Livro de Resumos - Dinossauros do Maranhão

Paleontologia 39
Aalenian (Early Jurassic) radiolarian fauna from Los Molles Formation,
Argentina, and its paleobiogeographic affinities
Karlos Guilherme Diemer Kochhann 1 , Simone Baecker-Fauth 1 , Gerson Fauth 1,2 & Ariane Santos da Silveira 1,2
Fossil radiolarians are useful as paleolatitudinal indicators due to the paleobiogeographic
control of some taxa, mainly during the Jurassic. The objectives of this work are to report
the occurrence of an Aalenian (Early Jurassic) radiolarian fauna in the Neuquén Basin,
Argentina, as well as infer its paleobiogeographic affinities. The micropaleontologic
analysis was carried out in fine-grained rocks that compose the turbiditic section of the Los
Molles Formation (Toarcian to early Bajocian in age), sampled during the description of a
lithostratigraphic profile in its better known section (road cut along the Ruta 46, near the
bridge over the La Jardinera Creek). These samples were firstly processed using hydrogen
peroxide (H2O2), and later treated with five percent hidrofluoridric acid solution (HF). The
second chemical treatment enabled the recuperation of a more abundant radiolarian
residue. The recovered fauna presents a low diversity and abundance, also a strong
dominance of spumellarians over nassellarians, being composed by the genera Paronaella,
Homoeoparonaella, Praeconocaryomma, Archaeocenosphaera, Orbiculiformella,
Praeparvicingula, and some unidentified spumellarians and nassellarians. The Aalenian age
of these rocks is based on biostratigraphic data of previous works in the same section. The
recovered fauna is in accordance with the Aalenian age, presenting species as
Homoeoparonaella reciproca, Praeparvicingula tlellensis and Praeparvicingula elementaria.
The paleobiogeographic affinities of the studied fauna were inferred according to a
paleobiogeographic model proposed to the early Jurassic and early Cretaceous and based
on morphological and ecological aspects. The main diagnostic characteristics presented by
the recovered fauna are: absence of low-latitude taxa; presence of high-latitude taxa;
presence of taxa with unknown affinities which may be endemics; general morphology of
taxa – large body size, presence of heavier, thick-walled smooth shells or shells with
heavily silicified walls; diversity gradient lower in high latitudes. These features led us to
infer a mid latitude origin to this fauna, possibly related to cold water currents. These data
suggest a bipolar distribution of some taxa, such as Praeparvicingula and
Praeconocaryomma, between the North and South hemispheres since the Early Jurassic
times. Apoio: Petrobras e Secretaría de Estado de Energía y Minería de la Provincia del
Neuquén (Argentina).
1 Laboratório de Micropaleontologia, Departamento de Geologia, Universidade Vale do Rio dos Sinos
(UNISINOS), Avenida Unisinos, 950 - Cristo Rei - 93022-000, São Leopoldo, RS, Brasil.
2 Programa de Pós-graduação em Geologia, UNISINOS.
E-mails: k.kochhann@gmail.com, sbfauth@unisinos.br, gersonf@unisinos.br
Micropaleontologia