After the enantiornithine lineage diverged, the ancestors of Patagopteryx and modern birds shortened the length of the back and fused the hip and sacral bones to form the synsacrum (Rayner, 2001). The most primitive branches of ornithine birds led to Patagopteryx and Vorona. Hesperornis, Ambiortus, and Ichthyornis arose afterwards (Chiappe, 2001). Patagopteryx was a hen-sized, flightless bird.  Growth rings in its bones were intermediate between dinosaurs and modern birds (Chiappe, 1995).


The fossil Ornithurine birds classified as Hesperornithiformes were a group of diving birds that propelled themselves with their feet. They were known from the Mid to Late Cretaceous in North America, Europe, Asia, and possibly South America and Antarctica as well. Their size ranged .5 to 1.5 meters in length. In addition to primitive members of the group (such as Enaliornia and Baptornis), several species are classified in the family Hesperornithidae (such as Hesperornis, Parahesperornis, and Asiahesperornis) (Dyke, 2006). They had primitive features such as teeth, the shape of the coracoid in the shoulder, the prominence and late fusion of the ascending process of astragalus, a pelvis which was not completely fused, and other features.  Their advanced features included a pygostyle, a synsacrum, a fused tarsometatarsus, a shortened trunk, and a carpometacarpus.   Hesperornis possessed foot modifications found in some modern birds.   Baptornis was loon-sized (Martin, )

BAPTORNIS baptornis 2

Ichthyornis and Apatornis from North America Late Cretaceous were capable of powerful flight (Swinton, 1975).


The earliest ornithurine bird which lacked teeth was Hongshanornis from the Early Cretaceous. It was a small bird which possessed 2 claws. Like other primitive ornithurine birds, it possessed a small predentary bone at the tip of its lower jaw (Zhou, 2005). Teeth were reduced or lost in several separate lineages of primitive birds. Teeth were completely lost in the primitive bird Confuciusornis, they were lost in the enantiornithine Gobipteryx and reduced in the enantiornithine Longirostravis, and they were lost in the primitive ornithurine Hongshanornis and reduced in Jeholornis (whose thick bill possessed only 3 teeth in the lower jaw and suggests that if fed on seeds) and Sapeornis (which lacked teeth in the lower jaw; Zhou, 2005). Cretaceous ornithurine birds were similar to modern birds in both their skeletal anatomy and their bone histology (such as the lack of growth rings) (Feduccia, 2006).


The primitive ornithurine bird Asparavis and many modern birds ossified tendons in their feet ( Hutchinson, 2002). Modifications of the wing bones identify the Late Cretaceous bird Limenavis as being more closely related to the crown group of modern birds than the enantiornithes or primitive ornithurine birds such as Ichthyornis (Clarke, 2001).

Unfortunately, fragmentary fossils and uncertain dating have made it difficult to determine whether modern bird orders originated after the extinction of the dinosaurs or whether some of these lineages arose just prior to this extinction in the Late Cretaceous. There are some fossils which may support the existence of four lineages prior to the end of the Cretaceous. Fossils at three sites have been identified as Late Cretaceous loons. Two fragmentary fossils attributed to the early duck relative Presbyornis have been identified from the Late Cretaceous. Other fragmentary fossils attributed to birds which live near aquatic environments (Order Charadriiformes which includes gulls and puffins and Order Procellariiforms which includes albatrosses) have also been found in the Late Cretaceous (Dingus, 1998; Cooper, 1997).

Studies that have moved embryonic tissue from the mouth of a chick and transplanted it into mouse and lizard embryos have demonstrated that chicken cells are capable of synthesizing teeth, although teeth have not existed in the modern bird lineage for 70-80 million years. Chickens with mutations in the talpid gene produce teeth similar to those of other archosaurs such as crocodiles (Harris, 2006). Although modern birds to not possess teeth, they continue to synthesize proteins which function in tooth development in nonavian embryos and vestigial structures temporarily form in the embryonic jaw. When mouse neural crest cells are transplanted in chick embryos, the chick embryos developed mineralized tooth-like structures (Mitsidais, 2003).

