Introducing Wulong bohaiensis, the dancing dragon

Wulong bohaiensis. From Poust et al., 2020

Birds are the most species-rich class of tetrapod vertebrates. They originated from a theropod lineage more than 160 million years ago. The evolutionary history of Birds is at the root of the paravian radiation, when dromaeosaurids, troodontids, and avialans were diverging from one another. Within the clade Paraves we found the morphology and soft tissue changes associated with the origin of modern avian flight. One of this key changes was the difference of nearly four orders of magnitude in body size, a pivotal element in the origin of powered avian flight. In recent years, several discovered fossils of theropods and early birds have filled the morphological, functional, and temporal gaps along the line to modern birds. Most of these fossils are from the Jehol Biota of northeastern China, dated between approximately 130.7 and 120 million years ago.
The Jehol Biota included two formations: the Yixian Formation, and the Jiufotang Formation, and contain the most diversified avifauna known to date. Among them are the long bony-tailed Jeholornis, only slightly more derived than Archaeopteryx, and many fossils of troodontids like Mei long, Sinovenator changii, Sinusonasus magnodens and Jinfengopteryx elegans. Now, the recently described Wulong bohaiensis, from the Jiufotang Formation, shed new light on the evolution of Birds. This small, feathered dromaeosaurid theropod lived in the Early Cretaceous (Aptian) and was discovered by a farmer more than a decade ago. The holotype (D2933) is a complete articulated skeleton (only some ribs are missing)and exhibits special preservation of keratinous structures.

An X-ray of Wulong showing wrist and vertebra detail on the right. (Poust et al., 2020)

Wulong (meaning “dancing dragon”) is distinguished by the following autapomorphic features: long jugal process of quadratojugal, cranially inclined pneumatic foramina on the cranial half of dorsal centra, transverse processes of proximal caudals significantly longer than width of centrum, presence of 30 caudal vertebrae producing a proportionally long tail, distally located and large posterior process of the ischium, and large size of supracoracoid fenestra (>15% of total area). The holotype has several gross osteological markers of immaturity like the unfused dorsal and sacral vertebrae, but mature feathers are present across the entire body of Wulong.

The feathered dinosaurs from the Jehol Biota are key to understand the origin of birds and dinosaur behavior. In modern birds development of ornamental feathers is generally timed to co-occur with sexual maturity. The presence of such elaborate feathers in the immature Wulong demonstrates that nonavian dinosaurs had a very different strategy of plumage development then their living relatives.

References:

Poust, AW; Gao, C; Varricchio, DJ; Wu, J; Zhang, F (2020). “A new microraptorine theropod from the Jehol Biota and growth in early dromaeosaurids”. The Anatomical Record. American Association for Anatomy. doi:10.1002/ar.24343

Junornis houi and the evolution of flight

Holotype of Junornis houi. (From Liu et. al; 2017)

Birds originated from a theropod lineage more than 150 million years ago. By the Early Cretaceous, they diversified, evolving into a number of groups of varying anatomy and ecology. The Enantiornithes are the most successful clade of Mesozoic birds. In the last decades, exceptionally well preserved avian fossils han been recovered from China. The most recent, Junornis houi, from the Yixian Formation of eastern Inner Mongolia, represents a new addition to the enantiornithine diversity of the Jehol Biota

The holotype (BMNHC-PH 919; Beijing Museum of Natural History), from the Early Cretaceous (~ 126±4 mya) of Yixian Formation,  is a nearly complete and articulated skeleton contained in two slabs, and surrounded by feather impressions defining the surface of its wings and body outline. The name Jun is derived from a Chinese character meaning beautiful; and ornis is Greek for bird. The species name, houi honors Dr. Hou Lianhai.

