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THE FIRST BIRDS
flock

JURASSIC BIRDS

Archaeornithine Birds

Archaeopteryx lithographica

      Originally, the crow sized Archaeopteryx was classified as a theropod dinosaur (or a pterosaur) until its feather impressions were noticed.  It was first named “Griphosaurus problematicus” , the mysterious griffin-reptile. Archaeopteryx contained a mix of dinosaur and bird characteristics.  Its birdlike traits include feathers, hollow pneumatized bones, a broad tail, the loss of the prefrontal bone, a femur which was less than 80% the length of the tibia, and a glenoid cavity which faced laterally, and a well developed furcula. Most of the avian features (with the exception of feathers) were also present in at least some theropods.  Its leg included birdlike features: the head of the femur was turned inwardly, the fibula was reduced, the metatarsals were elongate and partly fused, three toes faced forward and the first toe faced rearwards (it was opposable). An opposable first digit was one of the few traits which separated Archaeopteryx from its dinosaur relatives . The reptilian/dinosaurian characteristics of Archaeopteryx include teeth, limited fusion of skull bones, a long reptilian tail, claws on its functional fingers, a sclerotic ring of bones around the eye, gastralia (belly ribs from the sternum), the structure of the junction between the skull and vertebral column, and a reptilian pelvis (no synsacrum).  The feathers of the tail formed a “frond” shape rather than the “fan” shape of feathers around the pygostyle in modern birds.  The tail of Archaeopteryx would have been useful in maneuvering but would not have provided the lift, braking, and turning ability that the modern avian pygostyle provides (Ostrom, 1976; Wellnhofer, 1988; Britt, 1998; Gatesy, 1996; Chatterjee, 1997; Zhou, 2001). The feathers of dinosaurs were symmetrical while those of Archaeopteryx were asymmetrical (Qiang, 2001). Although a laterally facing glenoid cavity in the shoulder is a feature which united the first birds, it is also known in some of their maniraptoran relatives. After Archaeopteryx, the glenoid faced dorsolaterally (Padian, 2001).

The first digit did not become fully reversed until after Archaeopteryx (Padian, 2001). Archaeopteryx utilized digits I, II, and III in its wing, just like the hand of dinosaurs (Padian, 2001). Archaeopteryx still retained a small vestigial bony splint which represented its fifth toe, similar to theropod dinosaurs. Later birds do not possess a trace of the fifth toe. Modern birds begin their embryonic development with five toes. The first toe begins its development positioned similarly to its orientation in theropod dinosaurs only to later rotate to assume the avian position. The embryological structure of the fifth toe degenerates and is lost. In modern birds, the fused caropmetacarpus forms from separate finger elements which fuse and the tarsometatarsus forms from the fusion of the elements associated with three toes. Early in development, the fibula (a lower leg bone) extends all the way to the ankle as in dinosaurs and most vertebrates; its later development results in a short splint-like bone. Modern birds posesss a gland above their eyeball and the loss of the prefrontal bone from the skull in Maniraptorans may indicate that this gland was present in these dinosaurs.(Dingus, 1998).

ARCHAEOPTERYX tail feathers

--after Gatesy, 1996

     The feathers of Archaeopteryx had vane asymmetry as do those of modern birds, although studies have varied in their conclusions on whether the vane asymmetry of Archaeopteryx feathers are similar to modern birds which fly.  (Ostrom, 1976; Norberg, 1995).   Some analyses suggest that its muscles couldn’t have supported flight nor could its wrist have supported flapping.  It was first thought that the small medial supracoracoideus muscle in Archaeopteryx (as inferred from the absence of the process it is attached to in modern birds) would not have allowed flight but it has been since shown that modern birds are capable of flight after the muscle has been cut (Poore, 1997).  The robust furcula of Archaeopteryx suggests that it was capable of flight (Olson, 1979). The ability of Archaeopteryx to fly is further supported by the seventh specimen which includes a sternum (whose presence was suggested but not definite in previous specimens) which would have supported strong chest muscles (Padian, 1996).

