The Fossil Record of Insect Mouthparts: Innovation, Functional Convergence, and Associations with Other Organisms

Abstract

The mouthparts of insects are a phenomenal example of a multi-element, modular, feeding apparatus that repeatedly has been modified structurally to perform every feeding function imaginable in the terrestrial and freshwater realms, a process that began in the Early Devonian. Insect mouthparts have been structured to chew, pierce and suck, siphon, lap, sponge, bore, and mine on and within a wide variety of tissues, as well as filter, sieve, and collect particulate food such as plankton and pollen. Thirty-seven fundamental mouthpart classes perform these roles in the modern and fossil record, a result that has been expanded somewhat from earlier, phenetic cluster analyses of modern insect mouthparts. A broad survey of fossil insect mouthparts, in conjunction with the phenetic mouthpart analysis, revealed patterns of mouthpart innovation occurring in bursts of cladogenesis separated from intervals of rather static mouthpart morphology. For the Paleozoic Era, based on direct (body fossil) and indirect (trace fossil) evidence, and commencing during the Devonian Period, the four earliest mouthpart classes were present, accounting for 11.4% of all mouthpart classes in the fossil record. In the succeeding Mississippian Subperiod, no mouthparts are documented; the four mouthparts from the Devonian continue into the succeeding Pennsylvanian Subperiod. During Pennsylvanian time, there was a spectacular burst of new mouthpart classes, coincident with the appearance of approximately 15 major insect lineages. By the end of the period, 29.7% of all insect classes had appeared. The following Permian Period added another seven mouthpart classes, particularly those from early hemimetabolous and holometabolous lineages, resulting in 48.6% of all mouthpart classes present. The profound ecological crisis at the end of the Permian notably saw the near extirpation of only one mouthpart class, the Robust Beak of piercing-and-sucking paleodictyopteroid insects, which eventually was extinguished sometime during the ensuing Triassic Period. For the Mesozoic Era, the Triassic Period added another seven mouthpart classes, particularly involving aquatic naiads and larvae, and early dipteran mouthparts, resulting in 67.6% of all mouthpart classes at the end of the period. During the Jurassic, the Mesozoic Lacustrine Revolution had begun, reaching a peak in the invasion of freshwater ecosystems that commenced during the Late Triassic, but undergoing a major diversification of mouthparts in terrestrial lineages, resulting in 83.3% of all mouthpart classes present, notably before the ecological expansion of angiosperms in the subsequent Early Cretaceous. The Jurassic also was a time for the origin and initial innovation of mouthpart design in early Siphonaptera, and a largely parallel diversification event among hematophagous Diptera; both processes continued into the Early Cretaceous. The Cretaceous Period exhibits a considerable diversity in compression deposits and especially amber deposits, preserving relict lineages that bore mouthparts at a Permian and Triassic stage of evolution as well as new lineages with bizarre mouthpart structures that are difficult to place among existing mouthpart classes. During the Cretaceous, three new mouthparts classes are added, yielding 97.1% of all mouthparts at the end of the period. For the Cenozoic Era, no mouthpart classes are added during the Paleogene Period, and only one mouthpart class, lacking a fossil record, is added during the Neogene Period. During this time, there is modification and expansion of mouthpart classes established during the mid Mesozoic and the development of special mouthpart elements involved in leaf mining, blood feeding, and pollination.