Ichthyornis and the evolution of the avian skull.

 

Ichthyornis skull

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. Much of birds anatomical variety is related to their skulls and in particulary with their beaks.

Discovered in 1870 by Benjamin Franklin Mudge, a professor from Kansas State Agricultural College and good friend of Othniel Charles Marsh, Ichthyornis, which means‭ ‘‬fish bird‭’‭, was a small early ornithuromorph from the Late Cretaceous of North America. 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.

 

3D reconstruction of the skull of I. dispar (From Field et al., 2018)

The skull of I. dispar shows a transitional point in the evolutionary history of birds. The upper margin of the beak is concave in profile, a derived condition shared with living birds. The fused, toothless premaxillae have a terminal hook, and occupy the anterior quarter of the rostrum. Neurovascular foramina indicate the presence of a highly keratinized region of rhamphotheca called the premaxillary nail. The maxilla is plesiomorphically long. The dentition is extensive in both upper and lower jaws. A sulcus on the rostral half of the maxilla suggests a broad naso-maxillary contact and a correspondingly broad postnarial bar. The palatine is narrow and elongate, unlike that of Archaeopteryx and more stemward theropods. The quadrate exhibits two rounded capitular condyles that fit into cotyles on the prootic and squamosal bones to form a mobile joint with the cranium. The arrangement of the rostrum, jugal, and quadratojugal, the mobile suspensorium and the narrow, linear palatine all indicate that I. dispar possessed a fully functional avian cranial kinetic system.

The endocranial cavity appears essentially modern in sagittal section. The forebrain was enlarged and posteroventrally rotated while the optic lobes were inflated and laterally shifted, as in living birds. The squamosal exhibits an archaic, deinonychosaur-like morphology. The zygomatic process is deep and triangular in lateral view. The nuchal crest extends from the midline of the skull onto the zygomatic process, forming the upper edge of the squamosal bone, as in non-avialan theropods.

Darwin’s letter to Marsh (Yale Peabody Museum Archives)

Since its discovery, Ichthyornis has been viewed as a classical example of evolution, due to the combination of an advanced postcranial morphology and retention of toothed jaws. In a letter, dated August 31, 1880, Charles Darwin thanks Marsh for a copy of his monograph Odontornithes, which reported two contrasting bird genera: Hesperornis, which was about 1.8 metres tall, and Ichthyornis, which had an average wingspan of about 60 centimetres. In his letter, Darwin wrote: “I received some time ago your very kind note of July 28th, & yesterday the magnificent volume. I have looked with renewed admiration at the plates, & will soon read the text. Your work on these old birds & on the many fossil animals of N. America has afforded the best support to the theory of evolution, which has appeared within the last 20 years.”

 

References:

Daniel J. Field, Michael Hanson, David Burnham, Laura E. Wilson, Kristopher Super, Dana Ehret, Jun A. Ebersole & Bhart-Anjan S. Bhullar, Complete Ichthyornis skull illuminates mosaic assembly of the avian head, Nature (2018). nature.com/articles/doi:10.1038/s41586-018-0053-y
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.

 

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An early juvenile enantiornithine specimen from the Early Cretaceous of Spain

The slab and counterslab of MPCM-LH-26189

Mesozoic remains of juvenile birds are rare. To date, the only records are from the Early Cretaceous of China and Spain, from the mid-Cretaceous of  Myanmar, and from the Late Cretaceous of Argentina and Mongolia. The most recent finding from the Early Cretaceous of Las Hoyas, Spain, provide an insight into the osteogenesis of the Enantiornithes, the most abundant clade of Mesozoic birds. Previous records of Enantiornithes from the Las Hoyas fossil site include: Eoalulavis hoyasi, Concornis lacustris, and Iberomesornis romerali.

