Biology Reference
In-Depth Information
have to contend with the same seasonal severity in light and temperature
as their northern counterparts; it is warmer, and there is abundant vegeta-
tion. The plant community is a notophyllous broad-leaved evergreen forest
that includes some lianas, a few plants with drip tips, and
Guarea
, with its
distinctive fi glike fruits.
Tropicality increases southward from about 40°N, where the fauna in-
cludes turtle, crocodiles, and champsosaurs, which resemble small croco-
diles, although the relationship is obscure. The Cretaceous vegetation here
is a paratropical rain forest toward the west and a tropical forest toward the
east. At the southern limit of this zone, the MAT is around 23°C. The tran-
sition between notophyllous broad-leaved evergreen forest and paratropical
rain forest is marked by a change in faunas, with more eutherians (pla-
cental mammals) occurring to the north. As noted, these and other fossil
faunas are described in Michael Woodburne's
Late Cretaceous and Cenozoic
Mammals of North America
(2004) and Christine Janis and colleagues' two-
volume
Evolution of Tertiary Mammals of North America
(1998, 2008).
In Mexico, waters cover an even greater proportion of the land than in
northern North America. Proto-Baja California is a volcanic island arc lo-
cated in the Pacifi c Ocean, and barely emergent uplands extend along the
axis of the proto-Sierra Madres and the Tamaulipas Peninsula. The inter-
vening area of central Mexico is covered by shallow tropical water support-
ing corals, as around Coahuila, and ammonites swim in the deeper water
around Huasteca Canyon near Monterrey. The vegetation is suffi ciently
abundant that it accumulates in swamps and marshes faster than it can be
removed by microbial decay, so sequences of coal are forming in the state
of Coahuila. The Sierra Madres will rise especially fast between 100 and
45 Ma, so the epicontinental sea will drain from the interior; but when the
asteroid hits at 65 Ma, the Yucatán Peninsula, the Isthmus of Tehuantepec,
and the Balsas Depression will still be covered by shallow marine water. In
southwestern Mexico, the Sierra Madre del Sur in the Cretaceous is located
offshore in the Pacifi c Ocean as a volcanic island arc that will become part
of the mainland in the Late Cretaceous and Paleogene.
Only a glimpse of the vegetation is provided by the few Late Cretaceous
fl oras of Mexico. They include Aracauriaceae, Podocarpaceae, and Taxodia-
ceae gymnosperms, the widespread
Brachyphyllum
, a laurel (cf.
Persea
), and
some angiosperm fossils similar to the Haloragidaceae, together with ferns,
some of which (
Salvinia
) indicate the presence of a freshwater aquatic eco-
system, also found elsewhere in the New World during the Cretaceous. It
is likely that an early version of the low- to midaltitude part of a lower
to upper montane broad-leaved forest grew along the slopes of the moun-
