The fossil was originally found in a larger piece of Baltic amber and uneasy to examine. We polished the amber piece to a smaller one with 600 and 1200 grit wet sandpapers, to make detailed examination possible. After polishing, the specimen was examined using Olympus SZ61 binocular microscope under various light regimes both in dry and wet condition (submerged in glycerine). Photographs were taken using a Canon 850D camera attached to the binocular microscope by an AmScope adapter. Photographs were stacked from a series of original photos with different focus using Helicon Focus software; all photographs, including those not shown here, are available at the Zenodo archive under doi https://doi.org/10.5281/zenodo.7803930. Drawings are based on the photographs and were prepared in Clip Studio Paint software using a Wacom One graphical tablet. Morphological terminology follows Fikáček (2019), classification follows Short and Fikáček (2013). Comparison with modern specimens is based on material deposited in the Department of Entomology, National Museum, Prague. SEM micrographs of these specimens were taken using Hitachi S-3700N environmental electron microscope at the Department of Paleontology, National Museum (Prague, Czech Republic), using uncoated specimens in the low vacuum mode.

To reveal the impact of the fossil described in this study on the reconstruction of the ancestral distribution of the most recent common ancestor of Cymbiodyta, we performed a series of historical biogeography analyses. We used the dated phylogeny of Cymbiodyta published by Toussaint and Short (2019) into which we added the fossil species as a sister taxon to the European C. marginella, following the close relationship of the fossil and this species revealed by morphology. Since the timing of the divergence between C. marginella and the fossil remains unknown, we generated four trees corresponding to four alternative scenarios tested, the age of their MRCA being 38 mya, 50 mya, 70 mya or 90 mya, i.e., arbitrarily selected values ranging from the age slightly predating the yougest estimate for the Baltic amber age (34–48 mya, Seyfullah et al. 2018) to the age only slightly postdating the split of the European lineage of Cymbiodyta (96 mya, Toussaint and Short 2019). The distribution of the modern species and the fossil was coded as three areas: Europe (EU), North America (NA) and southern Asia (AS). The analyses were performed using BioGeoBEARS R package (Matzke 2013), using the tree and the distribution as the only data. Three models (DEC, DIVALIKE and BAYAREALIKE) available in the package and their +j alternatives allowing for jump dispersal were compared; the reconstruction based on the best performing model is presented here. The likelihoods of the three areas and their combinations for all four alternative trees are compared.

The external morphology of adult Cymbiodyta is very uniform, with only a few species being morphologically very distinct (e.g., C. bifida which was until recently classified as a separate genus Helocombus; Toussaint and Short 2019). Remaining species differ in the shape of the mesoventral projection, the presence or absence of elytral striae, the body size and coloration, the size and shape of the emargination on the abdominal apex, and in the morphology of male genitalia (Smetana 1974, Jia and Short 2010, Jia and Lin 2015). Moreover, the molecular phylogeny by Toussaint and Short (2019), albeit including only 13 of 33 known species, indicates that some of these characters originated multiple times independently. For example, the species with the large triangular mesoventral projection (i.e., part of the C. marginella group by Smetana 1974) form at least two clades: the European C. marginella is sister to all other Cymbiodyta, whereas C. vindicata (and hence very likely also the other larger-bodied American species with protruding mesoventrite: C. leechi and C. acuminata) are deeply nested among the American species.

Cymbiodyta samuelisp. nov. described here is un­doubtedly very similar to two modern species: the European C. marginella and the American C. minima. It corresponds with them by a small body size, the high mesoventral projection, the coloration, and the presence of the shallow emargination on abdominal apex. Of these two modern species, it seems to be closer to the European C. marginella in the shape of the mesoventral projection: this is highly elevated in C. marginella (Fig. 3E) and C. samueli (Figs 1D, 2D), but lowly elevated in C. minima (Fig. 3G). We hence consider the sister-position C. samueli and C. marginella as the most probable, further corroborated by the European distribution of both species. Our biogeographic analyses follow this assumption. Cymbiodyta minima is widely distribured through northern USA and southern Canada (Smetana 1974), but not included in the molecular analysis of Toussaint and Short (2019). Based on the morphology, we cannot exclude that it is in fact the member of the the earliest diverging lineage of Cymbiodyta (together with C. marginella and C. samuelisp. n.) but molecular data are needed to resolve this question.

Toussaint and Short (2019) reconstructed the historical biogeography of Cymbiodyta and revealed the American origin of the genus using the DEC model in Lagrange. In contrast, our re-analysis revealed wide ancestral range (North America + Europe) for MRCA of Cymbiodyta for all models compared, with DIVALIKE model fitting our data better than DEC in all cases. We argue that (1) the wider ancestral range corresponds better to data in hand, and (2) is congruent with the position of landmasses in the Late Cretaceous.

The narrower (North American) estimate of ancestral distribution of MRCA of Cymbiodyta by Toussaint and Short (2019) was likely caused by the inclusion of outgroup taxa (Notionotus liparus: South America, Enochrus testaceus: Palaearctic, E. ochraceus: North America). North America alone was not revealed as ancestral range in any of our analyses under any models or settings, with or without C. samuelisp. n. It is in fact suprising that the additional information about the lineage presence in Europe 38 mya changed the log-likelihoods only very slightly, without any effect on ancestral reconstructions. This indicates that under the models considered, only the inclusion of fossils contradicting the distribution of modern species affects the analysis significantly, but the information about historical occurrence in the same area is largely ignored. We expect that the effect may be larger in case of time-stratified analyses not performed here.

Our ancestral range of MRCA of Cymbiodyta seems to be also congruent with other available data. The stem age of Cymbiodyta (Late Cretaceous: Albian) corresponds to the time when eastern North America was situated very close to pieces of future Europe, with occassional land connections between eastern North America, Greenland and Scandinavia (DeGeer Bridge: Brikiatis 2014). The species with wide distribution across the area, with frequent gene flow during the times of emergent land bridges, may have really existed, similar to the widespread aquatic beetle species today (e.g., Helophorus sibiricus reaching from Scandinavia to Far East, but also inhabiting northern North America: Fikáček et al. 2011). The North American origin of Cymbiodyta proposed by Toussaint and Short (2019) cannot be fully excluded, but is not corresponding to the data available at the moment. A detailed molecular phylogeny of Enochrinae, revealing the sister group of Cymbiodyta and its modern distribution, will provide further data to reconstruct the origin of Cymbiodyta more reliably. The inclusion of Cymbiodyta minima into the molecular analyses may be also of interest, especially if it reveals its closer relationship to the European that to the American species; such a discovery would provide further support to the Euro-American rather than North American origin of the genus.

In any case, European Cymbiodyta is clearly an ancient relict lineage. Current data indicate that it survived in Europe since the Late Cretaceous, and our new fossil brings direct evidence of its presence in the Eocene. The fact that C. samuelisp. nov. clearly differs from the European species brings also evidence of a higher species diversity and subsequent extinction in Europe in the past.