Baltic amber is generally dated to the Eocene (55.8–33.9 Ma), although proposed age ranges extend from ~55 to 23 Ma due to repeated redeposition and the long persistence of amber-producing forests (Larsson 1978, Szwedo and Sontag 2009, Wolfe et al. 2016). Radiometric dating of glauconitic sediments suggests a middle Eocene age for the main Sambian deposits (Ritzkowski 1997), although younger ages have also been proposed (Kaplan et al. 1977). While Early Oligocene ages have been suggested based on insect ecology (Vitali and Daamgard 2016), most recent syntheses favor a middle Eocene age for both Baltic and Bitterfeld ambers (Wolfe et al. 2016).

The main mass of Baltic amber is hosted in the Eocene Prussian (Samland) Formation, where it is concentrated within the glauconite-rich “Blue Earth” (Blaue Erde) horizon of the Sambian Peninsula. With an estimated total mass of approximately 10⁵ tonnes, it represents the largest known accumulation of amber in the world (Szwedo and Sontag 2009). Eocene Baltic amber represents one of the most important Konservat-Lagerstätten, providing an unparalleled record of exceptionally preserved fossil organisms. Its rich inclusions, particularly insects, have made Baltic amber a cornerstone for studies in taxonomy, phylogenetics, paleoecology, paleoclimate, and paleobiogeography. This record is especially valuable for reconstructing biotic responses to major climatic changes during the Eocene, a period critical for the establishment of modern ecosystems (Zachos et al. 2001).

The term “Baltic amber” encompasses a wide range of secondary deposits distributed across northern and eastern Europe and beyond, from the Baltic Sea region to Arctic localities (Bogri et al. 2018, Szwedo and Sontag 2009). In a stricter sense, it refers to amber from Gdańsk Bay (Gdańsk, Poland) and the Sambian Peninsula (Russia), which together represent the classic Baltic amber sensu stricto (Alekseev and Alekseev 2016).

Baltic amber formed from plant resins originally deposited in forest soils and subsequently altered by diagenetic processes. Despite extensive study, the source of Baltic amber remains debated. Current evidence suggests that Baltic amber sensu stricto represents a mixture of resins derived from multiple source areas; erosion and fluvial transport, via the hypothetical Eridanos River system (Szwedo and Sontag 2009), later redistributed these resins into coastal and deltaic environments, resulting in predominantly allochthonous deposits. Alternative models propose southern source regions, including areas bordering the Eocene Paratethys Sea, with later redeposition of some amber types such as Bitterfeld amber (Szwedo and Sontag 2009, Wolfe et al. 2016). Geochemical data support a complex history involving multiple resin sources and transport pathways (Wolfe et al. 2016). Consequently, Baltic amber is no longer found at its original site of production (Weitschat and Wichard 2010).

During the Eocene, much of the Northern Hemisphere experienced warm, humid climates with vegetation comparable to modern subtropical forests (Szwedo and Sontag 2009). The Baltic amber forest is reconstructed as a thermophilous, humid mixed forest, broadly similar in structure and ecological parameters to the present-day forests of eastern and southeastern Asia (Alekseev and Alekseev 2016, Bogri et al. 2018).

Justification for the generic assignment to this superfamily.—Due to the diversity in the habitus of Eumastacoidea, it is difficult to propose a diagnosis applicable to the entire taxon, an issue already recognized in the first description by Burr (1899). According to that description, Eumastacoidea are small- to medium-sized orthopterans; they have an arolium between their tarsal claws; the antennae are very short (except for the genus Gomphomastax Brunner von Wattenwyl, 1898 [and the fossil genus Monolophomastax Schall, Bonino & Husemann, 2026a]); the head is short; the prosternum smooth (Burr used the Latin word muticum); the body is “normale”, neither bloated nor elongated; no tympanal organ at first abdominal segment. This diagnosis, as comprehensive as it may be, is consistent with the characters seen in the new specimen. However, it should be noted that Eumastacoidea are probably differentiated best by molecular methods rather than morphological ones within Acridomorpha (pers. opinion by OKO Schall).