.  Archaeopteryx probably would have had to run in order to take off however.  It is also possible that there was cartilage in the area of the sternum that provided some of the support that the keel in modern birds does (Burgers, 1999).The claw curvature of Archaeopteryx was similar to perching and climbing birds rather than running predators (Feduccia,1993). The name of the bird group after Archaeopteryx and Rahonavis is Pygostylia (Dyke, 2006).

archaeopteryx

protoavis

     A fossil known as Protoavis from the late Triassic of Texas was originally claimed to be the ancestor of birds.  Although there are some homologies with birds, many feel that the specimen too fragmentary for much analysis (Witmer, 2001). Others disagree, claiming that not only is Protoavis the oldest known bird, it is more advanced that Archaeopteryx proving that the first birds evolved well before the Late Triassic (Chatterjee, 1997).

Protoavis weighed about 0.6 kg. Its teeth indicate that it led a predatory lifestyle and its claws suggest that it was adapted for life in the trees. Its brain was larger than that of most theropods. It lacked a postorbital bone (unlike Archaeopteryx) and its zygomatic, quadratojugal, and quadrate bones are more similar to those of modern birds (Chatterjee, 1997).

nestling

 

 

 

     In 1997, a bird nestling was found in Spain, from135 million years ago (10 million years after Archaeopteryx).  Its wings and holes in bones (foramina) were very similar to those of modern birds.  Its skull retains some dinosaurian elements although they were reduced.  As in Archaeopteryx, the premaxillary bone was restricted to the tip of jaw and held 4 teeth while the maxilla held 5.  The postorbital bone was present (as in dinosaurs) but this bone is absent in modern birds.  There has been disagreement over whether this bone is present in Archaeopteryx upon analysis of the specimens; this find makes it more likely that it was.  Although the postorbital bone was present, it was reduced and didn’t contact the jugal bone, allowing the infratemporal fenesta to be continuous with the orbit (Sanz, 1997).  

     Iberomesornis was intermediate between Archaeopteryx and modern birds.  It had an unfused pelvic girdle and sacrum, an unfused tarsometatarsus, and the hind limbs were primitive while the coracoids and pygostyle are derived.  The number of dorsal vertebrae was 11 (Archaeopteryx had 13-4 while modern birds have 4-6).  It seems that there were 5 sacral vertebrae as in dinosaurs and Archaeopteryx.  The pygostyle (fused tail vertebrae) was made of 10-15 vertebrae, larger than in modern birds and a series of 8 free vertebrae separated it from the sacrum.  The pelvis was unfused and it possessed a dinosaur-like ilium similar to Archaeopteryx.  The astragalus (with the theropod ascending process) and calcaneum bones were free. The distal tarsals were unfused  (Sanz,1988).

Although the early theropods could possess 40-50 caudal vertebrae, the base of the tail became more mobile and the size of the tail was reduced. Most maniraptorans possessed between 23 and 26 caudal vertebrae and early birds possessed fewer than 23 caudal vertebrae (Padian, 2001). The number of caudal vertebrae in Archaeopteryx was numbered in the low 20s. After the evolution of the pygostyle, there were fewer than ten caudal vertebrae (Gatesy, 2001). The term Ornithurae is used to describe those birds whose tail is shorter than their femur because of the formation of the modern pygostyle. This lineage evolved after the enantiornithine birds diverged (Gauthier, 2001). The pygostyle evolved before Confuciusornis (Chiappe, 2001).

iberomesornis tails of birds
      Confuciusornis sanctus was pigeon sized bird of China; hundreds of specimens have been found. Like Archaeopteryx, it had long fingers with large curved claws and an unfused hand and wrist bones.  Like Archaeopteryx, it possessed a curved femur but unlike Archaeopteyx, it had a toothless, horny beak.  Its toothless beak is interesting—it isn’t the direct ancestor of modern birds since their close ancestors retained their teeth until 75 million years ago; thus toothless beaks in birds must have arisen more than once.  It was like the Cretaceous bird Gobipteryx (and unlike Archaeopteryx) in a number of features: a large, broad, heavy premaxilla with pitted holed for the nerves and vessels of the beak; a large external naris; a small antorbital fenestra; and a naso-frontal hinge for a modern pro-kinetic skull.  Other related birds (enantiornithine birds described shortly) do not share these characteristics and still have toothed maxillary and premaxillary jaw bones.   Confuciusornis had contour feathers on its leg (distinct from flight feathers) and males had very long tail feathers (Hou, 1995; Ackerman, 1998).Confuciusornis evolved a pygostyle, reduced the size of its tail, developed a stronger pectoral girdle, and modified its hand (Rayner, 2001).

