Quebec and stretched all the way to Arkansas.The rivers reached their maxi-
mum in the early Carboniferous, during times of low sea levels. However,
when sea levels rose, the rivers flooded the land and became vast swamps.
Huge buildups of peat, which later became coal, account for the vast coal
deposits in many areas of the eastern United States.
The sediments in the Tethys Sea separating Gondwana and Laurasia
buckled and uplifted into various mountain belts, including the ancestral
Hercynian Mountains of southern Europe.As the continents rose higher and
the ocean basins dropped lower, the land became dryer and the climate grew
colder, especially in the southernmost lands, which were covered with
glacial ice. All known episodes of glaciation occurred during times of low-
ered sea levels. The changes in the shapes of the ocean basins greatly influ-
enced the course of ocean currents, which in turn had a pronounced effect
on the climate.
The closing of the Tethys Sea eliminated a major barrier to the migra-
tion of species from one continent to another, and they dispersed to every
part of the world (Fig. 117). When all continents combined into Pangaea,
plant and animal life witnessed a great diversity in the ocean as well as on
land. The formation of Pangaea marked a major turning point in evolution
of life, during which the reptiles emerged as the dominant species, con-
quering land, sea, and sky.
A continuous shallow-water margin ran around the entire perimeter of
Pangaea.As a result, no major physical barriers hampered the dispersal of
marine life. Furthermore, the seas were largely confined to the ocean basins,
leaving the continental shelves mostly exposed.The continental margins were
less extensive and narrower, confining marine habitats to the nearshore
regions. Consequently, habitat areas for shallow-water marine organisms were
very limited, which accounted for the low species diversity. As a result, marine
biotas (flora and fauna) were more widespread but contained comparatively
fewer species.
In the northern latitudes, thick forests of primitive conifers, horsetails,
and club mosses that grew as tall as 30 feet dominated the mountainous
landscape. Much of the interior probably resembled a grassless version of
the steppes of central Asia, where temperatures varied from very hot in
summer to extremely cold in winter. Since grasses would not appear for
well over 100 million years, the scrubby landscape was dotted with bam-
boolike horsetails and bushy clumps of extinct seed ferns that resembled
present-day tree ferns.
Browsing on the seed ferns were herds of moschops, 16-foot reptiles
with thick skulls adapted for butting during mating season,a tactic similar to
the behavior of modern herd animals. They were probably preyed upon by
packs of lycaenops (Fig. 118), which were reptiles with doglike bodies andHistorical Geology