Marianella Talevi

Ophthalmosaurian (Ichthyosauria) records from the Aalenian–Bajocian of Patagonia (Argentina): an overview

The oldest ophthalmosaurian records worldwide have been recovered from the Aalenian–Bajocian boundary of the Neuquen Basin in Central-West Argentina (Mendoza and Neuquen provinces). Although scarce, they document a poorly known period in the evolutionary history of parvipelvian ichthyosaurs. In this contribution we present updated information on these fossils, including a phylogenetic analysis, and a redescription of ‘ Stenopterygius grandis ’ Cabrera, 1939. Patagonian ichthyosaur occurrences indicate that during the Bajocian the Neuquen Basin palaeogulf, on the southern margins of the Palaeopacific Ocean, was inhabited by at least three morphologically discrete taxa: the slender Stenopterygius cayi , robust ophthalmosaurian Mollesaurus periallus and another indeterminate ichthyosaurian. Rib bone tissue structure indicates that rib cages of Bajocian ichthyosaurs included forms with dense rib microstructure ( Mollesaurus ) and forms with an ‘osteoporotic-like’ pattern ( Stenopterygius cayi ).

Unexpected skeletal histology of an ichthyosaur from the Middle Jurassic of Patagonia: implications for evolution of bone microstructure among secondary aquatic tetrapods

During the Mesozoic, one of the most significant evolutionary processes was the secondary adaptation of tetrapods to life in water. Several non-related lineages invaded from the terrestrial realms and from the oceans of the entire world. Among these lineages, ichthyosaurs were particularly successful. Advance parvipelvian ichthyosaurs were the first tetrapods to evolve a fish-shaped body profile. The deep skeletal modifications of their bodies, as well as their biology, depict advance ichthyosaurs as the paradigm of secondary adaptation of reptiles to marine life. Functional inferences point to them as off-shore cruising forms, similar to a living tuna, and some of them were capable of deep diving. Bone histology of some genera such as Temnodontosaurus, Stenopterygius, Ichthyosaurus, and Caypullisaurus, characterized by overall cancellous bone, is consistent with the idea of a fish-shaped ichthyosaurs as fast and far cruisers. Here, we provide histological examination of the ribs of the Middle Jurassic parvipelvian Mollesaurus. Contrasting with the bone histology of other parvipelvian, Mollesaurus ribs are characterized by a compact and thick cortex. Our data indicate that the rib cage was heavy and suggest that not all advanced ichthyosaurs were fast cruisers. The compact and dense ribs in these parvipelvian show that advance ichthyosaurs were ecologically more diverse than previously thought and that the lightening of the skeleton reversed, as also occurred in the evolution of cetacean, at least once along the evolutionary history of ichthyosaurs.

Osteology of a perinatal aristonectine (Plesiosauria; Elasmosauridae)

Abstract Perinatal specimens give valuable information about the first stages of vertebrate ontogeny. Here, the morphology and palaeohistology of an aristonectine perinatal specimen from Seymour Island (Isla Marambio), López de Bertodano Formation are analysed. The palaeohistological analysis shows incomplete endochondral ossification (retention of a calcified cartilaginous core in the medullary region), predominance of primary bone tissue without secondary remodelling, lack of primary or secondary osteons and of growth marks in the cortical bone, and open vascular spaces not surrounded by a thin coat of lamellar bone tissue. General lines of morphological changes were inferred from comparing the fossil with an adult aristonectine specimen indicating i) a tendency of relatively high and broad posterior cervicals to decrease during ontogeny, ii) a decrease of relative size of the dorsolateral process and an increase of the glenoid ramus and iii) the existence of two separate stages in propodial growth divided into an initial elongation followed by a distal expansion. The presence of a perinatal specimen in the James Ross Archipelago indicates that the region was used as a breeding area by the aristonectines during the last part of the Cretaceous.

Remodelling of skeletal tissues bone and structural specialisations in an elasmosaurid (Sauropterygia: Plesiosauroidea) from the Upper Cretaceous of Patagonia, Argentina

Elasmosauridae were cosmopolitan Late Cretaceous plesiosaurs with conspicuous morphological diversity. Within this group, vertebral morphology is a criterion for estimating relative age in plesiosaur. On the other hand, the microstructure of plesiosaur bone is considered as indicative of ontogenetic stage. However, knowledge about ontogenetic tissue transformation in different elements of the skeleton is poorly known. Resorption and remodelling of skeletal tissues are required for development and growth, mechanical adaptation, repair and mineral homeostasis of the vertebrate skeleton. This contribution analyses different postcranial elements of a Late Cretaceous elasmosaurid from Patagonia. Characterisation of bone microstructure indicates the presence of compact bone inner organisation in an adult derived plesiosaur from the Cretaceous and that the distribution of bone specialisations depicts conspicuous variations within a single skeleton depending on the skeletal element considered. Bone compactness or degree of remodelling in elasmosaurids is not necessarily correlated with the ontogenetic age of the animal or to costal versus pelagic lifestyles. The available data are still scarce, but we propose a topic of discussion: perhaps the degree of remodelling and compactness also may be related to the activity level and increased mechanical load in different skeletal elements.

Morphology of the sacral region and reproductive strategies of Metriorhynchidae: a counter-inductive approach

Morphological and physiological features indicate Metriorhynchidae as the only group of crocodylomorphs with a pelagic lifestyle. Some of these features have evolved convergently in several clades of tetrapods secondarily adapted to aquatic life. One striking feature of metriorhynchids as compared to other crocodylomorphs is the morphology of the pelvic region (i.e., ventrally deflected sacral ribs and reduced pelvic girdle), which increases significantly the depth of this region. This morphology, as a whole, resembles that of other viviparous Mesozoic marine reptiles not phylogenetically related to metriorhynchids. We tested two alternative hypotheses of reproductive strategies in this clade: oviparity vs . viviparity. Given the lack of direct evidence supporting one or the other, we explored the use of evidence that may disconfirm either of these hypotheses. Using this counter-inductive approach, we found no cases contradicting viviparity in metriorhynchids, except for their phylogenetic position as archosaurs. A survey of reproductive modes amongst amniotes depicts the evolutionary plasticity of the transition to viviparity, and a widespread occurrence among tetrapods secondarily adapted to a marine life. Assuming oviparity for metriorhynchids implies egg-laying out of the water. However, their postcranial morphology (i.e., features of fore and hind limbs, pelvic girdle, and tail) contradicts this possibility. In this context, we rejected oviparity for metriorhynchids.