State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, People's Republic of China.
Department of Earth and Environmental Sciences, Palaeontology and Geobiology, Ludwig-Maximilians-Universität München, Richard-Wagner-Strasse 10, 80333 München, Germany.
Proc Biol Sci. 2020 Sep 30;287(1935):20201661. doi: 10.1098/rspb.2020.1661. Epub 2020 Sep 16.
The bivalved crustacean ostracods have the richest fossil record of any arthropod group and display complex reproductive strategies contributing to their evolutionary success. Sexual reproduction involving giant sperm, shared by three superfamilies of living ostracod crustaceans, is among the most fascinating behaviours. However, the origin and evolution of this reproductive mechanism has remained largely unexplored because fossil preservation of such features is extremely rare. Here, we report exceptionally preserved ostracods with soft parts (appendages and reproductive organs) in a single piece of mid-Cretaceous Kachin amber (approximately 100 Myr old). The ostracod assemblage is composed of 39 individuals. Thirty-one individuals belong to a new species and genus, gen. et sp. nov., exhibiting an ontogenetic sequence from juveniles to adults (male and female). Seven individuals are assigned to sp. (Cypridoidea, Candonidae, Paracypridinae) and one to sp. (Cytheroidea, Loxoconchidae). Our micro-CT reconstruction provides direct evidence of the male clasper, sperm pumps (Zenker organs), hemipenes, eggs and female seminal receptacles with giant sperm. Our results reveal that the reproduction behavioural repertoire, which is associated with considerable morphological adaptations, has remained unchanged over at least 100 million years-a paramount example of evolutionary stasis. These results also double the age of the oldest unequivocal fossil animal sperm. This discovery highlights the capacity of amber to document invertebrate soft parts that are rarely recorded by other depositional environments.
双壳甲壳类介形虫是节肢动物中化石记录最丰富的类群，它们具有复杂的繁殖策略，这有助于它们的进化成功。涉及巨型精子的有性生殖是现存的三个介形虫甲壳超科所共有的，是最引人注目的行为之一。然而，这种繁殖机制的起源和进化在很大程度上仍未得到探索，因为化石对这些特征的保存极为罕见。在这里，我们报告了中生代白垩纪缅甸琥珀中保存异常完好的具软部（附肢和生殖器官）的介形虫（约 1 亿年前）。该介形虫组合由 39 个个体组成。其中 31 个个体属于一个新的种和属， gen. et sp. nov. ，展示了从幼体到成体（雄性和雌性）的个体发育序列。7 个个体归入 sp. （Cypridoidea, Candonidae, Paracypridinae），1 个归入 sp. （Cytheroidea, Loxoconchidae）。我们的微 CT 重建提供了直接证据，证明了雄足钳、精子泵（Zenker 器官）、半阴茎、卵子和雌性精液接收囊具有巨型精子。我们的结果表明，与显著形态适应相关的繁殖行为谱在至少 1 亿年的时间里保持不变——这是进化停滞的一个重要例子。这些结果还将最古老的明确无误的化石动物精子的年龄增加了一倍。这一发现突出了琥珀记录无脊椎动物软体部分的能力，而这些部分在其他沉积环境中很少被记录。
