INSECT LEAF-MINING IN LATE TRIASSIC GYMNOSPERMOUS FLORAS FROM THE MOLTENO FORMATION OF SOUTH AFRICA

One of the most distinctive feeding strategies in insects is the consumption of internal tissues in the leaves of land plants by the larvae of holometabolous insects. This life-habit has originated numerous times among the four major orders of insects—Coleoptera, Diptera, Lepidoptera and Hymenoptera—but its earliest occurrences in the fossil record has been poorly documented and ignored. Currently, leaf mining is overwhelmingly dominant on angiosperms, although it has a sporadic, preangiospermous occurrence on extinct gymnospermous plants from several Middle Triassic to Lower Cretaceous deposits. In the Late Triassic (Carnian, ~225 Ma) Molteno Formation of the Karoo Basin, South Africa, seed-plant hosts represent seven major lineages that have mesophyll or palisade leaf mines constructed by polyphagan beetle and perhaps basal lepidopteran taxa. The seed-plant hosts are the umkomasialean Dicroidium odontopteroides, the broadleaved voltzialean conifer Heidiphyllum elongatum, the leathery-leaved cycadalean Pseudoctenis fissa, the ginkgoalean Paraginkgo antarctica, the hamshawvialean Sphenobaiera schenkii, the matatiellalean Dejerseya lunensis, and the herbaceous petriellalean Kannaskoppifolia sp. Additionally, thin, delicate mines are present on pinnules of the filicalean fern Cladophlebis sp. The four distinctive leaf-mine types exhibit host-plant specificity, and contain pelleted frass that is consistent with solid-feeding larvae. About 87 % of all mines occur on the conifer Heidiphyllum (currently 119 of 137 total leaves), the core taxon for mining activity. However, the greatest diversity of mine types and host plants is with the Ginkgoopsida, a clade of seed plants diverse during the early Mesozoic. The Molteno occurrences indicate that the origin and diversification of the leaf-mining habit occurred about 90 million years before the first appearance of fossil angiosperms. These preliminary data originate from an examination of 81,100 specimens from 49 localities, and are being supplemented by ongoing examination of additional Molteno material.