     Some modern birds still possess small claws on the tips of their digits.  One South American bird has clawed fingers as a hatchling.  Another South American bird possesses prominent claws as an adult:


claws 2


Based on the presence of a modified joint between the palatine and pterygoid or the ancestral condition, birds can be classified as Neognathidae or Paleognathidae. Paleognathous birds are divided into tinamous and ratites. Neognathous birds are divided into Galloanseridae (which contains the modern bird orders Galliformes and Anseriformes) and Neoaves (Gauthier, 2001).

Paleognathus Birds


     Many modern flightless birds are members of bird groups can fly.  For example, some evolved on oceanic islands where the absence of predators made flight less necessary. Other flightless birds are classified in the group of Paleognathus (“old mouth”) birds which is more primitive than the majority of modern birds (which or classified as neognathus or “new mouth” birds).  These flightless birds are known as ratites may not be closely related to each other.  The paleognathus condition is the combination of a number of shared characteristics associated with the mouth (the vomer is firmly attached to the pterygoid, the absence of joint between the pterygoid and palantine bones, the movable joint between braincase and pterygoid).  The retention of primitive characteristics does not necessarily indicate that the ratites share a common ancestry.  Paleognathus birds also differ from neognathus birds in other features such as their external auditory meatus, pharngotympanic tube, quadratocranial articulation, paths of cranial nerves, loss of paratympanic organ, and loss of chorda tympani.  These features suggest that they are a clade (Starck, 1995; Livezey, 2001) Mitochondrial DNA analyses indicate that tinamous and ratites are distantly related to other birds (Slack, 2003). There are several primitive “reptilian” features of the braincase (such as the basipterygoid processes and pterygoid articulation) which are not only present in paleognathus birds, but also the embryos of neognathus birds and even the adults of some groups. (Ericson, 1997).


The earliest fossil paleognath, Lithornis, is known from the Lower Eocene. This bird and its relatives of the family Lithornidae were chicken-sized birds capable of flight. This group was more closely related to modern tinamous than to the flightless ratites (Leonard, 2005). Paleognathus birds similar to tinamous were common during the Paleocene and Eocene. From these small, flying birds evolved several lineages of large flightless birds. The earliest ostriches are known from the Miocene, the earliest moas from the Pliocene, and the earliest elephantbirds from the end of the Pliocene (Feduccia, 2003). Kiwis, rheas, cassowaries, emus, and ostriches (plus extinct moas and elephant birds which were hunted to extinction when humans migrated to New Zealand and Madagascar) are restricted to the southern continents.  Fossil ostriches are also known from Europe. Titanis existed in North America only after Americas join (they probably migrated from South America).   Fossils indicate that crane-like Gruiforms gradually reduce third toe and similar to fossil ostriches (which have only 2 toes).  It is possible that ostriches should be included in Gruiforms and not with other ratites (flightless birds). The diversity of elephant birds varied in their sizes and the largest could reach heights more than 3 meters (10 feet). The largest elephant bird eggs measured 3 feet in circumference at their largest point and could hold a volume of 9 liters (Swinton, 1975).

Morphological and genetic evidence suggests that the kiwi is the sister group of the remaining ratites (with emus and cassowaries forming one group and ostriches and rheas forming a second) (Leonard, 2005).


Neognathus Birds



Most modern orders of birds were represented in the fossil record by the Lower Eocene, about 10 million years after the Cretaceous/Tertiary extinction (Feduccia, 2003).  Neornithurine skulls continued to evolve. For example, the condition of pterygoid bones contacting basipterygoid processes is known in reptiles, all paleognathus birds, the adults of two neognathus birds (some Gruiformes and common in Charadriiformes), and the young of several other groups of neognathus birds (Ericson, 1997).

