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.鞘亚纲头足类（Coleoid cephalopods）以壳体内化为典型特征，可分为十腕总目（Decabrachia，包含乌贼与墨鱼）和八腕总目（Vampyropoda，包含章鱼与吸血鬼乌贼）两大类。该类群拥有丰富的化石记录，其化石多产自自身有限的生物矿化骨骼结构：腕钩、平衡石、口器（口球，buccal mass）以及内壳（gladius 或称内鞘、pen 或称枪骨）；不过全球多处化石库（Lagerstätten）中已发现多例保存精美的鞘亚纲软躯体化石。 近期研究显示，尽管现存十腕类的内鞘与化石标本存在形态相似性，但目前尚未发现公认的化石十腕类标本。不过，分子钟（molecular clock）数据与系统发育括步法（phylogenetic bracketing）均表明，八腕类软躯体化石富集的化石库（如黎巴嫩白垩纪的Hâkel与Hâdjoula化石库）中理应存在十腕类化石。 我们提出，鞘亚纲类群间浮力调控方式的差异所带来的迄今未被认知的埋藏学偏差（taphonomic bias），会限制该类群的化石保存潜力。负浮力与中性浮力的十腕类均采用化学手段调控浮力（依赖氨），而八腕类则不具备该机制。在快速埋藏且满足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置换（authigenic calcium phosphate replacement）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，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留存易变质软组织的原因之一。