Confuciusornis was capable of powered flight. The primary metacarpal was fused to the semilunate bone, forming the basis for the modern carpometacarpus. Its sternum was larger than in Archaeopteryx. Although most Confusciusornis specimens lack a keeled sternum, some specimens possess a small keel (Gauthier, 2001). The furcula and humerus were also relatively larger. Its foot was more adapted to perching than was that of Archaeopteryx. Its supracoracoideus muscle was not well developed which probably made it less likely to take off from a standstill. (Zhou, 2001). While the supracoracoideus muscle in modern birds is important in taking off from a standstill, it is not essential and other muscles are sufficient to enable birds to take off while standing (Sokoloff, 2001).

confusciornis

      There is disagreement as to the position of Confuciusornis but it seems to have been a basal bird whose lineage predates the division of birds into enantiornithine and ornithine groups. sister group to enantiornithine birds (Serreno, 1999; Sereno, 2001; Livezey, 2001). The family Confuciusornithidae includes Confuciusornis and Changchengornis (Chiappe, 2001).     Ambiortus from Mongolia had advanced characteristics such as a keeled sternum, a furcula, a pillarlike coracoid and primitive characteristics such as teeth and an unfused hand.

     Concornis lacustris is also known from Spain.  It possessed some derived features such as a true tibiotarsus, a fan-shaped distal metatarsal zone, a phalangeal structure like modern birds, an avian furcula, and a pygostyle.  Its metacarpus was not fused distally (Sanz, 1992).

Jeholornis , Bapeornis, and Confusciornis are classified as basal birds which predated the split between enantiornithine and ornithurine birds (Feduccia, 2006).

     Rahona ostromi lived in the late Cretaceous but its pelvic bones resemble those of the first birds. (Rahona means “menacing cloud.)  It possessed a number of theropod characteristics (such as a pubic boot and the articulations between its vertebrae) but had the distinctive sickle claw and hyperextensible 2nd toe found in dinosaurs such as Velociraptor.  It had primitive theropod characteristics such as a long tail, a saurischian vertebral articulation (also seen in Patagonykus), a pubic boot (as in Archaeopteryx and the enantiornithine birds) and some pelvic features (similar to Archaeopteryx and Unenlagia).  It possessed a synsacrum which is an advanced feature (Forster, 1998; Gibbons, 1998).

Rahonavis may represent the descendants of a group of ancestral birds more primitive than Archaeopteryx ( Kirkland, 2005). Others feel it represents the most primitive bird lineage after Archaeopteryx (Chiappe, 2001). Because of hip and limb similarities between Unenlagia and Rahonavis, it is possible that Rahonavis represents a dinosaur lineage which evolved a capability for flight independent of the lineage which evolved into birds (Makovicky, 2005). The term Avialae has been given to the lineage of flying feathered birds, including Rahonavis (Gauthier, 2001).

 
rahona rahona foot
sinornis
     Sinornis was a sparrow sized, perching bird that still retained gastralia and teeth (Sereno, 1992).

Family Alvarezsauridae

The group Alvarezsauridae was originally classified as a primitive family of flightless birds and included the fossils Alvarezsaurus, Mononychus, Patagonykus, Parvicursor, and Shuvuuia. It is now classified as a group of dinosaurs close to the dromaeosaur lineage which gave rise to birds. Like dromaeosaurs and birds, Mononychus possessed a pubis which projected backwards (Sereno, 2001). Alvarezsaurids may be the sister group of birds with Caudipteryx, Protoarchaeopteryx, dromaeosaurs, and oviraptorids forming a descending order of related lineages (Rayner, 2001).

The term Carinatae is used to describe those birds which developed a keeled sternum, including enantiornithes and the lineage which would lead to modern birds (Gauthier, 2001). Only about a dozen Mesozoic carinate bird fossils are known which consist of more than a single bone (Clarke, 2001).

cladogram

Enantiornithine Birds

      The enantiornithine birds (“opposite birds”) were a diverse group of birds of the Jurassic and Cretaceous. Fossil evidence suggests that the enantiornithine birds were the most common, widespread, and diverse group of Mesozoic birds. Enantiornithine birds were the dominant group of land birds during the Cretaceous period. Their fossils are known from all continents other than Antarctica and ranged in size from that of sparrows to that of vultures. The ancestors of enantornithines and the lineage which led to modern birds possessed a fused carpus, wings better adapted for flight, and a supracoracoideus muscle capable of supporting takeoff from a standstill (Rayner, 2001; Feduccia, 2003). Enantiornithines also differ from ornithurine in the structure of their shoulder and sternum. Ambiortus and Hongshanornis were ornithurine birds known from the Early Cretaceous. Hongshanornis was small and possessed a beak (Feduccia, 2006).

