Balticoniscus Kästle & Ludwig, 2025, gen. nov.

Balticoniscus gen. nov. urn:lsid:zoobank.org:act: 4338CAC0-D29E-4571-B38D-EF36A222A20C Type species Balticoniscus walterludwigi gen. et sp. nov. Diagnosis Balticoniscus gen. nov. is characterized by the following unique combination of characters: body strongly convex. Dorsum smooth. Cephalon with frontal shield expanded into wide medial lobe occupying entire space between eyes and protruding over vertex. Anterior margin of cephalon strongly convex in dorsal view. Antennary tubercles present and projecting ventrally, not projecting anteriorly, and not visible in dorsal view of cephalon. Flagellum of antenna two-segmented with distal article three times as long as first. Eyes moderately large, consisting of multiple ocelli. Pereonite epimera without ventral structures for conglobation. Epimera of pereonites II–VII rectangular, anterior corner rounded. Protopodite of uropod cylindrical. Uropod exopodite short and acute, with apex shortly surpassing pleotelson. Endopodites shortly project past posterior tip of pleotelson. Pleotelson triangular, lateral margins weakly concave. Etymology The name Balticoniscus gen. nov. is a reference to the type specimen being preserved in baltic amber, combined with ‘-oniscus’, a suffix traditionally used for members of Oniscidea. Phylogenetic placement of Balticoniscus gen. nov. Unfortunately, as is common with fossil species, not all characters of generic and phylogenetic importance could be examined, such as the mouthparts, the number of pseudotracheae and the lung type. Only a comparison with recent taxa is possible at present, as previously described fossil crinochetan species are either not sufficiently described and of questionable validity or are placed in families which Balticoniscus walterludwigi gen. et sp. nov. certainly does not belong to. This comprises namely the crinochetan families Philosciidae Kinahan, 1857, Detonidae Budde-Lund, 1904, Eubelidae Budde-Lund, 1899, Olibrinidae Budde-Lund, 1912, Scleropactidae Verhoeff, 1938, Delatorreiidae Verhoeff, 1938 and, due to the faulty placement of Palaeoarmadillo Poinar, 2018, the family Armadillidae (Morris 1979; Schmalfuss 1980, 1984; Broly et al. 2017, 2018; Poinar 2018; Sánchez-García et al. 2021). Many oniscidean families are currently not defined by autapomorphies (Schmidt 2008), but the familial placement of Balticoniscus gen. nov. can only be discussed by comparisons with the few known autapomorphies, as well as the general morphotypes found within the different families and genera of Oniscidea. The character most helpful to assess the familial placement of Balticoniscus is the presence of rudimentary conoid processes on the anterior margins of pereonite epimera 2–4. According to Schmidt (2008) these processes, although present here only as subtle protuberances, place Balticoniscus inside an unnamed group of Crinocheta that includes the families Bathytropidae Vandel, 1952, Tendosphaeridae Verhoeff, 1930, Oniscidae Latreille, 1802, Trachelipodidae Strouhal, 1953, Cylisticidae Verhoeff, 1949, Agnaridae Schmidt, 2003, Porcellionidae Brandt, 1831 and Armadillidiidae Brandt, 1833. Unfortunately, most of these families are problematic and some likely not monophyletic, which results in unreliable or a lack of unique defining characteristics (Schmidt 2008). Balticoniscus cannot be placed in any of these families with certainty, as the new genus differs significantly from the known genera of these families by the small and strictly ventrally projecting antennary tubercles, which do not contribute to the anterior outline of the cephalon, unlike the large anteriorly projecting lateral lobes typically seen in most genera contained in this unnamed group. While this modification of the cephalon appears to be unique for the new genus, it is not present in any other genus and therefore does not indicate a close relationship to any of the known species. Furthermore, based on the respective groundpattern and apomorphies proposed by Schmidt (2003, 2008), the families Tendosphaeridae Verhoeff, 1930, Oniscidae, Bathytropidae, Cylisticidae and Armadillidiidae can be ruled out. Thus, Porcellionidae, Agnaridae, and Trachelipodidae are the remaining candidates. Although Schmidt (2003) questioned the monophyly of the Porcellionidae – characterized by monospiracular lungs on the first two pleopod exopods – molecular analyses support it as monophyletic (Mattern 2003; Schmidt 2008). Trachelipodidae is regarded as paraphyletic and characterized by ‘Trachelipus-type’ lungs, from which the monophyletic Agnaridae was split off to accommodate species with ‘Porcellio-type’ lungs (Schmidt 2008). The lung type in Balticoniscus gen. nov. remains undetermined due to the examination of such key internal characters typically requiring dissection. Following this, its placement within this group of families is uncertain. Conglobation in Balticoniscus gen. nov. Conglobation is a common antipredatory strategy and allows terrestrial isopods to conserve moisture (Smigel & Gibbs 2008; Hornung 2011; Tuf & Ďurajková 2022). This ability evolved multiple times within Oniscidea (Gruner 1953; Schmidt 2008) and can be found in many different families, such as Cylisticidae, Armadillidae, Armadillidiidae, Scleropactidae, Delatorreiidae, Eubelidae Budde-Lund, 1899 or Pudeoniscidae Lemos de Castro, 1973. The strongly convex dorsum of Balticoniscus walterludwigi gen. et sp. nov. is reminiscent of extant conglobating genera such as Cylisticus Schnitzler, 1853 and Porcellium Dahl, 1916, or the genera contained in the family Armadillidiidae. While these similarities are not indicative of a close relation to any of these genera, it strongly suggests that Balticoniscus walterludwigi was capable of conglobation. While the epimera are not as steeply angled as is seen in conglobating species such as Armadillidium vulgare (Latreille, 1804) or A. pulchellum (Zenker, 1799), they display a steepness similar to many other conglobating taxa like A. nasatum Budde-Lund, 1885 or Cylisticus convexus (De Geer, 1778). The small uropods, protruding only shortly past the posterior margin of the pleonites and pleotelson, reinforce the likeliness of Balticoniscus gen. nov. being a conglobating genus (Gruner 1953). Additionally, the short antennae and position of the small antennary tubercles suggest endoantennal conglobation, as they likely allowed the antennae to fold inward and be enclosed within the rolled-up body. While the ability to conglobate is likely, it cannot be entirely ruled out that Balticoniscus gen. nov. was a non-conglobating clinger or runner, as non-conglobating species with a convex dorsum are known as well, such as Porcellio gallicus Dollfus, 1904. However, runner type taxa are usually characterized by shorter epimera and relatively longer legs, and clinger type taxa feature a much flatter dorsum and nearly horizontally oriented epimera. The lack of ventral articulating structures, which are present in many genera of Armadillidae and Armadillidiidae (Gruner 1953; Taiti et al. 1998), might indicate limited or facultative conglobation but is not conclusive, as these structures are not obligatory for conglobating isopods. In conclusion, Balticoniscus gen. nov. shows several characteristics indicative of a conglobating taxon, but is lacking adaptions seen in dedicated conglobators. Therefore, the new genus may be comparable to the extant genus Cylisticus, which shows a similar degree of adaption to conglobation, but is more inclined to flee from potential threats than to make use of its ability to conglobate (Gruner 1953). Future discoveries of Balticoniscus may provide additional evidence for the ability to conglobate, as such a defensive behavior may also be employed in response to environmental hazards, which potentially includes the events that resulted in an entrapment in amber. Ecological considerations Baltic amber is one of three major Late Eocene amber deposits in Europe, alongside Rovno and Bitterfeld amber. It is distinct in having a lower δ²H signature, which indicates relatively cooler climatic conditions. Notably, Baltic amber exhibits high isotopic variability, which may reflect a broad catchment area or an extended period of resin deposition (Mänd et al. 2018). The likely catchment area was situated to the north, toward present-day Scandinavia, with average temperatures around 17° C. However, some amber specimens yield isotope values that indicate average temperatures below 15° C, unique among European amber deposits, while others reflect more tropical conditions (Wolfe et al. 2016). Without isotope analysis of the specific amber piece containing Balticoniscus gen. nov., a wide range of temperature and precipitation regimes is possible. Overall, beetle and plant species indicate a humid, thermophilic and polydominant forest in a flat to hilly landscape with primarily oligotrophic waters that can be interpreted as sea lowlands (Alekseev & Alekseev 2016). Comparable conditions can be found in the modern warmtemperate zonobiomes of East Asia and North America (Sadowski 2017).