Reefs and Mounds - Trace Fossils as Indicators of Sedimentary Environments - page 729

2.1.1 Worms
The polyphyletic group of morphologically similar “worm” borings refers espe-
cially to cavities produced by polychaete annelids and sipunculans, which
according to the literature are mainly cosmopolitan in distribution, but are more
likely to represent suites of cryptic species. Diverse boring polychaetes are
found on modern reefs belonging to several families that are not closely
related, suggesting that boring evolved several times during the evolution of
this old group. The dominant families with boring representatives are the Euni-
cidae, Dorvilleidae, Sabellidae, Cirratulidae, and the Spionidae. While the
precise methods of boring are still being investigated, two main methods
appear to be involved, chemical dissolution and/or mechanical removal of
the substrate ( Hutchings, 2008 ). A well-defined succession of borers occurs,
with initial colonizers being short-lived species followed by long-lived species
( Hutchings, 2008 ).
As polychaetes are soft-bodied animals, their fossil record is sparse.
However,theDorvilleidaehavewell-developed jaws that are well represented
as scolecodonts, starting in the Early Ordovician ( Rouse and Pleijel, 2001 )
and closely resemble modern-day boring species. The diversity of scole-
codonts and hence the clade Eunicida increased dramatically through the
Ordovician, followed by a major extinction at the end of the Permian
( Szaniawski, 1966 ). This raises the possibility that boring polychaetes were
present in fossil reefs.
The most common fossil wormmacroborings have a simple cylindrical form
( Trypanites ; Fig. 1A-C ) and may have a distal enlargement ( Palaeosabella ).
Although Trypanites is the oldest macroboring in the fossil record, appearing
in the Early Cambrian ( James et al., 1977 ), Bromley (2004) points out that
distinguishing it from Palaeosabella requires good exposure of the distal end,
which may result in underreporting of Palaeosabella . Taylor and Wilson
(2003) recognize the earliest Palaeosabella in the Ordovician. Sipunculans
and polychaetes are most frequently cited as potential tracemakers of Trypanites
and Palaeosabella .
Spionid polychaetes are responsible for modern equivalent structures of
Caulostrepsis and Maeandropolydora ( Bromley and D'Alessandro, 1983 ).
Both fossils include a U-shaped pouch, but whereas Caulostrepsis is stouter
with a single aperture, Maeandropolydora has elongated curved tunnels that
lead to more than one aperture. The type material of Caulostrepsis is Devonian
in age, but the origin of Maeandropolydora is contentious, with some evidence
pointing to a Silurian-Devonian beginning ( Bromley, 2004 ), or as late as the
Cretaceous ( Taylor and Wilson, 2003 ).
Talpina is a network of curved, branching tubular cavities that most easily
could be mistaken for sponge borings. Phoronid lophophorates generate such
cavities today, and their fossil record extends to the Devonian, and they are most
common starting in the Jurassic ( Taylor and Wilson, 2003 ).