First articulated stalked crinoids from the Mesozoic of South America: two new species from the Lower Cretaceous of the Neuquén Basin, west-central Argentina

Abstract

Two new isocrinids are described from the Lower Cretaceous Agrio Formation of the Neuquén Basin, west-central Argentina. Isocrinus (Chladocrinus) covuncoensis new species is based on several beautifully preservedspecimens from Valanginian beds of the Pilmatué Member. It is characterized by a small size, multiramose crownwith six arm divisions, 240 arm tips, mostly isotomous branching, seven (or rarely eight) secundibrachials, smoothand stout column, short noditaxis, and pentalobate columnals. The species occurs in a 30 mthick interval of cross-beddedsandstones and mixed clastic-carbonate sediments that represent the migration of large, tidally influenced, subaqueousdunes developed in the offshore. Sudden burial of crinoids that dwelled on the dune toes and interdunes, possibly bythe acceleration of the lee face migration, provided the exceptional preservation of specimens and thus this findincan be considered as a local crinoid Konservat Lagerstätte. Isocrinus (Chladocrinus) pehuenchensis new species isdescribed from a single articulated specimen preserved in a silty calcareous concretion collected from a late Hauterivianconcretion level of the Agua de la Mula Member. It is characterized by isotomous branching, eight or nine secundibrachials (IIBr), slender column ornamented with medial ridge of fine tubercles, interradius acuminated with fine tubercleson its tip, short noditaxis, and pentastellate columnals. It is associated with low-energy fall-out deposits in the offshore.The excellent state of preservation was due to an early cementation process by carbonate that enhanced lithificatioaround the specimen.