A transitional fossil sheds light on the early evolution of the Staphylinine group of rove beetles (Coleoptera: Staphylinidae)

Rove beetles of the Staphylinine group of subfamilies represent the most species-rich clade of Staphylinidae, which in turn is the most speciose beetle family. While several relationships within the Staphylinine group are well supported by morphological and molecular data alike, the basal relationships within the clade remain unresolved. In particular, understanding the largest radiation on the beetle tree of life is complicated by the absence of early-diverging transitional fossils. Here we describe a new genus and species of the Staphylinine group of subfamilies from the Lower Cretaceous Yixian Formation in eastern China (<i>c</i>. 125 Ma). <i>Paroxyporus fossilis</i> Cai gen. et sp. nov. shares a number of characters with the monogeneric subfamily Oxyporinae, namely its prognathous head with prominent apically curved mandibles, contiguous procoxae, moderately separated mesocoxae and abdominal intersegmental membranes with a ‘brick-wall’ pattern. It, however, shares oval procoxae and ‘U’-shaped abdominal impressions with the monogeneric subfamily Megalopsidiinae. A parsimony analysis of 211 morphological characters recovered <i>Paroxyporus</i> gen. nov. as sister to the whole Staphylinine group except Oxyporinae, linking Oxyporinae and other clades of the Staphylinine group, and it seems to be a stem member of the Staphylinine group. As such, <i>Paroxyporus</i> gen. nov. may represent an early-diverging isolated lineage within the Staphylinine group, a lineage that has been recognized on the basis of morphological data but challenged by some recent molecular studies, or alternatively a stem member of the Staphylinine group. Together with described fossil members of the subfamily Oxyporinae, Paederine and Staphylinine from the Yixian Formation, <i>Paroxyporus fossilis</i> gen. et sp. nov. is one of the earliest representatives of the Staphylinine group in the fossil record, providing an important calibration point for understanding the timing of the early diversification of rove beetles in the Mesozoic. http://zoobank.org/urn:lsid:zoobank.org:pub:40F8F256-7E9F-478E-AC96-1EB1BCE5A02B

隐翅虫（rove beetles）中的隐翅虫亚科群（Staphylinine group of subfamilies）是隐翅虫科（Staphylinidae）下物种最为丰富的演化支，而隐翅虫科本身即为物种多样性最高的甲虫类群。尽管隐翅虫亚科群内部的多个演化关系已得到形态学与分子数据的共同支持，但该演化支内部的基部演化关系仍未得到解析。尤为关键的是，由于缺乏早期分化的过渡化石，解析甲虫生命之树上这一最大的辐射演化事件变得困难重重。本研究记述了采自中国东部下白垩统义县组（Lower Cretaceous Yixian Formation）（约125 Ma）的隐翅虫亚科群一新属新种。*Paroxyporus fossilis* Cai 新属新种（gen. et sp. nov.）与单型亚科尖额隐翅虫亚科（Oxyporinae）共享诸多形态特征：前口式头部（prognathous head）具显著顶端弯曲的上颚（apically curved mandibles）、邻接的前足基节（procoxae）、中等间距的中足基节（mesocoxae），以及呈“砖墙”模式的腹部节间膜（abdominal intersegmental membranes）。但该新属同时与单型亚科大伪隐翅虫亚科（Megalopsidiinae）共享卵圆形前足基节（oval procoxae）与“U”形腹部压痕（U-shaped abdominal impressions）特征。基于211个形态学特征的简约性分析（parsimony analysis）结果显示，新属*Paroxyporus* gen. nov.为除尖额隐翅虫亚科外的整个隐翅虫亚科群的姊妹群，连接了尖额隐翅虫亚科与隐翅虫亚科群的其他演化支，且其大概率属于隐翅虫亚科群的茎群成员（stem member）。据此，*Paroxyporus* gen. nov.可能代表隐翅虫亚科群内一个早期分化的孤立演化支——该类群曾基于形态学数据被认定，但近年来遭到部分分子研究的质疑——亦或是隐翅虫亚科群的茎群成员。结合义县组中已记述的尖额隐翅虫亚科、伪隐翅虫亚科（Paederinae）及隐翅虫亚科（Staphylininae）化石类群，本新属新种*Paroxyporus fossilis* gen. et sp. nov.是化石记录中隐翅虫亚科群最早的代表类群之一，为理解中生代（Mesozoic）隐翅虫早期多样化的时间节点提供了重要的校准依据。http://zoobank.org/urn:lsid:zoobank.org:pub:40F8F256-7E9F-478E-AC96-1EB1BCE5A02B