Page 333 - Introduction to Paleobiology and The Fossil Record

320 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

and elsewhere. The trepostomes, however, lin- Bowerbankia has an erect colony with semi-
gered on until the Late Triassic. spirally arranged zooecia clustered around a
The cyclostomes have tube-shaped zooecia central branch. The cheilostomes, however,
and often grew as branching tree-like colonies dominate the class and are most diverse of all
or alternatively encrusting sheets or ribbons. the bryozoan groups (Box 12.8). Cheilostomes

The first representatives of the order are typically have polymorphic zooids, adapted
known in Lower Ordovician rocks, but the for different functions, which are usually
group peaked during the mid-Cretaceous in linked within the highly integrated colony.
spectacular style, with a diversity of over 70 This advanced group appeared during the
genera. Many genera such as Stomatopora, Late Jurassic; they are particularly common
consisting of a series of bifurcating, encrust- in shallow-water environments of the Late
ing branches, have very long stratigraphic Cretaceous and Paleogene of the Baltic and
ranges; moreover Stomatopora may have Denmark. Lunulites, for example, is discoidal
pursued an opportunist life strategy, rapidly and free-living, whereas Aechmella is an
spreading their zooids over hard surfaces. encrusting form often associated with sea
The Gymnolaemata are represented in the urchins.
fossil record by two orders, the ctenostomes
and the cheilostomes. The ctenostomes fi rst
appeared in the Early Ordovician and many
genera have since pursued boring and encrust- Ecology and life modes
ing life strategies. Penetrantia and Terebri- Virtually all bryozoans are part of the sessile
pora are borers whereas the modern genus benthos, mainly occurring from the sublitto-

Box 12.8 Competition and replacement in cyclostome and cheilostome
clades: what really happened at the KT boundary?

Perhaps one of the most obvious changes in bryozoan faunas through time involves the relative
decline of the cyclostomes and the diversification of the cheilostomes leading up to the Cretaceous–

Tertiary (KT) boundary. Since both groups occupied similar ecological niches and are comparable
morphologically, many workers have assumed that the cyclostomes, originating during the Ordovi-
cian and diversifying in the Cretaceous, were outcompeted by the cheilostomes at the end of the
Cretaceous. However Scott Lidgard (Field Museum of Natural History, Chicago) and his colleagues
have analyzed this transition in detail and the results are far from conclusive (Lidgard et al. 1993).
Both groups continued to participate together in bryozoan communities during the Cenozoic and
much of the apparent decline in the cyclostome numbers may be due to the greater diversifi cation
or expansion of the cheilostomes that began to dominate these assemblages in the Cenozoic. Perhaps
this expansion had already been seeded in the Jurassic, when the poor and sporadic bryozoan fauna
provided the ecological space for the expansion of the cheilostomes. A detailed statistical study based
on generic-range data from Sepkoski’s database (McKinney & Taylor 2001) has confi rmed that
origination within the cheilostome clades was the driving force behind the apparent takeover by this
group (Fig. 12.19).
See http://www.blackwellpublishing.com/paleobiology/.