Brachiopoda: Morphology, Geological History and Evolutionary Trends

  • They are exclusively marine and sessile forms.
  • Their fossils occur in most stratigraphic horizons and have been abundant in the past.

Morphology

  • The soft part of the organism include the pedicle, heart like organ, nerve ring, digestive system, genital and arms which are known as
  • The lophophore occur in pair with cilia and are used for gathering the food.
  • The lophophore is also known as brachia and hence the name Brachiopoda (from two greek word: Brachion = arm + podus = foot).
  • Major part of the mantle cavity is occupied by lophophore.
  • All the soft part are enclosed by a thin skin which is known as mantle and the mantle is, in turn, enclosed by a bivalve shell. The valves are equilateral and the shell is Two valves – the smaller valve is dorsal and the larger is ventral.
  • The posterior-terminal part of the both valves is produced into a beak or umbo.
  • The two valves are joined at a margin on the posterior side which is known as hinge line. The two valves are opened and closed by divaricator (diductor) and adductor muscles respectively. They pass from the centre of one valve to the centre of other valve. The place of attachment of these muscles are termed as muscle impression.
  • The main part of the mantle cavity is occupied by coiled arm-like processes composed of calcareous material and is known as brachial skeleton.
  • The brachial skeleton is in the form of two small curved plates in Rhynchonella, loop like in Terebratula or spirally coiled in Productus, Glassia, and Spirifer.
  • Some of the older genera were free moving type especially in old age, e.g., Productus, while some were attached to rock by one valve, e.g., crania.
  • The pedicle is short, stout and fibrous rod like structure which comes out through the pedicle opening in the pedicle or ventral valves, e.g., Terebratula.
  • In come cases it may came out through the opening between umbones, e.g., Lingula, or through the centre of the shell, e.g., Discina or through a triangular opening the delthyrium, e.g., Orthis.
  • The external surface of the shell may have concentric and radiating growth lines and tubercles .

Classification

  • Shrock and Twenhofel classified brachiopoda into Inarticulata and
  • Inarticulata – Sub divided into Atremata and Neotremata
    • Atremata – includes all those forms in which there is no pedicle opening e.g- Lingula , Trimerella, Age - Cambrian to Recent
    • Neotremata – Pedicle opening is present only in younger stage . e.g- Crania, Discina Age – Cambrian to recent
  • Articulata
    • Paleotremata- all those older forms with pedicle opening. It includes Rustella .
    • Protremata- e.g Orthis , productus age – Cambrian to cretaceous
    • Telotremata – highly modified pedicle opening and brachial skeleton g – spirifer , athyris, atrypa

Geological History

  • The earliest brachiopod are known from lower Cambrian beds of many parts of the world and on their earliest appearance they were quite diverse( Mostly inarticulates).
  • Well known articulates make their appearance in Ordovician.
  • Inarticulates remained a diverse group upto Ordovician after which they declined. In Silurian their number decreased abruptly and became extinct.Orthids,Pentamerids , Rhynconella all of these are Ordovician articulates. 
  • A drastic fall of Brachipods came after Permian and only a few groups such as Tetrabulids, rhynchonellids ,were able to cross the Paleozoic- Mesozoic Boundary.

Evolutionary trend

  • Brachipods reached its climax during paleozoic, but its number declined during Mesozoic.
  • During the Ordovician and Silurian periods , brachiopods became adapted to life in most marine environments and became particularly numerous in shallow water habitats.
  • During the mass extinction of Permian period, there was a major shift.
  • In the Mesozoic, the number of brachiopods and their diversity were drastically reduced and they were largely replaced by Lamaellibranchia.
  • Another possibility for their demise include increasing disturbances of sediments by roving deposit feeders, increased shell crushing predation .