The latest specimen, MPCM-LH-26189, a nearly complete and largely articulated skeleton (only the feet, most of its hands, and the tip of the tail are missing), is very small. The specimen died around the time of birth, a crucial moment to study the osteogenesis in birds. The skull, is partially crushed, and is large compared to the body size. The braincase is fractured. The frontals and the parietals form a uniformly curved cranial vault. The cerebrocast shows a very slight inflation, suggesting that the cerebral anatomy of MPCM-LH-26189 falls in between that of the Archaeopteryx, and the putative basal ornithurine Cerebavis, whose telencephalic expansion is close to most extant birds. The cervical series is composed of 9 vertebrae. There are 10  thoracic vertebrae, and the sacrum appears to be composed of 5–6 vertebrae. The prezygapophyses of the mid-thoracic vertebrae extend beyond the cranial articular surface. The thoracic ribs are joint to the thoracic vertebrae. The two coracoids, the furcula, and three sternal ossifications are preserved. The furcula is Y-shapped. Both humeri, ulnae, and radii are also preserved.

Reconstruction of MPCM-LH-26189 by Raúl Martín

The osteohistological analysis of the left humerus shows a dense pattern of longitudinal grooves. Those grooves correspond to primary cavities, which open onto the surface of the cortex in young and fast-growing bone. The shaft of the tibia and radius show very-thin cortices. In addition,  the primary nature of the vascularisation, the round shape of the osteocytes lacunae and the uneven peripheral margin of the medullary cavity (with no endosteal bone), strongly suggests that the bone was actively growing when the bird died.

Enantiornithines show a mosaic of characters, reflecting their intermediate phylogenetic position between the basal-pygostylians and modern bird. In this clade, the sternum adopts an elaborate morphology, and in adult Enantiornithes, no more than eight free caudal vertebrae precede the pygostyle. The differences observed in the ossification of the sternum and the number of free caudal vertebrae in MPCM-LH-26189, when it compared to other juvenile enantiornithines, reveal a clade-wide asynchrony in the sequence of ossification of the sternum and tail, suggesting that the developmental strategies of these basal birds may have been more diverse than previously thought.

References:

Fabien Knoll, et al., “A diminutive perinate European Enantiornithes reveals an asynchronous ossification pattern in early birds,” Nature Communications, volume 9, Article number: 937 (2018) doi:10.1038/s41467-018-03295-9

Chiappe, L. M., Ji, S. & Ji, Q. Juvenile birds from the Early Cretaceous of China: implications for enantiornithine ontogeny. Am. Mus. Novit. 3594, 1–46 (2007).

 

 

Introducing Caihong juji

Caihong juji holotype specimen (Hu, et al., 2018)

Over the last 10 years, theropod dinosaurs from the Middle-Late Jurassic Yanliao Biota have offered rare glimpses of the early paravian evolution and particularly the origin of birds. The first discovered Yanliao non-scansoriopterygid theropod was Anchiornis huxleyi, and since then several other extremely similar species have also been reported. Caihong juji, a newly discovered Yanliao specimen, exhibits an array of osteological features, plumage characteristics, and putative melanosome morphologies not previously seen in other Paraves. The name Caihong is from the Mandarin ‘Caihong’ (rainbow). The specific name, juji is from the Mandarin ‘ju’ (big) and ‘ji’ (crest), referring to the animal’s prominent lacrimal crests.

The holotype (PMoL-B00175) is a small, articulated skeleton with fossilized soft tissues, preserved in slab and counter slab, collected by a local farmer from Qinglong County, Hebei Province, China, and acquired by the Paleontological Museum of Liaoning in February, 2014. The specimen (estimated to be ~400 mm in total skeletal body length with a body mass of ~475 g) exhibits the following autapomorphies within Paraves: accessory fenestra posteroventral to promaxillary fenestra, lacrimal with prominent dorsolaterally oriented crests, robust dentary with anterior tip dorsoventrally deeper than its midsection and short ilium.

Caihong juji differs from Anchiornis huxleyi in having a shallow skull with a long snout, forelimb proportionally short, and forearm proportionally long. Caihong also resembles basal troodontids and to a lesser degree basal dromaeosaurids in dental features (anterior teeth are slender and closely packed, but middle and posterior teeth are more stout and sparsely spaced; and serrations are absent in the premaxilla and anterior maxilla).

