Modern squamates (lizards, snakes and amphisbaenians) are the world's most diverse group of tetrapods along with birds and have a long evolutionary history, with the oldest known fossils dating from the Middle Jurassic period-168 million years ago. The evolutionary origin of squamates is contentious because of several issues: (1) a fossil gap of approximately 70 million years exists between the oldest known fossils and their estimated origin; (2) limited sampling of squamates in reptile phylogenies; and (3) conflicts between morphological and molecular hypotheses regarding the origin of crown squamates. Here we shed light on these problems by using high-resolution microfocus X-ray computed tomography data from the articulated fossil reptile Megachirella wachtleri (Middle Triassic period, Italian Alps ). We also present a phylogenetic dataset, combining fossils and extant taxa, and morphological and molecular data. We analysed this dataset under different optimality criteria to assess diapsid reptile relationships and the origins of squamates. Our results re-shape the diapsid phylogeny and present evidence that M. wachtleri is the oldest known stem squamate. Megachirella is 75 million years older than the previously known oldest squamate fossils, partially filling the fossil gap in the origin of lizards, and indicates a more gradual acquisition of squamatan features in diapsid evolution than previously thought. For the first time, to our knowledge, morphological and molecular data are in agreement regarding early squamate evolution, with geckoes-and not iguanians-as the earliest crown clade squamates. Divergence time estimates using relaxed combined morphological and molecular clocks show that lepidosaurs and most other diapsids originated before the Permian/Triassic extinction event, indicating that the Triassic was a period of radiation, not origin, for several diapsid lineages.

It is widely accepted that ornithodirans (bird lineage) and some pseudosuchians (crocodilian lineage) achieved fully erect limb posture in different ways. Ornithodirans have buttress‐erected hindlimbs, while some advanced pseudosuchians have pillar‐erected hindlimbs. Analysis of the musculoskeletal apparatus of the early dinosauriform Silesaurus opolensis challenges this view. This ornithodiran had pillar‐erected hindlimbs like some pseudosuchians. This condition could be autapomorphic or represents a transitional state between adductor‐controlled limb posture of early dinosauromorphs and the buttress‐erected hindlimbs of dinosaurs. This sequence of changes is supported by Triassic tracks left by animals of the dinosaurian lineage. It was associated with the strong development of knee flexors and extensors. Furthermore, the forelimbs of Silesaurus were fully erect, analogously to those of early sauropods. Members of both lineages reduced the muscles related to the protraction, retraction and bending of the limb. They used forelimbs more as a body support and less for propulsion. A similar scapula and humerus construction can be found in the Lagerpetidae and Lewisuchus, suggesting that long, slender, fully erected forelimbs are primitive for all Dinosauromorpha, not just Silesauridae. Early dinosaurs redeveloped several muscle attachments on the forelimb, probably in relation to bipedality.

A principal component analysis (PCA) performed for a set of 24 measurements on 33 femora and 15 measurements on 20 ilia of Silesaurus opolensis from the early Late Triassic of Krasiej ow, southern Poland, shows that this sample is highly variable but probably monospecific. Most of the morphological variation is concentrated in the muscle attachments and proportions of bones, which significantly change in both size and position during ontogeny. Despite the small sample size, femora of smaller individuals have less flattened shafts and a more sinusoidal appearance. In many large specimens, proximal parts of muscle tendons are ossified at their attachment site on femora and remain attached to the bone in the largest specimens. The specimens with attached ossifications are interpreted as mature females that were statistically larger than proposed males. It is suggested that ossifications developed in females under calcitonin control. The intrapopulation variability of ilia is high, but less dependant on ontogeny. The population represented by a few specimens from the lower level at Krasiej ow may represent a different stage in the evolution of the species than that from the upper horizon.

Since 1990, several localities within the Keuper (upper Middle to Upper Triassic) strata in southern Poland have yielded remains of numerous terrestrial vertebrate species. Here we report a new Upper Triassic vertebrate assemblage from the rediscovered Kocury locality. An incomplete theropod dinosaur fibula named Velocipes guerichi described in 1932 was found there.The site was then forgotten and not explored until our excavations initiated in 2012, that yielded material of a lungfish, a proterochersid turtle, and a new typothoracin aetosaur Kocurypelta silvestris gen. et sp. nov. The new taxon is characterized by autapomorphies of the maxilla: an elongated edentulous posterior portion longer than 80% of the posterior maxillary process, a short medial shelf restricted to the posterior portion of the bone, an anteriorly unroofed maxillary accessory cavity, and lack of a distinct groove for choanal recess on the anteromedial surface of the bone. These new finds improve our knowledge on the vertebrate diversity of the Germanic Basin in the Late Triassic, evidencing the presence of yet unrecognized taxa. Additionally, the partial cranial aetosaur material emphasizes the issues with the aetosaurian taxonomy that is focused mostly on the osteoderm morphology.

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