“Early Angiosperm Diversification in the Cretaceous of Argentina: First Approach.”

ARCHANGELSKY, S.; BARREDA, V.; PASSALIA, M.; PRÁMPARO, M.B.; ROMERO, E.; ZAMUNER, A.; CUNEO, R.; GANDOLFO, M.A.; IGLESIAS, A.; LLORENS,M.; PUEBLA, G.G.; QUATTROCCHIO,M.E.; VOLKHEIMER,W

Bariloche.Argentina

Conferencia; VII International Organization of Paleobotany Conference; 2004

Museo Paleontológico Egidio Feruglio

One of the most interesting enigmas in the study of fossil floras is the one concerning the origin and evolution of angiosperms. Althought these types of studies are advanced worldwide, they are scanty in Austral South America. In this report, we analyze for the first time in an integrated form the known angiosperm fossil record (micro and megaflora) from the central (San Luis and Neuquen) and southern (San Jorge Gulf and austral) basins of Argentina. The study comprises the period between the Aptian (123 my) and the Coniacian (85.8my). The crhoronstratigraphic assignment of the units that bear the micro and megafloras were calculated by radiometric data, dinoflagellates and statigraphic correlation. The first and oldest evidence for the presence of angiosperms based on leaf remains and pollen grains is found in the earliest Aptian. Evidence from leaves corresponds to two basal morphotypes: nymphaeaphyll (1mf) and a micro-mesophyll lobaste serrate leaf with craspedodromous venation (2 mf). The evidence based on pollen is referred to microreticulate forms with poorly defined apertures of the species Clavatipollenites hughesii Couper (1p). this form was recognized at several important outcrops and its first records have an apparently synchronic distribution. In younger deposits within the Early Aptian, it is easy to recognize a new foliar type; hydrocotylephyll (3mf) as well as several species of Clavatipollenites pollen grains; and for the first time, the species Asteropollis astroides Hedlund & Norris (2p) associated with other monocolpate forms is documented. During de Late Aptian, fossil leaves of rosiphyll (4mf), pinnatilobate nanophyll (5mf) and possible a myrtophyll (6mf) forms are recorded. A particular pollen association composed of Brenneripollis peroreticulatus (Brenner) Juhasz & Goczan (4p), Stellatopollis (5p) and several species of Afropollis (6p) is recognized. Thse primitive angiosperms with very dinstictive morphologies and afgfinities, characterized the Ecuatorial Dicheiropollis etruscus/Afropollis Province. The presence of these forms at the sothermost section of South America indicates the necessity of a re-evauation of the extension of this province and the presence of transition zones as well.With the exception B. peroreticulatus, that was identified at younger sediments, Stellatopollis and Afropollis seem to have been restricted to the Late Aptian, possibly extending into the Early Albian in Argentina. Other pollen forms that become recognizable correspond to the genus Liliacidites (3p). Close to the Aptian/Albian boundary, the first record of microreticulate  tricolpate pollen grains related to the primitive eudicots (7p) were identified. This pollen form is associated with leaf remains that are possibly linked to the rosiphyll morphotype (7mf). Several leaf morphotypes are identifiable in equivalent or slightly younger sequences showing an early increment in the diversity of the angiosperms. These morphotypes are mostly elliptic leaves with entire margins and pinnate brochidodromous venation (Thorphyllum, 8mf), leaflets with strong intramarginal venation (Kachaikenia,  9mf), festooned leaves with brochidodromoius venation (Rogersia, 10mf), and finally, one leaf remain with eucamptodromous venation (12mf). Probably during the Late Albian, pollen evolved first into tricolpate psilate forms (8p) and later into tricolporoid types (9p). At this stage, it is possible to identify myrtophylls (13mf), a venation of the myrtophyll form with a double intramarginal vein (14mf) and cunoniophylls (15mf) types. Towards the end of the Albian and the beginning of the Cenomanian, there is an increase in the number of morphotypes with higher numbers of palmatilobed leaves of the platanophyll (16mf) and vitiphyll (17mf) types, elliptic leaves with entire margins and pinnate venation typical of the larophyll type (18mf), and acrodromophyll leaves (19mf). Coincinding with this change in the megaflora, there is a significant increase in angiosperm pollen diversity. It is possible to distinguish tricolpate forms with variable sculpture, such as psilate, gemate, verrucose (Verrutricolpites, 10p); other forms with variable lumens (Rousea, 11p); tricolporoidiod and clearly tricolporoid forms