Department of Earth and Planetary Sciences, University of California, Davis, California, USA.
J Morphol. 2023 Mar;284(3):e21564. doi: 10.1002/jmor.21564.
Molluscan shells display a high diversity of external sculpture. Sculptural elements may be symmetrical, where both edges of an element are morphologically similar, or asymmetrical, where one edge is steeper than the other. Asymmetrical sculpture can be ratcheted, with the leading edges (those in the direction of locomotion or growth) less steep than the trailing edges, or imbricated (leading edges steeper than trailing edges). While the ratcheted sculpture is better known, the diversity of imbricated sculpture has remained largely unexplored. In a survey of extant benthic shell-bearing molluscs, we document imbricated sculpture primarily in epifaunal bivalves or on the exposed sectors of shells of semi-infaunal bivalves. Imbricated sculpture is particularly widespread in pteriomorphian bivalves, but it is absent in the subclade Mytiloidea as well as in highly mobile Pectinidae. It also occurs in many carditid bivalves (Archiheterodonta) and in phylogenetically scattered euheterodonts. In several infaunal bivalves including species of Cardites (Carditidae), Hecuba (Donacidae), and Chione (Veneridae), comarginal elements on the posterior sector are imbricated whereas anterior comarginal ridges are ratcheted. Imbricated sculpture in bivalves tends to be concentrated on the upper (left) valves of pectinids or on the posterior sector of both valves in archiheterodonts and euheterodonts. Imbricated sculpture is uncommon in gastropods, even in epifaunal species, but does occur in the collabral ridges in some Vasidae and a few other groups. Expression of imbricated sculpture does not depend on shell mineral composition or microstructure. The ecological distribution and within-shell pattern of expression of imbricated sculpture point to the likelihood that this type of asymmetrical sculpture is both widespread and potentially functional. Additionally, we present a potential methodology whereby shell sculpture categories (symmetrical, ratcheted, and imbricated) may be quantified by comparing the lengths of corresponding leading and trailing edges across the shell surface.
软体动物壳显示出高度的外部雕塑多样性。雕塑元素可以是对称的，即元素的两个边缘在形态上相似，也可以是不对称的，其中一个边缘比另一个更陡峭。不对称的雕塑可以是棘轮式的，即前缘（在运动或生长方向上）比后缘更陡峭，也可以是叠瓦式的（前缘比后缘更陡峭）。虽然棘轮式雕塑更为人所知，但叠瓦式雕塑的多样性在很大程度上仍未得到探索。在对现存的底栖有壳软体动物的调查中，我们记录了主要在附生双壳类或半底栖双壳类暴露壳扇区的叠瓦式雕塑。叠瓦式雕塑在翼形贝类中特别普遍，但在 Mytiloidea 亚群以及高度移动的 Pectinidae 中不存在。它也存在于许多扇贝（Architerodonta）和系统分散的真异齿贝类中。在一些底栖双壳类中，包括 Cardites（Carditidae）、Hecuba（Donacidae）和 Chione（Veneridae）的物种，后扇区的后缘元素是叠瓦式的，而前缘后缘脊是棘轮式的。双壳类中的叠瓦式雕塑往往集中在翼形贝类的上（左）壳或 Archiheterodonta 和真异齿贝类的两壳的后扇区。在腹足类中，即使是在附生种中，叠瓦式雕塑也不常见，但在一些 Vasidae 和其他几个群组的 Collabral 脊中确实存在。叠瓦式雕塑的表达并不依赖于壳的矿物组成或微观结构。叠瓦式雕塑的生态分布和壳内表达模式表明，这种类型的不对称雕塑很可能广泛存在且具有潜在功能。此外，我们提出了一种潜在的方法，通过比较壳表面上相应的前缘和后缘的长度，可以对壳雕塑类别（对称、棘轮和叠瓦）进行量化。
