Data from: Buoyancy mechanisms limit preservation of coleoid cephalopod soft tissues in Mesozoic Lagerstätten

Coleoid cephalopods are characterized by internalization of their shell, and are divided into the ten-armed Decabrachia (squids and cuttlefish) and the eight-armed Vampyropoda (octopuses and vampire squid). They have a rich fossil record predominantly of the limited biomineralized skeletal elements they possess: arm hooks, statoliths, mouthparts (the buccal mass) and internal shell (gladius or pen), although exquisitely preserved soft tissue coleoids are known from several Lagerstätten worldwide. Recent studies have shown that although morphological similarities between extant decabrachian gladii and fossil examples exist, no known examples of fossil decabrachians are currently known. However, molecular clock data and phylogenetic bracketing suggest that they should be present in Lagerstätten that are rich in vampyropod soft tissue fossils (i.e. Hâkel and Hâdjoula Lagerstätten, Cretaceous, Lebanon). We propose that a hitherto unknown taphonomic bias pertaining to the differing methods of buoyancy control within coleoid groups limits preservation potential. Both negatively and neutrally buoyant decabrachians use chemical buoyancy control (ammonia) whereas vampyropods do not. In the event of rapid burial in an environment conducive to exceptional preservation, ammonia dramatically decreases the ability of the decabrachian carcass to generate the required pH for authigenic calcium phosphate replacement, limiting its preservation potential. Moreover, the greater surface area and comparatively fragile dermis further decrease the potential for fossilization. This taphonomic bias may have contributed to the lack of preserved labile soft-tissues in other cephalopods groups such as the ammonoids.鞘状头足类（Coleoid cephalopods）以壳内移为核心鉴别特征，可划分为十腕总目（Decabrachia，涵盖乌贼与墨鱼）及八腕的幽灵蛸总目（Vampyropoda，涵盖章鱼与幽灵蛸）。该类群拥有丰富的化石记录，其化石多源自自身有限的生物矿化骨骼结构：腕钩、平衡囊（statoliths）、口器（口球，buccal mass）以及内壳（角质鳍条或壳针，gladius or pen）。尽管如此，全球多处化石宝库（Lagerstätten）中均已发现保存极为精美的鞘状头足类软组织化石标本。近期研究显示，尽管现生十腕类的角质鳍条与化石标本存在形态相似性，但目前尚未发现公认的十腕类化石实例。不过，分子钟数据与系统发育括号法（phylogenetic bracketing）表明，十腕类应当存在于富含幽灵蛸总目软组织化石的化石宝库中，例如黎巴嫩白垩纪的哈克尔（Hâkel）与哈朱拉（Hâdjoula）化石宝库。我们提出，此前未被重视的、与鞘状头足类不同类群浮力控制方式相关的埋藏学偏差（taphonomic bias），限制了其化石保存潜力。带负浮力与中性浮力的十腕类均通过化学机制调控浮力（依赖氨），而幽灵蛸总目则不具备该特征。在快速埋藏且利于特异保存的环境中，氨会显著降低十腕类尸体生成自生磷酸钙置换所需pH值的能力，进而削弱其化石保存潜力。此外，十腕类更大的体表面积与相对脆弱的真皮进一步降低了其化石形成的可能性。该埋藏学偏差或许也是其他头足类类群（如菊石类）缺乏易降解软组织化石记录的原因之一。