Data from: Untangling phylogenetical, geometrical and ornamental imprints on Early Triassic ammonoid biogeography: a similarity-distance decay study.

Ammonoids are diverse and widespread fossil shelly cephalopods that flourished in the world ocean during more than 300 million years before their total extinction, 65 million years ago. In spite of two centuries of intensive scientific studies, their mode(s) of life, and most particularly long-distance dispersal abilities remain poorly known. Here we address this question by looking at the latitudinal distribution of Early Triassic (~250 Myr) ammonoids through similarity-distance decay analyses. We examine and compare how rates of similarity-distance decay differ between various systematic, shell geometry and ornamentation groups, during the same ~3.5 myr Early Triassic time interval, in order to untangle phylogenetical, geometrical and ornamental imprints on the observed biogeographical pattern. Our data do not support any phylogenetical and shell ornamentation control on the similarity-distance decay, but rather evidence a significant effect of (sub-)adult shell geometry: most evolute morphs tend to have been more endemic than most involute ones. This result contrasts with the classical hypothesis that long-distance ammonoid dispersal mainly occurred during the earliest planktonic young stages, and thus that (sub-)adult morphological characteristics should not constrain large-scale biogeographical patterns of ammonoids. While a direct control by Sea Surface Temperature can be discarded, this result may indicate that at least some adult Triassic ammonoid morphs were good active swimmers able to achieve long-distance migrations, as observed for some present-day coleoid cephalopods.

菊石（Ammonoids）是一类多样性丰富、分布广泛的带壳化石头足类，在距今6500万年前彻底灭绝前的3亿余年里，始终繁盛于全球海洋之中。尽管已有两个世纪的深入科学研究，其生活模式，尤其是长距离扩散能力，仍未得到充分解析。本研究针对约2.5亿年前的早三叠世菊石，通过相似性-距离衰减分析（similarity-distance decay analyses）探讨该科学问题。我们在相同的约350万年的早三叠世时间区间内，对比分析了不同分类单元、壳体几何形态及纹饰类群的相似性-距离衰减速率差异，以厘清观测到的生物地理格局背后的系统发育、几何形态与纹饰印记。研究数据并不支持系统发育与壳体纹饰对相似性-距离衰减的调控作用，反而证实了（亚）成体壳体几何形态存在显著影响：多数外旋壳体类群往往比内旋类群的分布更为局限。这一结果与经典假说相悖，经典假说认为菊石的长距离扩散主要发生在最早的浮游幼体阶段，因此（亚）成体的形态特征不应限制菊石的大规模生物地理格局。尽管可以排除海表温度的直接调控作用，该结果或表明，至少部分三叠纪菊石的成体形态类群具备较强的主动游泳能力，能够完成长距离迁徙，正如现生鞘形头足类（coleoid cephalopods）所观测到的特征。