Data from: A new approach using high-resolution computed tomography to test the buoyant properties of chambered cephalopod shells

The chambered shell of modern cephalopods functions as a buoyancy apparatus, allowing the animal to enter the water column without expending a large amount of energy to overcome its own weight. Indeed, the chambered shell is largely considered a key adaptation that allowed the earliest cephalopods to leave the ocean floor and enter the water column. It has been argued by some, however, that the iconic chambered shell of Paleozoic and Mesozoic ammonoids did not provide a sufficiently buoyant force to compensate for the weight of the entire animal, thus restricting ammonoids to a largely benthic lifestyle reminiscent of some octopods. Here we develop a technique using high-resolution computed tomography to quantify the buoyant properties of chambered shells without reducing the shell to ideal spirals or eliminating inherent biological variability by using mathematical models that characterize past work in this area. This technique has been tested on Nautilus pompilius and is now extended to the extant deep-sea squid Spirula spirula and the Jurassic ammonite Cadoceras sp. hatchling. Cadoceras is found to have possessed near-neutral to positive buoyancy if hatched when the shell possessed between three and five chambers. However, we show that the animal could also overcome degrees of negative buoyancy through swimming, similar to the paralarvae of modern squids. These calculations challenge past inferences of benthic life habits based solely on calculations of negative buoyancy. The calculated buoyancy of Cadoceras supports the possibility of planktonic dispersal of ammonite hatchlings. This information is essential to understanding ammonoid ecology as well as biotic interactions and has implications for the interpretation of geochemical data gained from the isotopic analysis of the shell.

现代头足类的分室壳体作为浮力装置，可使该类动物无需消耗大量能量以克服自身自重，即可进入水层。事实上，分室壳体普遍被视为一项关键适应性特征，使得早期头足类得以脱离海底、进入水层活动。然而，有学者提出，古生代与中生代菊石标志性的分室壳体所能提供的浮力，不足以抵消整个动物体的总重量，因此菊石基本只能营底栖生活，其生活方式与部分章鱼类群相似。
本文开发了一种基于高分辨率计算机断层扫描（computed tomography，CT）的技术，无需像过往研究那样将壳体简化为理想螺旋模型，或采用会抹杀固有生物学变异的数学模型，即可量化分室壳体的浮力特性。该技术已在珍珠鹦鹉螺（Nautilus pompilius）上完成验证，如今被拓展应用于现存深海枪鱿（Spirula spirula）以及侏罗纪菊石Cadoceras sp.的孵化幼体。
研究发现，当Cadoceras幼体壳体拥有3至5个气室时，其浮力接近中性甚至为正浮力。但研究同时表明，该动物也可通过游泳克服一定程度的负浮力，这与现代枪鱿的幼体阶段情况类似。这些计算结果对过往仅通过负浮力计算得出的菊石底栖生活习性推论提出了质疑。对Cadoceras浮力的计算结果，支持了菊石幼体存在浮游扩散的可能性。这一信息对于理解菊石生态学以及生物间相互作用均至关重要，同时也为通过壳体同位素分析获得的地球化学数据的解译提供了参考依据。