The Natural History Museum, Cromwell Road SW7 5BD, London, UK.
Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20013, USA.
Proc Biol Sci. 2023 Feb 8;290(1992):20221907. doi: 10.1098/rspb.2022.1907.
Evolutionary adaptation to novel, specialized modes of life is often associated with a close mapping of form to the new function, resulting in narrow morphological disparity. For bivalve molluscs, endolithy (rock-boring) has biomechanical requirements thought to diverge strongly from those of ancestral functions. However, endolithy in bivalves has originated at least eight times. Three-dimensional morphometric data representing 75 species from approximately 94% of extant endolithic genera and families, along with 310 non-endolithic species in those families, show that endolithy is evolutionarily accessible from many different morphological starting points. Although some endoliths appear to converge on certain shell morphologies, the range of endolith shell form is as broad as that belonging to any other bivalve substrate use. Nevertheless, endolithy is a taxon-poor function in Bivalvia today. This limited richness does not derive from origination within source clades having significantly low origination or high extinction rates, and today's endoliths are not confined to low-diversity biogeographic regions. Instead, endolithy may be limited by habitat availability. Both determinism (as reflected by convergence among distantly related taxa) and contingency (as reflected by the endoliths that remain close to the disparate morphologies of their source clades) underlie the occupation of endolith morphospace.
进化适应新的、专门的生活方式通常与形态与新功能的紧密映射相关，导致形态差异狭窄。对于双壳类软体动物，内栖生活（岩石钻孔）具有与祖先功能明显不同的生物力学要求。然而，双壳类动物的内栖生活已经至少起源了八次。代表大约 94%现存内栖属和科以及这些科中的 310 种非内栖种的三维形态计量数据表明，内栖生活可以从许多不同的形态起点进化而来。尽管一些内栖动物似乎趋向于某些壳形态，但内栖壳形态的范围与任何其他双壳类基质利用的范围一样广泛。然而，内栖生活在双壳类中今天是一个分类群较少的功能。这种有限的丰富性不是源于起源于起源率或灭绝率显著较低的源类群，而且今天的内栖动物并不局限于多样性较低的生物地理区域。相反，内栖生活可能受到栖息地可用性的限制。决定论（反映在远缘分类群之间的趋同）和偶然性（反映在仍然接近其源类群不同形态的内栖动物）都为内栖形态空间的占据提供了基础。
