Mating and aggregative behaviors among basal hexapods in the Early Cretaceous

Among the many challenges in paleobiology is the inference and reconstruction of behaviors that rarely, if ever, leave a physical trace on the environment that is suitable for fossilization. Of particular significance are those behaviors tied to mating and courtship, individual interactions critical for species integrity and continuance, as well as those for dispersal, permitting the taxon to expand its distribution as well as access new habitats in the face of local or long-term environmental change. In this context, two recently discovered fossils from the Early Cretaceous amber of Spain (ca. 105 mya) give a detailed view of otherwise fleeting ethologies in Collembola. These occurrences are phylogenetically spaced across the class, and from species representing the two major clades of springtails—Symphypleona and Entomobryomorpha. Specifically, we report unique evidence from a symphypleonan male (Pseudosminthurides stoechus Sánchez-García & Engel, 2016) with modified antennae that may have functioned as a clasping organ for securing females during mating on water’s surface, and from an aggregation of entomobryomorphan individuals (Proisotoma communis Sánchez-García & Engel, 2016) purportedly representing a swarming episode on the forest floor. We demonstrate that the mating behavioral repertoire in P. stoechus, which is associated with considerable morphological adaptations, likely implied elaborate courtship and maneuvering for guarantee sperm transfer in an epineustic species. These discoveries reveal significant behaviors consistent with modern counterparts and a generalized stasis for some ancient hexapod ethologies associated with complex mating and courtship and social or pre-social aggregations, so critical to specific constancy and dispersal.古生物学领域的核心挑战之一，在于推断并重建那些极少——甚至近乎无法留下适宜化石化的环境物理痕迹的生物行为。其中尤为关键的，是与交配、求偶相关的个体互动行为：这类行为对物种的完整性与存续至关重要；此外还有扩散相关行为，可帮助类群在面临局部或长期环境变化时，拓展分布范围并进驻新的栖息地。在此背景下，两件新近发现于西班牙早白垩世（约1.05亿年前）琥珀中的化石，为弹尾纲（Collembola）中稍纵即逝的行为习性提供了详尽的观测视角。这些化石在该纲内的系统发育分布跨度较大，分别来自弹尾虫两大主要演化支：愈腹亚目（Symphypleona）与长角跳虫亚目（Entomobryomorpha）的物种。具体而言，本研究报道了来自愈腹亚目雄性个体*Pseudosminthurides stoechus Sánchez-García & Engel, 2016*的独特证据：其特化的触角或曾作为抱握器，在水面上的交配过程中固定雌性；此外还有一件长角跳虫亚目类群*Proisotoma communis Sánchez-García & Engel, 2016*的聚集群化石，被认为代表了森林地表的一次群集事件。研究证实，*P. stoechus*的交配行为谱伴随着显著的形态适应性，这意味着该表水层（epineustic）物种为保障精子传递，演化出了复杂的求偶与操控行为。这些发现揭示了与现代类群行为高度一致的古生物行为，以及部分与复杂交配、求偶及社会性/前社会性聚集相关的古老六足动物（Hexapoda）行为习性的显著演化停滞，而这类行为对物种存续与扩散均至关重要。