Page 372 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 2: MOLLUSKS 359

to recover until a switch in images was pro- Paleozoic. On the other hand, close relatives
duced. Although such relationships are docu- of the mollusks, the annelids, may have been
mented for some Mesozoic (Box 13.9) and important predators equipped with an effi -
Cenozoic faunas, data are sparse for the cient jaw apparatus (Box 13.10).

Box 13.10 Fossil annelids and their jaws

The annelids are segmented protostomes that are represented today by animals such as the
earthworms and leaches. Recent species are important, widely distributed, benthic predators and
occur from intertidal to abyssal depths. Modern molecular studies suggest they form a sister group
to the mollusks and, in fact, share a number of morphological characters such as the possession of
chaetae. In general the group has a fairly sparse fossil record, appearing fl eetingly in Lagerstätte
deposits such as the Burgess Shale and Mazon Creek fauna. However many residues of acid-etched
Paleozoic limestones contain scolecodonts (Fig. 13.28). These were the jaws of ancient annelids and
are abundant and diverse at many horizons. They were similar to conodonts (see p. 429), forming

multielement apparatuses with similar functions but were composed of collagen fibers and various

minerals such as zinc. The group first appeared in the Lower Ordovician and diversifi ed rapidly to
become common in Upper Ordovician-Devonian carbonate facies. Scolecodonts were relatively rare
after the Permian, but nevertheless have proved useful in biostratigraphic and thermal maturation
studies.

Figure 13.28 Scolecodont morphology. Reconstruction of the polychaete jaw apparatus of the
Ordovician Ramp hoprion Kielan-Jaworowska. (Courtesy of Olle Hints.)