New Mexico Geological Society Annual Spring Meeting — Abstracts

Fossil aquatic insects of the order Hemiptera are well known in the Middle Jurassic Todilto Formation from the "Warm Springs" site. Discovered and worked by Kirkland and Bradbury in 1964, the site is in Sandoval County and has been relocated and reopened in the past year. At the site, fossil insects occur in a 33-cm-thick papery, calcareous shale that begins approximately 27 cm above the base of the Todilto Formation. The lower 60 cm of the Todilto (Luciano Mesa Member) here are a unique, localized facies that records two cycles of fining-upward lacustrine deposition. The overlying 83 cm of the Luciano Mesa Member are the characteristic kerogenic limestone facies found throughout the vast Todilto depositional basin. The insect bed is in the second cycle of deposition and thus predates development of the Todilto salina. Therefore, the ecology of the insects may have little or no direct bearing on interpreting the salinity and other paleoecological aspects of Todilto salina deposition.

At the Warm Springs site, there are two size variants of Hemiptera; the larger is assigned to Naucoridae, the creeping water bug family. Approximately one third of the fossils are preserved well enough to show legs, body segments, or other structure. These 4-mm-to 12-mm-long predaceous insects are usually evenly distributed on the bedding planes with a density of less than 1 to 2 per square foot. Rarely, isolated areas contain higher densities of up to 3 per square inch over several square inches. In addition to the previously reported water bugs, three new fossil types, consistent in appearance with non-naucorid insects, have been found at the site. Other invertebrates, possibly representing the classes Arachnida and/or Crustacea, are also present. Disarticulated fish scales and bones are common, and probable fish coprolites are also seen.