Purranisaurus potens Rusconi, an enigmatic metriorhynchid from the Late Jurassic–Early Cretaceous of the Neuquén Basin
Authors:
Herrera et al
Abstract:
The Tithonian record from northwestern Patagonia (Neuquén Basin) documents a complete succession of lower, middle, and late Tithonian marine reptiles. From the late Tithonian–early Berriasian of Patagonia, three marine crocodyliforms have been recorded: Dakosaurus andiniensis, Cricosaurus araucanensis, and Purranisaurus potens. Although P. potens includes the first metriorhynchid skull described from Patagonia, no detailed description and/or illustrations have been published. Since the mid-20th century, several authors have discussed the validity of P. potens. Recent preparation of the type material allowed a detailed description as well as exploration of its phylogenetic relationships. We consider P. potens as a valid taxon characterized by a unique combination of characters, such as medial and posterolateral processes of the frontal forming an angle of 60°; supratemporal fossae reaching the minimum interorbital distance, with the dorsal margin of the supratemporal arch being slightly concave in lateral view; thin bony lamina projecting from the lateral and medial alveolar margins of the maxilla; occipital surface ventral to occipital condyle parallel to transverse plane of skull; well-developed crest that extends along the entire height of the supraoccipital; and orbital process of the quadrate very conspicuous and acute, horizontal, and without bony attachment. Phylogenetic analysis recovers P. potens as deeply nested within Geosaurini and, contrary to previous proposals, not closely related to ‘Metriorhynchus’ casamiquelai and ‘M.’ westermanni. Internal relationships within Geosaurini are unresolved. Purranisaurus, as a monotypic genus, is restricted to the late Tithonian–early Berriasian.
A new metriorhynchoid (Crocodylomorpha, Thalattosuchia) from the Middle Jurassic of Oregon and the evolutionary timing of marine adaptations in thalattosuchian crocodylomorphs
Author:
Wilberg
Abstract:
Metriorhynchid thalattosuchians represent the most extreme archosaurian adaptation to the marine realm. Metriorhynchids possess aquatic adaptations throughout the skeleton. These adaptations were so extensive that some have suggested that they lost the ability to move on land, yet their evolutionary timing remains unresolved. The closest relatives of the metriorhynchoids, the teleosauroids, lack these aquatic adaptations, and the earliest metriorhynchoids are known exclusively from cranial material. Here I describe a partial skull with associated forelimb elements of a new marine crocodylomorph, Zoneait nargorum, gen. et sp. nov., of Aalenian–Bajocian age from the Snowshoe Formation of east-central Oregon. Phylogenetic analysis identifies Zoneait as the sister taxon to Metriorhynchidae. It possesses a derived skull with orbits that are more laterally directed and prefrontals that are more expanded than in other basal metriorhynchoids. The preserved forelimb elements are less derived. The humerus is elongate in comparison with that of other metriorhynchoids. The ulna is slightly reduced in length and flattened but resembles the teleosauroid condition more so than the plate-like element of metriorhynchids. This suggests that marine adaptations in metriorhynchoids were acquired in mosaic fashion, with modifications of the skull preceding forelimb reduction, with this forelimb reduction occurring first in the zeugopodial elements, prior to reduction of the humerus. This evolutionary timing has important implications for the transition from nearshore ambush predation to pelagic open-marine predation in Thalattosuchia, suggesting that adaptations related to prey detection and capture preceded the locomotor adaptations that allowed these organisms to fully invade the oceans.
