Page 411 - Introduction to Paleobiology and The Fossil Record

398 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 15.4 The age of crinoids: an Early Carboniferous diversity spike

Early Carboniferous (Mississippian) crinoids were abundant and diverse, so much so that this inter-
val is often called the “Age of crinoids”. Limestones of this age often consist of over 50% pelmato-
zoan debris, and are known as encrinites. Why then were crinoids so abundant at this time? Two
factors seem to have contributed to these extensive shoals of crinoids (Kammer & Ausich 2006).

Firstly, five major groups were in various states of recovery after the Frasnian-Famennian extinction
event, particularly the advanced cladids (Fig. 15.6). Secondly, with the disappearance of the shelf-
edge coral–stromatoporoid buildups at the end of the Devonian, platform geometries were quite
different. There was improved and unimpeded water circulation, which promoted stenohaline condi-
tions that encouraged the growth of crinoid communities. With new ecospace and a lack of predation
pressures, crinoid diversity exploded. Sadly, the good times came to an end with regression and the
cooler-water conditions associated with the Late Carboniferous glaciation. Crinoids were never again
so diverse.

200
2005
2002
160

Number of genera 120

80

40

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Figure 15.6 Diversity of Early Carboniferous crinoids. (From Kammer & Ausich 2006.)

wide range of strategies from a fi xed sessile the Late Devonian, and were probably
mode to free-living recumbent styles. The dip- replaced by the better adapted blastoids during
loporites were very widespread from the Early the Silurian and Devonian.
Ordovician to the Early Devonian and prob-
ably evolved from a Late Cambrian blasto- Blastoids
zoan ancestor.
The extinct blastoids were small, pentamer-
Rhombifera The rhombiferans appeared ally symmetric animals with short stems
during the Late Cambrian equipped with bra- and hydrospires adapted for respiration (Fig.
chioles and distinctive rhombic patterns of 15.8). They are represented by over 80 genera
respiratory pores crossing thecal plate sutures in rocks of Silurian to Permian age. The
(Fig. 15.7). They are classified according to blastoid cup or theca is usually globular and

the pattern and shape of their pores; these composed of a ring of three basal plates, sur-
separate the order Dichoporita from the Fis- mounted by a circle of fi ve larger radial plates.
tulipora. The rhombiferans became common The mouth is often surrounded by fi ve large
during the Early Ordovician and continued openings or spiracles associated with the
with a near cosmopolitan distribution until respiratory system. Although relatively rare,