Phylogeny and systematics of Sphaeriusidae (Coleoptera: Myxophaga): minute living fossils with underestimated past and present‐day diversity

Abstract

Sphaeriusidae (Coleoptera: Myxophaga) is a group of shiny, blackish and hemispherical riparian beetles, known for their miniaturized bodies. They are worldwide in distribution, but very limited information is available about taxonomic and morphological diversity, and natural and evolutionary history. The aim of this study is to help fill in these gaps. We examined the external morphology of modern representatives using scanning electron microscopy (SEM), and reconstructed the phylogeny of the family using five DNA markers (cytochrome oxidase I, 18S rRNA, 28S rRNA, CAD and wingless). Our results suggest a larger morphological diversity than previously expected, corresponding to the deep genetic divergences of principal lineages. We also examined two inclusions in 99‐million‐year‐old Burmese amber. The integration of all evidence allows us to recognize three genera: the extinct genus †Burmasporum Kirejtshuk, the newly defined genus Bezesporum gen.nov. preserved in Burmese amber (B. burmiticum sp.nov.) and present in the modern fauna of Southeast Asia, and the genus Sphaerius Waltl with a world‐wide distribution. Sphaerius species are morphologically highly uniform, with the exception of species from Australia and South Africa, which share some characteristics with Bezesporum gen.nov. despite being resolved as deeply nested lineages of Sphaerius by DNA data. The presence of Bezesporum gen.nov. in Burmese amber and in recent fauna indicates that Sphaeriusidae largely maintained their specific morphology and specialized riparian lifestyle for at least 100 million years. Therefore, they can be considered an exceptionally conserved group, with a minimum of evolutionary changes over a long period. Our study also demonstrates that the species numbers and fine‐scale morphological diversity of Sphaeriusidae are larger than expected in both the past and present‐day faunas. Both were apparently underestimated due to the minute body size and cryptic habits of these beetles.

New fossils of Sphaeriusidae from mid-Cretaceous Burmese amber revealed by confocal microscopy (Coleoptera: Myxophaga)

“…To evaluate the systematic placement of the two new fossils, a constrained morphology-based phylogenetic analyses was performed under Bayesian inference. The data matrix (Supplementary Data S1, S2) was mainly derived from a recently published dataset (Fikáček et al, 2022). The definitions of some characters were modified to fit the inclusion of the new fossils and additional outgroups.…”

“…Sphaeriusidae is a small but widespread family, with only two genera and about 25 species known in extant fauna (Lawrence & Ślipiński, 2013;Hall, 2019;Fikáček et al, 2022). The adults of Sphaeriusidae are easily characterized by the tiny and dorsally convex body, 11-segmented antennae, fused mesotrochanterofemora, large metacoxal plates, 3 (rarely 2)segmented tarsi, and abdomen with only three ventrites of which the second is short (Reichardt, 1973;Löbl, 1995;Fikáček et al, 2022). As pointed out by Beutel & Raffaini (2003), due to the small size of sphaeriusids, detailed morphology for extant species can only be properly documented when SEM (scanning electronic microscopy) techniques are applied.…”

“…As pointed out by Beutel & Raffaini (2003), due to the small size of sphaeriusids, detailed morphology for extant species can only be properly documented when SEM (scanning electronic microscopy) techniques are applied. However, only a few sphaeriusids have been imaged under SEM (e.g., Löbl, 1995;Liang & Jia, 2018;Yavorskaya et al, 2018;Fikáček et al, 2022), and some of them were not identified to species level (e.g., Beutel, 1999;Beutel & Raffaini, 2003;Kamezawa & Matsubara, 2012). As a result, the taxonomy of this group has been insufficiently studied and the its species-level diversity is likely strongly underestimated (Fikáček et al, 2022).…”

“…However, only a few sphaeriusids have been imaged under SEM (e.g., Löbl, 1995;Liang & Jia, 2018;Yavorskaya et al, 2018;Fikáček et al, 2022), and some of them were not identified to species level (e.g., Beutel, 1999;Beutel & Raffaini, 2003;Kamezawa & Matsubara, 2012). As a result, the taxonomy of this group has been insufficiently studied and the its species-level diversity is likely strongly underestimated (Fikáček et al, 2022).…”

“…Due to their minute body size, sphaeriusids are rarely documented and often overlooked in the fossil record. The only two fossils known to date are Burmasporum rossi Kirejtshuk and Bezesporum burmiticum Fikáček et al, both from mid-Cretaceous Burmese amber (Kirejtshuk, 2009;Fikáček et al, 2022). However, some important characters were not observed for these two fossils, partly due to the usage of traditional brightfield imaging technique only.…”