the genus Macrocnemus is a member of the Tanystropheidae, a clade of non-archosauriform archosauromorphs well known for their very characteristic, elongated cervical vertebrae. Articulated specimens are known from the Middle Triassic of Alpine Europe and China. Although multiple articulated specimens are known, description of the cranial morphology has proven challenging due to the crushed preservation of the specimens. Here we use synchrotron micro computed tomography to analyse the cranial morphology of a specimen of the type species Macrocnemus bassanii from the Besano Formation of Monte San Giorgio, Ticino, Switzerland. The skull is virtually complete and we identify and describe the braincase and palatal elements as well the atlas-axis complex for the first time. Moreover, we add to the knowledge of the morphology of the skull roof, rostrum and hemimandible, and reconstruct the cranium of M. bassanii in 3D using the rendered models of the elements. The circumorbital bones were found to be similar in morphology to those of the archosauromorphs Prolacerta broomi and Protorosaurus speneri. In addition, we confirm the palatine, vomer and pterygoid to be tooth-bearing palatal bones, but also observed heterodonty on the pterygoid and the palatine. The genus Macrocnemus was described in the early twentieth century by Baron von Nopcsa based on a fragmentary specimen from Monte San Giorgio 1,2. After subsequent expeditions to the Besano Formation, which crops out in the Monte San Giorgio area, resulted in the discovery of additional specimens, the taxon was more properly described 3. Unfortunately, the holotype, stored in Milan, has since been lost as it was destroyed during the Second World War 4. The phylogenetic affinities of Macrocnemus were initially unclear. Early researchers observed similarities in the shape of the cervical vertebrae with contemporaneous taxa and erected the taxon Protorosauria 5,6. Notable members of this formerly recognised 'superorder' were Tanystropheus, Protorosaurus, Macrocnemus and Prolacerta, and all these taxa have elongated cervical vertebrae. The elongation of the cervical vertebrae is expressed most extremely in the genus Tanystropheus, of which the neck is three times the length of its trunk 7. Researchers were unsure whether to place the clade closer to the Lepidosauria or to the Archosauromorpha (e.g. 7,8). Currently, the members of the Protorosauria are all considered non-archosauriform archosauromorphs (e.g. 9-11). However, the clade itself has been observed to be paraphyletic or even polyphyletic in recent phylogenetic analyses 9 and references therein. Nonetheless, the recent analyses noted above have all recovered a monophyletic Tanystropheidae at the base of Archosauromorpha that includes Macrocnemus as well as Tanystropheus, Amotosaurus, Langobardisaurus, and Tanytrachelos, with Jesairosaurus often recovered as sister taxon to the clade.

The availability of a large sample size from a range of ontogenetic stages makes Stenopterygius quadriscissus a good model to study ontogenetic variation in a fossil sauropsid. We qualitatively examined pre- and postnatal ontogenetic changes in the cranium of S. quadriscissus. The prenatal ossification sequence is similar to other diapsids, exhibiting delayed chondrocranial ossification compared to the dermatocranium. In the dermatocranium, the circumorbital area is more ossified earlier in development relative to other elements, especially those of the skull roof where ossification is comparatively weaker across prenatal stages. Perinatally all cranial elements are ossified, and many scarf and step joints are already closed. We propose four prenatal and three postnatal stages in S. quadriscissus on the basis of relative ossification, size and qualitative cranial characters pertaining to the jugal, parietal, frontal, pterygoid and surangular. These will provide a basis for determining ontogenetic stages in other ichthyosaurs. Moreover, our postnatal observations aid in refining ontogenetic characters for phylogenetic studies. Lastly, we observed that the antimeric sutures of the midline of the skull roof are open perinatally and that fusion of the midline only appears in the adult stage. We hypothesize that the loose connection of the midline functions as a fontanelle, limiting potential damage during birth.

The fossil record of the ichthyosaur genus Stenopterygius comprises a large number of specimens of all ontogenetic stages. This makes the taxon a good model for ontogenetic studies in a Mesozoic marine reptile. Here, we describe the morphology of the braincase elements of Stenopterygius over ontogeny and compare it with that of other known ichthyosaur braincases. Substantial ontogenetic changes were observed in most elements. These included negative allometry of the exoccipitals relative to the basioccipital, changes in the morphology of the parabasisphenoid and proportional reduction of the ridge separating the right and left internal carotid arteries, variable relative length of the paroccipital process of the opisthotic in juvenile specimens, and relative ossification of the basioccipital peg. Furthermore, we observed variability in relative braincase ossification between embryos within a single gravid female.