Palaeognathae represents a distinct and ancient lineage within the Class Aves. This group, whose name is derived from the Ancient Greek for “old jaws,” is one of the two primary living clades of birds, alongside the more diverse Neognathae. Together, these two clades form the Neornithes, which includes all modern birds. Palaeognathae are often considered basal, or more primitive, among living birds, setting them apart through unique anatomical characteristics.
Defining Traits
The defining characteristic of Palaeognathae is their unique palate structure, known as the “palaeognathous” palate. This skeletal arrangement involves a distinct fusion pattern of skull bones, restricting jaw movement and resembling the palates of some reptilian ancestors. This structure influences their feeding strategies, often limiting them to specific diets, such as seeds or vegetation.
Most Palaeognathae are predominantly flightless birds, collectively referred to as ratites. These species exhibit adaptations for a ground-dwelling existence, including strong legs suitable for running and significantly reduced wings. Their sternum, or breastbone, typically lacks the prominent keel found in flying birds, which normally serves as an anchor for powerful flight muscles.
The tinamous, a group found in Central and South America, are an exception as they retain the ability to fly, albeit weakly. Unlike other members of the group, tinamous possess a keeled sternum, a feature traditionally associated with flight. This highlights the varied adaptations within Palaeognathae, as not all members share the same locomotor capabilities.
Diverse Members and Habitats
Palaeognathae encompass a variety of species, each uniquely adapted to its environment, across different continents. The group includes the ostriches of Africa, known as the largest living birds, which can stand up to three meters tall and weigh around 135 kilograms. These powerful runners inhabit open savannas and deserts, using their speed and strong legs for defense.
South America is home to the rheas, large flightless birds resembling ostriches, that roam the grasslands and open areas. In Australia, the emu is a widespread inhabitant of various habitats, from woodlands to arid plains. The cassowaries, with their distinctive casque and vibrant neck coloration, are found in the rainforests of Australia and New Guinea.
New Zealand hosts the kiwis, small, nocturnal birds with long beaks and a shy demeanor. Unlike their larger relatives, kiwis are more secretive, foraging for invertebrates in dense forests. The tinamous are distributed across diverse habitats in Central and South America, including forests and grasslands. This geographical spread across southern continents reflects an ancient distribution pattern.
Evolutionary History
The evolutionary journey of Palaeognathae spans millions of years, with their fossil record extending back to the Paleocene epoch, approximately 60 million years ago. Scientific understanding of their diversification has evolved, particularly concerning flightlessness. While the group is considered monophyletic, meaning they share a common ancestor, molecular research indicates that flightlessness arose independently multiple times across different lineages within Palaeognathae. This pattern is known as parallel evolution.
Evidence suggests that the ancestors of modern Palaeognathae were likely capable of flight. Some early Cenozoic birds possessed the characteristic palaeognathous palate but also had a keeled sternum, implying they were able to fly. This indicates a transition from flighted to flightless forms over deep time.
Historically, the global distribution of flightless Palaeognathae across continents that were once part of the supercontinent Gondwana led to the “Gondwanan vicariance hypothesis.” This theory proposed that their current locations were primarily a result of the breakup of Gondwana. However, more recent molecular phylogenetic studies and divergence time analyses have challenged this simple model. These studies suggest that while their deep roots may align with Gondwanan patterns, many instances of flightlessness and large body size evolved more recently, indicating dispersal and independent adaptations after the continents had separated.