Earliest Record of Plant Preference in Caddisfly Larval Case Armor and of Caddisfly/Stromatolite Symbiosis from the Early Cretaceous Shinhudag Formation, Mongolia

Cory Dinter, Kurt Constenius, Cari L. Johnson, and Mark Loewen
University of Utah

Caddisfly larval cases have long been recognized in Cenozoic rocks and increasingly in the Mesozoic. Their association with stromatolites in lacustrine reefs is documented as early as the Aptian. We report on caddisfly larval cases as framework within lacustrine stromatolites from the Early Cretaceous Shinhudag Formation of Mongolia. The association occurs in a synrift succession that is Berriasian to Valanginian in age based on palynology (145.5 5 ± 4.0 Ma and 140.2 ± 3.0 Ma). Thin-sectioned samples reveal finely laminated stromatolites intercalated with straight round caddisfly larval cases of uniform size. The cases are filled with diagenetic calcite spar and display preferential armoring dominated by plant material, including woody tissue. Minor armor components include ostracod carapaces, ooids and quartz grains. Matrix surrounding the cases is largely micrite, with minor ooids and detrital silt. Plant material is rare except as case armor. This is the first documented occurrence of fossil caddisfly cases showing preferential choice of plant material for case armor, as is seen in living representatives of the family Phryganeidae. The stromatolite/caddisfly-bioherms suggest that they were deposited near the shore of a shallow, relatively alkaline lake. Likely the lake was periodically starved for calcite. These stromatolites might have formed when increased inflow brought in calcium-rich waters. Wave action and photosynthesis from the cyanobacteria of the stromatolites resulted in a carbonate-precipitating environment forming the stromatolite bioherms. The stable platform of the stromatolites provided an attachment site for the caddisflies in a well-oxygenated environment due to wave action, preferable to the shifting ooid forereef and backreef environments. The rare occurrence of the caddisfly/stromatolite reef association at only the greatest expansion of the lake within the formation is consistent with patterns and reefs present in the Laney Member of the Eocene Green River Formation. This represents the earliest occurrence of plant-preferring caddisfly larvae and the first documented occurrence of stromatolite bioherm/caddisfly symbiotic interactions.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013