Biology Reference
In-Depth Information
tinuing development of the drying and seasonal environments seen earlier,
for example, in the middle Eocene Green River fl ora of Colorado and Utah.
It further provides a picture of the mosaic of vegetation at this latitude in
the transitional times of the Oligocene (Pälike et al. 2006) that included a
shrubland/chaparral-woodland-savanna formation.
To the south in the state of Chiapas, just north of Tuxtla Gutiérrez and
San Cristóbal, there are microfossil-bearing lignites in the La Quinta For-
mation of Oligo-Miocene age around the village of Simojovel (fi gs. 6.2 and
6.3a-d. This was the fateful cemetery site where we were held at bay by
the residents until our intentions were made clear. The sediments contain
abundant fossil spores and pollen. Langenheim, Hackner, and Bartlett
(1967) studied material from the site, providing several biological identifi -
cations, and since then further studies have allowed reconstruction of the
vegetation and paleoenvironments in greater detail (Graham 1999; Gra-
ham and Palacios Chávez 1996). The lignites as sediments, and their con-
tent of pollen, including the brackish-water mangroves
Pelliceria
(fi gs. 6.4
and 6.5) and
Rhizophora
(fi g. 6.6; see also fi g. 2.24), place the Oligo-
Miocene shoreline about 90 km inland from its present position, refl ecting
high sea levels resulting from the absence of extensive continental glaciers.
Also present were spores of the fl oating fern
Ceratopteris
(fi gs. 6.3a, 2.25)
and the aquatic angiosperm
Pachira
that give a modern aspect to this com-
munity by the Oligocene. Pollen of some warm-temperate (e.g.,
Alfaroa-
Oreomunnea
; fi g. 6.3c) and tropical plants was present (e.g., the tree fern
Sphaeropteris-Trichipteris
, fi g. 6.3b;
Crudia
, fi g. 6.3d), but dominants of the
present-day lowland neotropical rain forest in Mexico were not found, so
the moist forests likely represented the wet phase of a lower to upper mon-
tane broad-leaved forest ecosystem. Most tropical trees comprised by the
canopy today derived from South America, and as noted in chapter 2, the
connection across the Isthmian region would not be completed until about
3.5 Ma. There was no extensive dry vegetation or grassland represented in
the La Quinta microfossil assemblage.
Also absent or poorly represented was pollen of
Abies
and
Picea
, suggest-
ing no high altitudes in the vicinity in Oligo-Miocene time. The present ele-
vation in the Front Ranges of Chiapas is about 2000 m, and the highest peak
is San Cristóbol de La Casas at 3004 m. Folded strata are early Miocene
through Pliocene-Pleistocene age, so the Oligocene and Oligo-Miocene el-
evations are estimated at about 1000-1200 m. Present climatic conditions
at Simojovel are a MAT of 24°C and a MAP of 2500 mm. It was warmer
and wetter in the Oligocene (fi g. 3.4) so the Oligocene temperature is es-
timated at 25°C-26°C and the precipitation at 2700-3000 mm. Given the
