VARIATION IN SPIRE INDEX OF SOME COILED GASTROPOD SHELLS, AND ITS EVOLUTIONARY SIGNIFICANCE
VARIATION IN SPIRE INDEX OF SOME COILED GASTROPOD SHELLS, AND ITS EVOLUTIONARY SIGNIFICANCE
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DOI：
10.1098/rstb.1977.0026
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发表时间：
1977-01-01
影响因子：
6.3
通讯作者：
CAIN, AJ
中科院分区：
文献类型：
--
作者：
CAIN, AJ
螺旋指数[高度/壳的最大直径（h/d）]是对大多数陆生和淡水腹足类壳的螺旋壳形状的相当充分的测量，但对于具有刺、凸缘和喷口的复杂的海洋壳来说则不那么充分。只有成年自由爬行的形式与几个轮，能够完全缩回到壳，被认为是。在西欧陆生动物区系的柄眼目中，螺旋指数的分布明显是双峰的，具有大约3和大约0.5的值的模式分别对应于具有高到非常高的螺旋（和小螺旋角）的壳和从或多或少球形或次摆线到非常扁平和盘状（螺旋角从60 °到100 °）变化的壳。180 °）。同样的2种模式在分类学上不同的美国陆生茎突动物中，以及在波多黎各（加勒比海）和新喀里多尼亚（西南太平洋）的动物群中发现。基索孔动物也显示2种，相当不同的模式。北美海洋古腹足类主要是等维的，但有一些盘状的形式和极少数的高spiral的，海洋中腹足类主要是高spiral的，但与盘状的形式，新腹足类高spiral，和相关的euthyneurans急剧双峰，如stylommatophorans。古生代的古腹足类化石比现代的种类更加多变。有一些迹象表明，随着新腹足类的丰富，它们的种类变得有限，但自古生代以来，海洋盘状壳的比例也明显减少。h/d的众数是大型分类群的特征，但不受分类学限制，因为给定的值可能以众数的特定、属或亚科变体出现，并且偶尔以不相关的形式出现。模态值和广义生态特征之间也没有广泛的联系。由于几乎所有的值都发生在某个组或另一个组中，因此没有机械要求可以用来解释这种变化。在陆地上，柱眼亚目对广泛的生态学有足够的了解，这表明在极端的栖息地中，大小或壳形非常不同的物种可能会出现在一起，而具有相似壳的广义饲养者可能会表现出生态或地理上的分离（但在这种情况下，也是生态上的分离）。由于不同形状的壳在被视为要搬运的负载时具有不同的机械特性，因此初步建议它们可能与不同物种通常行走的位置有关，从而与它们偏好的进食地点有关。这可以解释一种明显的趋势，即不同的分类类群在世界不同地区占据h/d散布的同一部分，同一地区的许多类群占据散布的不同部分，或许还可以解释自古生代以来，新腹足类明显地将古腹足类排除在散布的一部分之外。有人认为，所发现的分布只能用自然选择来解释。
The spire index [height/maximum diameter of shell (h/d)] is a fairly adequate measure of the shape of the coiled shell of most terrestrial and freshwater gastropod shells but less so in complex marine shells with thorns, flanges and spouts. Only adult free-crawling forms with several whorls, able to retract completely into the shell, are considered. In the Stylommatophora of the Western European terrestrial fauna the distribution of the spire index is markedly bimodal, the modes, with values of about 3 and about 0.5, corresponding respectively to shells with a high to very high spire (and small spire angle) and those varying from more or less globular or trochoid to very flattened and disk-like (spire angle from 60.degree.-180.degree.). The same 2 modes are found in the taxonomically different terrestrial stylommatophorans of the USA, and in the faunas of Puerto Rico (Caribbean) and New Caledonia (southwest Pacific). Basommatophorans also show 2, rather different, modes. North American marine archaeogastropods are mainly equidimensional but with a few disk-like forms and a very few high-spired ones, marine mesogastropods are mainly high-spired but with disk-like forms, neogastropods high-spired, and relevant euthyneurans sharply bimodal, like the stylommatophorans. Fossil archaeogastropods of the Paleozoic were much more variable than modern forms. There is some indication that they became restricted in variety as caenogastropods became abundant, but also that the proportion of marine disk-like shells has decreased markedly since the Paleozoic. Modes of h/d are characteristic of large taxonomic groups but not taxonomically restricted since given values may appear as specific, generic or subfamilial variants from a mode, and appear sporadically in unrelated forms. There is also no broad association between modal value and broad ecological characters. Since nearly all values do occur in some group or other, no mechanical requirement can be invoked to explain such variation. In the land Stylommatophora enough is known of the broad ecology to suggest that in extreme habitats species with very different size or shell-shape may occur together, and that generalized feeders with similar shells may show separation, ecological or geographical (but in that case, also ecological). Since different shapes of shell will have different mechanical characteristics when considered as burdens to be carried, it is suggested tentatively that they may be related to the positions in which different species normally walk and hence to their preferred feeding places. This would explain an apparent tendency for different taxonomic groups to occupy the same part of the scatter of h/d in different regions of the world, for many groups in the same region to occupy different portions of the scatter, and perhaps the apparent exclusion by caenogastropods of archaeogastropods from part of the scatter since the Paleozoic. It is argued that the distributions discovered are explicable only by natural selection.