Platelet-like nanostructures in Caihong juji and melanosomes in iridescent extant feathers (Hu, et al., 2018)

Feathers are well preserved over the body, but in some cases, they are too densely preserved to display both gross and fine morphological features. The contour feathers are proportionally longer than those of other known non-avialan theropods. The tail feathers resemble those of Archaeopteryx, and the troodontid Jinfengopteryx in having large rectrices attaching to either side of the caudal series forming a frond-shaped tail, a feature that has been suggested to represent a synapomorphy for the Avialae.

But, the most remarkable feature observed in Caihong, is the presence of some nanostructures preserved in the head, chest, and parts of its tail, that have been identified as melanosomes. They are long, flat, and organized into sheets, with a pattern similar of those of the iridescent throat feathers of hummingbirds.

Recovered as a basal deinonychosaur, Caihong shows the earliest asymmetrical feathers and proportionally long forearms in the theropod fossil record wich indicates locomotor differences among closely related Jurassic paravians and has implications for understanding the evolution of flight-related features.

References:

Hu, et al. A bony-crested Jurassic dinosaur with evidence of iridescent plumage highlights complexity in early paravian evolution. Nature (2018) doi:10.1038/s41467-017-02515-y

Godefroit, P. et al. A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds. Nature 498, 359–362 (2013).

Vegaviidae, a new clade of southern diving birds

Vegavis iaai by Gabriel Lio. / Photo: CONICET

The fossil record of Late Cretaceous–Paleogene modern birds in the Southern Hemisphere is fragmentary.  It includes Neogaeornis wetzeli from Maastrichtian beds of Chile, Polarornis gregorii and Vegavis iaai from the Maastrichtian of Antarctica, and Australornis lovei from the Paleogene of New Zealand. The phylogenetic relationships of these taxa have been variously interpreted by different authors. In a more recent analysis, Polarornis, Vegavis, Neogaeornis, and Australornis, are including in a new clade: Vegaviidae.

Vegaviids share a combination of characters related to diving adaptations, including compact and thickened cortex of hindlimb bones, femur with anteroposteriorly compressed and bowed shaft, deep and wide popliteal fossa delimited by a medial ridge, tibiotarsus showing notably proximally expanded cnemial crests, expanded fibular crest, anteroposterior compression of the tibial shaft, and a tarsometatarsus with a strong transverse compression of the shaft.

Histological sections of Vegavis iaai (MACN-PV 19.748) humerus (a), femur (b), polarized detail of humerus (c). Scale bar equals 10 mm for (a), (b) and 5 mm for (c). From Agnolín et al., 2017

The recognition of Polarornis, Vegavis, Neogaeornis, Australornis, and a wide array of isolated specimens as belonging to the new clade Vegaviidae reinforces the hypothesis that southern landmasses constituted a center for neornithine diversification, and emphasizes the role of Gondwana for the evolutionary history of Anseriformes and Neornithes.

The most informative source for anatomical comparison among Australornis, Polarornis, Vegavis as well as other southern avian is a recently published Vegavis skeleton (MACN-PV 19.748). Vegavis overlaps with Australornis in the proximal portion of the humerus, proximal part of the coracoid, scapula, and ulna; with Polarornis in the humerus, femur, and proximal end of the tibia; and with Neogaeornis in the tarsometatarsus.

Phylogeny with geographical distribution of Vegaviidae. From Agnolín et al., 2017.

The humerus is probably the most diagnostic element among anseriforms. In Vegavis and Australornis the humerus is notably narrow and medially tilted on its proximal half, and the deltopectoral crest extends for more than one third of the humeral length. The femur is well known both in Vegavis and Polarornis, and share a combination of characters absent in other Mesozoic or Paleogene birds, including strongly anteriorly bowed and anteroposteriorly compressed shaft (especially near its distal end)

Osteohistological analysis of the femur and humerus of V. iaai. shows a highly vascularized fibrolamellar matrix lacking lines of arrested growths, features widespread among modern birds. The femur has some secondary osteons, and shows several porosities, one especially large, posterior to the medullar cavity. The humerus exhibits a predominant fibrolamellar matrix, but in a portion of the anterior and medial sides of the shaft there are a few secondary osteons, some of them connected with Volkman’s canals, and near to these canals, there are a compact coarse cancellous bone (CCCB) with trabeculae. This tissue disposition and morphology suggests that Vegavis had remarkably high growth rates, a physiological adaptation that may be critical for surviving in seasonal climates at high latitudes, and  may also constitute the key adaptation that allowed vegaviids to survive the K/T mass extinction event.

