Osvaldo A. Reig


This paper anticipates some of the ideas of a more elaborated forthcoming paper dealing with problems of the origin and development of the South American tetrapod fauna The author proposes to improve the use of biocoenotic conceptions in the study of problems related with historical biogeography, and to introduce the concept of coenogenesis (Sukachev) into zoological biogeography to replace the vague ''historical zoogeography" as all evolutionary biogeographical inquiry must be historical, and because of the convenience of having in that the origin and development of fatinas can be adequately understood only as the process of development of communities, not of isolated animal or plant groups The author comments briefly on the present ideas on the origin of the fauna of South America, as elaborated mainly by Simpson, concluding that Simpson's scheme is a valid one for the Cenozoic, i e for the time of the main history of mammals: in spite of some secondary points, but one needs realize that the history of the faunas from Pennsylvanian (the time of the first record of tetrapods in South America) to Early Cenozoic may have had quite another pattern, including faunal relationships among Southern continental masses and the Southern origin of some of the groups incorporated to the South American biocoenosis The factual evidence for this conception is afforded mainly by fossil discovederies the Triassic of Brazil and Argentina and by agentinian fossil anurans recently discovered These data are not discussed being the subject of the mentioned more elaborated paper previously Thereafter the author discusses Simpson's concept of faunal strata; pointing out that in its original connotation it is strictly chronological, denoting a faunal group that invades a region in a definite time The adequacy of the term ''stratum” for this coenogenetic elements is discussed; the author proposes to reject this term, because the same term is used for a quite distinct synecological cqncept; and because it does not clearly indicates the merely temporal connotation of the concept He proposes the term coenochron for these coenogenetic units, defined as "an animal or plant group or community, whatever its origin, which enters into a given area between definite limits of “geological time" The author points out that in the analysis of the development of a given fauna or flora other aspects must be distinguished,- namely the original area of the invading elements arid t h e m i g t a t i c i n a l r o u t e t h e y u s e , w h i c h m a y o r m a y n o t b e c o i n c i d e r t t T h e f i r s t being a v e r y important aspect, it is proposed to distinguish the center of origin and of original dispersfon of the invading dements as an attribute of the coenochrons, discerning groups in relation withits lineage (Spanish: "abolengo", as used by Ringuelet). The author proposes further a new scheme of the main elements to be discerned in the coenogenesis of South American tetrapod fauna is deduced from a set of follr principal hypothesis of their subordinated hypothesis from the relevant data. This scheme distinguishes four principal coenochrones; namely: 1 Early coenochron, comprising groups that entered the continent till about the Middle Jurassic, and subdivided into: 1 1 Groups of Gondwanic lineage of the Early Coenochron, and 1 2 Groups of Boreal lineage of the Early Coenochron 2 Meso- and Neomesozoic Coenochron, comprising groups that entered the continent from about the Middle Jurassic till the end of the Cretaceous; and subdivided also into 2 1 Groups of Austral lineage of the Meso- and Neomesozoic Coenochron, and 2 2 Groups of Boreal lineage of the Meso- and Neomesozoic Coenochron 3 Mesocenozoic Coenochron, comprisfug groups of possibly diverse origin, that entered South America during its isolation, by means of surface-water or other fortuitous dispersal, and 4 Plio::Pleistocene Coenochron comprising the "Third Faunal Stratum" of Simpson i.e. groups that entered continent at the Late Cenozoic, inmediatly before and after the establishment of the Panamian bridge; being all of Boreal origin; this coenochron is subdivided into two stibordinated coenochrones, viz: 4.1 Oliocene Coenochron, comprising groups that entered the continent prior to the establishment of the bridge (island hoppers or surface-water dispersal), and 4.2 Pleistocene Coenochron, comprising groups arrived after the bridge was established, in the Early Pleistocene 
T'he author points out that differentiation in situ of distinct level groups, is another main process the development of the faunas, to be analized in a further work Some- evidence seems to indicate that 'South America has als o been a center of origin and of dispersal of important vertebrates groups.

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