

Shells of Polygyra septemvolva (Say 1818) are well-known for high within-populational variation, but have not previously been quantitatively assessed. Analyses of shell preapertural diameter, axial height, and whorl count were performed on 314 live adult shells collected from approximately 3 square meters of uniform habitat. All three measurements showed unimodal, leptokurtic, skewed, significantly non-normal distributions. Variation was enormous: diameter ranged 75% (max-min/min), height 77%, whorl-count 43%, and height/diameter 64%. Very large individuals were sparsely and disjunctly distributed from the general clusters of specimens in principal-component space; they appear to arise by increased whorl increment, which results in flat shells of either very tight or very loose coiling. Removal of large outliers had almost no effect on distributions of shell variables. Of the total measured variation, overall size (Principal Component 1) explained 76%, spire-height (PC2) explained 17%, and tightness of coiling (PC3) 7%; shells having extreme values of these principal components are illustrated. Possible causes of P. septemvolva s pattern of intrapopulational shell variation are discussed, but no conclusions are drawn because of a lack of basic biological data on this or any related species. 10 pp., 7 figs, gr. 8.…

In the southern Appalachian region of North America, the phylogenetically convergent shells of the polygyrid snails Triodopsinae Neohelix major (Binney) and Polygyrinae Mesodon normalis (Pilsbry) are even more convergent in size and shape in sympatry (7 sites) than in allopatry (23 and 10 sites). Environmental correlations account for 34% and 30% of size and shape variations in N. major (larger, taller, and more loosely coiled at northern, high-altitude, sheltered sites), but for only 14% and 9% in M. normalis (larger, flatter, and more loosely coiled at south-facing, exposed sites). The statistical significance of the sympatric convergence dropped out when these correlations were removed. This phenomenon helps account for the many cases in eastern North America of nearly identical land-snail shells in sympatry and questions the importance of competitive character displacement in the evolution of land-snail shell morphology. This apparently nonmimetic case of sympatric convergence provides an unusually precise and well-delimited, naturally replicated experiment in evolutionary morphology, which is analyzed for controlling factors in a follow-up paper. 7 pp., 4 figs, 4. …

In the first known attempt to use a sympatric, non-mimetic convergence (?a naturally controlled experiment in evolutionary morphology?) to quantitatively partition a morphology among its controlling factors, shells of the polygyrid land snails Neohelix major (Binney) and Mesodon normalis (Pilsbry) were analysed in a factorially designed rearing experiment. The complete absence of character displacement or character release confirmed previous field studies. Growth rate, a determinant of adult shell size, was controlled 50% by direct environmental induction (with temperature twice as effective as humidity), 10% by proximate natural selection (with a shorter growing season selecting for faster growth), 10% by genotype-environment interaction (with moisture generally inducing a greater growth surge in a drier climate), and 30% by within-cohort variation (apparently heritable). Spire height confirmed predictions that flatter shells make better estivators (shelterers from drought) and that taller shells make better climbers and tall/flat niches are readily filled: high within-cohort variation (70% of total) provides the raw material for niche-filling (but is reduced in severe climates by stabilizing selection); environmental induction (10%) is strictly by humidity, which makes shells taller for foraging more widely under more favourable conditions; proximate natural selection (10%) produces taller shells in moister climates (with intensity of effect dependent on evolutionary time); and genotype-environment interaction (10%) makes the drought-induction of flat shells stronger in more drought-prone areas. Coiling tightness, allied to whorl-expansion rate, shows distinct evidence of adaptive neutrality: 60% of total variance is explained by phylogenetic constraints despite an estimated 120 million years of genetic isolation and despite the demonstrated ability of related clades to converge; the 10% due to genotype-environment interaction is generally random and nonadaptive in its response; and the remaining 30% appears random, non-induced, and non-selected. This study s protocol of experimentally partitioning a ?closed universe? of morphology among a full set of controlling factors is recommended as an alternative to the usual practice of analysing the effects of one or a subset of factors on an ?open universe? of morphology. One advantage of this new protocol is that it obviates the ?nature-nurture debate? by quantifying the relative contributions of each, which in this snail-shell case differ drastically among the various components of a single morpholocgyy. 17 pp., 2 figs, gr. 8. …

The family of polygyrid land snails in North America is significant for its sympatric shell convergences, diversity of mating systems and complex zoogeography; its monophyly and its relation to other families has long been questionable. Cladistic analysis has been performed on one representative each of 17 subfamilies, including all three polygyrid subfamilies and one or more subfamilies each from all ten of the families that have been proposed as the polygyrid sister groups. Eighteen anatomical synapomorphies are used, of which eight are newly discovered, four are differently assessed from previous studies, and six are traditional. The resulting strict consensus tree of alternative maximum-parsimony cladograms is: (Acavidae (Ammonitellidae Corillidae ((Discidae Oreohelicidae) (Helminthoglyptidae Bradybaenidae Polygyridae (Thysanophoridae (Camaenidae Sagdidae)))))). According to this working hypothesis, the Polygyridae are monophyletic, and their sister group remains unresolved, although the Acavidae, Ammonitellidae, Corillidae, Discidae and Oreohelicidae can be ruled out. Of the five classifications of stylommatophoran families that have been proposed in the past 12 years, the consensus tree is closest to that of Nordsieck. For future morphological work, three regions are recommended as potentially rich in unused phylogenetic information: the fertilization pouch-seminal receptacle complex, the ureter at the pneumostome and the ventral-chain ganglia. Simultaneous dissection, with side-by-side comparisons, is recommended over other methods for detecting homologies in land snails. Molecular characters should be exploited, because of the scarcity and the frequent homoplasy of morphological characters. 18 pp., 5 figs, gr. 8.…

Madagascan acavids currently comprise 99 species, for 21 of which some anatomical data have been published. Cladistic analysis was performed on 19 of these species, based on 11 apomorphies (6 of them synapomorphies) found in illustrations of the genitalia and radulae, and in the embryonic sculptures of examined shells. The resulting maximum-parsimony tree (consistency index = 1.00) supported the conchologically defined genera Ampelita, Clavator, Helicophanta, and Leucotaenius. Extreme anatomical variation in Ampelita called for its division into four subgenera that are also conchologically distinct: Ampelita sensu stricto; Eurystyla; Vescona, subgen. n. (type A. robillardi); and Xystera, subgen. n. (type A. xystera). According to the cladogram, A. (Ampelita) is paraphyletic. Anatomically unknown species were tentatively classified based on shell morphology. Ampelita covani is removed to the Rhytididae, and is the first member of that family reported from Madagascar. Because of the Madagascan acavids are in great danger of extinction, immediate action is required to build on the scarce available data and to aid their survival. 17 pp., 6 figs, gr. 8.…

A fresh assessment of reproductive behaviour and anatomy, combined with new allozyme data, results in a phylogenetic hypothesis for the 23 polygyrid genera that differs substantially from previous attempts. Analyses based on this phylogeny suggest that (a) evolutionary transition from internal to external sperm exchange occurred only once in polygyrids, with unusual functional intermediates still extant; (b) polygyrid biogeographic history paralleled that of plethodontid salamanders, with hypothesized vicariance events at about 145, 120, 65 and 40 MA, and, dispersal events at about 55 and 40 MA; and (c) iterative shell evolution occurred despite a phylogenetic constraint on ontogenetic whorl-expansion rate. 31 pp., 9 figs, gr. 8.…

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42 pp., 17 figs, stapled gr. 8.

, 60 figs, stapled (Malacologia) 0.0.