Biology Reference
In-Depth Information
ward fewer trees and more grasses and herbs (Boraginaceae), anticipating
grasslands—possibly since about 23 Ma, as we saw in chapter 7—that will
become better defi ned and more extensive later in the Miocene around
13 Ma. In the Pliocene at 3 Ma, grasslands will appear east of the coastal
mountains—the Palouse Prairie, for example, of eastern Washington and
adjacent Idaho and Oregon. The nature and trend in the ecosystems of
the midcontinent region to increasingly open areas, with forest mostly in
upland-streamside-lakeside habitats, are refl ected by faunas of the Heming-
fordian NALMA at 19-16 Ma, which include the appearance or expansion
of mylagauline rodents, rabbits, beaver (small, probably burrowing repre-
sentatives of the family Castoridae), dromomerycids, antilocarpids (prong-
horns), rhinos, and a major radiation of the equine subfamily of horses.
These horses in North America increased from one species (
Parahippus leo-
nensis
) to seventy species during the Miocene (Maguire and Stigall 2008).
Slightly later, at the beginning of the Barstovian 16 Ma, hemicyonine bears
(longer-footed than modern bears, likely ambush predators) and probosci-
deans migrated into the New World from Asia with continuing expansion of
the shrubland-chaparral-woodland-savanna ecosystem.
Around 17 Ma in the southeastern United States, the principal commu-
nity is deciduous forest with some Asian exotics that will disappear in the
Pliocene, pine forest and beach/strand/dune plants on the edaphically drier
coastal sands, and swamp forests, aquatics, and herbaceous bog/marsh/
swamp plants growing in lowland areas beyond the infl uence of marine
waters. In Tennessee, the late Miocene to early Pliocene Gray Fossil Site
is revealing some of the mammals living in eastern North America at the
time—lesser panda, peccary, rhino, camel, badger, and tapirs. Several of
these have Asian affi nities, refl ecting the land bridge through Beringia in
the mid to late Tertiary.
In Mexico, the late Miocene Mint Canyon fl ora of southern California
(then of northwestern coastal Mexico), shows that shrubland/chaparral-
woodland-savanna is established, as in the drier areas of the western United
States. Grasses are beginning to coalesce into grassland and spread onto the
well-drained higher eastern slopes of the sierras. Since the middle to late
Eocene, there have been mangroves, now with
Rhizophora
, along the coasts
(
Pelliceria
disappeared from its northern limits by the middle Miocene).
Beach-bog-aquatic communities and pine-oak (coniferous) forest can be
found on drier sites, along with remnants of the lowland neotropical rain
forest in moister habitats, and a lower to upper montane broad-leaved for-
est just beginning to receive signifi cant northern deciduous components.
As yet there is no well-defi ned desert or páramo (the Transvolcanic Belt
