THE AMMONITE ERA
Coral reefs were the most widespread during the Cretaceous, ranging 1,000
miles away from the equator. In contrast, today they are restricted to the trop-
ics. The corals began constructing reefs in the early Paleozoic and built bar-
rier reefs and atolls, which were massive structures composed of calcium
carbonate lithified into limestone. The Great Barrier Reef,stretching more
than 1,200 miles along the northeast coast of Australia, is the largest feature
built by living organisms.
Bryozoans, corallike moss animals that encrusted shells and other hard
surfaces of the early Cretaceous seas, had been around for more than 300 mil-
lion years. Like most shallow-water marine invertebrates, bryozoans were rare
in the Triassic. However, they underwent widespread expansion in the Juras-
sic and Cretaceous until the late Cretaceous. Early in the Cretaceous, the bry-
ozoans evolved into two major groups, the cyclostomes and the newer
cheilostomes, whose higher growth rate and greater diversity crowded out the
older group, which was relegated to inferior status. The typical encrusting
forms had become a major group of bryozoans by the end of the Cretaceous,
a position they maintain today. Living species occupy seas at various depths,Figure 159Limestones
of the Hawthorn and
Ocala formations, Marion
County, Florida.
(Photo by G. H.
Espenshade, courtesy
USGS)
Historical Geology