Arkell.1956.Jurassic..

MEXICO
knopi (Neum.), H. cf. hybonotum (Oppel) and H. cf. autharis (Oppel),
a remarkably south European assemblage of the Beckeri Zone. With
them occur the earliest Mazapilites, Physodoceras and a few Perisphinctids.
The Lower Kimeridgian is the most richly fossiliferous and characteristic
stage in Mexico, with an immense fauna of Idoceras and many other genera.
Two zones are recognizable, an upper with Idoceras durangense and
Glochiceras fialar, a lower with Idoceras of the group of /. balderum. Both
must be mainly Lower Kimeridgian, for the higher zone contains many
Involuticeras and Streblites, with Ochetoceras and Sutneria, also Pararasenia
zacatecana (Burckhardt). Besides the genera mentioned there are
numerous Aspidoceras, Nebrodites, Taramelliceras, Haploceras, etc., with
subordinate Perisphinctids and some Phylloceras and Lytoceras. (For
lists see Imlay, 1939, tables 4 and 5; numerous figures in Burckhardt's
Mazapil (1906), San Pedro (1912), and Symon (1919-21) monographs
and in Imlay, 1939). The Glochiceras fialar fauna also occurs in the
Taman formation (altogether over 1000 m. thick) in the southern Sierra
Madre Oriental (Heim, 1926).
Many of the later Mexican Idoceras are clearly related closely to Ataxioceras,
from which the ventral smooth band alone provides a doubtful
distinction (Burckhardt, 1912, pis. xxvi-xxxi). Peculiar local genera are
Subneumayria, type S. ordonezi Burckhardt sp. (1906, pis. i, ii), and
Epicephalites, type E. epigonus Burckhardt sp. (1906, pi. iii), both probably
Rasenids related to Involuticeras.
OXFORDIAN
Ochetoceras beds (100 m.±) at San Pedro del Gallo : shales and sandy
marls with nodules and a black limestone band containing O. canaliculatum
(d'Orb.), O. pedroanum Burck., O. mexicanum Burck., Taramelliceras
neohispanicum Burck., Discosphinctes virgulatus (Quenst.) var. carribbeanum
(Jaworski), Euaspidoceras (Burckhardt, 1912, pi. v, 5, 8, 9; pi. vii, 4-22).
The date of this assemblage is probably Bimammatum Zone.
Perisphinctes beds (150 m.) at San Pedro del Gallo: marls and shales
with beds of sandstone and limestone and many pelecypods, including
Trigonia hudlestoni Lycett. All the Perisphinctes figured from these beds
are typical Dichotomosphinctes, strongly resembling common forms of the
Plicatilis Zone, with which the beds certainly correlate (Burckhardt,
1912, pis. ii-vii). There are also Taramelliceras spp. and a Creniceras
(pi. vii, 15-17). A characteristic Cuban form occurs: P. plicatiloides
O'Connell (Burckhardt, 1912, pi. iii, 3-6).
Below these beds are 600 m. or more of shales and sandstones, with
Nerinean and coralline limestone, resting on red shales, marls and
sandstones, all without ammonites. In Zacatecas the Nerinean and coralline
limestones are 500-1000 m. thick (Burckhardt, 1930, table 6). In
eastern Mexico are up to 420 m. of red beds of uncertain date but approximately
Lower Oxfordian, the Huizachal formation (Imlay & others, 1948).
This formation unconformably overlaps all previous Jurassic beds on to
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