Box The Wonderful Birds Of Liaoning

The first reports of spectacular bird fossils from Liaoning Province in north-east China came out in 1984. Farmers and school children had excavated specimens from limestone quarries in their fields and these were sent piecemeal to palaeontologists in

Beijing and Nanjing. More concentrated research began in the 1990s and so far some 15 genera of birds have been described, seven of which are enantiornithines (Zhou and Hou, 2002; Zhou et a/., 2003). The fossiliferous horizons are in the Yixian and Jiufotang formations of the Jehol Group, some 2600 m of sediments, and bird fossils have come from all levels through the succession. Early workers suggested these beds might be Late Jurassic in age, but radiometric dating and biostratigraphy show they are Early Cretaceous (late Hauterivian to early Aptian, 128—110 Myr ago).

The fine limestones, laid down by slow accumulation of sediments in ancient lakes, have produced rich floras and faunas. The flora is dominated by conifers, but many other groups, including angiosperms, are represented by leaves, flowers, fruits, stems and roots. Invertebrates include insects (mayflies, dragonflies, cockroaches, bugs, flies), spiders, ostracods, concostracans, crayfish, bivalves and gastropods. Other than birds, the vertebrates include bony fishes, frogs, salamanders (see p. 103), turtles, choris-toderes, lizards, pterosaurs and dinosaurs, including the feathered theropods (see Box 8.2), and mammals (see pp. 305, 311).

Some of the vertebrates from the Jehol Group are relicts, late-surviving members of groups that had died out much earlier elsewhere, such as Sinosauropteryx (close relative of Compsognathus from the Upper Jurassic of Germany) and an anurognathid pterosaur (otherwise known also only from the Late Jurassic). In addition, some of the plants, fishes, turtles, the psittacosaurid dinosaurs and the confuciusornithid birds are also unique to eastern Asia. Are the Jehol biotas freaks, assemblages of unusual and bizarre plants and animals that do not represent the mainstream? Seemingly not. Most of the organisms are typical of Early Cretaceous localities elsewhere, and this is true of the majority of the birds.

The book of the touring exhibition of dino-birds from China (Milner, 2002) contains colour photographs of specimens and life restorations.

The Early Cretaceous bird Confuciusornis: (a) complete (?) male specimen, with long tail; (b) lateral view of the skull. (Courtesy of Zhou Zhonghe.)

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Excellent web coverage of the Liaoning bird localities and some of the specimens may be seen at http://www.peabody.yale.edu/exhibits/cfd/CFDintro.html, http://www.nhm.ac.uk/dinobirds/, http://www.austmus.gov.au/chinese_dinosaurs/feathered_dinosaurs/index.htm,

http://www.carnegiemuseums.org/cmnh/exhibits/feathered/, http://eebweb.arizona.edu/courses/ecol484/conf.html, http://www.dlnm.org/english/003.htm and http://www.nhm-wien.ac.at/NHM/Geolog/Mesozoic/02.htm and a digital three-dimensional scan of the skull of Confuciusornis at http://digimorph.org/specimens/Confuciusornis_sp./skull/.

specimen preserves dozens of seeds in the stomach area, direct evidence of diet.

9.3.2 Confuciusornithidae: toothless birds from China

The confuciusornithids, consisting of two genera, Confuciusornis and Changchengornis,are basal pygostylians (Chiappe et al., 1999; Zhou and Hou, 2002). Specimens were first reported in 1995 and ever more material is being found in the spectacular Liaoning deposits of China (see Box 9.3). Confuciusornis was about the size of a rook and it is known from hundreds of specimens; Changchengornis was starling-sized and is known only from one specimen.

Confuciusornithids (Figure 9.5(b)) have no teeth and they have a horn beak (probably absent in Ar-chaeopteryx). The nostril is large and only separated from the antorbital fenestra by a thin bar of bone composed of the nasal and maxilla. The antorbital fenestra in turn is separated from the huge round orbit by only a thin boomerang-shaped lacrimal. The temporal openings appear primitive, with the jugal fused to the postorbital bar. The quadratojugal is much reduced and the quadrate appears to be streptostylic. The lower jaw is slender, with a downturned pointed tip and a large mandibular fenestra.

