Heterodonta

Sand gaper ( Mya arenaria )

The Heterodonta ( Verschiedenzähner or Wechselzähnige shells on German called ) are a large group of mussels ( Bivalvia ), which is conducted in the newer classifications as an order within the Autolamellibranchiata. It is a large, very diverse and abundant in many habitats Shell Group. It is considered the sister group of the Palaeoheterodonta. The first shells, which can be attributed to this group appear in the Middle Ordovician.

Characterization

The housing of the Heterodonta are very diverse. You can equivalve ( bilaterally symmetrical), inequivalve ( asymmetric) to aberrantly (eg Hippuritoida ). The shells are aragonitisch with kreuzlamellaren or prismatic microstructures. There are no pearl ring microstructures. The Hippuritoida layers are superposed with fibro- prismatic calcite crystals on the outer prismatic layer. The castle was originally heterodont, but can also greatly enlarged ( pachydont ) or be reduced; or can also be turned into a periodontal ligament castle. The ligament is external.

The Heterodonta are isomyar, that is, the two sphincters are approximately equal, anisomyar ( unequal size ) or monomyar, that is, the original two sphincters are merged into a sphincter. The foot is well developed, as a rule, in some taxa it can also be reduced. It is generally used for digging, crawling and rarely also for drilling. The siphons can be quite large in grave shapes. The gills are eulamellibranchiat.

Way of life

The Heterodonta are predominantly marine bivalve molluscs, which have opened up a very wide range of habitats. A few superfamilies are also penetrated into brackish and fresh water. Other forms have been recently discovered in the vicinity of the so-called black smokers in the deep sea. It forms occur, the different depths dig into soft sediments, drilling in hard substrates, are rooted with a calyx-like flap and probably lived in symbiosis with algae ( Hippuritoida ), the shell almost completely reduced and how slugs crawling around. The Heterodonta point within the shells probably the largest range of shapes and lifestyles. Most species feed on "Shell - typical " as filter feeders. There are also some forms that breed bacteria. The extinct Hippuritoida have most likely lived in symbiosis with photosynthetic organisms drifting.

System

The Heterodonta be considered in cladistics as the sister group of the Palaeoheterodonta. The classification changes so far in the recent work than that the superfamily Carditoidea ( order Veneroida ) was raised to rank order, while the former superorder Anomalodesmata was " demoted " on trim level. This demotion is not followed here:

• Order Carditoida Dall, 1889 ( this order is not yet recognized by the Zoological Record and the majority of malacologists, the group is usually done as a family of the order Veneroida )
• Order Myoida Stoliczka, 1870
• Order † Hippuritoida Newell, 1965
• Order Veneroida Adams & Adams, 1856
• ( Order Anomalodesmata Dall, 1889; .. These regulations is considered by most editors as an order or even subclass Harper et al (2006) advocate a downgrade to rank order )