While modern consensus and multiple studies support the thesis that birds evolved directly from theropod dinosaurs, historical and alternative minority hypotheses have proposed independent origins from other archosaur groups.
Therizinosaurs were a taxon of unusual theropods from North America and Asia. The derived therizinosaur Nothronychus graffami possessed a synsacrum and hip convergent with extant birds. The osteology is figured and described in detail supplemented with material from Nothronychus mckinleyi. Both species exhibit traits convergent with extant birds, ornithischian dinosaurs, and titanosaurs. As preserved, the ilia and sacrum are minimally distorted and apomorphically modified into a synsacrum. The pubes are modified into a mesopubic condition, wherein the pubes are partially retroverted. The femoral shaft has an oval cross-section. As these characters are absent from the basal therizinosaur Falcarius utahensis, they evolved more than once within the maniraptoran lineage. An herbivorous ecology is supported. Both specimens were skeletally mature, based on fusion of the neural spines and scapulocoracoid.
DURING the period 1926–73 most ornithologists and vertebrate palaeontologists supported Heilmann's theory of avian origins. Heilmann1 argued that all dinosaurs and pterosaurs were too specialised to have been ancestral to birds. Instead he chose to derive birds directly from a primitive group of Triassic archosaurs, the Pseudosuchia. Heilmann's theory has recently been challenged by Walker2–4, who has suggested that birds evolved from an early crocodilian, and by Ostrom5–10, who argued that birds descended from theropod dinosaurs. We recently began a study of the otic region in archosaurs and birds, in search of advanced, homologous features which would give definitive support to one theory or the other. The results of this investigation, discussed below, strongly support a hypothesis of a common pseudosuchian origin for birds and crocodiles, independent of dinosaurs.
The origin of flight in avian dinosaurs has been historically an ideal framework for proposing the evolutionary relationship between form and function in limb proportions under the hypothesis of specialized locomotor modules. However, other hypotheses suggest that the evolution of the forelimb and the hindlimb is strongly influenced by shared developmental constraints, entailing that limbs evolve in an integrated manner and keeping the scientific debate open. To assess this, we used an alternative morphometric approach to compare and statistically analyse limb morphological covariation in a phylogenetically broad context across non-avian maniraptoran theropods and modern birds. Our results show that the maniraptoran lineage shares a strong covariation between limb proportions, a pattern indicating that consistent morphological integration has constrained the forelimb and hindlimb evolutionary transformation. Different evolutionary grades within Maniraptora, both volant and non-volant lineages, display distinct and weaker covariation patterns, suggesting the emergence of independent evolutionary trends within such underlying patterns of integration. These findings are consistent with a developmental hypothesis in which the evolutionary transformation of limbs in maniraptoran dinosaurs was influenced by its serial homology, underscored by shared developmental programmes. Thus, limb evolution was not solely driven by modular (functional) specialization for flight.
Evidence relating to dinosaur origins, dinosaur physiology and bird ancestry is reviewed. The thesis that birds evolved directly from theropod dinosaurs is perfectly acceptable. There is no convincing evidence to support the view that dinosaurs were endotherms, with an avian system of thermo-regulation, and the biological success of dinosaurs is explicable in terms of straightforward anatomical adaptations (particularly the development of an enarthrodial hip joint). There is considerable evidence in favour of dinosaurian polyphyly. The dinosaur order Saurischia is apparently diphyletic - sauropods and prosauropods having descended from proterosuchian thecodonts, and theropods probably having evolved from pseudosuchian thecodonts. The ancestry of the dinosaur order Ornithischia remains a matter for speculation. The tarsal and pelvic structures of pseudosuchian thecodonts are not irreconcilable with those of dinosaurs, and the differences that do exist are not sufficient to debar pseudosuchians from consideration as dinosaur ancestors. The common pattern of limb joint structure in Triassic dinosaurs is not conclusive evidence of dinosaur monophyly, and is probably nothing more than a consequence of parallel evolution across the thecodontian- dinosaurian boundary. A new classification of archosaurs and birds is presented, wherein the theropod ancestors of birds are transferred to the class Aves while all other dinosaurs (sauropods, prosauropods and ornithischians) are retained i
66 mya. Dinosaurs can therefore be divided into avian dinosaurs—birds—and the extinct non-avian dinosaurs, which are all dinosaurs other than birds. Dinosaurs
Dinosaurs are a diverse group of warmblooded reptiles of the clade Dinosauria. They existed through most of the Mesozoic era, first appearing early in the Triassic period. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201.3 mya and their dominance continued throughout the Jurassic and Cretaceous periods. The fossil record shows that birds are feather
Dinosaurs belong to a group known as archosaurs, which also includes modern crocodilians. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodilians sprawl out to either side.
