Birdlike growth and mixed-age flocks in avimimids (Theropoda, Oviraptorosauria)

Avimimids were unusual, birdlike oviraptorosaurs from the Late Cretaceous of Asia. Initially enigmatic, new information has ameliorated the understanding of their anatomy, phylogenetic position, and behaviour. A monodominant bonebed from the Nemegt Formation of Mongolia showed that some avimimids were gregarious, but the site is unusual in the apparent absence of juveniles. Here, a second monodominant avimimid bonebed is described from the Iren Dabasu Formation of northern China. Elements recovered include numerous vertebrae and portions of the forelimbs and hindlimbs, representing a minimum of six individuals. Histological sampling of two tibiotarsi from the bonebed reveals rapid growth early in ontogeny followed by unexpectedly early onset of fusion and limited subsequent growth. This indicates that avimimids grew rapidly to adult size, like most extant birds but contrasting other small theropod dinosaurs. The combination of adults and juveniles in the Iren Dabasu bonebed assemblage provides evidence of mixed-age flocking in avimimids and the onset of fusion in young individuals suggests that some of the individuals in the Nemegt Formation bonebed may have been juveniles. Regardless, these individuals were likely functionally analogous to adults, and this probably facilitated mixed-age flocking by reducing ontogenetic niche shift in avimimids.

Core-and-belt organisation of the mesencephalic and forebrain auditory centres in turtles: expression of calcium-binding proteins and metabolic activity

The distribution of immunoreactivity to the calcium-binding proteins parvalbumin, calbindin and calretinin and of cytochrome oxidase activity was studied in the mesencephalic (torus semicircularis), thalamic (nucleus reuniens) and telencephalic (ventromedial part of the anterior dorsal ventricular ridge) auditory centres of two chelonian species Emys orbicularis and Testudo horsfieldi. In the torus semicircularis, the central nucleus (core) showed intense parvalbumin immunoreactivity and high cytochrome oxidase activity, whereas the laminar nucleus (belt) showed low cytochrome oxidase activity and dense calbindin/calretinin immunoreactivity. Within the central nucleus, the central and peripheral areas could be distinguished by a higher density of parvalbumin immunoreactivity and cytochrome oxidase activity in the core than in the peripheral area. In the nucleus reuniens, the dorsal and ventromedial (core) regions showed high cytochrome oxidase activity and immunoreactivity to all three calcium-binding proteins, while its ventrolateral part (belt) was weakly immunoreactive and showed lower cytochrome oxidase activity. In the telencephalic auditory centre, on the other hand, no particular region differed in either immunoreactivity or cytochrome oxidase activity. Our findings provide additional arguments in favour of the hypothesis of a core-and-belt organisation of the auditory sensory centres in non-mammalian amniotes though this organisation is less evident in higher order centres. The data are discussed in terms of the evolution of the auditory system in amniotes.

Early evolution of avian flight and perching: new evidence from the lower cretaceous of china

Fossil bird skeletons discovered in Lower Cretaceous lake deposits in China shed new light on the early evolution of avian flight and perching. The 135 million-year-old sparrow-sized skeletons represent a new avian, Sinornis santensis, n. gen. n. sp., that preserves striking primitive features such as a flexible manus with unguals, a footed pubis, and stomach ribs (gastralia). In contrast to Archaeoperyx, however, Sinornis exhibits advanced features such as a broad sternum, wing-folding mechanism, pygostyle, and large fully reversed hallux. Modern avian flight function and perching capability, therefore, must have evolved in small-bodied birds in inland habitats not long after Archaeopteryx.

Evolutionary origin of the feather epidermis

The formation of scales and feathers in reptiles and birds has fascinated biologists for decades. How might the developmental processes involved in the evolution of the amniote ectoderm be interpreted to shed light on the evolution of integumental appendages? An Evo-Devo approach to this question is proving essential to understand the observation that there is homology between the transient embryonic layers covering the scale epidermis of alligators and birds and the epidermal cell populations of embryonic feather filaments. Whereas the embryonic layers of scutate scales are sloughed off at hatching, that their homologues persist in feathers demonstrates that the predecessors of birds took advantage of the ability of their ectoderm to generate embryonic layers by recruiting them to make the epidermis of the embryonic feather filament. Furthermore, observations on mutant chickens with altered scale and feather development (Abbott and Asmundson [1957] J. Hered. 18:63-70; Abbott [1965] Poult. Sci. 44:1347; Abbott [1967] Methods in developmental biology. New York: Thomas Y. Crowell) suggest that the ectodermal placodes of feathers, which direct the formation of unique dermal condensations and subsequently appendage outgrowth, provided the mechanism by which the developmental processes generating the embryonic layers diverged during evolution to support the morphogenesis of the epidermis of the primitive feather filament with its barb ridges.

Cranial anatomy and relationships of a new Triassic bird from Texas

The oldest known fossil bird from the Late Triassic (about 225 million years ago) Dockum Formation of Texas, provides insights into the anatomy, evolution and phylogenetic relationships of early birds. In life, this adult bird was about the size of a pheasant ( Phasianus ) counting its long bony tail. Many characters of the skull show that the Texas species is more closely related to other birds than to any known group of archosaurs. The skull is lightly built, pneumatized, with an enormous orbit and expanded temporal region. The teeth are restricted to the tip of the jaws, the posterior teeth having been lost. The temporal region is modified from the diapsid condition, as in modern birds, where the orbit is confluent with the upper and lower temporal openings because of the breakdown of the temporal arcades. The relatively large brain size and the modification of the brain architecture in avian fashion show neurosensory specializations that may be associated with balance, coordination, flight, agility and high metabolic activity. The new species had binocular vision, which suggests that it was a visually oriented predator. Auditory acuity may be associated with vocal behaviour. The quadrate was streptostylic and the whole upper jaw was moved prokinetically as in modern birds. Among current hypotheses for the relationships of birds among archosaurs, both theropod and crocodilian hypotheses have been supported by shared apomorphies. Some of the avian features in the crocodilian skull may have been acquired convergently because of homoplasy. Conversely, the highly akinetic skull and monimostylic quadrate along with primitive brain architecture in early crocodylomorphs negate its close phyletic relationships with birds. Within archosaurs, the theropods are closest to birds, but just what taxon is the sister group among theropods is uncertain at this time. Numerical cladistic analysis of 30 cranial characters generated a hypothesis of the phylogenetic pattern of early avian evolution. By using theropods and sphenosuchids as comparative outgroups and root for the tree, the analysis confirms the monophyly of the class Aves. Archaeopteryx is the most primitive taxon and is sister group to all other birds. Archaeoptryx, Avimimus and the Texas bird are suc cessively closer to the remaining avian taxa or Ornithurae. Hesprom, Ichthyornis and Gobipteryx are the Cretaceous representatives of the Ornithurae. The Triassic bird extends the known avian record back at least 75 million years and documents an early stage in the evolution of modern birds. The avian skull evolved in response to two functional requirem ents: efficient feeding mechanism, leading to the development of cranial kinesis, and neurosensory specializations leading to the enlargement of the braincase and orbit. Cranial kinesis, braincase inflation and otic specialization greatly modified the architecture of early avian skulls from the theropod condition. Phylogenetic analysis suggests that theropods shared a common ancestry with birds, but it is indeterminate from the fossil record whether or not the immediate common ancestor itself was a theropod.