Fossils from the San Hipolito Formation were first mentioned by Mina (1957). He states that poorly preserved gastropod fragments which could not be identified were found in the upper part of the San Hipolito Formation. Pelecypods were collected by R. Gordon Gastil and Gael H. Troughton shortly before the author began a study of the San Hipolito Formation. The pelecypods were identified as Halobia? or Monotis? by David L. Jones (written communication, 1973) of the United States Geological Survey, Menlo Park, California. Better preserved pelecypods from new fossil localities were found by the author and identified by Jones and Norman L. Silberling as Monotis cf. M. subcircularis of Late Norian age and Halobia? sp. of Ladinian? through Mid-Norian age (Jones, written communication, 1975).
Monotis and Halobia? occur in beds made up of accumulations of great numbers of individuals in layers up to several centimeters thick. This mode of occurrence is characteristic of these and other pelecypods found in Triassic rocks around the world (Moore, 1969). According to Williams (1974), who has studied Monotis subcircularis and other thin-shelled bivalves in rocks from west-central Nevada,
Fossils from limestone boulders in the breccia member of the San Hipolito Formation were sent to David L. Jones and examined by his colleague, Augustus K. Armstrong, of the United States Geological Survey, Menlo Park, California. The fossils include corals, gastropods, pelecypods, crinoid debris and foraminifera, but none could be identified beyond doubt to genus and according to Armstrong (written communication, 1975), the "age of the limestone samples in the collection . . . is not readily determinable." Fossils identified include: (1) corals which "appear to belong to the order scleractinia," (2) "shell fragments of what are probably Inoceramus?" (3) foraminifera which are "small and not abundant," including "fragments of encrusting types and nodosarids," and (4) "abundant calcareous algae including fragments of dasycladaceae, coralline algae and possible Solenapora and Girvanella." Armstrong concludes that
Radiolarians from a radiolarian, Halobia?-bearing limestone from the limestone member and from chert and tuffaceous chert from the chert member have been identified by Emile A.Pessagno, Jr. of the University of Texas at Dallas. According to Pessagno (written communication, 1975), the radiolarian assemblage from interbedded red chert and green tuffaceous chert "consists entirely of spumellarina and is of rather low diversity (14 species)."A more diversified fauna, including spumellarina and nasselarina, is present in the radiolarian and Halobia? bearing limestone collected in the limestone member. At least one new genus and four new species have been identified and it appears that the radiolarians represent the most diverse Triassic assemblage in the world. Plans are being made for the publication of more complete information concerning the radiolarians.
The general distribution of radiolarians is in open marine waters. The suborders nassellina and spumellina are the most common of preserved remains (Moore, 1954). Reproductive activity of radiolarians often takes place after local temporary enrichment of seawater with silica (Moore, 1954). Volcanic activity in or near the basin in which the San Hipolito Formation was deposited provided abundant silica to the sea. A great part of the volume of red chert within the formation is made up of radiolarians but the assemblage is of rather low diversity. The red chert contains little or no tuffaceous material which suggests that volcanic activity was quiescent during its accumulation. Accumulations of Halobia? and Monotis are sometimes found between or within tuffaceous layers and radiolarian-rich beds in the limestone member suggesting that the introduction of tuffaceous material may have contributed to the death of the pelecypods and the subsequent radiolarian bloom.