It is the modern consensus that neognathus lineages evolved to produce a series of aquatic groups, birds of prey, and then a diversity of terrestrial groups of modern birds (ending with the largest group, the passerines that include about 5000 species).  Earliest Passerines known from the Southern hemisphere in the Late Cretaceous.  The more primitive of the two groups of modern passerine birds, the suboscines, are most abundant in Southern continents.  Unfortunately, the fossil record of southern continents has not received the same attention as northern (Barker, 2004).


Anatomical traits and DNA evidence suggest that the most primitive branch of the Neognathus birds produced the modern groups Galliformes and Anseriformes (Cracaft, 2001). Other analysis suggests that Galliformes is the most primitive branch followed by Anseriformes which forms the sister group to all other birds (Ericson, 2001). Mitochondrial DNA analyses indicate that Galliformes and Anseriformes are a sister group to other neognathus birds (Slack, 2003).

      Hoatzins (Cuculiforms) are thought to be a descendent of species near the base of the land bird lineage; its nestlings they retain clawed digits on wing, as do some ratites.  A number of fossils Cenozoic birds known and most bird families known by the end of the Eocene. 

Presbyornis is a fossil Anseriform closely related to the duck family Anatidae which is known from the Paleocene and Eocene (Ericson, 1997; Elzanowski, 1995).  Parrots are known from the Miocene (Stidham, 1998).  Falcons are known from the Miocene, and 62 extinct species of hawks and eagles are known from as early as the Late Eocene. 



The Cretaceous loon Polarornis is the only Cretaceous bird classified in a modern bird family (Chatterjee, 1997). Presbyornis has a morphology intermediate between ducks and flamingos (Chatterjee, 1997).

The earliest known penguin Waimanu was intermediate between loon-like birds and higher penguins.

fossil penguin

The 17 species of penguins represent the largest modern family of flightless birds. After the diverse marine reptiles became extinct at the end of the Cretaceous, penguins adapted to aquatic life and became the dominant endothermic marine predators from 40 to 25 million years ago. Early penguins were living in Peru in the Eocene, during one of the Cenozoic’s warmest periods (Clarke, 2007). Perudyptes devriesi possesses cranial features which link it to loons and its wing was intermediate between Waimanu and modern penguins (Clarke, 2007). Icadyptes salasi was a giant penguin, standing 1.5 meters tall. There were at least 40 species of fossil penguin, half of which were larger than the largest modern penguins. The species Anthropornis nordenskjoeldi reached a height of more than 5 ½ feet and may have weighed 300 pounds. Modern penguins range in size from the Little or Fairy Penguin which measures 16 inches long and weighs 1 ¾ to 3 pounds to the emperor penguin which measures almost four feet in length and weighs 55-84 pounds (Lynch, 1997). Athough there are a number of primitive penguins known from the eary Cenozoic, the earliest member of the modern family Spheniscidae is first known from fossils 8 million years old. The early penguins Perudyptes, Waimanu and Icadyptes possessed longer bills than any modern penguin (Clarke, 2007).


Enormous fossil raptors are known from North and South America.  Teratorns such as Teratornis merriami and Teratornis incredibilis from Norht

America had wingspans of 12 and 16 feet while Argentavis magnificens from South America

had a wingspan of 25 feet and may have weighed 250 pounds. 



