Archaeopteryx, as we have seen, had many features that are far from the condition found in living birds, including teeth, an unfused hand, a bony tail, no synsacrum, and gastralia. How and when did the changes take place that distinguish living birds from Archaeopteryx? Here our interest will be within Avialae, the clade that includes Archaeopteryx, Aves (living birds), and everything in between.
Within Avialae, very close to Archaeopteryx is Rahonavis from the Late Cretaceous of Madagascar. We've opted to emphasize its avian features in tentatively considering it more derived than Archaeopteryx. Slightly larger than Archaeopteryx (the size of a crow; Figure 11.1), recent work places Rahonavis as a dromaeosaurid theropod, but its position above or below Archaeopteryx remains uncertain. It possessed an enlarged sickle-shaped claw on its feet (similar to that of dromaeosaurids and troodontids), and a long, Archaeopteryx-like tail. Younger that Archaeopteryx by 25 million years, it had forward-looking features such as pneumatic foramina leading into pleurocoels in its thoracic vertebrae, which as we've seen implies unidirectional breathing and possibly a more efficient metabolism (see Box 8.1 and Chapter 12), along with a series of other bird-like characters (Figure 11.2). Time and further analyses will eventually consolidate its position.
Flight proficiency seems to have been a driving force in avialan evolution. Subsequent events included the formation of a pygostyle as well as the development of the synsacrum and other features for a rigid trunk, all of which contribute to the efficient flight that characterizes modern birds. Confuciusornithidae, a group containing Confuciusornis (Figure 11.3) and Changchengornis from the Early Cretaceous of China, is characterized by these relatively modern features (see Figure 11.2).
Figure 11.2. Cladogram of Mesozoic birds, depicting some of the steps in early avialan evolution. Derived characters include: at 1, pygostyle; at 2, reduction in number oftrunk vertebrae, flexible furcula, strut-like coracoid, alula, carpometacarpus, fully folding wings; at 3, further reduction in number oftrunk vertebrae, loss of gastralia, final rotation of pubis to lie parallel with ilium and ischium; at 4, reorientation of pubis to lie parallel to ilium and ischium, reduction of number oftrunk vertebrae, decrease in size of acetabulum, patellar groove on femur; at 5, loss of teeth.
Thereafter, birds reduced the number of trunk vertebrae, altered the shoulder joint, and fused the digits of the hand into a carpometacarpus, among other flight-related features. All birds with these transformations are united within Ornithothoraces (ornitho - bird; thora - "thorax, or chest"). Ornithothoracans have two main divisions: on the one hand, Enantiornithes (enant - opposite), and on the other Ornithuromorpha (ornitho - "bird;" uro - "tail;" morpha- "form"), the lineage leading to Aves (see Figure 11.2).
Enantiornithes. Enantiornithes ("opposite bird") were all sparrow-sized, small, arboreal birds (Figure 11.4) that had well-developed flight capabilities. They were the most diverse clade of Mesozoic birds, yet all went extinct before the close of the Era.
Enantiornithes modified the wrist joint to allow, for the first time, the wing to fold tightly against the body, and developed adaptations indicative of perching: the first toe is positioned opposite to the others. The perching foot is a clue that these Mesozoic birds lived in trees, reinforcing other evidence that flight was an integral part of their life habits.
Still, one would hardly call Enantiornithes modern birds - they had gastralia across their belly (a carry-over from the primitive archosaurian condition), relatively numerous back vertebrae (a number intermediate between the 13 or 14 found in Archaeopteryx and the 4-6 found in living birds), an unfused tarsometatarsus, and an unfused pelvis.
Enantiornithes apparently had a worldwide distribution. Nanantius is from Australia, Iberomesornis hails from Spain, and Sinornis comes from China. Others include Kizylkumavis and Sazavis from Uzbekistan (Asia), Alexornis from Mexico, Enantiornis itself and Avisaurus, known from both Argentina and the USA.
Ornithuromorpha. Returning to the line leading to Aves, Ornithomorpha is represented by Vorona from the Late Cretaceous of Madagascar, Patagopteryx (Figure 11.5) from the Late Cretaceous of Argentina, and all remaining birds, the clade known as Ornithurae (ornith -bird; uro - tail).
Ornithurae is one of the most robust of all avialan clades, united by at least 15 unambiguous characters (see Figure 11.2) Not surprisingly, ornithurans include not only the closest relatives to living birds (Hesperornithiformes and Ichthyornithiformes), but also Aves as well.
Hesperornithiform (hesper - western) birds were a monophyletic clade of large, long-necked, flightless, diving birds that used their feet to propel themselves, much like modern loons or cormorants (Figure 11.6). They had highly reduced arms and developed powerful hindlimbs for propulsion. The hindlimbs were oriented to the side of the creature, and could not be brought under the body. For this reason, locomotion on land must have been a kind of seal-like waddling (at best).
In the water, on the other hand, hesperornithiform birds were supremely adapted for marine life. The long, flexible neck must have been useful in catching fish, a behavior indicated from coprolites preserved with their skeletons. In many respects, the group is quite close to modern birds, yet they retained teeth in their jaws. Like modern diving birds, some of
Figure 11.5. Patagopteryx, from the Late Cretaceous of Argentina.
the pneumaticity in the bones was lost. Presumably because strength for flight was no longer needed, the furcula, coracoid, and forelimbs were highly reduced.
All of these adaptations indicate that this group had a long evolutionary history prior to their appearance in the Late Cretaceous. Hesperornithiforms include Enaliornis from the Early Cretaceous of England, Hesperornis (Figure 11.6) and its smaller relative Baptornis, and Parahesperornis, all from North America.
Closer related yet to Aves were the toothed Ichthyornithiformes (see Figure 11.2). Unlike hesperornithiforms, ichthyornithiforms were excellent flyers (Figure 11.7). Ichthyornis, from the Late Cretaceous of North America, had a massive keeled sternum and an extremely large deltoid crest that was probably an adaptation for powerful flight musculature. In other respects, it shared many of the adaptations of modern birds including a shortened, fused trunk, a carpometacarpus, a pygostyle, a completely fused tarsometatarsus, and a synsacrum formed of 10 or more fused vertebrae. Found exclusively in marine deposits, ichthyornithiforms must have been rather like Mesozoic sea gulls - but with teeth.
Aves is a well-supported clade, involving as many as 11 characters of the skull, pelvis, and ankle. The Mesozoic fossil record of Aves -all of it restricted to the Late Cretaceous - is very fragmentary and scattered, although tantalizing.
The group includes screamers and waterfowl (Anceriformes), loons (Gaviiformes),
and possibly shorebirds such as sandpipers, gulls, and auks (Charadriiformes), landfowl (Galliformes), wing-propelled divers such as modern petrels (Procellariiformes), and parrots (Psittaciformes). Clearly, these early records of modern birds speak, however incompletely, to the origin, initial radiation, and establishment of Aves in the closing moments of the Cretaceous.
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