Description of ichthyosaur remains from the Lower Cretaceous Agrio Formation (Neuqu
en Basin, west-central Argentina) and their paleobiological implications

LAZO, D.G., TALEVI, M., CATALDO, C.S., AGUIRRE-URRETA, M.B. AND B. AGUIRRE-URRETA

CRETACEOUS RESEARCH (PRINT)

ACADEMIC PRESS LTD-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2018

0195-6671

Ichthyosaur remains from the Lower Cretaceous of the Neuquen Basin, west-central Argentina, are hereindescribed and put into the broader context of their mode of occurrence and facies relationships. Theichthyosaur material was collected from six stratigraphic levels within the Agrio Formation. Five recordscome from upper Valanginian black and dark-gray shales, and one comes from upper Hauterivian finegrainedgrayish sandstones. According to current paleogeographic schemes, all the studied ichthyosaurlocalities had a distal position on the marine shelf of the Agrio Formation, i.e., towards the offshore andbasin. Ichthyosaurs were recorded in second-order TST and HST deposits, being better preserved in blackshales of the TST. The best preserved material is a series of seven partially articulated posterior thoracicvertebral centra along with an ischiopubis (plus femur?) of an adult individual found in close associationwith a partially articulated fin and phalanges of a perinatal specimen. This finding might provide evidenceof viviparity and also would indicate that at least some ichthyosaurs were able to pass through thevolcanic arc from the Pacific Ocean to reproduce and give birth in an epeiric sea (the Neuquen Basin),similarly to what some large marine mammals do in the present. Given that the upper ValanginianeHauterivian is particularly devoid of ichthyosaur records worldwide, the remains herein presentedcan shed light on the Early Cretaceous distribution of these marine reptiles in the Southern Hemisphere.The apparent local absence of ichthyosaurs from Berriasianemiddle Valanginian deposits may be due tothe overall prevalence of shallower settings in the Neuquen Basin in this time window that deterred theentrance of ichthyosaurs from the open ocean.