TAXONOMY AND ZOOGEOGRAPHIC RELATIONSHIPS OF THE SOUTH AMERICAN NAIADES (PELECYPODA: UNIONACEA AND MUTELACEA)! J. J. Parodiz and A. A. Bonetto Carnegie Museum, Pittsburgh, Pennsylvania, U. S. A. and Consejo Nacional de Investigaciones Cientificas Santa Fe, Argentina ABSTRACT A natural system of classification is proposed for the South American fresh-water pearly mussels which were formerly all grouped in the superfamily Unionacea. The systems recognized since the end of the last century which were based mainly on con-chological, and partly on anatomical characters, are here discussed in relation to recent embryological and phylogenetic research, especially as regards the structure and de-velopment of the different types of larvae. Researches made by the authors during the last decade have confirmed the existence of the "lasidium" larva, discovered by Ihering in 1891, but not observed since by other authors. This larva is typical of the South American genera Anodontites, Mycetopoda, Monocondylaea and Leila. At the same time the research of other workers on African species of Mutela has revealed the existence of a larva which, if not entirely similar to the lasidium, is similar in its basic structural features. Comparative studies of such structures and their development show a family differentiation between Mutelidae of Africa and Mycetopodidae in South America. On the other hand, the close relation between these two families, and their extraordinary embry-ological divergence from those other fresh-water mussels characterized by the well known "glochidium" larva, warrants the distinction of a new superfamily, MUTELACEA. All other South American fresh-water mussels with larvae of the glochidium type remain in UNIONACEA. The Mutelacea are living today in the southern hemisphere, excepting Australasia, Whether South American groups are derived from African groups, or vice versa, is not known. The anatomical and embryological differences between Mutelidae and the more advanced Mycetopodidae seem to indicate an ancient separation. Paleontological records are rare: none exist for Africa or Australasia; In North America, fossil casts from the Triassic of Pennsylvania were referred to by Pilsbry as a Mycetopoda -like mussel; Pleiodon priscus described by Ihering from the Cretaceous of Brasil is not a mutelid, as was assumed, but belongs to the genus Paxyodon (Hyriidae). Some references to Anodontites-\ik.e fossils from the Cretaceous of Bahia, Brasil are very doubtful. The South American Mutelacea, i.e., the Mycetopodidae, are divided into 3 subfamilies: Mycetopodinae, Anodontitinae and Monocondylaeinae; another subfamily, Leiliinae, might be accepted in view of more recent anatomical and embryological research. Other groups, at the subfamily level, indicated in previous classifications, cannot be main-tained, their characters being insufficient and the intergradations numerous. The South American Unionacea belong to the family Hyriidae, which also occurs in Australia but is absent in the rest of the world; the exclusively South American forms belong in the subfamily Hyriinae and are divided into 3 tribes: Diplodontini, Castaliini and Prisodontini. Especially the Diplodontini, largely formed by the genus Diplodon, are more closely related to forms of Australia and New Zealand, Here again, attempts to separate a number of subgenera have failed on account of the difficulty of defining constancy of characters. From the embryological point of view, however, we may dis-tinguish two entities of subgeneric value: Diplodon s.S., with parasitic glochidia, and Rhipidodonta with non-parasitic glochidia, i.e., having direct development. There is paleontological evidence of Hyriidae in the North American Triassic, the Paleocene of Southern Argentina, and the Eocene of Chile, the latter fossils being very similar to the species now living in the region as well as to related groups from Australia. All these fossils belong to the genus Diplodon, of which other species are known from the strata, at different levels of the middle and upper Tertiary, scattered over the continent of South America. The monotypic genus Bartlettia of the so called "fresh-water oysters", currently included within the Etheriidae, very probably belongs to a polymorph species of Mutelacea, Anodontites tenebricosus. Larval stages of Etheriidae are unknown, but further investigation may prove that the family, if it should be maintained as such, inte-grates with the Mutelacea. Comparative tables are given of the different systems of classification since 1900 as well as of the new system adopted here, from the superfamüy down to subgeneric level. ^Research supported by a research grant, NSF-15032, to the senior author from the National Science Foun-dation, Washington, D. C, U. S, A. (179)
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