The specimen CNU–DIP–MA2016125 is deposited in the College of Life Sciences, Capital Normal University, Beijing, China (CNUB; Dong Ren, Curator). The amber material analyzed in this study was collected from the Hukawng Valley, Tanai Township, Myitkyina District, Kachin State, Myanmar, a locality renowned for its exceptionally diverse and abundant insect inclusions (Kania et al. 2015). All newly examined specimens were acquired by Mr. Fangyuan Xia in 2015 and donated for scientific study in 2016, ensuring full ethical compliance for Kachin amber research (Engel 2020). The deposit has been dated to approximately 99 Ma (earliest Cenomanian) based on U–Pb analyses of zircons from the volcaniclastic matrix enclosing the amber (Shi et al. 2012).

The amber pieces containing the specimens were ground and polished to an appropriate size to facilitate photography and morphological examination. Specimens were examined and photographed using a Nikon SMZ18 stereomicroscope equipped with a Nikon DS-Ri2 digital camera. Measurements are provided in millimeters (mm). Terminology follows the one used by Hauser et al. (2017).

Abbreviations are as follows: A1, acanthophorite A1 spines; A2, acanthophorite A2 spines; spines al, alula; an lb, anal lobe; a plp, apical plpomere; cerc, cercus; CuA, cubitus anterior vein; CuP, cubitus posterior vein; d, discal cell; dc: dorsocentral macrosetae; flg: flagellum; h, humeral crossvein; lbl: labellum; lbr: labrum; M, medial vein; m, medial cell; np, notopleural macrosetae; pa, postalar macrosetae; ped, pedicel; plp, palpus; R, radius vein; r, radial cell; r-m, radial–medial crossvein; Rs, radial sector vein; sa, supraalar macrosetae; Sc, subcostal vein; sc, subcostal cell; scp, scape; ST1–ST8, sternite 1 to 8; T1–T10, tergite 1 to 10.

The phylogenetic analysis yielded four most parsimonious trees (MPTs) with a tree length of 69, a consistency index (CI) of 0.52, and a retention index (RI) of 0.81. One MP tree with a fully bifurcating topology is used to illustrate the internal relationships of Therevidae, which is nearly identical to the consensus tree (Fig. 1). In the results, all four subfamilies were recovered as monophyletic, in agreement with prior studies (Winterton and Ware 2015; Winterton et al. 2016). Phycusinae was identified as the earliest-diverging lineage, and sister to the remaining subfamilies. Four extinct species—†Dasystethos hoffeinsi, †Glaesorthactia magnicornis, †Kroeberiella pinguis, and †Palaeopherocera scudderi are assembled in this subfamily—as suggested in earlier works (Hauser and Irwin 2005a; Hauser 2007; Winterton and Ware 2015). The monophyly of Phycusinae is supported by one synapomorphy: a sensory pit at the tip of the palpus (char. 6:1). Xestomyzinae represents the second earliest-diverging subfamily and is sister to the clade Agapophytinae + Therevinae. †Arctogephyra agilis and †Peratrimera mexicana were placed with Xestomyzinae, consistent with previous studies (Hauser and Irwin 2005b; Hauser 2007). The monophyly of Xestomyzinae is supported by a synapomorphy: the presence of macrosetae on sternite 8 in female (char. 29:1). The sister-group relationship of Agapophytinae + Therevinae was recovered as reported earlier (Winterton et al. 2016). This clade is supported by four synapomorphies: a circumambient costal vein (char. 14:3), the presence of a sclerotized ridge between T8 and T9+10 (char. 25:1), the presence of a gonocoxal ventral lobe (char. 26:1), and well-developed acanthophorite spines (char. 27:1).

Monophyly of Therevinae is supported by three synapomorphies: scutellum with 2 pairs of macrosetae (char. 8:1) and multiple types of femoral vestiture (char. 9:1), and a non-forked ventral apodeme of the aedeagus (char. 22:1). †Ambradolon grimaldii was resolved as a member of Therevinae, agreeing with earlier findings (Metz and Irwin, 2000). Agapophytinae is supported as monophyletic by one synapomorphy, suggested in earlier studies (Winterton and Ware, 2015; Winterton et al. 2016): three spermathecae that join the spermathecal duct to form a common duct (char. 29:1). The new species †Paleothereva longicoxagen. et sp. nov. was recovered within Agapophytinae and is closely related to the Taenogera genus-group in this subfamily. Its placement is supported by two synapomorphies: absence of setae along the medial surface of the scape (char. 5:2) and the presence of a single seta anteroventrally at the apex of the hind femur (char. 12:1).

