Characterization of a new Early Jurassic fossiliferous lacality with crinoids and ammonites in eastern central Mexico (Temapá, Hidalgo)
The general geological and paleontological aspects of the Huayacocotla Group, México have been previously investigated by other authors. We report here the ﬁrst outcrop with ammonoids (Paltechioceras cf. mexicanum, P. rothpletzi, Ortechioceras incaguasiense, Coroniceras? (Metophioceras) aff. conybeari auct, Phylloceras sp., Sulciferites? cf. stenorhynchus) and isocrinids, some of them benthic (Isocrinidae) and some pseudoplanktonic (Pentacrinitidae). This is the ﬁrst time the isocrinids are reported from Sinemurian sediments (Lower Jurassic) in Mexico. The supposed habitats for the families of these ammonoids are considered, in support to sedimentary environment. In this outcrop, we also identiﬁed a bivalve Posidontis semiplicata and an unidentiﬁed gastropod.
Three types of fossil assemblages are repetitively observed in the Temapá outcrop, which belongs to the Despí Formation. The types of fossil associations are identiﬁed by (1) the differences in the ammonoids diameters; (2) its faunal compositions as demonstrated by Simpson index application; and (3) by the fragmentation level of shells. Additionally, we established a criterion of deposition (in situ/transported) on the basis of size distribution.
In the stratigraphic sequence with three types of associations are observed sandy siltstone with milimetric ﬂasser-wavy layers of ﬁne sands. The ammonoids are present in both sediments, although the largest and most complete fossils are found in the sandy siltstone (assembalge 1). The smaller ammonites are found only in the ﬁne sand forming the assemblage two, and crinoids with fragmentary ammonoid bioclastic material, also in ﬁne sand, compose the third assemblage.
In addition to the macrofaunal associations as mentioned above, we report the lithologic description, the sedimentary petrography and grain size distribution of sediments to establish the characteristics of the Temapá outcrop.
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