A Dendroctonus bark engraving (Coleoptera : Scolytidae) from a middle Eocene Larix (Coniferales : Pinaceae): Early or delayed colonization?

An engraving made by a scolytid bark beetle, assigned to the genus Dendroet onus of the tribe Tomieini, has been identified on a mummified, middle Eoce ne (45 Ma) specimen of Larix altoborealis wood from the Canadian High Arcti c. Larix altoborealis is the earliest known species of Larix, a distinctive lineage of pinaceous conifers that is taxonomically identifiable by the mi ddle Eocene and achieved a broad continental distribution in northern North America and Eurasia during the late Cenozoic. Dendroctonus currently consi sts of three highly host-specific lineages that have pinaceous hosts: a bas al monospecific clade on Pinoideae (Pinus) and two sister clades that consi st of a speciose clade associated exclusively with Pinoideae and six specie s that breed overwhelmingly in Piceoideae (Picea) and Laricoideae (Pseudots uga and Larix). The middle Eocene engraving in L. altoborealis represents a n early member of Dendroctonus that is ancestral to other congeneric specie s that colonized a short-bracted species of Larix. This fossil occurrence, buttressed by recent data on the phylogeny of Pinaceae subfamilies and Dend roetonus species, indicates that there phylogenetically congruent colonizat ion by these bark-beetle lineages of a Pinoideae + (Piceoideae + Laricoidea e) host-plant sequence. Based on all available evidence, an hypothesis of a geochronologically early invasion during the Early Cretaceous is supported over an alternative view of late Cenozoic cladogenesis by bark beetles ont o the Pinaceae. These data also suggest that host-plant chemistry may be an effective species barrier to colonization by some bark-beetle taxa over ge ologically long time scales.