Ontogenetic development captured in amber - the first record of aquatic representatives of Isopoda in Cretaceous amber from Myanmar

Abstract Two fossils from Burmese amber are the subject of this study. The specimens differ in size; yet, they appear to be conspecific because of the profound morphological similarity. The fossils are interpreted as representatives of Isopoda, more precisely of the group Cymothoida, due to the presence of a triangular basipod of the uropod. Cymothoida comprises parasitic forms of Isopoda as well as many other types of feeding-habits. The morphology in the studied fossils suggests that they are not representatives of any of the parasitic ingroups of Cymothoida. Since there are no other findings of Isopoda from the Cretaceous with the same morphological features, the fossils at hand are described as a new species - Electrolana madelineae sp. nov. The smaller specimen lacks well-developed walking appendages on trunk segment seven; it can thus be interpreted as a manca stage (immature) individual. The systematic affinity and the functional morphology of the herein described fossils, as well as three seed shrimps (Ostracoda) in close proximity to one of the specimens, and the presence of pyrite in the amber piece points towards an aquatic lifestyle and a preservation in moist conditions. In addition, we review the fossil record of immature forms of Isopoda.

摘要 本研究聚焦于两件产自缅甸琥珀的化石标本。两件标本尺寸存在差异，但因形态特征高度相似，被判定为同一物种。基于尾肢（uropod）具三角形基肢节（basipod）这一特征，该化石被归为等足目（Isopoda），更确切地说属于缩头水虱亚目（Cymothoida）类群。缩头水虱亚目包含等足目中的寄生类群，同时也涵盖多种其他取食习性的类群。本次研究的化石形态特征表明，它们并不属于缩头水虱亚目中的任何寄生类群分支。由于白垩纪时期尚未发现具有相同形态特征的等足目化石标本，本次研究将这两件化石描述为一新种——Electrolana madelineae 新种（sp. nov.）。尺寸较小的标本在第七躯干体节上缺乏发育完全的步足，因此可被判定为幼体阶段（manca stage）的未成熟个体。本次描述的化石的系统发育亲缘关系与功能形态特征，以及其中一件标本旁伴生的三件介形类（Ostracoda）标本，连同琥珀标本中检出的黄铁矿（pyrite），均指向该类群曾营水生生活，且标本保存在湿润环境中。此外，本文还对等足目未成熟个体的化石记录进行了综述。