 

References:

Agnolín, F.L., Egli, F.B., Chatterjee, S. et al. Sci Nat (2017) 104: 87. https://doi.org/10.1007/s00114-017-1508-y

Jordi Alexis Garcia Marsà, Federico L. Agnolín & Fernando Novas (2017): Bone microstructure of Vegavis iaai (Aves, Anseriformes) from the Upper Cretaceous of Vega Island, Antarctic Peninsula, Historical Biology, DOI: 10.1080/08912963.2017.1348503

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

Sapeornis and the flight modes of birds

Sapeornis chaoyangensis (DNHM-3078) showing well-preserved primary (P) and secondary (S) feathers. From Serrano and Chiappe, 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. Most of these fossils, like Sapeornis chaoyangensis (125 to 120 Ma), are from the Jehol Biota of northeastern China. Sapeornis shows a combination of derived and primitive features, like a short, robust non-strut-like coracoid and a fibula reaching the distal end of the tarsal joint, a pygostyle, reduced manual digits, and a well-fused carpometacarpus. All of these features indicates a mosaic pattern in the early evolution of birds and confirm the basal position of Sapeornis near Archaeopteryx and Jeholornis in the phylogeny of early birds.

The evolution of flight involved a series of adaptive changes at the morphological and molecular levels, that included 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, 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.

Morphofunctional fitness of the wing shape for soaring as depicted by the relation between the lift surface and the wingspan in modern soaring birds and Sapeornis (From Serrano and Chiappe, 2017)

The flight modes of modern birds are a reflection of their different strategies to reduce the energetic costs of a highly demanding style of locomotion. Among these features are wing shape, and the use of thermals and tail winds. Flapping flight is energetically more costly  than gliding and soaring flight, consequently, large birds have either elongated wingspans that allow them to gain height through air currents and to glide for long distances with much lower transit costs than flapping.

Fossil evidence suggests that S. chaoyangensis was a specialized flier that used continental soaring as its main flight mode. Computational models of S. chaoyangensis are also congruent with other morphological similarities between S. chaoyangensis and modern soaring birds including the shape of the furcula and the proportions of the forelimbs. Modern soaring birds include dynamic soarers that exploit air velocity gradients over sea waves, and thermal soarers that use ascending air currents mainly generated in continental areas. Because, exceptionally well preserved fossils of S. chaoyangensis have revealed seeds and/or fruits in its intestinal tract, this interpretation of the flight capabilities of S. chaoyangensis is consistent with the energetic disadvantages from a herbivorous diet, because soaring is a less demanding flight mode than continuous flapping.

References:

Serrano FJ, Chiappe LM. 2017 Aerodynamic modelling of a Cretaceous bird reveals thermal soaring capabilities during early avian evolution. J. R. Soc. Interface 14: 20170182. http://dx.doi.org/10.1098/rsif.2017.0182

Butler PJ. 2016 The physiological basis of bird flight. Phil. Trans. R. Soc. B 371, 20150384 doi:10. 1098/rstb.2015.0384

Zhou, Zhonghe & Zhang, Fucheng (2003): Anatomy of the primitive bird Sapeornis chaoyangensis from the Early Cretaceous of Liaoning, China. Canadian Journal of Earth Sciences 40(5): 731–747. doi 10.1139/E03-011

A mid-Cretaceous enantiornithine frozen in time

Overview of HPG-15-1 in right lateral view. (From Xing et al., 2017)

Overview of HPG-15-1 in right lateral view. (From Xing et al., 2017)

Amber from the Hukawng Valley in northern Myanmar, called Burmese amber, has been commercially exploited for millennia. Of the seven major deposits of amber from the Cretaceous Period, Burmese amber has probably the most diverse paleobiota, including the tail of a non-avian coelurosaurian theropod, and three juvenile enantiornithine birds. The third specimen, HPG-15-1, is the most complete fossil bird discovered in Burmese amber. It comes from the Angbamo site, and measures approximately 86 mm x 30 mm x 57 mm, and weighs 78 g. It  was encapsulated during the earliest stages of its feather production, and  plumage preserves an unusual combination of precocial and altricial features unlike any living hatchling bird.