Justification for the tentative assignment to this family.—With respect to the difficulties of assigning fossil Eumastacoidea to lower-level taxa in the absence of the most important morphological diagnostic character (i.e., the structure of the male phallic complex [Descamps, 1973]), all fossils of monkey hoppers, for now, should be placed within the extant subfamilies only tentatively and until more knowledge of their systematics is acquired. With that said, we suggest that Baltoparalethus may belong to the Episactidae family of Eumastacoidea based on the 1) absence of wings (apterous); 2) male subgenital plate not membranous; 3) median ocellus well-developed; 4) carinae of frontal ridge continue below median ocellus, diverging basally to form a supraclypeal trapezium; 5) supra-anal plate triangular; 6) terminal segment of antennae long and tapering to a rounded point, bearing the antennal organ; 7) presence of short medial carina just below the fastigium (characters after Rowell and Perez-Gelabert 2006).

Justification for the tentative assignment to this subfamily.—Based on characters 1–5 from above, we believe that the new specimen may belong to the subfamily Episactinae (Rowell and Perez-Gelabert 2006).

Baltoparalethus alessioboninoigen. et sp. nov. also has interesting implications for the evolution of monkey hoppers. Eumastacoidea is divided into seven mostly extant families (Chorotypidae Stål, 1873, Episactidae Burr, 1899, Eumastacidae Burr, 1899, Euschmidtiidae Rehn, 1948, Mastacideidae Rehn, 1948, Morabidae Rehn, 1948 and Thericleidae Burr, 1899) (Cigliano et al. 2026). Additionally, there is one extinct family Promastacidae Kevan & Wighton, 1981, containing only two genera and species from the Paleocene to the Eocene of Canada (Handlirsch 1910, Kevan and Wighton 1981).

Any systematic assignment of fossil monkey hoppers to one of the families of Eumastacoidea should be understood as preliminary until more knowledge of the group’s morphological apomorphies is acquired. However, if we assume that the hypotheses on the placement of fossil discoveries of Eumastacoidea made so far are correct, we find that four of the seven extant families were already present before the end-Cretaceous extinction event (Table 2). These include the Chorotypidae (Sharov 1971), Eumastacidae (Lin 1980; Husemann et al. 2025), Thericleidae (Schall et al. submitted), and possibly the Mastacideidae (Schall et al. 2026a; Monolophomastax disantoi Schall et al., 2026a was speculated to belong either to this family or the Eumastacidae).

With the discovery of Baltoparalethus alessioboninoi, we present what is possibly the oldest occurrence of the family Episactidae, pre-dating the previous oldest finding by ca. 25 Ma (Perez-Gelabert et al. 1997). Currently, there are no described fossils in the families Morabidae and Euschmidtiidae. Especially for the latter, this is surprising because Euschmidtiidae comprise 239 extant species, representing about 22% of overall monkey hopper diversity. While it is likely that the first Euschmidtiidae fossil will be unearthed sooner or later, it is also possible that this family radiated only later in Eumastacoidea evolution.

Unfortunately, among the many mysteries that still surround monkey hoppers, one of them is their evolutionary history. Matt et al. (2008) were unable to apply a molecular clock in their analyses that focused specifically on the phylogeny of Eumastacoidea. In that study, depending on the type of input data (i.e., 18s, 28s, 12s + 16s or all genes concatenated), different analyses identified different families of monkey hoppers as the earliest diverging lineage within the clade. Chorotypidae were frequently recovered as the most basal member of Eumastacoidea (this result was found by the 12s + 16s [fig. 2] and the concatenated tree [fig. 5] by Matt et al. 2008), which is corroborated by the suggested early occurrence of this family in the fossil record. However, for the other families, the evidence is less clear.

Further confusion arises when considering the comprehensive phylogeny of Orthoptera performed by Song et al. (2015, 2020). Based on data from complete mitochondrial genomes and four nuclear loci, the earlier study found Eumastacoidea consisting of two clades with Episactidae and Chorotypidae as the early diverging lineage of each clade, respectively. However, this study sampled only one species of Episactidae. In contrast, based on transcriptomic and mitochondrial genome data, Song et al. (2020) found the Episactidae is most closely related to Proscopiidae Serville, 1838, the two taxa representing a lineage nested deeply within Eumastacoidea as one of their more recently diverging clades.

The findings of this study show how hypotheses based on fossil findings can complement molecular results and provide new information on the systematic position of a nowadays species-rich taxon (here, Eumastacoidea). However, they also highlight how little is known about monkey hoppers, both in terms of their morphological diversity as well as how the families are phylogenetically related and when their lineages first evolved.