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Abstract
Sauropterygia was the most successful marine reptile radiation in history, spanning almost the entire Mesozoic and exploiting a wide range of habitats and ecological niches. Here we report a new, exceptionally preserved skull of a juvenile stem placodont from the early Middle Triassic of the Netherlands, thus indicating a western Tethyan (European) origin for Placodontia, the most basal group of sauropterygians. A single row of teeth on an enlarged palatine supports this close relationship, although these are small and pointed instead of broad and flat, as is the case in placodonts, which demonstrate the strongest adaptation to a durophagous diet known in any reptile. Peg-like, slightly procumbent premaxillary teeth and an 'L-shaped' jugal also confirm a close relationship to basal placodonts. The new taxon provides insight into the evolution of placodont dentition, representing a transitional morphology between the plesiomorphic diapsid condition of palatal denticles and the specialized crushing teeth of placodonts.
The interplay of evolution and development has been at the heart of evolutionary theory for more than a century. Heterochrony—change in the timing or rate of developmental events—has been implicated in the evolution of major vertebrate lineages such as mammals, including humans. Birds are the most speciose land vertebrates, with more than 10,000 living species representing a bewildering array of ecologies. Their anatomy is radically different from that of other vertebrates. The unique bird skull houses two highly specialized systems: the sophisticated visual and neuromuscular coordination system allows flight coordination and exploitation of diverse visual landscapes, and the astonishing variations of the beak enable a wide range of avian lifestyles. Here we use a geometric morphometric approach integrating developmental, neontological and palaeontological data to show that the heterochronic process of paedomorphosis, by which descendants resemble the juveniles of their ancestors, is responsible for several major evolutionary transitions in the origin of birds. We analysed the variability of a series of landmarks on all known theropod dinosaur skull ontogenies as well as outgroups and birds. The first dimension of variability captured ontogeny, indicating a conserved ontogenetic trajectory. The second dimension accounted for phylogenetic change towards more bird-like dinosaurs. Basally branching eumaniraptorans and avialans clustered with embryos of other archosaurs, indicating paedomorphosis. Our results reveal at least four paedomorphic episodes in the history of birds combined with localized peramorphosis (development beyond the adult state of ancestors) in the beak. Paedomorphic enlargement of the eyes and associated brain regions parallels the enlargement of the nasal cavity and olfactory brain in mammals. This study can be a model for investigations of heterochrony in evolutionary transitions, illuminating the origin of adaptive features and inspiring studies of developmental mechanisms.
The fossil record is our only direct means for evaluating shifts in biodiversity through Earth's history. However, analyses of fossil marine invertebrates have demonstrated that geological megabiases profoundly influence fossil preservation and discovery, obscuring true diversity signals. Comparable studies of vertebrate palaeodiversity patterns remain in their infancy. A new species-level dataset of Mesozoic marine tetrapod occurrences was compared with a proxy for temporal variation in the volume and facies diversity of fossiliferous rock (number of marine fossiliferous formations: FMF). A strong correlation between taxic diversity and FMF is present during the Cretaceous. Weak or no correlation of Jurassic data suggests a qualitatively different sampling regime resulting from five apparent peaks in Triassic-Jurassic diversity. These correspond to a small number of European formations that have been the subject of intensive collecting, and represent 'Lagerstätten effects'. Consideration of sampling biases allows re-evaluation of proposed mass extinction events. Marine tetrapod diversity declined during the Carnian or Norian. However, the proposed end-Triassic extinction event cannot be recognized with confidence. Some evidence supports an extinction event near the Jurassic/Cretaceous boundary, but the proposed end-Cenomanian extinction is probably an artefact of poor sampling. Marine tetrapod diversity underwent a long-term decline prior to the Cretaceous-Palaeogene extinction.
The largest and most diverse collection of Pachypleurosauridae (Nothosauria, Reptilia) comes from Monte San Giorgio, Switzerland. Several hundred complete skeletons were collected from four distinct horizons of bituminous limestones and shales of Anisian-Ladinian boundary to early Ladinian age (Middle Triassic). Serpianosaurus mirigiolensis comes from the oldest strata, the Grenzbitumenzone Beds. The three younger strata, all in the Lower Meride Limestone, yield three species of Neusticosaurus. Neusticosaurus pusillus comes from the Cava Inferiore horizon, Neusticosaurus peyeri, new species, from the Cava Superiore horizon, and Neusticosaurus edwardsii, new combination, from the Alla Cascina horizon. Neusticosaurus pusillus is biostratigraphically important because it is one of the rare species reported from both the Germanic and the Alpine Triassic. Neusticosaurus pusillus and N. peyeri are small and very similar in their anatomy. Neusticosaurus species are easiest separated by their number of presacral vertebrae. Ornamentation of the bone surface is distinctive for all four pachypleurosaurids. Soft parts are rarely preserved, except for one partial squamation. The biological age of Neusticosaurus individuals can be determined by skeletochronology (aging by bone annuli). Small species of Neusticosaurus were sexually mature after three to four years and lived for six to nine years. Taphonomic analysis of the small species indicates attritional mortality and suggests weak bottom currents in the Monte San Giorgio basin during early Ladinian times. Morphometric comparison of all four pachypleurosaurids indicates that the changing vertebral numbers between species are largely due to a change in number of segments. All Monte San Giorgio pachypleurosaurids are sexually dimorphic in forelimb development. Sex x has poorly differentiated and relatively short humeri whereas sex y has well differentiated and relatively long humeri. The sexes are of about the same size and represented in roughly equal numbers. Identification of gender was not possible. Good growth series, especially of Neusticosaurus peyeri, from embryo to large adult permitted qualitative and quantitative study of ontogeny. The skull grows with negative allometry; the humerus grows isometrically or with positive allometry, depending on sex and species; the femur grows isometrically. The adult size range in N. peyeri is considerably larger than in modern reptiles. The Monte San Giorgio pachypleurosaurids are a monophyletic group. The phylogeny of this group is congruent with the stratigraphic distribution of its members.
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