Caddisfly cases from the early Eocene of Chubut, Patagonia, Argentina

GENISE, J. F. Y PETRULEVICIUS, J. F.

Cracovia

Congreso; II International Congress on Palaeoentomology (Fossil insects); 2001

Fossil insect cases in subaquatic environments, such as those of caddisflies and chironomids have been little studied in comparison with their potential as paleoenvironmental indicators. Although the former use coarser materials to construct their cases, comparative studies are lacking to distinguish similar fossil cases from both main groups of subaquatic constructors. The material described herein represents the second record of fossil caddisfly cases from South America and the first for Argentina. It came from a well-studied paleobotanical locality from Chubut, Patagonia, Argentina, known as Tufolitas Laguna del Hunco (early Eocene). These rocks are part of a volcanic-pyroclastic complex interpreted as a caldera deposit. The Hunco flora, insects and caddisfly cases were deposited in a lacustrine moat in fine-grained, reworked, pyroclastic debris. The flora shows elements of subtropical as well as subantartic forests developed under a warm temperate and humid climate, with a dry season. The caddisfly cases seem to be demic of this lake to which the different vegetal remains and terrestrial insects would have arrived from land by means of wind or watercourses. The cases are mostly composed of small bits of leaves and other unidentifiable vegetal debris; consequently they can be attributed to the ichnogenus Folindusia Berry. Eighty-three specimens in different state of preservation were examined. The cases are straight (in two cases curved), elongated and trapezoidal, with the posterior end smaller and rounded, whereas the anterior is larger and truncated. Well-preserved specimens show a darker layer laterally and at the rear suggesting this extreme of the case to be closed. The most complete specimens are 14 to 23 mm long, although the maximum length measured was 25 mm in an incomplete specimen, suggesting that the largest specimens would have been even longer. It also suggests that the sample include cases constructed by larvae in different stages of development. The widths of the ends show a great dispersion. The differences between both ends of the same case result in subtriangular to subrectangular morphologies. The cases show different arrays of constructional material. The simplest cases are composed of 65 to 90 small (0.1 to 0.5 mm), rounded bits of vegetal debris with a regular distribution. Some of them also show two lateral rows of thin, elongated bits, directed mid-posteriorly and displaying an open V-shaped pattern. The more complex specimens show, in addition, large (up to 10 mm long) bits of vegetal debris or coleopteran elytra arranged like fringes at both laterals mostly in the anterior part of the case and oriented toward the posterior end. Fringes are present in 15 of 78 specimens examined, even in small cases of more simple morphology. Accordingly, more complex morphologies could not be related definitely with the larger cases, although the incomplete specimen measuring 25 mm shows the best-developed fringe-like structure and the larger bits of vegetal debris.The presence of Folindusia caddisfly cases indicates eutrophic and lentic conditions for the Laguna del Hunco lake, in agreement with other paleontological and sedimentological evidences. * Contribution of PICT 99 6156