MicroCT data provide evidence correcting the previous misidentification of an Eocene amber beetle (Coleoptera, Cicindelidae) as an extant species

Abstract

The fossil record suggests some insect species have a marked longevity. The oldest fossils purported to represent extant insect species are from the Oligocene and Eocene. One of the most cited fossils is the extant tiger beetle Tetracha carolina (Coleoptera: Cicindelidae) that was identified over a century ago by Walther Horn in Eocene Baltic amber. We examined this and compared it to the previously described cincindelid Baltic amber fossil Palaeoiresina cassolai using X-ray microscopy and 3D imaging techniques. We conclude that Horn's fossil tiger beetle specimen is conspecific with the Eocene P. cassolai and is a member of an extinct stem group lineage of Cicindelidae. Based on a review of all the tiger beetle fossils described from Cretaceous and Paleogene deposits, we found that the assignment of these fossil species to extant lineages is not supported. There are currently no synapomorphies known from fossils that can provide evidence for Cretaceous Manticorni or Megacephalini nor is there evidence for Eocene Iresina. We provide evidence that rejects the idea of a recent beetle species persisting since the Eocene period, which is crucial for using the currently known fossil Cicindelidae species to calibrate divergence dating of beetle phylogenies.

Figure 1

World map showing the current distribution of species of the genus Tetracha (green region) and the source locality of Horn’s tiger beetle fossil in the Kaliningrad region (green circle). The included images are of a specimen of Tetracha carolina carolina (left), the somewhat oddly cut piece of amber with the cicindelid fossil inclusion (right). The interactive PDF of the microCT reconstruction of Horn’s tiger beetle fossil as an embedded 3D image is available at Supplementary figure 1.

Figure 2

Volume rendering of Horn’s tiger beetle fossil (Palaeoiresina cassolai Wiesner et al., 2017) with specimen ID MB.J 1647 in the NMB. (A) beetle body, left lateral view. (B) pronotum and anterior part of elytra. (C) head, frontal view. (D) labrum and clypeus, dorsal view. (E) right maxilla, ventral view. (F) anterior part of head, ventral view. (G) labium, dorsal view. (H) meso- and metathorax, right latero-ventral view. (I) apical part of elytra and abdomen, caudal view. Abbreviations: aed-ap, apex of aedeagal median lobe; al, hindwings (highlighted by dotted line); as, antennal scape; ant, antenna; cd, cardo; ce, compound eye; cl, clypeus; cl-s, clypeal setae; el-ai, elytral anterior impression; el-ap, apex of elytra; el-ep, elytral epipleuron; el-pi, elytral posterior impression; fm-p, -m, -t, pro-, meso-, metafemur; ga, galea; gu, gula; hu, humerus; la, labrum; la-as, -ls, apical resp. lateral setae of labrum; lap, labial palpus; lapm1, 2, 3, labial palpomeres 1, 2, 3; lc, lacinia; lc-as, apical spur of lacinia; li, ligula; map, maxillary palpus; mapm1, 2, 3, 4, maxillary palpomeres 1, 2, 3, 4; mcx, mesocoxa; md, mandibles; msem, mesepimeron; mses, mesepisternum; msst, mesosternum; mt, mentum; mtes, metepisternum; mtst, metasternum; mtt, median tooth of mentum; pcx, procoxa; pn-am, anterior margin of pronotum; pn-bm, basal margin of pronotum; pn-ai, pronotal anterior impression; pn-pi, pronotal posterior impression; ps-am, anterior margin of prosternum; sc, scutellum (highlighted by dotted line); so-s, insertion points of the supraorbital setae (only shown on left side of head); st, stipes; su, elytral suture. The interactive PDF of the microCT reconstruction of the head of Horn’s tiger beetle fossil as an embedded 3D image is available at Supplementary figure 2.

Figure 3

Volume rendering of the holotype specimen of Palaeoiresina cassolai Wiesner et al., 2017 (A, B, D) and Horn’s tiger beetle fossil (C). (A) anterior part of beetle body, dorsal view, with (A’), enlarged section of part of the elytra, showing puncture. (B, C) mandibles, dorsal view. (D) ventral surface of labrum. Abbreviations: ant, antenna; bfb, basal face brush of mandible; ce, compound eye; eph, epipharynx, dorsal surface; hu, humerus; it, incisor tooth; la-as, -ls, apical resp. lateral setae of labrum; pn, pronotum; ppr, parapedial ridge; pps, parapedial setae; rt, retinacular tooth; rt1, 2, 3, retinacular tooth, cusp 1, 2, 3; sc, scutellum (highlighted by dotted line); su, elytral suture; tta, ttb, anterior resp. basal terebral tooth; vm, ventral microtrichia of mandible. The interactive PDF of the microCT reconstruction of the forebody of the holotype specimen of P. cassolai as an embedded 3D image is available at Supplementary figure 3.

Figure 4

Molecular genetic based phylogeny of Cicindelidae as presented by Duran and Gough with Mantica added, and with reference to the morphological character states of the basal clades and the fossil representatives of the family, together with their minimum ages (see text, for details). Numbers in white boxes refer to synapomorphies of the clades: 1 basal terebral tooth tricuspidate. 2 mandibular diastema developed. 3 anterior epipharyngial parapedial setae terminally detached from the parapedial ridge. 4 labial palpiger not covered by the mentum. 5 absence of the latero-marginal seta both sides of labrum. 6 presence of a supplementary retinacular tooth. Of the six currently-known fossil species of cicindelids, one is identified as being stem group member of the family (red box), one of the MCCCO clade (blue box), while the systematic positions of four taxa are unknown (black box).