In the skeleton, the confuciusornithid sacrum is composed of seven fused vertebrae, and it can be termed a synsacrum. The tail is also much modified, forming a pygostyle, the bony element formed from fused caudal vertebrae, eight or nine in the case of confuciusornithids. Confuciusornis may have been a slightly better flyer than Archaeopteryx: it has a somewhat larger sternum with a slight keel and the wrist was more flexible, useful in flexing (folding) the wing for the recovery stroke. The wing also retains three long fingers with claws, presumably used in climbing. The pelvis and hindlimb are also like those of Archaeopteryx.

Most spectacular of course are the feathers. These are exquisitely preserved in all specimens (see Box 9.3) and show short feathers over the neck, body, upper legs, the front of the wings and the top of the tail. Long flight feathers extend behind the wings. The tail feathers are most extraordinary, being generally short and radiating like a fan from the pygostyle, as in modern birds. But half the specimens, perhaps males, have two extremely elongated tail feathers, each longer than the body and forming dramatic pennants that may have been used as display structures.

9.3.3 Oviraptorosauria: egg-brooders

The placement of oviraptorosaurs among birds is highly controversial. The group includes the ovirap-torids and ingeniids from the Upper Cretaceous of Mongolia and China and the caenagnathids from the Upper Cretaceous of Canada. Oviraptorosaurs were first thought to be birds, but were then generally treated as aberrant theropods, related to the ornithomimids, troodontids,or therizinosaurids (e.g. Clark et al.,2002). In some recent analyses, however (e.g. Maryariska et al., 2002), they have been located firmly among birds, perhaps falling between confuciusornithids and enantiornithines in the cladogram (see Box 9.2). Oviraptorosaurs share with birds a number of characters, including fused premaxillae, a mobile quadrate-quadratojugal articulation, 13 cervical vertebrae, more than eight sacral vertebrae, ilia that are close together dorsally, as well as the pygostyle, consisting of the last five of the 24 caudals.

Oviraptor from the Upper Cretaceous of Mongolia is typical of the group. It has an odd-looking skull (Figure 9.5(c)), high and full of openings. The snout is very much shortened and it lacks teeth. Recent collections have shown that this dinosaur had been unfairly given a bad name. Oviraptor means 'egg thief', and that name was coined because the type skeleton was found in 1923 lying on top ofa nest containing eggs. A further skeleton of Oviraptor was found in 1993 (Norell et al., 1995),also located on top of a nest, but this time an embryo was found inside one of the eggs and it turned out to be an unhatched Oviraptor.Far from being an egg thief, these Oviraptor individuals were apparently brooding their own eggs.

9.3.4 Enantiornithes: most diverse Cretaceous bird clade

The Enantiornithes were a major Cretaceous bird group, consisting of 40 or more species, distributed worldwide, from the Lower Cretaceous of China, Australia and Spain and the Upper Cretaceous of Argentina, Mexico, the USA, Mongolia, Uzbekistan, Madagascar, Australia and France (Chiappe and Walker, 2002; Sereno et al., 2002). They have been found rarely in near-shore marine deposits, but occur mainly in freshwater settings, and they ranged in size from Sinornis, the size of a sparrow, to Enantiornis, with a wingspan of 1 m. Most of them had teeth, although Gobipteryx from the Upper Cretaceous of Mongolia was toothless.

The Enantiornithes are the basal group of the Or-nithothoraces (see Box 9.2), characterized (Chiappe, 2002b; Sereno et al., 2002) by having a short back with fewer than 13 thoracic vertebrae. They have a strut-like coracoid, interpreted as a support for the flight apparatus (Figure 9.3). They also have a triosseal foramen, essential for passage of the supracoracoideus muscle, which effects the upstroke of the wing in modern birds (see section 9.2), as well as an alula, a supplementary winglet (see below).

Enantiornithines were recognized first in 1981 from the Upper Cretaceous of South America,where isolated limb bones pointed to a new group of birds characterized by a tarsometatarsus (Figure 9.5(d)), the fused distal tarsals and metatarsals, in which there is a deep groove distally and in which metatarsal 4 is very thin. The humerus of enantiornithines (Figure 9.5(e)) shows diagnostic characters at the proximal end: a concave portion in the middle of the articular face and a prominent bicipital crest. During the 1990s a whole flock of new enantiornithine species came to light. Some, such as Gobipteryx from the Upper Cretaceous of Mongolia,had been misidentified long before as related more directly to modern birds, others had been announced as theropod dinosaurs and yet others were entirely new finds, most importantly from Lower Cretaceous deposits of China and Spain (Chiappe and Walker, 2002).

