Morphology of the Devonian mandibulate arthropod Rhyniognatha and its implications for the timing of insect evolution

Abstract

Insects are phenomenally diverse, yet their evolutionary origins remain poorly understood. The fossil Rhyniognatha hirsti from the Early Devonian Rhynie chert (ca. 408 Ma) has been widely accepted as the earliest insect and a crown-group pterygote (flying insect), based on the interpretation of short, dicondylic mandibles. For over two decades, this fragmentary specimen has served as a key calibration point in insect molecular phylogenies. An alternative hypothesis has proposed a myriapod affinity, citing ocular, antennal and maxillary structures. Here, we re-examine Rhyniognatha using confocal laser scanning microscopy. We find no evidence for an anterior mandibular acetabulum, refuting true dicondyly and undermining its placement within Dicondylia (silverfish–pterygote clade). We re-assess the proposed centipede affinity, demonstrating that certain diagnostic features are more plausibly interpreted as exoskeletal folds, and that the mandibles lack a clear myriapod-type base plate. Comparative analysis reveals similarities with apterygote hexapods, including bifurcated maxillary endites and comb-like superlinguae, and myriapods, such as an edafopodan-like elongate suspensorium, but does not support assignment to any specific mandibulate lineage. Consequently, Rhyniognatha cannot be placed within crown-group Insecta and is unsuitable as a calibration fossil for insect evolution, shifting the minimum age of winged insects by approximately 80 million years.