Page 314 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 1: LOPHOPHORATES 301

change their shape and life mode during The linguliformeans, for example, have phos-
ontogeny from being attached to the seabed phatic material as part of their shell fabric.
to lying untethered in the mud. The shells of rhynchonelliformean brachio-
pods are composed of low-magnesian calcite;
these shells may have fi brous, laminar or
cross-bladed laminar shell fabrics in their sec-
Ultramorphology: brachiopod shell
ondary layers. The mineral fabrics themselves,
The brachiopod shell is a multilayered when investigated at the nanoscale, may be of
complex of both organic and inorganic mate- particular ecological importance. Those with
rial that has proved of fundamental impor- calcite seminacre, rather like mother-of-pearl,

tance in classification. The shells of most can cement directly to the seafl oor whereas
rhynchonelliformean brachiopods consist of those with fi brous shells can not (Pérez-Huerta
three layers (Fig. 12.6). The outer layer (peri- et al. 2007).
ostracum) is organic, and underneath are the Many shells are perforated by small holes
mineralized primary and secondary layers. or punctae, in life holding fi nger-like exten-
These layers are sequentially secreted by cells sions of the mantle or ceca. Their function is
within the generative zone of the mantle, uncertain but they increased the amount of

forming first a gelatinous sheath followed by the brachiopod’s soft tissue. Some strophom-
the organic periostracum, and then the granu- enates have pseudopunctae, with fi ne inclined
lar calcite of the primary layer. The subse- calcite rods or taleolae embedded in the shell
quent secondary layer is thicker and composed fabric.
of calcite fibers, and in some brachiopods a The relatively stable brachiopod shell sub-

third prismatic layer is secreted. There are a stance can tell much about the secretion of the
number of variations of this basic template. shell but also about environmental conditions

Box 12.1 Brachiopod classifi cation

Recent cladistic and molecular phylogenetic analyses have shown that the traditional split of the
phylum Brachiopoda into the Inarticulata and Articulata is incorrect, and instead there are three
subphyla, the Linguliformea, Craniiformea and Rhynchonelliformea. All three have quite different

body plans and shell fabrics (Fig. 12.2). The linguliformeans contain five orders united by organo-
phosphatic shells; the inclusion of the paterinides is the most problematic since the group shares
some morphological characters with the rhynchonelliforms. The craniiformeans include three rather
disparate groups with quite different morphologies but which together possess an organocarbonate
shell. Most scientists now accept 14 articulated orders in the rhynchonelliformeans, not counting
the chileides, dictyonellides, obolellides and kutorginides, mainly based on the nature of the cardi-
nalia and the morphology of the other internal structures associated with the attachment of muscles
and the support of the lophophore. Recently the more deviant chileides, obolellides and kutorginides
have been added to the subphylum. In addition, the articulated taxa have been split into those with
deltidiodont (simple) and cyrtomatodont (complex) dentitions; the former group includes the orthides
and strophomenides whereas the latter include the spire bearers.
Cladistic-based investigations have developed a phylogenetic framework for the phylum (Williams
et al. 1996), supporting the three subphyla (Fig.12.2); their defining characters are based on shell

structure and substance. The mutual relationships among these groups are still unclear as are the
relationships between the many primitive articulated and non-articulated groups that appeared
during the Cambrian explosion together with the origin of the phylum as a whole (Box 12.2).
A data matrix containing all the data from Williams et al. (1996) is available at http://www.
blackwellpublishing.com/paleobiology/.

Continued