Photograph and interpretative drawing of the forelimb of Junornis houi (From Liu et. al; 2017)

Junornis exhibits the following combination of characters: rounded craniolateral corner of sternum; distinct trough excavating ventral surface of mediocranial portion of sternum; triangular process at base of sternal lateral trabecula; sternal lateral trabecula broad and laterally deflected; sternal intermediate trabecula nearly level with mid-shaft of lateral trabecula; sternal xiphoid process level with lateral trabeculae; costal processes of last two penultimate synsacral vertebrae three times wider than same process of last synsacral vertebra; and very broad pelvis. Non-pennaceous, contour feathers cover much of the skeleton except the wings and feet.

Based on the well-preserved skeleton and exquisite plumage of Junornis, it was possible  make some estimation of its flight capacity. The body and wings of this bird were similar to those of modern passeriforms such as Alauda arvensis and to other small-sized birds that fly using intermittent bounds. The low aspect ratio (AR = 5.5) wings of BMNHC-PH 919 suggest that it may have been adapted to rapid take-offs, given that modern birds with proportionally short, broad wings tend to maximize thrust during slow flight. The low wing loading (WL = 0.18 g/cm2) of this fossil indicates that this bird would have been able to generate a large magnitude of lift at low speeds because for a given speed and angle of attack, birds with greater wing area (and therefore lower WL) generate more lift than those with small wing areas. This value also suggest that this bird would have been highly maneuverable and able to perform tight turns.

References:

Liu D, Chiappe LM, Serrano F, Habib M, Zhang Y, Meng Q (2017) Flight aerodynamics in enantiornithines: Information from a new Chinese Early Cretaceous bird. PLoS ONE12(10): e0184637. https://doi.org/10.1371/journal.pone.0184637

Jianianhualong and the evolution of feathers.

Jianianhualong tengi holotype (From Xu, X. et al., 2017)

In recent years, several discovered fossils of theropods and early birds have filled the morphological, functional, and temporal gaps along the line to modern birds. Most of these fossils are from the Jehol Biota of northeastern China, dated between approximately 130.7 and 120 million years ago. Among them are many fossils of troodontids, which are considered as the closest relatives of birds. Previous reported troodontid species include Mei long, Sinovenator changii, Sinusonasus magnodens and Jinfengopteryx elegans. Now a new troodontid, Jianianhualong tengi gen. et sp. nov., has anatomical features that shed light on troodontid character evolution.

The holotype (DLXH 1218) is a nearly complete skeleton with associated feathers, and is inferred to be an adult. It is estimated to be 112 cm in total skeletal body length with a fully reconstructed tail, and its body mass is estimated to be 2.4 kg, similar to most other Jehol troodontids, such as Sinovenator. The skull and mandible are in general well preserved, and  has a relatively short snout and highly expanded skull roof. There are probably 21 maxillary teeth and 25 dentary teeth on each side of the jaw. The vertebral column is nearly completely represented and  the tail is 54 cm long. The furcula is poorly preserved, and the humerus is 70% of femoral length. The manus is typical of maniraptoran theropods, and measures 112 mm in length. The pelvis is in general similar to those of basal troodontids, with a proportionally small ilium, a posteroventrally oriented pubis, and a short ischium. A phylogenetic analysis places Jianianhualong in an intermediate position together with several species between the basalmost and derived troodontids.

Plumage of Jianianhualong tengi (Adapted from Xu, X.  et al, 2017)

The tail frond of Jianianhualong preserves an asymmetrical feather, the first example of feather asymmetry in troodontids. Feathers were once considered to be unique avialan structures. Since the discovery of the feathered Sinosauropteryx in 1996, numerous specimens of most theropod groups and even three ornithischian groups preserving feathers have been recovered from the Jurassic and Cretaceous beds of China, Russia, Germany, and Canada. These feathers fall into several major morphotypes, ranging from monofilamentous feathers to highly complex flight feathers.

Evidence indicates that the earliest feathers evolved in non-flying dinosaurs for display or thermoregulation, and later were co-opted into flight structures with the evolution of asymmetrical pennaceous feathers in Paraves, therefore, the discovery of tail feathers with asymmetrical vanes in a troodontid theropod indicates that feather asymmetry was ancestral to Paraves.

