Early angiosperm diversification in the Cretaceous of Argentina: first approach

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

S.C. de Bariloche

Congreso; 7º International Organization of Paleobotany Conference; 2004

One of the most interesting enigmas in the study of fossil floras is the one concerning the origin and evolution of angiosperms. Although 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 Neuquén) and southern (San Jorge Gulf and Austral) basins of Argentina. The study comprises the period between the Aptian (123 my) and the Coniacian (85.8 my). The chronostratigraphic assignment of the units that bear the micro and megafloras were calculated by radiometric data, dinoflagellates and stratigraphic 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 lobate serrate leaf with craspedodromous venation (2mf). 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, components of the Asteropollis group (2p) associated with other monocolpate forms are documented. During the 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) Júhasz & Góczan (4p), Stellatopollis (5p) and several species of Afropollis (6p) is recognized. These primitive angiosperms with very distinctive morphologies and affinities, characterized the Ecuatorial Dicheiropollis etruscus/Afropollis Province. The presence of these forms at the southernmost section of South America indicates the necessity of a re-evaluation of the extension of this province and the presence of transition zones as well. With the exception of 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) was 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 brochidodromous 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 variation of the myrtophyll form with a double intramarginal vein (14mf) and cunoniophylls (15 mf) 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 laurophyll type (18mf), and acrodromophyll leaves (19 mf). Coinciding 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 (Dryadopollis, 12p); some grains with more complex apertures; and finally, other forms with developed colpi costae related to Nyssapollenites (13p). Within the Cenomanian, it is possible to discern the polyporate forms of the genus Cretacaeiporites (14p) and pollen grains with a remarkable reduction of the polar axis and shortening of the colpi (15 p), probably anticipating the emergence of the triporate forms. In the Turonian-Coniacian levels, elements derived probably from hydrocotylephyll (20 mf) and cunoniophyll types (21 mf) are conspicuous, as well as leaves with serrate margins of the hamamelididioid type (22mf). Finally, in the Coniacian, a very diverse angiosperm association is identifiable. In it, some of the morphotypes already identified in older sequences are present (myrtophyll, laurophyll, nymphaeaphyll, acrodrophyll and platanophyll) in conjunction with new ones, such as fagophyll (23 mf), pinnatilobulophyll (24 mf) and pentalobulophyll (25 mf). Based on this analysis, four significant phases in the evolution of the angiosperms in the Southernmost section of South America are evident: a) Early Aptian, oldest records for angiosperm leaves and pollen grains, the angiosperms are the subordinate component of the flora dominated by gymnosperms, pteridophytes and bryophytes; b) Late Aptian- Early Albian, the first eudicots appear; c) Late Albian- Early Cenomanian, a first pulse of diversification of leaf morphotypes and pollen grains with complex apertures and sculpture; and d) Turonian-Coniacian, very diverse angiosperm associations comprising leaf morphotypes already recognized in older sequences and new morphotypes with possible affinities to extant taxa.