The following groups of birds are some of the earliest of the birds we know today:-
- Tinamous have prospered in South America - they have adapted to live in habitats ranging from 500 to 4000 metres elevation.
- Ostriches now favour open spaces in Africa but are unlikely to evolve further - little scope for separated populations.
- Swans, Geese and especially Ducks have prospered world-wide because they evolved a variety of feeding techniques.
- Giant Penguins used to exist – our modern species have been evolving since about 35 mya.
These phrases support and demonstrate two key points about evolution which are expanded upon in the following review:-
1. The environment and habitat into which birds evolve plays a major part in determining which offspring will survive and prosper.
2. Circumstances which facilitate separation of species populations promote the development of new species.
Ancient Ancestors of our modern birds.
Birds have been evolving for at least 150 million years. We can learn more about our modern birds by looking back in time to some of their ancient ancestors. The time period seems long to us but is short in geological and evolutionary terms. Changes and events during the time period are important to an understanding, or at least an appreciation, of how they have influenced evolution.
The environment plays a major part in determining the success or failure of bird species. As species populations develop, physical changes to the topography may facilitate population separation and such changes may eventually result in the formation of new species. That’s what evolution is all about.
If we consider a bird which is alive today its ancestors, which evolved millions of years ago, will have originated in a particular geographic location and habitat in which it has been able to prosper. Where it is today will depend on the geographic movements of successive generations (many of them) and the adaptations which have occurred to enable it to continue to survive in what must have been a series of changing habitats.
The purpose of this review is to highlight how some of the changes have influenced the evolution of our modern birds. There seems to be general agreement that the first two groups of our modern birds evolved about 90 million years ago somewhere on the part of the giant land-mass known as Pangea. This land mass was breaking up to form Gondwanaland as the Southern Hemisphere and Laurasia as the Northern Hemisphere.
Tinamous and Ostriches.
It has been suggested that birds in these two groups shared a common ancestor which may have been a member of the Lithornithidae family (now extinct but dates back to about 88 mya). Fossils of birds in this family dating from the period 65 to 35 mya appear to be similar to the Tinamous.
South America became home to the Tinamous.
The part of Gondwanaland we now know as South America was drawing away from the part which was to become Africa and Europe. Various rifts and land movements were leading to the formation of the mountainous Andes. Much of the land was heavily forested with plenty of lush vegetation which provided food for plant eating dinosaurs. It was into this habitat that the Tinamous evolved.
Ability to adapt to a range of habitats at various elevations favoured speciation.
The Tinamous are large bodied, secretive, ground dwelling, birds which favour wooded and forest habitats where they can run for cover. The Tinamou is a weak flier which perhaps explains why they have not spread beyond Trinidad and Costa Rica. Evidence suggests that they began to radiate strongly from about 30 mya onwards, filling habitats ranging from dense forest to areas of open grassland which were becoming available by then. Their ability to live at elevations ranging from 500 to 4000 metres will have been favourable to separation of populations and formation of new species.
Currently 49 species are found in a variety of habitats. Open grasslands account for 7 species which evolved about 23 mya, woodlands and forest account for a further 22 species evolving 16 mya. The remaining species appear to have evolved in the period 5 mya to the present day.
Africa and Asia suited the Ostrich.
At about the same time as the Tinamous were evolving a group of very different birds, which we call the Ratites, were coming on the scene. The Ratites are flightless ground foragers. Living members of the Ratite lineage include the Ostrich of Africa, Rhea of South America, Emu and Cassowary of Australia and New Guinea, and the New Zealand Kiwi. The extinct Elephant bird of Madagascar and the Moas of New Zealand were Ratites. These birds went wherever their piece of Gondwanaland was going and because they had become flightless they were isolated on these land-masses.
There seems to be some doubt about which came first; the Rhea or the Ostrich. Which ever it was the Rhea went with the South American land-mass and the Ostrich went with the parts of Gondwanaland which became Africa and Asia.
Ostrich moved from forest to open country habitat and prospered at first but….
Initially the Ostrich and probably the Rhea occupied forest habitats but they were to follow a totally different line of evolutionary development to the Tinamous. The trigger for this change if direction may well have been the impact of a giant asteroid on the earth 65 mya (the K/T event) caused global mass extinction of an estimated 75% of animals and plants, most notably the dinosaurs!
An abundance of food (plants and invertebrates) and a lack of predators, following the extinction of dinosaurs, saw these birds grow larger and become flightless. So Ostriches, Rheas (and Emus which evolved in Australia) are large, robust birds which prefer open spaces where their good vision enables them to spot potential predators and run if necessary. In contrast to the Tinamous the Ratites rarely go above 100 metres in elevation.
The open landscapes in which we see these birds today did not appear until about 35 mya. Some birds in the modern Ostrich genus Struthio are known from 23 mya onwards. The habitats which Ratites occupy now don’t encourage separate populations to develop so further speciation would appear to be extremely unlikely. Only two species of Ostrich and Rhea and one of the Emu remain today.
Wildfowl and Waterfowl – land and inland waters encourage diversification.
I use the term Wildfowl for the ground dwelling birds and Waterfowl for the Swans, Geese and Ducks, which have evolved in an aquatic environment. These two groups, known collectively as Galloansers, are basal to all of our other modern birds. This group appears to have evolved about 77 mya, with Wildfowl in order Galliformes appearing first followed by the Waterfowl in order Anseriformes appearing 72 mya.
Wildfowl or Game Birds.
