Lecture 2 Brachiopods

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Lecture 2 - Brachiopods
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PHYLUM BRACHIOPODA Brachiopods are bivalved
organisms which evolved very early in the
Cambrian period, at a time when creatures with
mineralised shells became exceptionally abundant
in the fossil record (the so called Cambrian
Explosion). They are still living today and are
sometimes referred to as lamp shells as they
resemble a roman oil lamp in profile. They were
very prominent components of the marine benthos
(bottom dwellers) during the Palaeozoic.
Brachiopods live inside a pair of valves which
are symmetrical, but unequal in shape and size
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Morphology
POSTERIOR
pedicle (ventral) valve
pedicle foramen
(b)
palintrope
hinge
delthyrium
umbo
growth lines
(a)
ribs
External shell morphology of Magellania
flavescens, (a) dorsal view, (b) lateral view.
brachial (dorsal) valve
commissure
ANTERIOR
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Internal morphology
Stylised section through an articulate
brachiopod, illustrating general internal
morphology
pedicle
diductor muscle
gonad
adductor muscle
mantle
stomach
spiral part of Lophophore
side arm of Lophophore with cross section
mantle cavity
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In many cases the brachiopod is attached to the
sea floor via a fleshy stem (the pedicle), which
emerges from a small opening in the rear of the
shell (the pedicle foramen).
Other forms were designed to simply sit on or
into the sediment and some even had fine spines
on the outside of their shells which stuck into
the substrate to help anchor the
brachiopod. Either way, nearly all brachiopods
have/had a fixed mode of life (sessile) and are
epi- or quasi-infaunal.
diductor muscle
adductor muscle
The shell is actively opened and closed by the
antagonistic action of a pair of muscles the
adductors and diductors.
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Brachiopod shell morphology.
Outlines of brachiopods as viewed from dorsal or
ventral sides.
Brachiopod classification is quite complex and
studies on living brachiopods utilise DNA
analysis and details of the soft part morphology
in helping to group them. Palaeontologists have
to rely solely on information provided by the
empty shells. The following 2 figures illustrate
some of the terminology applied to the
description of brachiopod shells
Ovoid
Circular
Rounded-polygonal
Lobed
Triangular
Semicircular
Alate
Parallel-sided
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Terminology for brachiopod lateral
profiles. Note dorsal valve is white, ventral
valve is shaded grey.
Convexoconcave
Resupinate
Concavoconvex
Geniculately concavoconvex
Biconvex
Geniculately biconvex
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Classification
Brachiopods can broadly (and informally) be
subdivided into two groups Inarticulate
brachiopods (subphyla Craniiformea
Linguliformea)
Lack tooth and socket structures for articulation
of shells, which are often held together by
complex musculature. Lingula (opposite) is extant
and an active burrower. Its basic shape hasnt
changed much since Cambrian times. It is often
known as a living fossil.
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Inarticulate brachiopods Lingula (left)
Discina (right)
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Articulate brachiopods (subphylum
Rhynchonelliformea)
Possess a bivalved calcareous shell with tooth
and socket structures for valve articulation near
the hinge. Two sets of muscles actively open and
close the valves (adductors and diductors). There
are six major orders Order Terebratulida Order
Rhynchonellida Order Spriferida Order
Productida Order Pentamerida Order Orthida
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order Terebratulida

• Astrophic biconvex
• Commonly circular or ovoid in outline
• Circular pedicle foramen located in beak
• "lamp-shells"

Terebratula insignis (Jurassic)
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order Terebratulida
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order Rhynchonellida

• Biconvex, astrophic shells
• Strong fold and sulcus developed

• Generally strongly ribbed
• Pedicle foramen present

Pleuropugnoides pleurodon (Carboniferous)
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order Rhynchonellida
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order Spiriferida

• Shells characteristically strophic and have a
  winged appearance
• Fold and sulcus sometimes developed
• Internal spiral support for lophophore
  (spiralia) characteristic feature

Spirifer striata (Carboniferous)
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order Spiriferida
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order Productida
- Strophic hinge - Pedicle lost in adult -
spines NB - Generally planoconvex profile -
may also be concavoconvex - Ventral valve
usually heavily enlarged
Productus muricatus (Carboniferous)
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order Productida
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order Pentamerida

• Generally strongly biconvex valves with
  inpointed umbos

-Usually astrophic

• Frequently have a pentagonal shell outline
  (hence name)

Pentamerus galeatus (Silurian)
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order Orthida

• Generally biconvex and strophic
• Delthyrium usually open
• Radiating ribs common

Orthis rustica (Silurian)
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Geological History