Based on the results of phylogenetic analysis and morphological comparisons, †Paleotherevagen. nov. is well associated to the Taenogera genus group of Agapophytinae, representing a derived therevid lineage (Fig. 1). This discovery unequivocally provides critical new evidence for calibrating the timing of crown therevid divergence and for reconstructing their early biogeographic evolution. Its occurrence indicates that Agapophytinae had already diverged from Therevinae by at least the mid-Cretaceous. Together with the widespread Cenozoic record of therevid fossils across the Northern Hemisphere (Fig. 6A), it supports the hypothesis that Agapophytinae and Therevinae underwent a rapid radiation beginning in the Cretaceous (Winterton et al. 2016). Moreover, the geological age of †Paleotherevagen. nov. closely aligns with the Early Cretaceous divergence of Therevidae inferred from molecular data (Winterton et al. 2016), suggesting an earlier origin of the family than previously recognized.

The extant Agapophytinae, are now restricted to Australasia and South America (Winterton et al. 1999, 2001; Hauser et al. 2017). Nevertheless, its Cretaceous relative, †Paleotherevagen. nov. occurs in the West Burma Block—an isolated island during mid-Cretaceous (Fig. 6B). The disjuctive distribution pattern between †Paleotherevagen. nov. and extant Agapophytinae implies a complex evolutionary scenario of this lineage. The West Burma Block once formed part of eastern Gondwana during the Middle to Late Jurassic supported by paleontological, paleomagnetic, and stratigraphic evidence (Ezcurra and Agnolín 2012; Seton et al. 2012; Westerweel et al. 2019). It is likely that the ancestors of Agapophytinae, or of the broader clade comprising Agapophytinae + Therevinae, originated in Gondwana prior to the rifting of the West Burma Block. Given the disjunct distributions of these therevid flies, this family may have experienced a more complex evolutionary history, as has been suggested for many other insect lineages (Jiang et al., 2021; Ma et al., 2022; Feng et al., 2025). Considering the scarcity of Mesozoic fossil records for stiletto flies, their evolutionary history is far from being understood. While †Paleotherevagen. nov. offers direct evidence and valuable new data for testing earlier hypotheses of therevid evolution and Cretaceous paleobiogeography, additional wellpreserved Mesozoic fossils and further integration of multiple lines of evidence will be required to fully clarify the early evolutionary history of Therevidae.

†Paleotherevagen. nov., with exceptionally preserved female terminalia, provide direct morphological evidence for reconstructing its oviposition behavior (Fig. 5). Its well-developed acanthophorite spines on tergite 10 closely resemble with their extant relatives, indicating that the new genus likely employed a similar oviposition strategy. This finding demonstrates that the acanthophorite spine-assisted substrate-anchoring oviposition mechanism characteristic in Therevidae was already established by the mid-Cretaceous.

Female terminalia within Therevidae exhibit notable structural diversity, which can be classified into three main types. Results of ancestral character state reconstruction indicated that their occurrence across subfamilies, under the phylogenetic framework of Therevidae, is outlined in Fig. 6C. Type I (Fig. 6C) representing as a plesiomorphic state within therevid flies, found in the subfamily Phycinae, is associated with an oviposition behavior in which females excavate pits using their hind legs, deposit eggs at the bottom, and then bury them through abdominal movements—a process that requires no anchoring device (Irwin 1976). This oviposition strategy resembles that of vermileonid flies (Hemmingsen and Nielsen 1971) and is regarded as the plesiomorphic condition within lower Brachycera. Type II (Fig. 6C), present in Xestomyzinae, involves modified macrosetae on abdominal sternite 8 that function both as the primary digging apparatus and as anchors during oviposition (Hauser 2012). Type III (Fig. 6C) is remarkably characterized by well-developed acanthophorite spines and represents the most common female terminalia within Therevidae, occurring in †Paleotherevagen. nov. as well as in Agapophytinae and Therevinae. In these subfamilies, females perform successive abdominal contortions to insert and stabilize the abdomen in the substrate, with the acanthophorite spines providing crucial anchoring support during oviposition. The structural diversity of female terminalia in Therevidae likely play an important role in adapting to different environments.

Extant therevids possessing well-developed acanthophorite spines (Type III) are most diverse in arid or semiarid regions with sandy soils (Winterton et al. 2001; Gaimari and Webb 2009; Mortelmans and Bree 2022; Marchiori 2023). The presence of similar structures in †Paleotherevagen. nov. suggests it inhabited a comparable environment. Notably, substrate-piercing oviposition mechanisms—mediated by acanthophorite spines or analogous terminal structures—occur in multiple lineages of Asiloidea preserved in mid-Cretaceous Kachin amber (Grimaldi et al. 2011; Dikow and Grimaldi 2014; Grimaldi 2016; Zhang et al. 2018; Ye et al. 2019; Ngô-Muller et al. 2020). It is implied that the West Burma Block possibly had a wide range of arid or semiarid regions with sandy soils, an environment that would have promoted the diversification of lineages employing this convergent oviposition strategy.