A few fragmentary fossils of putative Galliformes from the period ending 65 mya exist, one of which is Austinornis. A specimen from Argentina is also a plausible Galliformes candidate. Its general shape and anatomic details resembles the more basal lineages of Galliformes. The period 65 to 23 mya had several now extinct Galliformes families.
The Galliformes order as we know it today contains five families which tend to favour terrestrial habitats in various parts of the world. Megapodiidae has the Megapodes which are primitive mound nesters, Cracidae has Chachalacas, Guans and Curassows, Numididae has Guineafowl and Odontophoridae has New World Quail. Pheasants in Phasianidae appear to have evolved some considerable time after the original ancestors. They seem to have started radiating about 30 mya.
The Horned Guan represents the sole survivor of a distinct and ancient lineage and DNA data suggests that the Cracidae originated before 65 mya. Curassows diverged from the Guans in the period 35 to 20 mya.
The wide-spread distribution and the variety of habitats in which Wildfowl prosper, mainly because their camouflage helps to keep them safe, have provided scope for various species populations to become separated and this in turn has encouraged further speciation and evolutionary development.
Waterfowl.
The oldest known waterfowl, Vegavis iaai, dates from about 65 mya. Early water-birds clearly roamed the Earth with animals such as Tyrannosaurus rex. They were the first of our modern birds to favour aquatic, mainly inland water, environments.
Although the “Gondwanaland break-up” effect has clearly been responsible for the location in which the Screamers and Magpie Goose are found, the other birds are nearly all strong fliers and are distributed quite widely throughout the world. The Anseriformes order as we know it today contains three families which are highly adapted for an aquatic existence; Anhimidae the Screamers and Anseranatidae the Magpie Goose were eventually followed by the Anatidae Ducks, Geese and Swans.
The Screamers of South America and the Magpie Goose of Australia show features of both Game Birds and Waterfowl. The ancestors of the Anseriformes developed the characteristic bill structure which is used to feed on plant materials although some (Sawbills) catch fish. Many have subsequently adopted alternative feeding strategies.
The earliest direct ancestors are not yet documented by fossils. Presbyornithidae were a family of water-birds with an apparently global distribution that lived until about 34 mya but are now extinct. The Cygnus genus of Swans evolved in Europe or western Eurasia during the period 23 to 5 mya, spreading all over the Northern Hemisphere.
Speciation, either because of alternative feeding techniques and/or geographical separation and distribution has clearly played a large part in the evolution of the Waterfowl.
Sea and Ocean Birds – marine environment does not encourage diversification.
This third group of birds, which were almost certainly evolving at about the same time as the Galloansers, are mainly found in a marine environment. Compared to the birds discussed above these birds have a low species diversity. This may well be because their environment offers less opportunity for isolation of various populations.
Penguin.
Penguins are only found in the Southern Hemisphere on the southern coasts of Australia, New Zealand, South Africa and South America and Antarctica.
The most recent common ancestor of Penguins and their sister clade (Albatross and possibly Loons) dates back to around 70–68 mya. By 65 mya the lineage was considered to be evolutionarily distinct, though it is suggested that they were not yet entirely flightless.
The basal Penguins lived around the time of the K/T event (65 mya) somewhere in the general area of New Zealand and Byrd Land, Antarctica. The oldest known fossil Penguin species is Waimanu manneringi, which lived in New Zealand about 62 mya. While they were not as well-adapted to aquatic life as modern Penguins, these were Loon-like birds but already flightless, with short wings adapted for deep diving. They swam on the surface using mainly their feet, but were already adapting to underwater locomotion.
During the period 40–30 mya some lineages of gigantic Penguins existed but they had disappeared by around 25 mya. Their decline and disappearance coincided with the spread of primitive, fish-eating toothed whales, which certainly competed with them for food, and were ultimately more successful. A fossil from Argentina indicates that by about 39–38 mya, primitive Penguins had spread to South America and were in the process of expanding into Atlantic waters. At about this time genus Aptenodytes the King and Emperor Penguins diverged and became basal to the group which are our modern Penguins. Further divergences resulted in new species in the period 34 to 25 mya and again 5 mya.
Albatross, Petrels and Shearwaters.
A prehistoric seabird, Tytthostonyx, is known from the period 70 to 65 mya. This was closely related to the ancestor of birds listed in the orders Procellariiformes and Pelecaniformes. The earliest fossil Albatrosses were found in rocks dating back to about 55 mya. The fossil record in the Northern Hemisphere is more complete than that of the Southern Hemisphere and many fossil forms of Albatross have been found in the North Atlantic, which today has no albatrosses.
The main radiation of birds in the order Procellariiformes, to which the Albatrosses belong, occurred in the period 35–30 mya. By about 15 to 12 mya Albatrosses in genera Phoebastria and Diomedea had already diverged.
Petrels and Shearwaters are tentatively dated around 30 mya.
Loons.
Loons seem to have originated by 65 mya but modern Loons are only known with certainty since about 55 mya. Nearly ten prehistoric species have been named to date, and a similar number of undescribed ones await further study. The modern birds in genus Gavia are known from 23 mya.
Concluding Comments.
The three groups discussed above have clearly originated from ancestors which are amongst the earliest of our modern birds. Land-mass movements and the subsequent development of flightlessness, were such that some of these birds are only found in specific regions of the world.
The key birds highlighted here, Tinamous, Ostrich and possibly the Rhea, Wildfowl and Waterfowl, Albatross and related birds and possibly the Loons, have varied considerably in their ability to evolve to form new species and families. The various environments, in which they found themselves, during the period from their initial evolution to the present day, must have played a part in their diversity.