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.鞘形亚纲头足类（Coleoid cephalopods）以壳体内化为典型鉴别特征，可划分为十腕总目（Decabrachia，含乌贼与墨鱼）和八腕总目（Vampyropoda，含章鱼与吸血鬼乌贼）两大类群。该类群拥有丰富的化石记录，其化石多由自身有限的生物矿化骨骼结构构成：腕钩、平衡囊、口器（buccal mass，口球）以及内壳（gladius或pen）；不过全球多处特异埋藏化石库（Lagerstätten）中，均发现过保存极为精美的鞘形亚纲头足类软组织化石。近期研究显示，尽管现生十腕类的内壳与化石标本存在形态相似性，但目前尚无已确认的十腕类化石标本被报道。然而，分子钟数据与系统发育括号法（phylogenetic bracketing）的推断结果表明，这类类群应当存在于富含八腕类软组织化石的特异埋藏化石库中——即黎巴嫩白垩纪的Hâkel与Hâdjoula特异埋藏化石库。本研究提出，此前未被重视的、与鞘形亚纲不同类群浮力调控方式差异相关的埋藏学偏差，限制了其化石保存潜力。负浮力与中性浮力的十腕类均依靠氨实现化学浮力调控，而八腕类则不具备该机制。在快速埋藏且具备特异保存条件的环境中，氨会显著降低十腕类尸体生成自生磷酸钙交代作用所需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更脆弱的真皮层，进一步降低了其化石形成的可能性。这一埋藏学偏差或许也是菊石类（ammonoids）等其他头足类类群中难以发现易变质软组织化石的重要原因之一。