Longirostrine crocodylians from the Bartonian of Morocco and Paleogene climatic and sea level oscillations in the Peri-Tethys area

The Eocene–Oligocene transition was a period of high faunal and floral turnover, often correlated with climatic deterioration. Crocodyliforms are climate sensitive, and they have been often used for reconstruction of paleoclimates. The description of crocodylian material from the Bartonian of Aaiun-Tarfaya Basin (Morocco) allows the recognition of at least two longirostrine crocodylians, including a gavialoid. This identification is important, because the migration of gavialoids to South America probably occurred during the late Eocene. Close relationships between late Eocene–early Oligocene Eogavialis africanum from Egypt, Argochampsa krebsi from the Paleocene of Morocco, and the South American gavialoid clade suggests that Morocco could have had a particular place in the gavialoid dispersal route to South America. The resemblance between Moroccan material described herein and E. africanum is thus important in this context. Analysis of the distribution of longirostrine crocodylians in the Peri-Tethys area through the Eocene–Oligocene shows a strong difference in the evolution of the distribution between longirostrine crocodylians and Diplocynodon. Whereas the freshwater Diplocynodon has a continuous distribution in western Europe, the mainly marine longirostrine crocodylians have a northern maximal expansion that moves southward during the middle Eocene, being restricted to North Africa during late Eocene–early Oligocene. European freshwater Asiatosuchus-like taxa also disappear during the late Eocene. Climatic deterioration, helped by sea level oscillations, may have strongly impacted the diversity and latitudinal distribution of the crocodylians, but we highlight a possible differential tolerance in range of climatic conditions between crocodylians. This could help to refine the use of crocodylians as paleoclimatic proxies.

始新世-渐新世过渡期（Eocene–Oligocene Transition）是一次发生了大量动植物类群更替的时期，该事件常与气候恶化相关联。鳄形类（Crocodyliformes）对气候敏感，常被用于古气候（paleoclimate）重建工作。对产自摩洛哥阿尤恩-塔尔法亚盆地（Aaiun-Tarfaya Basin）巴顿阶（Bartonian）的鳄类化石材料的研究，确认了至少两种长吻鳄类（longirostrine crocodylians），其中包括一种长吻鳄类群（gavialoid）。这一鉴定结果具有重要意义，因为长吻鳄类群（gavialoids）向南美洲的迁徙大概率发生在始新世晚期。产自埃及的始新世晚期-渐新世早期非洲始长吻鳄（Eogavialis africanum）、产自摩洛哥古新世（Paleocene）的克雷布斯阿尔戈鳄（Argochampsa krebsi），与南美洲长吻鳄类演化支（clade）之间存在密切的亲缘关系，这表明摩洛哥或许在长吻鳄类群向南美洲的扩散路径中占据了特殊地位。本文所描述的摩洛哥化石材料与非洲始长吻鳄（E. africanum）之间的形态相似性，在这一研究背景下因此具有重要价值。对始新世-渐新世时期特提斯周缘（Peri-Tethys）区域内长吻鳄类分布情况的分析显示，长吻鳄类与双孔鳄属（Diplocynodon）的分布演化模式存在显著差异。淡水生的双孔鳄属在西欧拥有连续的分布范围，而以海生为主的长吻鳄类则在始新世中期达到最北的分布范围后逐渐南撤，并在始新世晚期-渐新世早期仅残存于北非地区。产自欧洲的淡水亚洲鳄属（Asiatosuchus）类群也在始新世晚期灭绝。气候恶化叠加海平面波动的影响，或许对鳄类的多样性及其纬度分布范围造成了强烈影响，但本研究指出，不同鳄类类群对气候条件的耐受范围可能存在差异。这一发现有助于优化鳄类作为古气候代用指标（paleoclimatic proxy）的应用方式。