The ancient lake deposits of the Jehol Group of Liaoning, north-east China (Hauterivian-Aptian, 128-110 Myr ago), have yielded skeletons of the enan-tiornithines Sinornis, Otogornis, Boluochia, Liaoxiornis and Longipteryx, as well as numerous other bird taxa (Sereno et al., 2002; Zhou and Hou, 2002). These were sparrow-sized birds that could fly actively and their feet show that they were well adapted for perching on branches. Sereno et al. (2002) conclude that Sinornis (Figure 9.5(f)) lived mainly in the trees and that it was capable of sustained flight, as it flitted around in search of insects. Sinornis shares primitive features with Archaeopteryx, such as a flexible hand with claws, but it has the pygostylian features of a larger ossified sternum, a pygostyle and a fully reflexed hallux, as well as the ornithothoracine features noted above.

Spectacular bird skeletons have been reported from the Las Hoyas Formation of central Spain (Barremian, 130Myr ago.), including three enantiornithines, Iberomesornis, Concornis and Eoalulavis (Sanz et al., 2002). Iberomesornis is a sparrow-sized bird (Figures 9.5(g) and 9.6) with eight free caudal vertebrae, a strutlike coracoid and a large plate-like pygostyle. The foot is specialized for perching, with a reversed hallux. Concornis (Sanz et al., 1995, 2002) is based on an incomplete skeleton, lacking the skull. The hindlimb (Figure 9.5(h)) is comparable to modern perching birds, with a

Iberomesornis Skeleton
Fig. 9.6 Exceptionally preserved skeleton of Iberomesornis from the Las Hoyas Formation, Cuenca Province, Spain. (Courtesy of José L. Sanz.)

reflexed hallux, long curved claws and a largely fused lower limb. This fused portion consists of a tibiotarsus (astragalus and calcaneum fused to tibia) and an enan-tiornithine tarsometatarsus, although the metatarsals are fused only proximally. The wing shows 'modern' proportions, but the fingers are still equipped with claws. Eoalulavis is represented by the wings and thorax, but these show a key feature relating to flight. The first finger is separate from the other two and bears its own tuft of feathers, lying in front of the main portion of the wing. This is the first record of the alula, or bastard wing, a structure seen in all modern flying birds that is used to improve their manoeuvrability at slow flying speeds. Normally, the alula lies parallel to the leading edge of the wing, but the thumb can move forward, creating a slot between the alula and the wing. This extra winglet allows the bird to avoid stalling at slow speed and at a steep angle of attack, for example when landing or taking off. Similar devices are used in aeroplanes.

9.3.5 Patagopteryxand Vorona

Two birds, Patagopteryx from the Upper Cretaceous of Argentina and Vorona from the Upper Cretaceous of Madagascar, appear to be more derived than the Enan-tiornithes, but less so than the Ornithurae (see Box 9.2). Patagopteryx (Chiappe, 2002a) is a hen-sized flightless bird, known from three specimens that represent the whole skeleton except the tip of the snout and the end of the tail (Figure 9.7(a)). The hindlimbs are much heavier than in any other Cretaceous bird hitherto, and the wings are too small to have been able to sustain this bulky bird in flight. Patagopteryx was a terrestrial bird, but it does not show adaptations for fast running or large size, as seen in modern ostriches and emus. Despite some early suggestions, Patagopteryx is not an early representative of the ratites, the flightless birds (see section 9.4).

Vorona is less well known (Forster etal., 2002),being represented only by elements of the hindlimb. The tar-sometatarsus shows, however, the derived character of nearly complete fusion of metatarsals 2-4 and nearly complete enclosure of a vascular canal between metatarsals 3 and 4 (Figure 9.7(b)).

