Biology Reference
In-Depth Information
to
Araucarioxylon
, which makes up much of the wood in the Triassic Chinle
Formation of the Petrifi ed National Forest in Arizona. Also present was the
gymnosperm
Brachyphyllum
, an extinct heterogeneous genus of small trees
with fl attened, mostly microphyllous, helically arranged leaves widespread
in Mesozoic deposits of the New World, and the
Persea
-like angiosperm
(Lauraceae, laurels). Little can be said about the fl oras except that the fos-
sils and the setting are an extension of the mesothermal to megathermal
tropical fl ora and the seasonally dry environments described for the south-
ern United States.
Inland, the vegetation on the low proto-Sierra Madre Occidental is also
poorly known, although the Maestrichtian Huepac Chert Member of the
Tarahumara Formation contains fossils described as
Opispocaulis
(stem)
and
Tarahumara
(infl orescence) that are part of an aquatic angiosperm of
the family Haloragidaceae (Hernández-Castillo and Cevallos-Ferriz 1999).
Together with forms in the Late Cretaceous of the central United States
(
Isöetes
,
Azolla
,
Salvinia
,
Equisetum
,
Lycopodium
,
Selaginella
), this estab-
lishes the presence of freshwater aquatic and marsh habitats with some
angiosperms at this time, but without present-day
Typha
(cattail),
Thalia
,
and aquatic grasses that characterize the modern communities.
Farther east, in Cohuila, southwest of Saltillo, is the late Campanian
Cerro del Pueblo fl ora (Estrada-Ruiz et al. 2009). Dating from 73.5 Ma, it
contains seeds of the aquatic
Decodon tiffneyi
(Lythraceae) and
Ceratophyl-
lum lesii
(Ceratophyllaceae). The Maestrichtian Olmos fl ora in the Sabinas
region of Coahuila represents a fl ood plain or delta environment, and many
of the fossils are fragmentary, suggesting transport by rivers from the foot-
hills of the proto-Sierra Madre Oriental. They include the aquatic ferns
Salvinia
and
Dorfi ella
(extinct), several extinct conifers including
Brachy-
phyllum
, and fossil woods of
Podocarpoxylon
(similar to
Podocarpus
, at pres-
ent a warm-temperate genus of conifers) and
Taxodioxylon
(Weber 1973,
1975, 1976). There is also wood of the angiosperm families Fagaceae and
Malvaceae (Estrada-Ruiz et al. 2007). The woods have poorly developed
growth rings, like those from the Paleocene of the Big Bend region of Texas,
indicating moderate seasonality in temperature and rainfall. This view of
the climate is supported by context information from isotopic analysis
of carbonate nodules in fossil soil horizons (paleosols) that suggest a MAT
of at least 15°C, possibly reaching 20°C-25°C in the warmer intervals (Fer-
guson et al. 1991; Lehman 1990).
Still farther east is the Maestrichtian Piedras Negras microfossil fl ora
north of Sabinas (Martínez-Hernández et al. 1980). Most of the spores and
pollen are too old to be identifi ed to modern families and genera, and an
