Page 311 - Introduction to Paleobiology and The Fossil Record
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298 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
What do lampshells, moss animals and the (microns in length) to the massive gigantopro-
rare tube-dwelling phoronids, or horseshoe ductids (nearly 0.5 m wide). Although only
worms, have in common? They may look very about 120 genera of brachiopods, also known
different, but these three phyla, the Brachiop- as lampshells, survive today, they occupy a
oda, Bryozoa and Phoronida, all possess a wide range of habitats from the intertidal
complex feeding organ, the lophophore, and zone to the abyssal depths. The brachiopods
have similar body cavities or celoms. Never- are entirely marine, bilaterally symmetric
theless the relationships among the three are animals with a ciliated feeding organ, or loph-
not yet fully resolved, although the phoronids ophore, contained within a pair of shells or
probably lie close to or may even be part of valves. Internal structures such as teeth and
the group, the bryozoans are more distantly sockets, cardinal processes and various muscle
related. Our understanding has not changed scars are all associated with the opening and
much since 1890, but new molecular studies closing of the two valves during feeding cycles.
may help resolve these uncertainties in the Brachiopods have featured in many paleoeco-
next 10 years. logical studies of Paleozoic faunas, when they
The phoronids are tube-dwelling, worm- dominated life on the seabed in terms of
like lophophorates, with the 10 or so described numbers of both individuals and species.
species divided between two genera, Phoronis Their use in paleobiogeographic analysis is
and Phoronopsis. These animals lack a min- well documented (see Chapter 4). Neverthe-
eralized skeleton and pursue burrowing or less brachiopods have also been widely used
boring life strategies with near-cosmopolitan in regional biostratigraphy and, during the
distributions. The phylum has a long though Silurian, a number of orthide, pentameride
questionable geological history, as some and rhynchonellide lineages show good pros-
authors suggest that Precambrian and Lower pects for international correlation.
Paleozoic records of the vertical burrow Despite their relative low diversity today,
Skolithos (see p. 523) may possibly be the living brachiopods are actually quite wide-
work of phoronids. The ichnogenus Talpina, spread, represented mainly by forms attached
present as borings in both Cretaceous belem- by pedicles to a variety of substrates across a
nite rostra and Tertiary mollusk shells, may spectrum of water depths. At high latitudes
also have been constructed by phoronids. brachiopods range from intertidal to basinal
environments at depths of over 6000 m. They
are most common in fjord settings in Canada,
Norway and Scotland and in the seas around
BRACHIOPODA
Antarctica and New Zealand. The associa-
It is no valid objection to this conclusion, tion of the brachiopod Terebratulina retusa
that certain brachiopods have been but growing on the horse mussel, Modiolus modi-
slightly modified from an extremely olus, a bivalve, is widespread in the northern
remote geological epoch; and that certain hemisphere. In the tropics, however, many
land and fresh-water shells have remained species are minute, exploiting cryptic habi-
nearly the same, from the time when, as tats, hiding in reef crevices or in the shade of
far as is known, they fi rst appeared. corals and sponges. Larger forms live in
deeper-water environments, out of the range
Charles Darwin (1859) of predators, like sea urchins, that graze on
On the Origin of Species the sumptuous meadows of newly attached
larvae.
The brachiopods are one of the most suc-
cessful invertebrate phyla in terms of abun-
dance and diversity. They appeared fi rst in the Morphology: brachiopod animal
Early Cambrian and diversifi ed throughout The brachiopod soft parts are enclosed by
the Paleozoic to dominate the low-level, sus- two morphologically different shells or valves
pension-feeding benthos; a wide range of that are opened and closed by a variety
shell morphologies and sizes characterize of muscles; this arrangement is modifi ed
the phylum, from the tiny acrotretides differently across the three subphyla – the
