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; IV International Palaeontological Congress; 2014

International Palaeontological Association

The aims of the present work are to present a preliminary morphological description andpalaeobiogeographical affinities of stalked crinoids recorded from the Lower Cretaceous of theNeuquén Basin, west-central Argentina. The studied specimens proceed from the Agrio Formationin two different stratigraphic positions and facies relationships. Most of them come from abundantcrinoid 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 theassociated ammonoids and trigonioid bivalves. Besides, there are two specimens preserved in acalcareous nodule recorded from dark-grey shales of the Agua de la Mula (upper) Member ofthe Agrio Formation and dated as late Hauterivian by the associated ammonoids. The studiedcrinoids have a long and slender heteromorphic stem. Nodals and internodals are petaloid intransverse section. The articulation between internodals is symplexy. Axial canal is narrow androunded. Internodals are 6-7 in number. Nodal plates are twice as high as internodal plates. Calyx iscryptodicyclic, 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 isstraight cryptosyzygial. Arms are more slender toward distal ends, but almost uniform in diameterwithin 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 (FamilyIsocrinidae, Suborder Isocrinina, Order Isocrinida) with small basal plates not forming a contiguouscirclet, low columnals, large cirrus sockets as high as nodals and synarthry articulation betweenprimibrachials 1 and 2. The genus has been previously recorded in the Lower Cretaceous mostlyin the Northern Hemisphere from Germany, France, Poland, Russia and Japan. In the SouthernHemisphere, they are extremely rare and complete specimens are only reported from Antarcticaand 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 AgrioFormation in the Neuquén Basin is remarkably important in terms of crinoid evolutionary historyand palaeobiogeography.