Inferences on the development and morphology of extinct brachiopods must be informed by the ontogeny and shell ornamentation of extant brachiopods. Although the adult shells of extant brachiopods are well studied, detailed descriptions of the embryonic and juvenile shells of extant lingulides are lacking. Here, we describe in detail the shells of juveniles of Lingula anatina Lamarck, 1801 from Vietnam and the Republic of the Philippines. The following previously unknown properties of the lingulide shell are described: (1) a distinct border between the protegulum and the brephic shell; (2) drapes that develop on both the protegulum and brephic shell; and (3) the notched anterior margin of the brephic shell. The drapes and cogs on the brephic shell may be caused by the formation of setal follicles during the planktonic stage. Specimens of L. anatina from the Philippines have larger brephic shells than those from Vietnam, probably because the former have a longer planktonic stage. Based on comparisons of the first-formed shells of extant brachiopods with published data on fossil brachiopods, we suggest that the life cycle of extant lingulides, in which planktotrophic juveniles with a shell hatch from the egg envelope, is the most evolutionarily advanced brachiopod life cycle and appeared in the early Silurian. We suggest criteria for determining the type of life cycle based on the structure of the first-formed shell of brachiopods. Finally, we consider hypothetical scenarios of life cycles of fossil brachiopods, including true planktotrophic larvae in the Cambrian linguliforms.

中文翻译：

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Lingula anatina (Brachiopoda: Linguliformea) 幼体壳的分析提供了对腕足动物化石生命周期演变的深入了解

对已灭绝腕足动物发育和形态的推断必须以现存腕足动物的个体发育和壳纹饰为依据。尽管对现存腕足动物的成虫壳进行了充分研究，但缺乏对现存舌足类的胚胎和幼体壳的详细描述。在这里，我们详细描述了少年的贝壳凤梨拉马克，1801 年来自越南和菲律宾共和国。描述了舌内酯壳的以下以前未知的特性：（1）前舌壳和胸骨之间的明显边界；(2) 在前壳和脐壳上发育的窗帘；(3) 臀骨前缘有缺口。蝶骨上的褶皱和嵌齿可能是由浮游阶段刚毛囊的形成引起的。标本L. anatina菲律宾的贝壳比越南的贝壳大，可能是因为前者的浮游阶段更长。基于对现存腕足动物最初形成的壳与已发表的腕足动物化石数据的比较，我们认为现存舌形动物的生命周期是进化上最先进的腕足动物生命周期，其中从卵壳孵化的浮游营养幼体并出现在志留纪早期。我们建议根据腕足动物最初形成的壳的结构来确定生命周期类型的标准。最后，我们考虑了腕足动物化石生命周期的假设情景，包括寒武纪舌形动物中真正的浮游营养幼虫。