The Middle Jurassic Sundance Formation, of Callovian age (160 Ma), is comprised of a finely-laminated, platy lithographic limestone that represented a benthos of lime mud. An allochthonous palynoflora includes pollen from the conifer families Araucariaceae (Araucariacites) and the extinct Cheirolepidiaceae (Classopollis), as well as Eucommiidites, of possibly gnetalean affinities. Whereas the vertebrate fauna consists of one fish species, the nektonic insect fauna consists of about 15 species of dominantly Hemiptera and Coleoptera. Sedimentological, paleobiological, and geochemical evidence indicates a shallow basinal environment near sea level that periodically was infilled with fine-grained terrestrial sediment. There was no resident infauna or epifauna, as bottom conditions probably were inhospitable to respiring organisms.

Unlike other mid-Mesozoic, lime-rich deposits, the Sundance Formation lacks land-derived insect taxa. The most abundant insects are heteropterous Belostomatidae, Corixidae, Naucoridae, and extinct Enicocoridae. The Coleoptera are represented principally by Dytiscidae and possibly extinct Parahygrobiidae. Rare caddisfly cases constructed of quartzose sand grains also are present. Adult and subadult taxa are present, occurring in sufficient numbers to allow recognition of instar-related size differences. Soft-bodied preservation of insects by calcium phosphate impregnation reveals overall body form, including segmented appendages, setae, sclerotized elements such as mouthparts and perhaps genitalia, occasional surface ornamentation, and even premortem color patterns.

This deposit and the poorly known Todilto fauna in New Mexico are the only diverse, mid-Mesozoic, fossil insect deposits known in North America. As Sundance taxa are resolved with greater certainty, their taxonomic affinities to contemporaneous deposits in Eurasia and Brazil should reveal important biogeographic patterns.