Micro-CT reveals hidden morphology and clarifies the phylogenetic position of Baltic amber water scavenger beetles (Coleoptera: Hydrophilidae)

ABSTRACT

Water scavenger beetles (Coleoptera: Hydrophilidae) are a broadly distributed group with high extant diversity. However, reliable records from Baltic amber (Eocene) are scarce and limited to two undescribed species previously reported in the literature. Here we study these two specimens plus four additional ones. All specimens were sub-optimal in terms of preservation and visibility of their morphology using traditional light microscopy. Opaque bubbles, cracks in the amber pieces and contracted appendages obscured most of the crucial characters, preventing their formal description and making them prone to misidentification. Here we used X-ray micro-computed tomography (μCT) to reconstruct the morphology of the embedded specimens and described three new species: Anacaena morla sp. nov., Crenitis profechuyi sp. nov. and Helochares fog sp. nov. Micro-CT reconstructions allowed us to visualise the morphology of the specimens to an extent where it was possible to hypothesise possible phylogenetic relations of the new taxa as well as the biogeographic implications of these relationships.

KEYWORDS:

• Eocene
• Hydrophiloidea
• Chaetarthriinae
• Acidocerinae
• X-ray μCT

Acknowledgments

We are grateful to collectors and museum curators for making specimens available for our study, Christel and Hans Werner Hoffeins (Hoffeins collection), Andrzej Górski (Andrzej Górski collection), Elżbieta Sontag (MAIG), Daniel Kubisz (ISEA), and Stephan Blank and Lutz Behne (SDEI). Micro-CT imaging of amber inclusions was performed by Brian Metscher (University of Vienna) and Jana Bruthansová (National Museum, Prague). We are thankful to Andrew E.Z. Short (University of Kansas) for discussing with us the possible relations of Helochares fog sp. nov. within Acidocerinae. Additionally, we appreciate the comments and suggestions of Bruno Clarkson Matos and one anonymous reviewer that helped us improving the manuscript. This work was supported by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 642241 to EAV, the A-base funding of AJB, and by the Ministry of Culture of the Czech Republic (DKRVO 2019-2023/5.l.a and 2018/06, National Museum, 00023273) to MF and JB. The work of EAV at the Department of Zoology, Charles University, Prague was partly supported by grant SVV260434/2018.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the H2020 Marie Skłodowska-Curie Actions [642241] the A-base funding (Canada) and the Ministry of Culture of the Czech Republic (DKRVO 2019-2023/5.l.a and 2018/06, National Museum, 00023273).