 Details of the head in HPG-15-1. A, x-ray µCT reconstruction in left lateral view

Details of the head in HPG-15-1. A, x-ray µCT reconstruction in left lateral view (From Xing et al., 2017)

The skull was split when the amber was cut. The rostrum is preserved in one section and the neck and most of the braincase in the other. The skull is mesorostrine. A  single tooth is visible in the left premaxilla. As in Early Cretaceous enantiornithines, the premaxillary corpus is short, forming approximately one-third of the rostrum. The exoccipitals contributed to the dorsal portion of the condyle and were unfused at the time of death. The frontals articulate for most of their length with a small gap between their rostral ends as in Archaeopteryx.  The inner ear and its semicircular canals are preserved. There are at least six articulated cervical vertebrae, including the atlas and axis, preserved in articulation with the skull. The post-axial vertebrae are rectangular with large neural canals, low and caudally displaced neural spines, and a ventral keel as in many enantiornithines. The articulated skull and series of cervical vertebrae bear plumage in dense fields. The individual feathers  are dark brown in color, and appear to consist of tufts of four or more barbs. Skin is preserved as a translucent film in unfeathered regions of both the head and neck.

Microstructure and pigmentation of feathers on wing and body of HPG-15-1. Scale bars equal 1 mm in (A, C); 0.5 mm in (B, D). From Xing et al., 2017

The new specimen also preserves a partial distal wing, the distal right tibiotarsus and complete right foot as well as part of the left pes. Both skeletal material and integumentary structures from the wing’s apex are well-preserved. The plumage consists of fragments of some of the primaries, and alula feathers, some of the secondaries and coverts, and traces of contours from the wing base. The hind limbs preserve feathers and traces of skin. The absence of fusion between the tarsals indicates that the specimen is ontogenetically immature. The proportions of the pedal digits suggest an arboreal lifestyle. Plumage within the femoral and crural tracts consists of neoptile feathers with a short or absent rachis. These feathers are nearly transparent, suggesting that they were pale or white. The skin beneath the crural tract is thin and smooth. The tip of the tail clearly preserves the remains of a single large sheathed rectrix.

The slow post-natal growth results in a protracted period of vulnerability, which is reflected in the Enantiornithes by the large number of juveniles found in the fossil record, whereas young juveniles of other Cretaceous bird lineages are unknown.

 

References:

Lida Xing, Jingmai K. O’Connor, Ryan C. McKellar, Luis M. Chiappe, Kuowei Tseng, Gang Li, Ming Bai , A mid-Cretaceous enantiornithine (Aves) hatchling preserved in Burmese amber with unusual plumage, (2017), doi: 10.1016/j.gr.2017.06.001

High variation in postnatal development of Early Dinosaurs.

Cleveland Museum of Natural History Coelophysis block, originally AMNH Block XII collected in 1948 by Colbert and crew

Cleveland Museum of Natural History Coelophysis block, originally AMNH Block XII collected in 1948 (From Wikimedia Commons)

Birds originated from a theropod lineage more than 150 million years ago. Their evolutionary history is one of the most enduring and fascinating debates in paleontology. They are members of the theropod dinosaur subgroup Coelurosauria, a diverse clade that includes tyrannosauroids and dromaeosaurids, among others. Features like “hollow” bones and postcranial skeletal pneumaticity, feathers, a unique forelimb digit formula, endothermy, and rapid growth rate arose in non-avian dinosaurs in a gradual process occurring over tens of millions of years.

In contrast with all other living reptiles, birds grow extremely fast and possess unusually low levels of intraspecific variation during postnatal development, suggesting that this avian style of development must have evolved after its most recent common ancestor with crocodylians but before the origin of Aves. Most studies indicates that the low levels of variation that characterize avian ontogeny were present in close non-avian relatives as well.

