THE RADIATION OF INSECTS INTO LAKES IN THE EARLY JURASSIC, AS RECORDED BY TRACE FOSSILS FROM THE HARTFORD AND DEERFIELD BASINS OF MASSACHUSETTS

Insects colonized lakes during the Late Triassic and Early Jurassic to form the first lacustrine entomofaunas. Body fossils are the primary evidence for this colonization event, but trace fossils can fill important gaps in the record of the radiation in regions with sparse body fossils. In particular, the lacustrine rocks of the Hartford and Deerfield basins of Massachusetts span the time of the radiation, but insect body fossils in these basins are rare and fragmentary, with the exception of the beetle larva Mormolucoides. Despite the rarity of body fossils in rocks from these basins, insect trackways are abundant and occur in sediments that are interpreted to represent shallow-water environments. The occurrence of the trackways in lacustrine rocks, however, does not indicate that they were made by aquatic insects because terrestrial insects could have colonized the lakebeds as the lakes dried. Nevertheless, two ichnotaxa, Bifurculapes laqueatus and Lunulipes obscurus, are thought to provide evidence for this radiation. Bifurculapes laqueatus is likely of aquatic origin because some specimens are associated with current lineations, and turns in the same direction as the lineations suggest that the still unidentified makers interacted with the current. Lunulipes obscurus was identified as being made by a swimming insect (possibly a water boatman, Family Corixidae) due to its morphology, which most resembles trackways produced by modern water boatmen made in laboratory experiments. Given the large number of insect ichnotaxa described from these basins, it is hypothesized that as more of them are reexamined, more will be identified as having been produced under water and will thus provide additional evidence for the Late Triassic and Early Jurassic aquatic insect radiation.