Evolutionary and developmental patterns of Cretaceous uncoiled ammonites

异形菊石(即偏离平面螺旋、规则缠绕的菊石)属于最神秘的化石群，但其起源、古生物学和功能形态仍存在争议。人们对菊石反复偏离正常卷曲的可能原因知之甚少，而是意识到海螺结构在浮力控制和运动方面可能是不利的。此外，在分类多样性和形态差异方面，尤其是在白垩纪，人们对它们为什么成为成功的支系知之甚少。在拟议的项目中，将使用相关特征，如海螺几何形状、贝壳装饰和缝合线，分析这些未盘绕的菊石的形态空间占据情况。从白垩纪中选择的分类群(取决于材料的可获得性)将被研究，包括可能的性别二型性、种内变异性和个体发育特征的变异。它们广泛的地理分布和高丰度，快速的演化速度，以及由于壳体增长而保存下来的整个个体发育，使特别是中生代菊石成为研究古生态扰动演化情景的理想化石群。该项目的主要目的是(1)检验关于白垩纪异形菊石起源的各种假说，(2)评估物种和属的概念以及高等分类实体及其系统发育关系的有效性，(3)调查古生态参数及其对海螺几何形状的可能影响，(4)比较个体发育特征的多样性和差异模式。在这里，我们首次尝试分析白垩纪异形菊石的种内变异性，并讨论它们在卷曲和海螺形式上与单形菊石的偏差。通过这项研究，我们将提高对白垩纪无卷曲菊石的系统发育关系、古生物学、古生态和地层范围的理解。白垩纪以海侵/海退、变暖/降温事件和海洋缺氧事件为特征，这使得研究特定的异形贝壳形态的出现和分布并有望检测到特征模式成为理想的方法。与同时代沉积物中正常卷曲的菊石相比，可以检验不同的古环境变化(水深、温度、氧气)的影响及其对异形壳体几何形状的影响。

The heteromorph ammonoids (i.e., ammonoids that deviate in their coiling from the planispiral, regular coiling) belong to the most enigmatic fossil groups, but their origin, palaeobiology, and functional morphology is still a matter of debate. Little is known about possible reasons why ammonoid iteratively deviated from normal coiling and instead realise a putatively disadvantageous conch construction with respect to buoyancy control and locomotion. Furthermore little is known why they became successful clades in terms of taxonomic diversity and morphological disparity especially during the Cretaceous. Within the proposed project, these uncoiled ammonoids will be analysed with respect to their morphospace occupation by the use of relevant characters such as conch geometry, shell ornamentation, and suture line. Selected taxa (depending on availability of material) from the Cretaceous will be studied including possible sexual dimorphism, intraspecific variability, and variation in ontogenetic traits. Their wide geographical distribution and high abundances, fast evolutionary rates, and the preservation of the entire ontogeny due to accretionary shell growth made especially Mesozoic ammonoids a perfect fossil group to study evolutionary scenarios with respect to palaeoecological perturbations. The main goal of this project is (1) to test various hypotheses of the origin of Cretaceous heteromorph ammonites, (2) to evaluate the species and genus concepts as well as validity of higher taxonomic entities and their phylogenetic relationships, (3) to investigate palaeoecological parameters and their possible influence on the conch geometry, and (4) to compare diversity and disparity patterns with respect to ontogenetic traits. Here we make a first attempt to analyse the intraspecific variability of Cretaceous heteromorph ammonites and discuss them with respect to their deviation in coiling and conch form from monomorph ammonoids. With this study we will improve the understanding of the phylogenetic relationships, palaeobiology, palaeoecology, and stratigraphic range of the Cretaceous uncoiled ammonites. The Cretaceous period is characterised by transgressions/regressions, warming/cooling events and oceanic anoxic events which made it ideal to study the appearance and distribution of specific heteromorph shell morphologies and hopefully detected characteristic patterns. In comparison with normally coiled ammonites from contemporaneous deposits the influence of distinct palaeoenvironmental changes (water depth, temperature, oxygen) and their effect on the heteromorph shell geometry can be tested.