Two C. bauri casts mounted at the Denver Museum of Nature and Science (From Wikimedia Commons)

Two C. bauri casts mounted at the Denver Museum of Nature and Science (From Wikimedia Commons)

Compared with birds, the theropod Coelophysis bauri possess a large amount of intraspecific variation. Coelophysis bauri is the type species of the genus Coelophysis, a group of small, slenderly-built, ground-dwelling, bipedal carnivores, that lived approximately 203 million years ago during the latter part of the Triassic Period in what is now the southwestern United States. Using this taxon to interpret development among early dinosaurs, geoscientists Christopher Griffin and Sterling Nesbitt discovered that the earliest dinosaurs had a far higher level of variation in growth patterns between individuals than crocodiles and birds. The presence of scars on the bones left from muscle attachment and marks where bones had fused together helped the researchers assess how mature the animals were compared with their size.

Body size and extinction risk have been found to be related in various vertebrate groups, therefore a high level of variation within a species may be advantageous in an ecologically unstable environment and may have contributed to the early success of dinosaurs relative to many pseudosuchian clades in the latest Triassic and through the End-Triassic Mass Extinction into the Early Jurassic.

References:

Christopher T. Griffin and Sterling J. Nesbitt, Anomalously high variation in postnatal development is ancestral for dinosaurs but lost in birds. PNAS 2016 : 1613813113v1-201613813.

Brusatte SL, Lloyd GT, Wang SC, Norell MA (2014) Gradual assembly of avian body plan culminated in rapid rates of evolution across the dinosaur-bird transition. Curr Biol 24(20):2386–2392

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.

An avian vocal organ from the Mesozoic.

The Vegavis iaai specimen showing the location of the syrinx. (Adapted from Clarke et al., 2016)

The Vegavis iaai specimen showing the location of the syrinx. (Adapted from Clarke et al., 2016)

Birds originated from a theropod lineage more than 150 million years ago. Their evolutionary history is one of the most enduring and fascinating debates in paleontology. 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. 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. Today, with more than 10500 living species, birds are the most species-rich class of tetrapod vertebrates.

In the mid-nineteenth century, T. H. Huxley recognized that birds were most closely related to dinosaurs. He also named the unique vocal organ in birds as the syrinx. Located at the base of a bird’s trachea, the syrinx consists of specialised cartilaginous structures, connective tissue masses, membranes and muscles. The oldest known remains of a syrinx was found within the fossilised, partial skeleton of a bird, known as Vegavis iaai, from the Late Cretaceous (66 mya) of Antarctica.

Vegavis iaai by Gabriel Lio. / Photo: CONICET

Vegavis iaai by Gabriel Lio. / Photo: CONICET

The Vegavis iaai holotype specimen from Vega Island, western Antarctica, was discovered in 1992 by a team from the Argentine Antarctic Institute, but was only described as a new species in 2005 (Clarke et al., 2005). It belonged to the clade Anseriformes, a group that includes ducks, geese and swans. Vegavis exhibits the fusion of cartilage rings and asymmetry between the left and right sides of the syrinx, that are useful for making comparisons with structural data from the present-day birds. Fused rings in Vegavis form a well-mineralized pessulus, a derived neognath bird feature, proposed to anchor enlarged vocal folds or labia. Although mineralized structures of the syrinx in Vegavis and many parts of extant Anatidae show asymmetry, Presbyornis, Chauna and Galliformes lack this feature. The absence of known tracheobronchial remains in all other Mesozoic dinosaurs may be indicative that a complex syrinx was a late arising feature in the evolution of birds, well after the origin of flight and respiratory innovations.

 

References:

Julia A. Clarke, Sankar Chatterjee, Zhiheng Li, Tobias Riede, Federico Agnolin, Franz Goller, Marcelo P. Isasi, Daniel R. Martinioni, Francisco J. Mussel and Fernando E. Novas. Fossil evidence of the avian vocal organ from the Mesozoic. Nature, 2016 DOI: 10.1038/nature19852

Clarke, J. A., C. P. Tambussi, J. I. Noriega, G. M. Erickson, and R. A. Ketcham. 2005. Definitive fossil evidence for the extant avian radiation in the Cretaceous. Nature 433:305-308.

Larsen, O. N.; Franz Goller (2002). “Direct observation of syringeal muscle function in songbirds and a parrot”. The Journal of Experimental Biology. 205 (Pt 1): 25–35.

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.

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.