Page 318 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 1: LOPHOPHORATES 305

“Spirifer” - type “Atrypa” - type “Terebratula” - type

frequent variations

increasing hydroenergy

Figure 12.5 Morphological variation in Terebratalia from the San Juan islands related to changing
hydrodynamic conditions. (From Schumann 1991.)

Periostracum

Primary shell layer
External periostracal layer
Inner bounding membrane Secondary shell layer
Mucoprotein
Protein cement Primary shell Nucleus Cellular epithelium

Axis of rotation

Generative zone
Vacuole

Outer bounding membrane Secretion droplet Mucopolysaccharide
Figure 12.6 Shell secretion at the margins of Notosaria. (Based on Williams, A. 1968. Lethaia 1.)

These brachiopods were members of
Distribution in time: extinctions and radiations
a variety of loosely-structured, nearshore
The make up of the Cambrian, Paleozoic and paleocommunities.
Modern brachiopod faunas are fundamen- During the Ordovician radiation, the delti-
tally different, represented by a dominance of diodont orthides and strophomenides domi-

different orders; some key representatives are nated faunas. These first evolved around Early
illustrated in Fig. 12.7. Cambrian faunas Ordovician island complexes and came to
were dominated by a range of non-articulated dominate the shelf benthos, where they began
groups together with groups of disparate to move offshore and diversify around
articulated taxa such as the chileides, nauka- carbonate mounds. These communities
tides, obolellides, kutorginides, billingsellides, formed the basis of the Paleozoic brachiopod
protorthides, orthides and pentamerides. fauna.