Haast’s eagle was the world’s largest eagle (at 30 pounds) which lived in New Zealand and preyed on the flightless moas (Weidensaul, 1996). One of the eagles known from La Brea tar pits, Wetmoregyps, also possessed long legs (Kurten, 1988)


Extinct pelicans could reach wingspans of 18 ft  and the largest penguins are estimated more than 6 ft.  The earliest passerines are known from the end of the Oligocene (Colbert, 1991).  There are more than 9000 species of modern birds. Most modern groups of birds appeared in the early to mid-Tertiary including grebes (Oligocene), albatrosses (Oligocene), Pelicans (Eocene), penguins (Eocene), herons (Eocene), flamingoes (Eocene), ibises (Eocene), ducks (Eocene), true geese (Miocene although the possible ancestor Romainvillea is known from the Eocene), rails (Eocene), pigeons (Miocene), parrots (Oligocene), cuckoos (Oligocene), owls (Eocene), swifts (Oligocene), and earliest passerines (Oligocene) (Swinton, 1975).

hawk swans
dove geese

After the Cretaceous/Tertiary extinction, the ancestors of modern ornithurine birds (Neornithes) found themselves in a world which suddenly lacked the dominant bird groups of the Cretaceous, the enantiornithine and primitive ornithurine birds. The modern ornithurine birds began to diversify. Some evolved into the giant terror birds such as Diatryma and Gastornis (Feduccia, 2003).

    At least one giant bird is known from the Cretaceous, Gargantuavis, an ostrich-sized bird from France (Bartlett, 1998).  In the following photo, a stork’s display of aggression has managed to frighten off a small deer.  A number of birds evolved to much larger sizes and actually preyed on mammals.


Modern ostriches can reach 8 feet in height and the recently extinct New Zealand moas could reach 15 feet.  At least 12 genera are known from the fossil record which range in height from 1 to 3 meters (one almost reaching 10 feet in height).  The oldest known member of the family of phororhacoids, Paleopsilopterus,  is known from just after the extinction of the dinosaurs and was only about a meter in height.  Another member of this family, Ameghinornis, was probably capable of short flights.   There are two foot tall running birds in South America today (Cariama and Chunga) which are considered to be closely related to this family.  Fossil species such as Diatryma, Andalgalornis, and  Phororachos could reach 6 feet in height.   Diatryma’s skull was the size of a horse’s skull and an adult bird probably weighed over 380 pounds.  Because of their size, some fossils of terror birds have been misidentified as belonging to horses or other animals (Marshall, 1994; Case, 1987).  Although the limbs of Diatryma would have allowed an active hunting lifestyle, its jaws were so powerful that it could have crushed bone and specialized in scavenging (Witmer, 1991)

diatrymaterror bird

     After North and South America fused 4 million years ago, the number of these birds dwindled.  Titanis walleri (6 ft.) apparently colonized North America from South America and it still retained the ancestral trait of clawed fingers and strong arms (Brodkorb, 1963; Zimmer, 1997).  Terror birds might have survived until the point that humans colonized the Americas.   The giant bird Genyornis became extinct at the time when humans colonized Australia (about 50 thousand years ago) (Miller, 1999).  New Zealand separated from Antarctica and Australia about 80 million years ago. A group of flightless birds known as moas diversified to produce at least 64 species which ranged in weight from 20 to 250 kg and(which could reach 15 feet in height. The hunting of the moa began in the 13th century, peaked about 700 years ago and ended 400 years ago (Holdaway, 2000; Baker, 2005).

Elephant birds could weigh 450 kg and were the heaviest birds in history (Dewar, 2007). Elephant birds laid eggs which measured a foot long. The last one was killed in Madagascar in 1658 (Chatterjee, 1997).

elephant bird

diatryma phororhacis gastornis
     Some modern birds still have aspects of their anatomy which link them to their ancestral past.  Hoatzins and some ratites possess clawed digits when they hatch.  Ratites and some tinamous lack a pygostyle (although this might have been lost after it developed).  Bird embryos still possess the remnants of a finger which degenerates without forming part of the adult hand.  Bird embryos develop tooth buds and can even be manipulated so they actually develop teeth.  One leg muscle begins with its ancestral attachment points before developing the modern pattern.  Some birds carry variations in their femurs (winged lesser trochanters and a separate greater trochanter) which resemble the ancestral form.   The bird of paradise has modified feathers which resemble the scale-like feathers of Protopteryx.