POSSIBLE MINUTE BRANCHIOPOD MANDIBLES (?CLADOCERA) FROM LAKE DEPOSITS (CINIZA LAKE) IN THE UPPER TRIASSIC CHINLE FORMATION (NEW MEXICO) AND THE TEMPORAL RECORD OF SMALL CARBONACEOUS FOSSILS (SCFS)

The Upper Triassic Chinle Formation is a widespread, non-marine rock unit in the western US. Amongst the variable facies it exhibits, it contains some notable lake deposits known as the Ciniza Lake Beds (CLBs), exposed near Fort Wingate, New Mexico. The CLBs are correlative with the upper Blue Mesa or lower Sonsela Member of the Chinle Formation in northern Arizona. The CLBs contain horizons that composed of a very organic-rich black shale dominated by clam shrimp (‘conchostracans’). When this shale is broken down in HF, it yields a rich residue of ‘palynodebris’ which contains not only bits of the clam shrimp, but also the mandibles of an as-yet unidentified crustacean. The mandibles are of the right morphology and size to be cladocerans.

This discovery helps bridge the gap of small carbonaceous fossils (SCF) between the Cambrian and Quaternary. It suggests that other organic-rich units may also contain microarthropod remains—only visible in the organic-rich residue left after HF processing.