Unionoida

Common swan mussel ( Anodonta anatina )

The Unionoida ( "river clam -like " ) are an order of mussels. They belong to the superorder Palaeoheterodonta. First representatives are known from the Middle Devonian. The Unionoida are probably the sister group of the Trigonioida. Currently, about 700 extant species are found in this group. Add to that a previously unknown number of fossil species.

Characteristics

The order Unionoida mainly comprises the so-called " freshwater mussels ." However, other groups have penetrated into the freshwater mussel, so that the reverse conclusion, all " freshwater mussels " are among the Unionoida is not correct. The housings are equivalve usually. The shell is made of a thick layer of nacre, a thinner prismatic layer which is overlain by a thick layer of organic Periostrakums. The castle is heterodont, schizodont or greatly reduced. The ligament is usually external. Almost all forms have more or less distinct isomyare sphincters. The surface line is integripalliat.

System

The classification of the order Unionoida is not yet uniform. While some malacologists still favor a subdivision in one or two superfamilies (plus two exclusively fossil groups), listen to others, only the families. The phylogeny within the order Unionoida is still very much in flux.

• Superfamily freshwater mussel similar ( Unionoidea Rafinesque, 1820 ) Family freshwater pearl mussels ( Margaritiferidae Haas, 1940)
• Family river mussels ( Unionidae Fleming, 1828)

• Family Etheriidae Deshayes 1830
• Family Hyriidae Swainson, 1840
• Family Mycetopodidae Gray, 1840
• Family Iridinidae Swainson, 1840

• Family † Archanodontidae Weir, 1969

• Family † Anthracosiidae Amalitsky, 1892
• Family † Palaeomutelidae Weir in Vokes, 1967
• Family † Prilukiellidae Starobogatov, 1970
• Family † Nyassidae Hall, 1885
• Family † Ferganconchidae Martinson, 1961
• Family † Pseudocardiidae Martinson, 1961

Phylogeny

The phylogeny of the extant groups of Unionoida was recently examined by Graf & Cummings (2006) who found the highest probability for the following cladogram:

Unionidae

Margaritiferidae

Hyriidae

Etheriidae

Mycetopodidae

Iridinidae

Trigonioida

Unionoidea and Etherioidea likely to form sister groups. Within the Unionoidea turn Unionidae and Margaritiferidae form sister groups. Within the Etherioidea to Hyriidae and a still unnamed taxon are composed over from Etheriidae ( " fresh oysters " ), Mycetopodidae and Iridinidae.