Although they were certainly birds, they left no modern descendants and are not closely related to modern birds. These birds form a distinct group based on the opposite fusion of three tarsal (foot) elements.  (In enantiornthines, the developmental fusion of tarsal elements was proximodistal, as opposed to the distal to proximal fusion in modern birds; also the canal for the ligament for the wing upstroke is formed by a distinctive bony configuration.)      The enantiornithine birds also have a reduced outer metatarsal, a highly modified pectoral girdle, and, in some, a characteristic perforation of the humerus and an extreme modification of the tarsometatarsus.  There are a number of characteristics that set them apart from both the Jurassic birds (Archaeornithes) and more modern Odontornithes and Neornithes.     Enantiornithine birds are known from at least 5 continents and disappeared with the dinosaurs without leaving any descendents.  The earliest had an Archaeopteryx-like skull.  Most had teeth and fleshy tails.  They include Enatornis, Nanantius, Neuquenornis,  Cathayornis, Eoalulavis, Gobipteryx, Avisaurus, and Protopteryx.  Neuqueornis, Soroavisaurus, and Avisaurus are classified together as a group of related enantiornithine birds. While some were flightless birds, others were capable of flight (Chiappe, 1994;Chiappe, 2001; Walker, 1981; Molnar, 1986; Padian, 1996; Elzanowski, 1976; Feduccia, 1995). Archaeopteryx and the enantiornithine birds probably lacked the complete air-flow system of modern avian lungs given that they lacked thoracic modifications typical of modern birds (Jones, 2001).

      Protopteryx is the most primitive enantiornithine bird known.  Its primitive characteristics include its teeth, a long quadrate, the shape of the postorbital bone, a procoracoid process (which is unknown in other enantiornithine birds), the size of the lateral process of the coracoid, an unfused carpometacarpus, and unfused tibiotarsus, the long alular digit, a pubic boot, and an ilium which resembles that of Archeaopteryx.  Protopteryx possesses three kinds of feather: downy feathers, flight feathers, and primitive, scalelike feathers in the center of the tail.  This type of feather is also known in some specimens of Confusciornis, a more advanced enantiornithine bird and at least 4 other enantiornithine birds (Zhang, 2000).  Vorona and Patagopteryx may be a separate group closely related to the enantiornithines (Forster, 1996). 

cathayornis cathayornis 2
     Vescornis hebeiensis possessed teeth and its wing claws were reduced in size (Zhang, 2004).  Cathayornis is a bird from the early Cretaceous of China which retained characteristics of the Archaeopteryx skull. It had short, toothed premaxillaries bones, separate nasal bones that meet in the midline, and a submaxillary fossa in the antorbital fenestra.  There was no postorbital bar (Martin, 1997).
     Eoalulavis hoyasi or “dawn alula bird” is known from 115 million years ago and was about the size of a goldfinch.  It had the alulae or “bastard wings” --the thumblike components of the wings of modern birds that help them make successful landings and takeoffs.  Some feel that it belonged to a sister group of the enantiornithine birds (Sanz, 1996). 

Analysis of skeletal tissues indicates that the growth patterns and metabolic rates of enantiornithine birds were not substantially different from those of modern birds (Cambra-Moo, 2006).

 

In the Cretaceous, primitive ornithurine birds diversified to form a number of lineages on land, a common group of loon-like diving birds with toothed jaws (Hesperornithiforms), and a tern-like group (Ichthyornithiforms). The Ichthyornithiforms included a number of species (Ichthyornis, Apatornis, Apsaravis, Limeavis) and were known from North and South America, Europe, and Asia. Some primitive ornithurine birds evolved into giant forms such as the giant Gargantuavis, known from France in the Late Cretaceous. Not a single modern ornithurine bird fossil is known until after the Cretaceous/Tertiary extinction.

The Cretaceous/Tertiary extinction event apparently wiped out the enantiornithine birds and the primitive ornithurine birds (Feduccia, 2003).