Nesbitt et al. 2016 discuss the early evolution of bird-line archosaurs.
From the Nesbitt et al. abstract:
“Bird-line archosaurs (= Avemetatarsalia, the clade containing dinosaurs, pterosaurs, and their kin) (1) had their origin in the Triassic Period. However, that origin is poorly documented (2) as fossils from their early evolutionary history are extremely rare and consist mostly of postcrania. (3) Here, we report the discovery of a new reptile (femoral length = 17 cm) (4) from the lower portion of the Middle Triassic Lifua Member (Manda beds) of the Ruhuhu Basin, southwestern Tanzania. Material referred to the new taxon includes a partial skeleton of a single individual including cervical, trunk, and caudal vertebrae, pectoral, pelvic, forelimb, and hind limb material (= ‘Teleocrater’ of A. Charig), and parts (skull elements, vertebrae, pectoral, pelvic, and limb elements) of a minimum of three individuals collected from a bonebed discovered in 2015 very close to Charig’s original partial skeleton. Character states of the limbs, vertebrae, and ilium indicate a close relationship with early dinosauromorphs including: elongated cervical vertebrae, an ilium with a slightly concave ischial peduncle and clear anterior crest, a weakly developed anterior trochanter of the femur, an anteriorly compressed fibula with long strap-like iliofibularis crest, and absence of osteoderms (5). Many character states suggest that the new reptile taxon falls outside of the pterosaur-dinosaur clade (= Ornithodira). (6) However, the distributions of some of these character states at the base of Archosauria are unclear and some character states of the new taxon suggest a more basal relationship outside Archosauria (e.g., absence of two medial tubera of the proximal femur). No matter the position within or outside Archosauria, the new Lifua taxon shares seemingly unique character states with the poorly known Dongusuchus from the Middle Triassic of Russia (known from femora) and Yarasuchus from the Middle Triassic of India (known from partial skeletons), rather than with other archosauriforms. As a result, these forms appear to represent a globally distributed clade of early diverging avemetatarsalians. (7) The larger body size of the Manda form and its potential phylogenetic position outside of pterosaurs and dinosauromorphs indicates that there was a size decrease at the origin of Ornithodira. (8) This new taxon, and other new discoveries from the Middle to Late Triassic, are elucidating the sequence of character acquisitions in Avemetatarsalia and fill a crucial gap in the evolutionary history that led to the flourishing of dinosaurs later in the Mesozoic”. (9)