The earliest beetle †Coleopsis archaica (Insecta: Coleo­ptera) – morphological re-evaluation using Reflectance Transformation Imaging (RTI) and phylogenetic assessment

Abstract

The earliest known fossil beetle †Coleopsis archaica is re-examined using Reflectance Transformation Imaging (RTI). The morphological observations are evaluated with respect to phylogenetic implications and the early evolution of Coleoptera. †Coleopsis archaica belongs to an early Permian branch of beetles, outside a monophyletic unit comprising Coleoptera (in the widest sense) excluding †Tshekardocoleidae. This clade is mainly characterized by a complex of apomorphic features: elytra with epipleura and with a close fit with the posterior body, thus forming a tightly sealed subelytral space. In contrast to this, the elytra of †C. archaica and †Tshekardocoleidae cover the metathorax and abdomen in a loose tent-like manner and posteriorly distinctly surpass the abdominal apex. So far, no synapomorphies of the two taxa from the first half of the Permian have been identified. The very short and transverse pronotum is likely an autapomorphy of †C. archaica. A thorough documentation of the structural features of early beetle fossils should have high priority. RTI is a very promising tool to obtain new and well-founded morphological data, which will allow a thorough phylogenetic evaluation of Permian beetles in future studies. We extended the conventional RTI workflow by focus merging and panoramic stitching, in order to overcome previous limitations. Taxonomic re-arrangements of stem group beetles including †C. archaica were suggested in recent studies by A.G. Kirejtshuk and co-workers. As they are not based on shared derived features they are irrelevant in a phylogenetic and evolutionary context.

When a key innovation becomes redundant: Patterns, drivers and consequences of elytral reduction in Coleoptera

“…Reduction of the elytra, either partially or completely, is unknown in Permian stem groups of Coleoptera (Boudinot et al, 2023a;Ponomarenko, 1969). The most ancestral condition in Coleoptera was outlined by Boudinot et al (2023a) and Schädel et al (2022): elytra broader than the abdomen and surpassing the abdominal apex posteriorly, covering the posterior body loosely in a tent-like manner (see also Goczał & Beutel, 2023). Elytra fitting closely with the abdomen forming a tightly sealed subelytral space had evolved by the Middle Permian (Boudinot et al, 2023a).…”

Beetle elytra: evolution, modifications and biological functions

“…Schematic drawings from [ 14 ]. ( b,c ) Reconstruction of postcephalic body of a Lower Permian beetle (†Tshekardocoleidae/† Coleopsis ), based on [ 14 , 15 ]: ( b ) dorsal view; ( c ) ventral view (note that window punctures (in red) are present on entire elytral surface). ( d,e ) Postcephalic body of † Peltosyne triassica Ponomarenko (†Peltosynidae), modified from [ 16 ]: ( d ) dorsal view, ( e ) ventral view.…”

“…Elytra of early Permian stem group beetles (Coleoptera sensu lato ), †Tshekardocoleidae and † Coleopsis [ 14 , 15 ], distinctly surpassed the elytral apex posteriorly, lacked epipleura folded inwards, and did not fit tightly with the abdomen, thus only loosely covering the posterior body [ 14 , 15 ] ( figure 1 a–c ). Moreover, they had partially maintained the original longitudinal wing venation ( figure 1 b ), lacking the parallel arrangement of later extinct groups and modern beetles [ 15 , 21 – 23 ] ( figure 1 d ).…”

“…The earliest known elytra [ 15 , 23 ] already differ distinctly from membranous wings suitable for flight, lacking a large anal field ( figure 1 b ), and displaying a straight posterior edge and a pointed apex (e.g. [ 18 ]; figure 1 b,c ).…”

“…[ 18 ]; figure 1 b,c ). A crucial transformation was the formation of epipleura folded inwards ( figure 1 e,g,i ), instead of broad and flat lateral flanges [ 14 , 15 ] ( figure 1 b,c ), and an adjustment of the length and width to the shape of the metathorax and abdomen. An intermediate condition was present in †Permocupedidae [ 14 , 18 , 27 , 28 ], but all following groups (Metacoleoptera) [ 14 ] are characterized by a close fit of elytra and posterior body, and consequently a secluded sub-elytral space.…”