The pteropods (sea butterflies) and heteropods (sea elephants) are two groups of holoplanktonic gastropods that are important components of the modern, open ocean, plankton. The geological record of the pteropods (Pteropoda) and heteropods (Pterotracheoidea) extends to at least the earliest Eocene (55 million years ago), although there are records of taxa in the earliest Jurassic and mid-Late Cretaceous that may be planktonic gastropods. The more complete fossil record of pteropods and heteropods begins in the early Paleogene, coinciding with the transition from a calcitic ocean to an aragonitic ocean and it is possible that these aragonitic gastropods either appeared at that time or certainly commenced their wider diversification. While known diversity or species richness are subject to taxonomic ‘distortion’, especially when comparing biological data with palaeobiological data, pteropod and heteropod diversity appears to have been affected by changes in global climate. The heteropods may have been the product of our modern, thermohaline-driven ocean, while the pteropods diversified during the hyperthermal events of the early to mid-Eocene but also survived the transition to the modern, thermohaline-driven ocean and the onset of the present ‘icehouse world’.

翼足类（海蝶类）和异足类（海象类）是两类整体浮游腹足纲动物，它们是现代开放海洋浮游生物的重要组成部分。翼足类（Pteropoda）和异足类（Pterotracheoidea）的地质记录至少可以追溯到最早的始新世（5500万年前），尽管在侏罗纪和中晚白垩世有类群的记录，可能是浮游的腹足类动物。更完整的翼足类和异足类化石记录始于古近纪，与从钙质海洋向古生物海洋的过渡相吻合，这些古生物腹足动物可能在那时出现，或者肯定开始了其更广泛的多样化。虽然已知的多样性或物种丰富度会受到分类学上的“扭曲”，尤其是将生物学数据与古生物学数据进行比较时，翼足类和异足类的多样性似乎已受到全球气候变化的影响。异足类可能是我们现代的热盐驱动海洋的产物，而翼足动物在始新世中期至中新世的高温事件中多样化，但在过渡到现代的热盐驱动海洋和现在的开始期间仍然幸免于难。 “冰屋世界”。