Taxonomical diversity and palaeobiogeographical affinity of belemnites from the Pliensbachian - Toarcian GSSP (Lusitanian Basin, Portugal)

For the first time, high-resolution analysis of the Late Pliensbachian - Early Toarcian belemnite assemblages from the Peniche section (Lusitanian Basin) is presented. The systematic analysis allowed the recognition of eight taxa of the suborder Belemnitina, previously reported from contemporaneous NW Tethyan and Arctic sections. The presence of Bairstowius amaliae sp. nov. in the Late Pliensbachian (Emaciatum Zone) represents a novelty since hitherto the genus Bairstowius was only known from Late Sinemurian and Early Pliensbachian deposits.  Additionally, the replacement of Bairstowius amaliae by Catateuthis longiforma, during the latest Pliensbachian, suggests an evolutionary relationship between the two taxa. This relationship suggests a new scenario for the subsequent development of endemic Toarcian Boreal-Arctic faunas, characterised by the occurrence of Catateuthis. 

The comparison of the Peniche belemnite fauna with coeval faunas from the Mediterranean/Submediterranean and Euro-Boreal domains indicates taxonomic uniformity during the Late Pliensbachian – Early Toarcian (Emaciatum and Polymorphum zones), in the NW Tethys. Despite the lack of a marked taxonomic turnover, the Pliensbachian-Toarcian boundary corresponds to a slight decrease in diversity observed not only in the Lusitanian Basin but also in coeval NW European basins. Ordination and cluster analyses indicate that the largest changes in belemnite diversity and palaeogeographic distribution occurred rather during the Toarcian Oceanic Anoxic Event (base of Levisoni Zone). This event is marked by the extinction of taxa, affecting more severely the Mediterranean/Submediterranean domain and resulting in a provincial differentiation amongst NW European and Arctic belemnite faunas.

本研究首次对卢西塔尼盆地佩尼谢剖面产出的普林斯巴阶晚期-托阿尔阶早期箭石组合开展高分辨率分析。通过系统分类研究，共识别出8个箭石亚目（Belemnitina）类群，此类群此前仅在同时期的西北特提斯与北极剖面中被报道。在普林斯巴阶晚期埃马西亚通带（Emaciatum Zone）中发现的Bairstowius amaliae新种（sp. nov.）为本次研究的新发现——此前Bairstowius属仅记录于西涅缪尔阶晚期与普林斯巴阶早期地层中。此外，在普林斯巴阶最晚期，Bairstowius amaliae被长形卡塔箭石（Catateuthis longiforma）取代，这暗示两类群间存在演化关联。该关联为后续以北欧-北极特有托阿尔阶生物群（以卡塔箭石属出现为特征）的演化提供了全新研究场景。

将佩尼谢剖面的箭石动物群与地中海/亚地中海域以及欧北域的同期动物群进行对比，结果表明，在普林斯巴阶晚期至托阿尔阶早期（埃马西亚通带与多形带（Polymorphum Zone））的西北特提斯区域内，类群组成具有分类学统一性。尽管未出现显著的类群更替，但普林斯巴阶-托阿尔阶界线处的物种多样性出现小幅下降，这一现象不仅见于卢西塔尼盆地，在同时期的西北欧洲盆地中也有观测。排序分析与聚类分析结果显示，箭石多样性与古地理分布的最大变化发生在托阿尔阶海洋缺氧事件（Toarcian Oceanic Anoxic Event）期间（莱维索尼带（Levisoni Zone）底部）。该事件以类群灭绝为标志，对地中海/亚地中海域造成更为严重的冲击，并最终导致西北欧洲与北极箭石动物群出现地理分异。