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 本研究以两件缅甸琥珀化石为研究对象。两件标本尺寸存在差异，但因形态相似性极高，故被归为同一物种。依据尾肢基节呈三角形这一特征，该化石被归入等足目（Isopoda），更具体地隶属于扇肢下目（Cymothoida）。扇肢下目不仅包含等足目中的寄生类群，还涵盖多种其他取食习性的类群。本次研究的化石标本形态特征显示，它们并不隶属于扇肢下目中的任何寄生亚类群。由于白垩纪时期尚无其他具有相同形态特征的等足目化石被发现，因此本次研究将这两件标本命名为新种：Electrolana madelineae sp. nov.。体型较小的标本第七躯干节未发育出完整的步足，因此可被认定为曼卡期（未成熟个体）。本次描述的化石的系统亲缘关系与功能形态学特征、其中一件标本附近伴生的三件介形类（Ostracoda）标本，以及该琥珀标本中黄铁矿（pyrite）的存在，均指向该类群曾营水生生活习性，且保存于湿润环境中。此外，本研究还梳理了等足目幼体化石的化石记录。