From Simple English Wikipedia, the free encyclopedia
The
Western Interior Seaway
[1]
was a huge
inland sea
. It split
North America
into two halves for most of the mid- and late-
Cretaceous
Period
. It was up to 2,500 feet (760 m) deep, 600 miles (970 km) wide and over 2,000 miles (3,200 km) long.
Sea level
was high in the Cretaceous, and the middle part of North America sank down. A shallow
subducting
plate
pulled on the bottom of the
lithosphere
, causing it to move down. The sea entered both north and south, and came and went during the course of the Cretaceous. Widespread
carbonate
deposition suggests that the Seaway was warm and
tropical
, with abundant calcareous
algae
.
[2]
The shrunken, regressive phase of the Western Interior Seaway is sometimes called the
Pierre Seaway
.
[3]
The long, narrow western border of the seaway called
Laramidia
had one of the most diverse collections of dinosaurs anywhere on Earth.
[4]
The Western Interior Seaway was a shallow sea filled with abundant marine life. This included predatory marine
reptiles
such as
plesiosaurs
, and
mosasaurs
that grew up to 18 meters long. Other marine life included
sharks
such as
Squalicorax
and the giant shellfish-eating
Ptychodus mortoni
(believed to be 10 meters long).
[5]
There were advanced
bony fish
such as the massive 5-meter long
Xiphactinus
? a fish larger than any modern bony fish. Other sea life included
invertebrates
such as
molluscs
,
ammonites
, squid-like
belemnites
, and
plankton
including
coccolithophores
that secreted the chalky platelets that give the Cretaceous its name,
foraminiferans
and
radiolarians
.
The Western Interior Seaway was home to early
birds
also, including the flightless
Hesperornis
which had stout legs for swimming through water and small wing-like appendages used for marine steering rather than flight; and the
tern
-like
Ichthyornis
, an early avian with a toothy beak.
Icthyornis
shared the sky with large
pterosaurs
such as
Nyctosaurus
and
Pteranodon
.
Pteranodon
fossils are very common and it was likely a major component of the surface
ecosystem
, though it was only found in the southern reaches of the Seaway.
[6]
On the bottom, the giant
clam
Inoceramus
left common
fossilized
shells in the
Pierre Shale
. This clam had a thick shell paved with prisms of
calcite
deposited at right angles to the surface, giving a pearly luster in life. Paleontologists suggest that its giant size was an
adaptation
for life in the murky bottom waters, where a correspondingly large gill area would have allowed the animal to cope with oxygen-depleted waters.