Indentification keys

The classification of brachiopods is mainly founded on the characters of the shell. The lexicon of the paleontologists thus contains several hundreds of terms used for the description of the various species.

Anterior commissure

 

 

uniplicated
paraplicated

 

sulciplicated
rectimarginated

Lateral commissure

The lateral commissure is either straight, or more or less curved.

Beak angles

Straight, nearly straight, suberect, erect, incurved...

The outline

Globulous, punt, spherical, lengthened, triangular, pentagonal, round, large, small, horned...many alternatives exist as for the shape and the general aspect of the shell.

Other external criteria :

  • position and size of foramen,
  • presence and form of deltidium,
  • number of growth lines,
  • shell ornamentation,
  • outline,
  • form of the beak,
  • height of pedicle valve,
  • height of brachial valve,
  • breadth of pedicle valve,
  • breadth of brachial valve ,
  • thickness of pedicle valve ,
  • thickness of brachial valve,
  • microstructural aspects of shell...

Internal criteria

For some Articulata, the internal characters of the shell is added to the other criteria to define the species.They are mainly the characteristics of the lophophore (aspect of the brachial system)

Atrypa
Terebratula
Terebratella
Spirifer

and of the muscular prints...

Polymorphism

Brachiopods are by definition very polymorphic organisms and can thus present variations inside a species.

It is often necessary to gather, as far as possible, several samples of the same species to be able to indentify them with precision. All the more reason that the shape of the shell changes with age.

The foramen

 
 
Terebratulida with large foramen
Zeilleriidae with small foramen
The foramen corresponds to the opening by which the pedicle of the animal passes to fix itself on the sediment.This peduncular opening can be of different form and structure. Several cases can arise: - either the stalk is extruded between the hooks (Lingula) - or through a small triangular opening (delthyrium) between the hook and the edge of the valve, or, it is the case in certain advanced brachiopods (Discina), the opening is in the middle of the valve. The genus Zeilleria probably did not have a pedicle.

When mathematicians meet biologistes...

...they talk about measurements, of course!

Morphometry may be very important to identify brachiopods, because their shells are often very similar in shape.

Recently, a scientist (Tort,A., 2000) used Fourier analyses in order to study the morphological space of the different living and fossil populations, also examining intra and interspecific internal features.

.

A caliper gauge

is very useful to identify brachiopodes.

Glossary

Lophophore : brachial apparatus.

Morphomettry : Measurement of the form of organisms or of their parts.

Foramen : opening by which the foot (pedicle) passes to fix the brachiopod on the sediment.
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