Fins to Feet


Flightless Wonders
January 20, 2014, 2:19 am
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NOTE: This post is about birds that lost the ability to fly and evolved to occupy ecological niches typically associated with big mammals. In the sections that follow, we will study birds as large, terrestrial grazers (moas), as sea-going, flipper-possessing hunters of fish and krill (penguins) and as fleet-footed, land predators (phorusrhachids).

The Moa 

The first voyagers touched land on the coast of New Zealand some time before 1300 AD. The coming of man signaled the beginning of a devastating mammalian assault on the island’s ecosystem. This isolated land-mass in the southern Pacific, where all land mammals but bats had been extinct for millions of years, was suddenly overrun with human, canine and rodent invaders from Polynesia. A wave of deforestation and extinction ensued.

Richard Owen with the skeleton of a Moa

New Zealand’s earliest colonists belonged to a great seafaring culture with an impressive history of settling remote island chains. As they explored the land, they encountered massive flightless birds and the largest species of eagle in the world. Descriptions of these spectacular creatures survived in oral legends, centuries after they had been extirpated by hunting and habitat change.

In the near complete absence of mammals, birds dominated the vertebrate fauna of New Zealand prior to human contact. Instead of ungulate browsers and grazers, there were different species of moa. Instead of small mammals foraging in the leaf-litter at night, there were nocturnal kiwis. The island’s largest predator was a raptor with a wingspan of three meters, the Haast’s eagle. In short, it was home to a truly astonishing range of avian species, from penguins to parrots. In this section, I will focus on the biology of the Moa – the most famous of New Zealand’s extinct birds.

New Zealand was once home to 9 species of moa. These birds were ratites – flightless relatives of ostriches, emus and cassowaries. The defining characteristic of this group is the absence of a keel on the breastbone which, in flighted birds, serves as an attachment surface for powerful wing muscles. Ratites (and tinamous) branched off relatively early in the evolution of modern avians (Neornithes). Much controversy has surrounded the timing and nature of their divergence from the rest of bird-kind: did ratites evolve in the cretaceous, prior to the extinction of non-avian dinosaurs, or after?

During the late cretaceous, the supercontinent of Gondwana (itself the southern fragment of an earlier, larger and more famous supercontinent: Pangea) split into a number of smaller continents and islands that today account for almost all of the landmass in the southern hemisphere (namely, Africa, South America, Antarctica and Australia)*. Tellingly, all modern ratites live in these southern bits of gondwana: ostriches in Africa, rheas and cassowaries in South America, emus in Australia and kiwis in New Zealand. This distribution suggests that the most recent – and presumably flightless – ratite common ancestor arose in Gondwana during the Cretaceous period. As the continents drifted apart, descendant ratite lineages “rode” the crustal fragments to their present locations, evolving in geographic isolation. However, more recent DNA evidence complicates and challenges this picture. No published molecular evolutionary tree describing the branching events between different ratite species conforms exactly to what we would expect based simply on the order of separation of the Gondwanan continents. Furthermore, an important paper (Harshman et al. 2008) nests tinamous, which are weak-flying birds from South and Central America, within the ratite clade, and identifies ostriches as the most deeply diverging ratite group. At first blush, that might appear to indicate that tinamous are ratites that somehow re-evolved flight. However, no avian group is yet known to have lost and then regained the ability to fly. A number of such phylogenetic studies have raised an intriguing possibility: perhaps the last shared ancestor of all living ratites was a bird fully capable of flight. This would imply that flightlessness evolved more than once among ratites – and that the keel-less breastbones and non-functonal wings of ostriches and emus may actually be an astonishing example of parallel evolution. The global distribution of ratites may be best explained by volant ancestors dispersing across bodies of water and by independent losses of flight in different lineages. 

Polynesians hunting a giant moa. Painting by Heinrich Harder.

The moa head was small relative to body size. They had long necks and stout legs. Moas were unique among flightless birds in lacking even vestigial wings. At a height of 3.6 meters, the Giant Moa towers over the biggest of its extant relatives, the ostrich. The only birds that are known to have surpassed the largest moas in weight were the elephantbirds of Madagascar – another group of giant, island ratites that went extinct within the last millennium. The smallest species of moa approximated the size of a turkey. 

Moa were herbivores that lived in or on the edges of forests, feeding on twigs and branches from low trees and shrubs. Like all other birds, they possessed a gizzard – a stomach chamber with thick, muscular walls containing stones that the bird swallowed to aid in digestion. Driven by powerful muscular contractions, these stones helped grind down ingested plant material. Gizzard stones are the functional equivalent of mammalian teeth. Moa did not live in a world free of natural predators. In the 1870s, half a century after the first moa bones were described by European scientists, a number of bones were discovered at a moa-excavation site that seemed to belong to giant bird of prey. This bird, called the Haast’s eagle, had a wingspan that exceeded that of any living raptor or vulture. It is thought to have preyed on moa. Watching one of these eagles swoop down on and dispatch a moa several times its size would have been a sight indeed!

The Haast’s eagle preyed on moa. Artwork by John Megahan.

Within a century of polynesian arrival, the Moa was hunted to extinction. The decimation of New Zealand’s forests also played a role. By the time the first Europeans set foot on the island, the moa was only a distant mythical-cultural memory to the Maori.

