Exaggerated morphologies present challenges for understanding the factors that enable, prevent or compel lineages to evolve unusual functional traits. Here, I examine the case of gastropods whose adult shells are bottom heavy by virtue of a thick, often broad, convex ventral callus deposit on the inner lip and adapical to the aperture. Data on the spatial, temporal and phylogenetic distribution of callus-bearing gastropods were gleaned from the literature and from personal observations on fossil and living gastropods in collections. An exaggerated ventral callus has evolved in ≥ 72 lineages of gastropods, all in warm, shallow waters, beginning in the Late Ordovician but mostly during and after the Late Cretaceous. The phylogenetic distribution of these gastropods is highly clumped, with multiple acquisitions in some clades (e.g. Stromboidea and Buccinoidea) and none in others (e.g. Conoidea). Functional considerations and comparisons indicate that the enhanced gravitational stability provided by a ventral callus evolved only in conditions of low-cost calcification and when predators capable of dislodging or overturning gastropods or causing them to flee were important agents of selection.

中文翻译：

---

平衡的生命：腹足动物腹壳重量的演变

夸张的形态对理解使、阻止或迫使谱系进化出不寻常的功能特征的因素提出了挑战。在这里，我研究了腹足类动物的情况，其成虫的壳底部很重，这是由于内唇上厚的、通常是宽的、凸出的腹侧愈伤组织沉积物位于开口的顶端。有关带有愈伤组织的腹足动物的空间、时间和系统发育分布的数据是从文献和对收藏中的化石和活腹足动物的个人观察中收集的。在 ≥ 72 个腹足动物谱系中已经进化出夸张的腹侧愈伤组织，它们都在温暖的浅水中，始于晚奥陶世，但主要是在晚白垩世期间和之后。这些腹足类动物的系统发育分布高度聚集，在一些进化枝（例如 Stromboidea 和 Buccinoidea），而在其他（例如 Conoidea）中没有。功能考虑和比较表明，腹侧愈伤组织提供的增强的重力稳定性仅在低成本钙化条件下进化，并且当能够驱逐或翻转腹足动物或导致它们逃跑的捕食者是重要的选择因素时。