Paleobiogeographic affinities of the Early Ordovician graptolites of western Gondwana

TORO, B.A; VENTO B.A.; MALETZ J.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

CCT/CONICET, Mendoza

The Central Andean Basin (Northwestern Argentina and Southern Bolivia) evolved at the western margin of Gondwana during the Early Paleozoic. A number of analyses dealing with the paleobiogeographic affinities of the Early Ordovician graptolite faunas from Northwestern Argentina were carried out in the last decade. Nowadays it is widely accepted that the region was located in middle to high latitudes, included in the cold water faunal realm. Recent studies used statistic treatments to quantitatively interpret the paleobiogeographical relationships between the different regions of the world. Most of them focused on new records representing taxa from uppermost Tremadocian to Floian deposits, which are extensively exposed in the Cordillera Oriental of Argentina and Bolivia. In contrast, paleobiogeographic affinities of the early and middle Tremadocian graptolite assemblages from the Central Andean Basin are still poorly discussed, as they are composed of a few taxa of worldwide distribution. The present work includes a cluster analysis to deal with the occurrence of 17 graptolite taxa recorded throughout the whole Tremadocian (Rhabdinopora flabelliformis parabola, Anisograptus matanensis, R. f. anglica, Adelograptus, Bryograptus kjerulfi, Aorograptus victoriae, Kiaerograptus supremus, Araneograptus murrayi and Hunnegraptus copiosus biozones) from Northwestern Argentina and Bolivia. The resulting dendrogram, which included five further regions of the world, highlights the remarkable graptolite faunal affinities between Central Andean Basin and Baltoscandia with a similarity index of 0.75. This clade is successively related to Laurentia, with a similarity index of approximately 0.65, and to South China (Jiangnan Region) through an affinity index of 0.55. This fact could be interpreted as a closer relationship with the warm water faunal realm than previously established for the Foian graptolite faunas. On the other main clade, the Yangtze Region and Great Britain appear to be separated from other regions through a low affinity index of 0.40. This noticeable difference to the close relationships previously established between graptolite faunas from Northwestern Argentina and those of the Yangtze Region during the middle-upper Floian could be attributed to the influence of water depth, relating to the different paleoenvironment, in which both Floian and Tremadocian graptolite faunas were respectively developed, rather than to the paleolatitudinal thermal gradient.