Brachiopods & Bivalves: Revision
Brachiopods
Bivalves
This table summarises the similarities and differences between brachiopods and
bivalves.
Articulate brachiopods Bivalves
1. Shell shape
Two valves, one usually bigger
Two valves, usually similar shape
2. Shell composition
Calcite
Calcite or aragonite
3. Shell symmetry
Bilateral symmetry.
Plane of symmetry along mid-line of
each valve
Bilateral symmetry.
Plane of symmetry along plane
where 2 valves meet
4. Valves
Brachial & pedicle valves. (Pedicle
valve bigger than brachial valve)
Left & right valves. (Usually
mirror images of each other)
5. Shell ornament
Ribs, spines, growth lines
Ribs, spines, growth lines
6. Hinge line
Curved or straight
Teeth on pedicle valve, sockets on
brachial valve
Curved or straight
Teeth & sockets (if present) on
hinge line of each valve
7. Hinge mechanism
Teeth fit into sockets in opposite
valve
No ligament
Teeth & sockets interlock
Ligaments (external or internal)
8. Valve operation
Two sets of muscles (adductor &
diductor muscles) work together to
open/close shell
Ligaments hold valves under
tension & one set of muscles
(adductor muscles) contract (to
close shell) or relax (to open
shell)
9. Muscle scars
At least two sets of muscle scars in
each valve
One or two muscle scars in each
valve
10. Soft body parts &
internal shell features
Small body (& no simple pallial line
or pallial sinus)
No foot
Most of shell cavity filled by filter
feeding mechanism (lophophore), so
shows brachidium (supports
lophophore)
Usually attached by pedicle (thick
stalk or a series of fine threads) so
shows a foramen
Large body fills most of shell
cavity (marked by pallial line)
Siphons (position of retractable
siphons marked by pallial sinus)
Foot (for locomotion)
No brachidium
No pedicle, so no foramen
(If attached - used byssal threads
or cement for attachment)
11. Environment
Marine
Marine, freshwater, brackish
water
12. Mode of life
Epifaunal (attached by pedicle),
Epifaunal (unattached & lying on
sediment surface)
Infaunal (burrowing)
Semi-infaunal (unattached & partly
submerged in sediment)
Epifaunal (attached by
cementation or byssal threads)
Epifaunal (unattached & lying on
sediment surface)
Infaunal (deep burrowing or
shallow burrowing)
Free swimming
Boring
13. Geological history
Early Cambrian to Recent
More common than bivalves until the
mid-Triassic. After the mid-Jurassic
less common than bivalves.
Lower Cambrian to Recent
Only more common than
brachiopods in mid-Triassic.
More common than brachiopods
since the mid-Jurassic