 

 

References:

Xu, X. et al. Mosaic evolution in an asymmetrically feathered troodontid dinosaur with transitional features. Nat. Commun. 8, 14972 doi: 10.1038/ncomms14972 (2017).

Xu, X. et al. An integrative approach to understanding bird origins, Science, Vol. 346 no. 6215 (2014). DOI: 10.1126/science.1253293

Introducing Zhongjianosaurus.

 

Photograph of Zhongjianosaurus yangi holotype (From Xu & Qin, 2017).

Dromaeosaurids are a group of carnivorous theropods, popularly known as “raptors”. Most of them were small animals, ranging from about 0.7 metres in length to over 7 metres. They had a relatively large skull with a narrow snout and the forward-facing eyes typical of a predator. They also had serrated teeth, and their arms were long with large hands, a semi-lunate carpal, with three long fingers that ended in big claws. The earliest known representatives are from the Lower Cretaceous Jehol Group of western Liaoning, China. The most recent described dromaeosaurid is Zhongjianosaurus yangi. The new taxon was named in honor of  Yang Zhongjian, who is the founder of vertebrate paleontology in China.

The Early Cretaceous Jehol dromaeosaurids not only display a great size disparity, but also show a continuous size spectrum. Zhongjianosaurus represents the ninth dromaeosaurid species reported from the Jehol Biota. It was first reported in 2009, and is notable for its small size (about 25 cm tall), compact body, and extremely long legs.

Zhongjianosaurus yangi holotype. A. right scapulocoracoid in lateral view and furcula in posterior view; B. right humerus in anterior view; C. left ulna and radius in lateral view; D. ‘semilunate’ carpal, metacarpals II and III in ventral view and phalanges II-1 and -2 in lateral view; scale bars equal 5 mm (From Xu & Qin, 2017)

The holotype is an adult individual distinguishable from other microraptorines in possessing many unique features, most of them are present in the forelimbs. For example, the humerus has a strongly offset humeral head, a large fenestra near the proximal end, and a large ball-like ulnar condyle. Zhongjianosaurus also displays several other features which are absent in other Jehol dromaeosaurids. For instance, the uncinate processes are proportionally long and fused to the dorsal ribs, the caudal vertebral transitional point is located anteriorly, and the pes exhibits a full arctometatarsalian condition.

The coexistence of several closely related Jehol dromaeosaurids can be interpreted as niche differentiation. Tianyuraptor have limb proportions and dental morphologies typical of non-avialan carnivorous theropods, suggesting that they were ground-living cursorial predators, meanwhile Microraptor are more likely to have been arboreal or even gliding animals.

References:

Xu X , Qin Z C, 2017, in press. A new tiny dromaeosaurid dinosaur from the Lower Cretaceous Jehol Group of western Liaoning and niche differentiation among the Jehol dromaeosaurids. Vertebrata PalAsiatica

Xu X, 2002. Deinonychosaurian fossils from the Jehol Group of western Liaoning and the coelurosaurian evolution. Ph.D thesis, Beijing: Chinese Academy of Sciences. 1–322

The First 100 Million Years of Avian History.

The basal avian Sapeornis chaoyangensis (From Wikimedia Commons)

The basal avian Sapeornis chaoyangensis (From Wikimedia Commons)

Birds originated from a theropod lineage more than 150 million years ago. By the Early Cretaceous, they diversified, evolving into a number of groups of varying anatomy and ecology. In recent years, several discovered fossils of theropods and early birds have filled the morphological, functional, and temporal gaps along the line to modern birds. Most of these fossils are from the Jehol Biota of northeastern China, dated between approximately 130.7 and 120 million years ago. The Jehol Biota is formed from two formations: the Yixian Formation, and the Jiufotang Formation, and contain the most diversified avifauna known to date. Among them was the long bony-tailed Jeholornis, only slightly more derived than Archaeopteryx, that lived with Sapeornis, Confuciusornis, and the earliest members of Enantiornithes and Ornithuromorpha. The last two groups, form the clade Ornithothoraces, characterized by a keeled sternum, elongate coracoid, narrow furcula, and reduced hand.

