Paper No. 162-44
Presentation Time: 9:00 AM-6:30 PM
MICROBIAL- CADDISFLY BIOHERM ASSOCIATION FROM THE LOWER CRETACEOUS SHINEHUDAG FORMATION, MONGOLIA: EARLIEST RECORD OF PLANT ARMORING IN FOSSIL CADDISFLY CASES
Caddisflies (Insecta Trichoptera) are fully aquatic in their larval and pupal stage and most species build cases for protection and respiration reasons. The ability of caddisfly larvae to construct cases with their own silk glands using common surrounding materials is interesting biological behavior, as well as an environmental indicator in modern and ancient examples. The recent discovery of caddisfly-dominated microbial-bioherms from Early Cretaceous lake systems of western Mongolia calls into question several aspects of the lacustrine paleo ecosystem and paleoenvironment. Characteristics of these bioherms were studied in detail including outcrop mapping and detailed description, hand sample description, and microscopic analysis. Petrographic analyses reveal that bioherms are characterized by intercalation of finely laminated stromatolites with tens to hundreds of non-uniform sized caddisfly cases. Cases are straight to slightly curved, circular in cross section, and elliptical shaped, slightly tapered to the posterior end in long section. Cases are filled with grainy limestone, micrite, and calcite spar, and are commonly armored with largely thin, elongate plant fragments, woody tissue, and ostracod valves. Larvae also used elongate mica, and rare fine feldspar grains for armoring. Common allochems excluding caddisfly cases are ostracods, oncoids, rarely plant fragments, gastropod, feldspar and a few fine quartz grains. The microbial-caddisfly association as well as oncoids resulted from wave action suggest that bioherms were deposited within the wave base in a shallow nearshore, carbonate-rich environment. The bioherms are found in a discrete stratigraphic interval between thick oil shale deposits in the Berriassian-Valanginian (145.5 5 ± 4.0 Ma to 140.2 ± 3.0 Ma) Shinehudag Formation in the Bayankhongor province of Mongolia, suggesting rapidly changing lake dynamics. This is the earliest documented fossil record of caddisfly case preferentially armored by plant fragments, and a rare occurrence of microbial-caddisfly bioherm association in the late Mesozoic. Through analyzing both caddisfly case architecture and bioherm character and stratigraphic variability, it will be possible to elucidate controls from the larger scale processes that impacted lake basin development.