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

Abstract

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