Palaeognathae

Palaeognathae is a group of birds known as paleognaths. They belong to a special group called an infraclass within all birds, called Aves. There are two main groups of birds, and Palaeognathae is one of them. The other group is called Neognathae.

Palaeognathae includes five groups of birds today. Four of these groups are flightless birds called ratites. Examples include kiwis, cassowaries, emus, rheas, and ostriches. The fifth group is the tinamous, which can fly and live in South America.

Scientists have found that paleognaths share a common ancestor, but they used to think that all flightless birds were closely related. Now we know that tinamous are related to ratites, and that being unable to fly happened many times in different groups. There were also ancient flightless birds like moas and elephant birds that are now extinct. The name paleognath means "old jaws" because these birds have a jaw structure that looks more like that of reptiles than other birds. Even though they have some ancient traits, these birds have continued to change and evolve over time.

Origin and evolution

The Palaeognathae, also called paleognaths, are a group of birds that includes flightless birds like ostriches and emus, as well as flying birds called tinamous. Scientists have long wondered when these birds first appeared. While there are no clear fossil records of paleognaths until the Cenozoic period, some fossils from earlier times suggest they might have evolved during the Cretaceous period.

Studies combining molecular and fossil data suggest that modern bird groups, including paleognaths, began to diverge in the Early Cretaceous, around 110 to 120 million years ago. However, the oldest known fossils are from about 70 million years ago, leaving a gap in the fossil record. Some scientists think these birds evolved earlier than the fossils show, while others believe bird evolution sped up after a major event called the Cretaceous–Paleogene boundary.

Before genetic analysis, some scientists questioned whether all paleognaths were closely related, suggesting they might look similar because they evolved flightlessness independently. Recent studies show that paleognaths form a natural group, but the flightless birds within them evolved flightlessness separately. Tinamous, which can fly, are closely related to the extinct moa of New Zealand. Modern paleognath orders originated after the Late Cretaceous, with ostriches appearing first, followed by rheas, kiwis, elephant birds, cassowaries, and finally tinamous and moas.

Description

Paleognaths are birds with a special bone structure in their upper jaw. They also share similar pelvis shapes, with a large opening in the bones. These birds have a special pattern of grooves on their beaks.

In paleognaths, the males take care of the eggs. Sometimes, females that are not their mates may leave eggs in the male's nest.

Tinamous, found in Central and South America, are small birds that can fly a little. They have short tails and look like quails. Their feathers have a unique way of connecting.

Ratite birds cannot fly and are built for life on the ground. They have flat breastbones, unlike flying birds which have a special bone for strong wings. Ratites have strong legs and simple wings. Most do not have a gland that produces oil for their feathers.

Living paleognaths range in size from about 6 inches to over 9 feet tall. Ostriches are the largest, with long legs and necks. Emus and rheas are also tall, while cassowaries and kiwis are smaller. Tinamous are the smallest, about the size of a chicken. Many of these birds can run very fast, with ostriches being the quickest.

Biogeography

Today, most flightless birds called ratites live in the Southern Hemisphere, but they used to be found in Europe, North America, and Asia too. Long ago, during a time called the Cretaceous, all these southern lands were joined together as one big continent named Gondwana. This is important for understanding how these birds evolved.

There are two main ideas about how these birds developed. One idea says they evolved once on Gondwana and then spread to the lands that moved apart. The other idea says they evolved after a big event that changed the Earth, from flying ancestors in many places. Scientists are still learning more about which idea is right. A study in 2016 suggested these birds may have started in Laurasia.

Relationship to humans

Humans and ostriches lived close together in Africa long ago. When humans moved to other places, they still saw ostriches in Arabia and parts of Asia. Later, people in New Guinea, Australia, Central and South America, and other areas met different large birds like tinamous, rheas, elephant birds, and moa. These big birds had grown up without many predators, so they had a hard time when humans arrived. Many of these large birds are no longer with us today.

Now, ostriches and other large birds are sometimes raised on farms or kept as pets. They are important in many cultures and can be found in zoos, used for food, racing, and other purposes.