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Placodus gigas is an unarmored placodont marine reptile (Diapsida, Sauropterygia) known from the Middle Triassic of Europe, most commonly found in the shallow marine carbonate facies of the German Muschelkalk (lower Anisian to middle Ladinian, approximately 243–235 Ma). Generally, the morphology of the skull is well understood, with the exception of the braincase, which is partially obscured by dermatocranial bone. Two skulls that display well-preserved and intact chondrocranial elements were scanned using industrial micro-computed tomography (μCT), thus revealing parts of the braincase that were previously obscured and allowing a new three-dimensional reconstruction of the region to be constructed. This includes a complete osteological description, the first reconstruction of a sauropterygian vestibular apparatus, and a new virtual cranial endocast. The morphology of the braincase and sphenoid region has been revised, revealing the position of the hypophyseal pit. The enigmatic ‘alisphenoid bridge’ has been reinterpreted as being a dorsally extended dorsum sellae. The vestibular apparatus is shown to have strongly dorsoventrally compressed vertical semicircular canals, a commonly observed morphology of other marine reptiles, and an ‘alert’ head position of about 20°, indicating a highly aquatic lifestyle. Because placodonts are the sister group to all other sauropterygians, these new data are of great comparative and phylogenetic significance, providing insight into some of the morphological and functional changes that occurred on the stem leading to the more derived sauropterygians. [ABSTRACT FROM AUTHOR]

Fossil soft-shelled turtles (Trionychidae) have so far been recognized in all continents except Antarctica, based largely on remains preserving their diagnostic sculptured shell bones. The origin of the group is generally assumed to be in the Early Cretaceous of Asia, whereas they first appear in North America and Europe during the Late Cretaceous and Paleocene, respectively. Here we describe the first record of an indeterminate trionychid from the late early Campanian from southern Sweden, a part of the paleobiogeographically isolated Fenno-Scandian Shield, thus extending the stratigraphic record of the group in Europe back about 15 Ma into the Late Cretaceous. Our finding provides evidence against the currently favored dispersal scenario in which trionychid turtles are interpreted to have come to Europe first during the Paleocene either directly from North America or via Asia. The described indeterminate trionychid possibly represents a relic of a pre-Cretaceous endemic radiation of North European trionychids living mainly on the Fenno-Skandian Shield or it may indicate a potential lower latitude dispersal route of trionychids from Asia to North America via Europe during the Late Cretaceous. [ABSTRACT FROM AUTHOR]

Evolutionary changes in lifestyle (e.g., terrestrial vs. aquatic habits) influence tetrapod limb morphology. Similar evolutionary trends in osteogenetic sequences (order of bone ossification) and bone microstructure are often shared in extant tetrapods with similar habit preferences: such data are sometimes available from extinct taxa. The pachypleurosaurids from Monte San Giorgio (Switzerland, Italy) are Triassic marine reptiles with high quality, complete ontogenetic series. We studied osteogenesis and bone histology in the four species from this locality, comparing these with data from Recent terrestrial lizards and secondarily aquatic reptiles in order to determine if the osteogenetic sequences of pachypleurosaurids were similar to either the hypothesized plesiomorphic condition for terrestrial eureptilians or those of Recent aquatic reptiles. Pachypleurosaurian limb osteogenesis occurs in two steps: (1) developmental sequences of ossification during embryology and/or in early neonates, (2a) additional primary periosteal compaction processes, and (2b) additional primary and secondary endosteal compaction processes during neonate ontogeny. Taphonomic patterns reveal information on the order of the initiation and termination of these steps, which are (A) onset of ossification, (B) onset of additional compaction processes (early phase), and (C) termination of additional compaction processes (final phase). An event pairing analysis found that ossification of forelimb elements in pachypleurosaurids precedes that of the hind limb elements in all osteogenetic stages except for initiation of ossification (A). The order of their early phase compaction processes is similar to the hypothesized plesiomorphic eureptilian condition, whereas their final phase compaction processes varies among pachypleurosaurids, with S. mirigiolensis showing minor heterochronic shifts and N. edwardsii showing many heterochronic shifts relative to the hypothesized ancestral condition. Pachypleurosaurids from Monte San Giorgio increase the number of heterochronic shifts with decreasing stratigraphic age, showing a transition from more ‘terrestrial’ to more ‘aquatic’ osteogenetic sequences in comparison to data on ossification sequences of Recent aquatic reptiles. [ABSTRACT FROM PUBLISHER]