Cephalopoda is the only class of molluscs in which virtually all its modern representatives have a pair of powerful jaws. There is little doubt that jaws have contributed to the evolutionary success of cephalopods, but their origin still remains a mystery. Though cephalopods appeared at the end of the Cambrian, the oldest unequivocal jaws have been reported to date from the Late Devonian, though they were initially interpreted as phyllopod crustaceans of the suborder Discinocarina. After their relation with ammonoids was proven, they were considered as opercula, and only later their mandibular nature was recognized and widely accepted. Finds of discinocarins from Silurian deposits are still considered as opercula of ammonoid ancestors - nautiloids of the order Orthocerida. However, according to modern ideas, there is no place within their soft body for the location of such large opercula. Moreover, the repeated appearance of very similar structures in the same evolutionary line at least twice, but in different places of the body and for different purposes seems highly improbable. A new hypothesis is proposed herein, in which the Silurian fossils, earlier assigned to Discinocarina, are not specialized opercula, but protective shields, to defend orthocerids not from the predators, but from their own prey. The chitinous plates around the mouth likely appeared in the Silurian orthocerids for protection from such damage and later, during Silurian and Devonian, most likely gradually evolved into the jaws.

中文翻译：

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早古生代Discinocarina：头足类颌骨出现的关键

头足类是唯一一类软体动物，其​​中几乎所有现代代表都有一对有力的下颚。毫无疑问，下颚对头足类动物的进化成功做出了贡献，但它们的起源仍然是一个谜。尽管头足类动物出现在寒武纪末期，但据报道，迄今为止最古老的明确颌骨来自晚泥盆世，尽管它们最初被解释为盘尾亚目的叶足类甲壳类动物。在它们与菊石的关系被证实后，它们被认为是鳃盖，直到后来它们的下颌特性才被认可并被广泛接受。从志留纪矿床中发现的 discinocarins 仍然被认为是菊石祖先的鳃盖 - Orthocerida 目鹦鹉螺。然而，按照现代观念，如此大的鳃盖在它们柔软的身体中是没有位置的。此外，非常相似的结构在同一进化路线中至少重复出现两次，但在身体的不同部位和用于不同目的似乎极不可能。本文提出了一个新的假设，其中志留纪化石，早先被分配到 Discinocarina，不是专门的鳃盖，而是保护盾，不是为了保护直角科动物免受捕食者的侵害，而是保护它们自己的猎物。嘴部周围的几丁质板很可能出现在志留纪的正切科动物中以保护免受这种损害，后来，在志留纪和泥盆纪，很可能逐渐演变成颌部。但在身体的不同部位和不同的目的似乎极不可能。本文提出了一个新的假设，其中志留纪化石，早先被分配到 Discinocarina，不是专门的鳃盖，而是保护盾，不是为了保护直角科动物免受捕食者的侵害，而是保护它们自己的猎物。嘴部周围的几丁质板很可能出现在志留纪的正切科动物中以保护免受这种损害，后来，在志留纪和泥盆纪，很可能逐渐演变成颌部。但在身体的不同部位和不同的目的似乎极不可能。本文提出了一个新的假设，其中志留纪化石，早先被分配到 Discinocarina，不是专门的鳃盖，而是保护盾，不是为了保护直角科动物免受捕食者的侵害，而是保护它们自己的猎物。嘴部周围的几丁质板很可能出现在志留纪的正切科动物中以保护免受这种损害，后来，在志留纪和泥盆纪，很可能逐渐演变成颌部。