Pygostyle

Abstract: 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.

Abstract: Two almost complete skeletons are presented for the theropod dinosaur Deinocheirus mirificus, revealing a humpbacked form with a duckbill-like skull. For almost 50 years, the strange theropod dinosaur Deinocheirus mirificus has been regarded as one of the most intriguing puzzles in palaeontology. Known only from two gigantic forelimbs unearthed in Mongolia in 1965, it had been variously described as an ornithomimid, a therizinosaur or a member of a completely new theropod clade. New excavations have now revealed two almost entire skeletons of the creature, enabling Yuong-Nam Lee et al. to offer a detailed picture of D. mirificus. What emerges is a gigantic creature with a duckbill-like skull and a humped back, the largest known member of the ornithomimosaurs, living in a damp environment, with plants and fish as part of the diet of D. mirificus. The holotype of Deinocheirus mirificus was collected by the 1965 Polish–Mongolian Palaeontological Expedition at Altan Uul III in the southern Gobi of Mongolia1. Because the holotype consists mostly of giant forelimbs (2.4 m in length) with scapulocoracoids2, for almost 50 years Deinocheirus has remained one of the most mysterious dinosaurs. The mosaic of ornithomimosaur and non-ornithomimosaur characters in the holotype has made it difficult to resolve the phylogenetic status of Deinocheirus3,4. Here we describe two new specimens of Deinocheirus that were discovered in the Nemegt Formation of Altan Uul IV in 2006 and Bugiin Tsav in 2009. The Bugiin Tsav specimen (MPC-D 100/127) includes a left forelimb clearly identifiable as Deinocheirus and is 6% longer than the holotype. The Altan Uul IV specimen (MPC-D 100/128) is approximately 74% the size of MPC-D 100/127. Cladistic analysis indicates that Deinocheirus is the largest member of the Ornithomimosauria; however, it has many unique skeletal features unknown in other ornithomimosaurs, indicating that Deinocheirus was a heavily built, non-cursorial animal with an elongate snout, a deep jaw, tall neural spines, a pygostyle, a U-shaped furcula, an expanded pelvis for strong muscle attachments, a relatively short hind limb and broad-tipped pedal unguals. Ecomorphological features in the skull, more than a thousand gastroliths, and stomach contents (fish remains) suggest that Deinocheirus was a megaomnivore that lived in mesic environments.

Abstract: Modern birds have extremely short tail skeletons relative to Archaeopteryx and nonavialian theropod dinosaurs. Long- and short-tailed birds also differ in the conformation of main tail feathers making up the flight surface: frond shaped in Archaeopteryx and fan shaped in extant fliers. Mechanisms of tail fanning were evaluated by electromyography in freely flying pigeons and turkeys and by electrical stimulation of caudal muscles in anesthetized birds. Results from these experiments reveal that the pygostyle, rectrices, rectricial bulbs, and bulbi rectricium musculature form a specialized fanning mechanism. Contrary to previous models, our data support the interpretation that the bulbi rectricium independently controls tail fanning; other muscles are neither capable of nor necessary for significant rectricial abduction. This bulb mechanism permits rapid changes in tail span, thereby allowing the exploitation of a wide range of lift forces. Isolation of the bulbs on the pygostyle effectively decouples tail fanning from fan movement, which is governed by the remaining caudal muscles. The tail of Archaeopteryx, however, differs from this arrangement in several important respects. Archaeopteryx probably had a limited range of lift forces and tight coupling between vertebral and rectricial movement. This would have made the tail of this primitive flier better suited to stabilization than maneuverability. The capacity to significantly alter lift and manipulate the flight surface without distortion may have been two factors favoring tail shortening and pygostyle development during avian evolution.

Abstract: Oviraptorosaur caudal osteology is unique among theropods and is characterized by posteriorly persistent and exceptionally wide transverse processes, anteroposteriorly short centra, and a high degree of flexibility across the pre-pygostyle vertebral series. Three-dimensional digital muscle reconstructions reveal that, while oviraptorosaur tails were reduced in length relative to the tails of other theropods, they were muscularly robust. Despite overall caudal length reduction, the relative size of the M. caudofemoralis in most oviraptorosaurs was comparable with those of other non-avian theropods. The discovery of a second Nomingia specimen with a pygostyle confirms that the fused terminal vertebrae of the type specimen were not an abnormality. New evidence shows that pygostyles were also present in the oviraptorosaurs Citipati and Conchoraptor. Based on the observed osteological morphology and inferred muscle morphology, along with the recognition that many members of the group probably sported broad tai...