Taxonomy and palaeobiogeographical affinites of Early Cretaceous stalked crinoids from the Neuquén Basin, west-central Argentina

LAZO, DARÍO G.; BRESSAN, GRACIELA S.; SCHWARZ, ERNESTO; VEIGA, GONZALO D.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

The aims of the present work are to present a preliminary morphological description and palaeobiogeographical affinities of stalked crinoids recorded from the Lower Cretaceous of the Neuquén Basin, west-central Argentina. The studied specimens proceed from the Agrio Formation in two different stratigraphic positions and facies relationships. Most of them come from abundant crinoid beds alternating with cross-bedded sandstones recorded at the basal third of the Pilmatué (lower) Member of the Agrio Formation. These beds have been dated as late Valanginian by the associated ammonoids and trigonioid bivalves. Besides, there are two specimens preserved in a calcareous nodule recorded from dark-grey shales of the Agua de la Mula (upper) Member of the Agrio Formation and dated as late Hauterivian by the associated ammonoids. The studied crinoids have a long and slender heteromorphic stem. Nodals and internodals are petaloid in transverse section. The articulation between internodals is symplexy. Axial canal is narrow and rounded. Internodals are 6-7 in number. Nodal plates are twice as high as internodal plates. Calyx is cryptodicyclic, composed by five radial plates and five basal plates not forming a contiguous circlet. Articulation between primibrachials 1 and 2 is synarthrial and between secundibrachials 3 and 4 is straight cryptosyzygial. Arms are more slender toward distal ends, but almost uniform in diameter within each brachitaxis. They display isotomous branching with at least 6 bifurcations in line per ray. Pinnules are arranged in one alternating series. Maximum crown height is approximately 80 mm. The studied specimens has been identified as belonging to the genus Isocrinus Von Meyer (Family Isocrinidae, Suborder Isocrinina, Order Isocrinida) with small basal plates not forming a contiguous circlet, low columnals, large cirrus sockets as high as nodals and synarthry articulation between primibrachials 1 and 2. The genus has been previously recorded in the Lower Cretaceous mostly in the Northern Hemisphere from Germany, France, Poland, Russia and Japan. In the Southern Hemisphere, they are extremely rare and complete specimens are only reported from Antarctica and Australia and as far as we know no records from South America have been published so far. Therefore, the first finding of well-preserved stalked crinoids from the Early Cretaceous Agrio Formation in the Neuquén Basin is remarkably important in terms of crinoid evolutionary history and palaeobiogeography.