Ornithuromorphs, include Gansus, Patagopteryx, Yixianornis, and Apsaravis, which form a grade on the line to Ornithurae, a derived subgroup that includes modern birds and their closest fossil relatives (Brusatte et al., 2015).

The single best record of a Cretaceous neornithine is the partial skeleton of Vegavis from the latest Cretaceous (around 68–66 million years ago) of Antarctica.

Zhenyuanlong suni (photo by Junchang Lu¨ ) from the Jehol Biota.

Zhenyuanlong suni (photo by Junchang Lu) from the Jehol Biota.

Anatomical features like aspects of egg shape, ornamentation, microstructure, and porosity of living birds trace their origin to the maniraptoran theropods, such as oviraptorosaurs and troodontids. In addition, some preserving brooding postures, are known for four oviraptorosaurs, two troodontids, a dromaeosaur, and one basal bird providing clear evidence for parental care of eggs.

In birds, particularly their forebrains, are expanded relative to body size. Birds also exhibit the most advanced vertebrate visual system, with a highly developed ability to distinguish colors over a wide range of wavelengths.

Feathers were once considered to be unique avialan structures. The megalosaurus Sciurumimus, the compsognathus Sinosauropteryx, and a few other dinosaurs, document the appearance of primitive feathers. Zhenyuanlong suni, from the Yixian Formation, provides the first evidence of well-developed pennaceous feathers in a large, non-flying dromaeosaur. Evidence indicates that the earliest feathers evolved in non-flying dinosaurs, likely for display and/or thermoregulation, and later were co-opted into flight structures in the earliest birds (Brusatte et al., 2015).

The basal avian Jeholornis prima.

The basal avian Jeholornis prima.

The evolution of flight involved a series of adaptive changes at the morphological and molecular levels, like the fusion and elimination of some bones and the pneumatization of the remaining ones. Archaeopteryx lacked a bony sternum and a compensatory specialized gastral basket for anchoring large flight muscles (O’Connor et al., 2015), while Jelohornis had several derived flight-related features of modern birds like fused sacral vertebrae, an elongated coracoid with a procoracoid process, a complex sternum, a narrow furcula, and curved scapula. In Enantiornithines, their robust pygostyle appears to have been unable to support the muscles that control the flight feathers on the tail in modern birds. The increased metabolism associated with homeothermy and powered flight requires an efficient gas exchange process during pulmonary ventilation. Recent anatomical and physiological studies show that alligators, and monitor lizards exhibit respiratory systems and unidirectional breathing akin to those of birds, which indicate that unidirectional breathing is a primitive characteristic of archosaurs or an even more inclusive group with the complex air-sac system evolving later within Archosauria.

The earliest diversification of extant birds (Neornithes) occurred during the Cretaceous period and after the mass extinction event at the Cretaceous-Paleogene (K-Pg) boundary, the Neoaves, the most diverse avian clade, suffered a rapid global expansion and radiation. A genome-scale molecular phylogeny indicates that nearly all modern ordinal lineages were formed within 15 million years after the extinction, suggesting a particularly rapid period of both genetic evolution and the formation of new species. Today, with more than 10500 living species, birds are the most species-rich class of tetrapod vertebrates.

 

References:

Brusatte, S. L., O’Connor, J. K., and Jarvis, E. D. 2015. The origin and diversification of birds. Current Biology, 25, R888-R898

Padian, K., and Chiappe, L.M. (1998). The origin and early evolution of birds. Biol. Rev. 73, 1–42.

Puttick, M. N., Thomas, G. H. and Benton, M. J. (2014), HIGH RATES OF EVOLUTION PRECEDED THE ORIGIN OF BIRDS. Evolution, 68: 1497–1510. doi: 10.1111/evo.12363 A.

Xing Xu, Zhonghe Zhou, Robert Dudley, Susan Mackem, Cheng-Ming Chuong, Gregory M. Erickson, David J. Varricchio, An integrative approach to understanding bird origins, Science, Vol. 346 no. 6215, DOI: 10.1126/science.1253293.