Collectively, dinosaurs as a clade are divided into two primary branches, Saurischia and Ornithischia. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia, while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia. Anatomically, these two groups can be distinguished most noticeably by their pelvic structure. Early saurischians—"lizard-hipped", from the Greek sauros (σαῦρος) meaning "lizard" and ischion (ἰσχίον) meaning "hip joint"—retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward. This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus, therizinosauroids, dromaeosaurids, and birds). Saurischia includes the theropods (exclusively bipedal and with a wide variety of diets) and sauropodomorphs (long-necked herbivores which include advanced, quadrupedal groups).
By contrast, ornithischians—"bird-hipped", from the Greek ornis (ὀρνίς) meaning "bird" and ischion (ἰσχίον) meaning "hip joint"—had a pelvis that superficially resembled a bird's pelvis: the pubic bone was oriented caudally (rear-pointing). Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of species that were primarily herbivores.
Despite the terms "bird hip" (Ornithischia) and "lizard hip" (Saurischia), birds are not part of Ornithischia. Birds instead belong to Saurischia, the "lizard-hipped" dinosaurs—birds evolved from earlier dinosaurs with "lizard hips".
The following cladogram showing the relationships of dinosaur groups is simplified after Michael Benton, 2015:
Abstract Powered flight has required birds to undergo numerous dramatic and coordinated evolutionary responses across the entire body, yet studies are limited to a small number of traits and often exclude a critical component of the vertebrate skeleton – the vertebrae. The neck is a critical region of the avian spine as it operates in tandem with the head as a surrogate forelimb across a diverse array of behaviours. However, the drivers of cervical vertebral evolution remain poorly understood. Here, we model shifts in adaptive optima and evolutionary rates of the neck, forelimb and head of extinct dinosaurs and extant birds to test if these modules co-evolved. We observe a co-occurrence of adaptive optima shifts for neck and forelimb proportion at the base of Avialae – to vertebrae adapted for stability and a forelimb better adapted for flight. These patterns are due to shifts in neck and forelimb allometry and suggest that heterochrony is an important factor in avian neck and forelimb evolution. Further, we find lower rates of both neck and forelimb evolution in birds compared to their non-avian theropod ancestors. The coordinated evolutionary response of the neck and forelimb is a derived feature of Avialae that initially evolved to stabilise in-flight vision. This axio-appendicular co-evolution has contributed to avian macroevolutionary dynamics by facilitating the evolution of a novel locomotory mode without sacrificing the grasping capability needed to directly interact
No area of paleontology has changed more in recent years than the history of birds, both during the Mesozoic Era and the Tertiary Period. The most controversial issue in the study of birds for several decades has been their origin, and the origin of avian flight and feathers, and clearly too much emphasis has been placed on the earliest known bird, the late Jurassic Archaeopteyx , an arboreal form that was already well on its way to becoming a modern bird (Feduccia, 1993a). Over the past two decades this urvögel has mistakenly been characterized as an earth-bound, feathered theropod, and a number of rather bizarre scenarios have been envisioned to account for the origin of flight and feathers in birds. However, the debate has centered on two main themes. Were birds derived directly from small, bipedal theropod dinosaurs (Ostrom, 1991), and therefore evolved via the scenario of the cursorial (ground-up) theory (Ostrom, 1986); or were they derived earlier in time from small, arboreal “thecodonts” or basal archosaurs, and therefore evolved via the scenario of the arboreal (trees-down) theory (Bock, 1986; Martin, 1991; Feduccia and Wild, 1993).
Birds Although the belief that birds evolved from dinosaurs or pterosaurs became pretty popular … new theory not only says that birds evolved from dinosaurs, but also identifies a particular … problem. Fossils tell us that birds evolved directly from theropod dinosaurs. Yet common
infant birds prior to their eventual modification in birds into structures that support flight. Since scientific research began on dinosaurs in the early
A feathered dinosaur is any species of dinosaur possessing feathers, including all species of birds. In recent decades, evidence has accumulated that many non-avian dinosaur species also possessed feathers or feather-like structures in some shape or form. The extent to which feathers or feather-like structures were present in dinosaurs as a whole is a subject of ongoing debate and research.
It has
A feathered dinosaur is any species of dinosaur possessing feathers, including all species of birds. In recent decades, evidence has accumulated that many non-avian…
of bird evolution is represented by many of the extinct reptiles that are called dinosaurs . These animals are not the direct ancestors of the birds , but
Everything we examined (11) — 10 independent sources
This check searched the claim as stated. It did not run a separate search for evidence against it.