Page 327 - Introduction to Paleobiology and The Fossil Record

314 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 12.5 Tethyan brachiopods in Greenland: a Cretaceous Gulf Stream
current?

Brachiopods can give clues about ancient ocean currents. Today, the Gulf Stream runs out from the
Caribbean, sweeps up the eastern seaboard of North America, and then detaches from the coast just
north of New York and heads across the Atlantic to wrap the shores of Britain and western Europe

in warmer-than-expected waters. Has the Gulf Stream always flowed the same way? Some Cretaceous
brachiopods give us a clue. David Harper and colleagues (2005) showed how some Early Cretaceous
brachiopod faunas from East Greenland were a mix of animals from two ocean provinces, Tethyan
(low latitude) and Boreal (high latitude). The Boreal, shallow-water assemblage is dominated by
large terebratulids and ribbed rhynchonellids, and occurs adjacent to a fauna containing Tethyan
elements, more typical of deeper water, including Pygope (see p. 311). How did these exotic, tropical
visitors travel so far north? Harper and colleagues suggested that an Early Cretaceous out-of-Tethys
migration was helped by the early and persistent northward track of a proto-Gulf Stream current
(Fig. 12.14). These kinds of studies of changing patterns of paleobiogeography through time are
critical for understanding modern climate and ocean patterns.

Land area Boreal Ocean
Present-day
coastline Arctic Canada
Migration route North Siberia

Greenland

Russian
Platform
?
? Polish Furrow ?
Mediterranean
Tethys

Gondwana

Paleoequator

Figure 12.14 Tethyan brachiopods in East Greenland: Pygope and the proto-North Atlantic
current (arrows), one of its possible migration routes. The star indicates the Lower Cretaceous,
East Greenland locality.

Bryozoans are the only phylum in which all (Box 12.6). Superfi cially resembling the corals
species are colonial. Many skeletons are and hydroids, the bryozoans (“moss animals”)
exquisitely designed, but fragment very easily are like minute colonial phoronids (see p.
after death. Although relatively common, 298) with tiny individuals or zooids, com-
bryozoans are among the least well-known monly less than 1 mm in diameter. Each zooid
invertebrates. There are about 6000 living is celomate with a separate mouth and anus
and 16,000 fossil species, and most are marine together and a circular or horseshoe-shaped