Fig. 9.7 Two Late Cretaceous birds: (a) Patagopteryx from Argentina (black areas are unknown); (b) left tarsometatarsus of Vorona from Madagascar; df, pf, distal and proximal foramina of enclosed vascular canal. [Figure (a) courtesy of Luis Chiappe; (b) courtesy of Cathy Forster.]

Fig. 9.7 Two Late Cretaceous birds: (a) Patagopteryx from Argentina (black areas are unknown); (b) left tarsometatarsus of Vorona from Madagascar; df, pf, distal and proximal foramina of enclosed vascular canal. [Figure (a) courtesy of Luis Chiappe; (b) courtesy of Cathy Forster.]

9.3.6 Hesperornithiformes: flightless divers

Hesperornis (Figure 9.8(a)) is more than 1 m tall and has a long neck, reduced tail and long powerful legs. The forelimb is much reduced, being represented by only a pointed humerus that looks like a hat-pin. Like Patagopteryx, the hesperornithiforms probably evolved from flying ancestors, and it seems that wing reduction has been a persistent feature of bird evolution. Hesper-ornithiforms share a pointed anterior process of the quadrate (Figure 9.8(c)), and other features, with later birds (see Box 9.2).

The remains of Hesperornis and the related smaller Baptornis (Figure 9.8(b); Martin and Tate, 1976) have been found abundantly in the Upper Cretaceous Niobrara Chalk Formation of Kansas, USA, which was deposited in the shallow warm waters of the great sea channel that ran from north to south through North America at the time. Enaliornis from the Mid-Cretaceous of England may be the earliest hesperornithiform.

The hesperornithiforms were clearly flightless, and they are interpreted as foot-propelled divers that swam rapidly by kicking their feet. The toes are long and could spread widely. In life, they were probably linked by webs of skin or at least bore lobes to increase the surface area for swimming (Figure 9.8(b)). The tiny

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Fig. 9.8 The Late Cretaceous toothed birds: (a) skeleton ofHesperornisin standing pose; (b) restoration ofBaptornisswimming; (c) skull of Hesperornis; (d) Ichthyornis skeleton and tooth. [Figures (a, d) after Zittel, 1932; (b) used by permission of the Smithsonian Institution Press from Martin and Tate, 1976; (c) after Martin, in Carroll, 1987.]

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Fig. 9.8 The Late Cretaceous toothed birds: (a) skeleton ofHesperornisin standing pose; (b) restoration ofBaptornisswimming; (c) skull of Hesperornis; (d) Ichthyornis skeleton and tooth. [Figures (a, d) after Zittel, 1932; (b) used by permission of the Smithsonian Institution Press from Martin and Tate, 1976; (c) after Martin, in Carroll, 1987.]

wing stumps may have had a modest function in steering. Parts of the jaws are lined with small pointed teeth, and hesperornithiforms ate sea fishes, as is shown by their coprolites.

9.3.7 Ichthyornithiformes: toothed fishers

Ichthyornis, also from the Niobrara Chalk Formation of Kansas, as well as from other sites in North America and in Europe, is smaller than Hesperornis,being the size of a small gull (Figure 9.8(d)). The wings are fully developed and there is a deeply keeled ossified sternum, as in modern birds. The tail is more reduced than in Hesperornis and the body is deeper. The head is large and the massive jaws are lined with short pointed teeth set into a groove as in Hesperornis. Ichthyornis presumably caught fishes in the Niobrara sea by diving into the water from the wing, as terns do.

9.3.8 Other ornithurine birds from the Cretaceous

Apart from the reasonably diverse Hesperornithi-formes and Ichthyornithiformes, the Ornithurae (see Box 9.2) diversified further in the Cretaceous. Remains include Ambiortus from Mongolia, Gansus from China and others from France and Romania, all of which have from time to time been assigned to modern bird groups, and so regarded as evidence for an early radiation of Ne-ornithes. None of these identifications can be sustained and they are at best ornithurines or carinates (Chiappe, 2002b; Hope, 2002; Dyke and Van Tuinen, 2004). Newly described forms from the Late Cretaceous, Apsaravis from Mongolia (Norell and Clarke, 2001) and Limenavis from Argentina fit in the cladogram between Hesperornithiformes and Ichthyornithiformes, and between Ichthyornithiformes and Neornithes respectively.

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