Penguins

The six living genera of penguins (and their various extinct relatives) together constitute the avian order Sphenisciformes. They are undoubtedly the most aquatically-adapted of all birds. Studies have revealed the body-form of a penguin to be among the most hydrodynamic shapes in the animal kingdom. Its wings have evolved into stiffened flippers that are optimized for generating thrust underwater. Their webbed feet, which serve as the primary source of propulsion in many other diving sea-birds, are used for steering rather than paddling when underwater. It has densely-arranged, short feathers that serve to insulate and make the animal waterproof. It has dense bones that allow it to resist buoyancy and dive deep in pursuit of prey. Emperor penguins, for example, are capable of diving down to over 1,800 feet. Many species forage for krill, squid and fish hundreds of kilometers away from the location of their home colonies. While penguins are typically imagined to be remote denizens of frigid Antarctic coasts, they are actually found throughout the Southern Hemisphere. Consider that Galapagos penguins cross the equator on a regular basis!

The oldest known Sphenisciforme fossils date to the Paleocene, around 60 million years ago, not long after the demise of the non-avian dinosaurs. These bones belong to the species Waimanu manneringi, a long-billed, flightless water-bird with forelimbs that show some signs of adaptation toward wing-propelled swimming. It probably used its feet to actively propel itself, rather than employing them simply as a rudder like modern penguins. Its skeleton presents an early stage in the anatomical evolution of penguins.

Left: Waimanu manneringi, Right: Icadyptes salasi, Artwork by Nobu Tamura

Left: Waimanu manneringi, Right: Icadyptes salasi, Artwork by Nobu Tamura

Interestingly, a number of gigantic penguin species have been discovered from the Eocene (56-33 mya) and Oligocene (33-23 mya) periods. Like ratites, the Sphenisciformes were able to experiment with larger body-sizes once they abandoned flight. Anthropornis, the tallest of them, stood at about 5 feet and 7 inches. Its wings were not as straight as those of modern penguins. Icadyptes, another one of these fossil giants, possessed an elongate, spear-like beak for skewering fish and may have been a strong diver. One interesting, if largely speculative, hypothesis posits that rising competition for food stocks and predation pressure imposed by the emergence of new lineages of whales and pinnipeds (i.e. seals) during the Oligocene drove giant penguins to extinction. By a similar token, perhaps the earlier extinction of large marine reptile groups after the cretaceous period opened up fresh new watery niches for early penguins to exploit. While the Sphenisciformes as a whole are certainly a very ancient group, all living penguin taxa trace their descent to an ancestor that lived only 10-11 million years ago.

In an interesting evolutionary parallel, auks in the northern hemisphere independently evolved wing-propelled swimming, an upright posture and black-and-white colors. Living auks are generally inefficient fliers – having traded in much of their flying ability for swimming prowess – but none of them are flightless. The great auk is an extinct member of the group that was flightless and disappeared only 150 years ago. Curiously, the word “penguin” was originally applied to this species of auk, prior to the discovery of what know today as penguins by western explorers.

A stuffed great auk

The Phorusrachids

South America was an island continent for nearly all of the Cenozoic era (i.e. the last 65 million years). While elephants, horses, camels, cats, dogs and bears were evolving in and dispersing throughout the Old world and North America, South America’s mammal fauna remained isolated and evolved in its own distinct fashion. The land mass was once home to elephant-sized sloths, the marsupial equivalents of ‘saber-toothed’ cats,  and many strange and unique ungulates.  These fascinating creatures will be the subject of a future post. South America was also home to a clade of large, flightless birds with a predilection for meat- the Phorusrhacids.  Their closest living relatives are two species of seriema  - long-legged, mostly-terrestrial, carnivorous birds native to the same continent.

Titanis walleri, a large North American Phorusrhacid, artwork by Dmitry Bogdanov

Unlike some other candidate “Terror birds” from the fossil record, like the Mihirungs of Australia or the Gastornithiformes, there has been relatively little debate about whether or not Phorusrhacids were predators. They had large skulls with tall, laterally-compressed and strongly- hooked beaks. The neck was not built to withstand side-to-side stresses (so these birds could not snag and then violently shake their prey), but could mete out powerful downward strikes. The biggest known Phorusrhacids were around 3 meters tall. Large phorusrhacids are often described as being agile pursuit hunters. Their legs might have also been employed in kicking prey to death, a tactic used to great effect by the secretary bird, a living terrestrial bird of prey in Africa.

South America’s long isolation ended when the Isthmus of Panama formed 4.5 million years ago, connecting the continent to North America. This inaugurated a fascinating period of inter-continental animal migrations that permanently changed the faunal composition of South America. Northern species were generally more successful in invading the south than vice-versa. The phorusrhacids did manage to spread north of the isthmus and the remains of one impressive species, Titanis walleri, have been recovered from Texas and Florida.  They went extinct not long after this event (perhaps succumbing to competition from a host of new carnivorous mammalian rivals) about 2 million years ago, well before humans entered the New World.

References

Phillips, Matthew J., et al. “Tinamous and moa flock together: mitochondrial genome sequence analysis reveals independent losses of flight among ratites.”Systematic biology 59.1 (2010): 90-107.
Smith, Jordan V., Edward L. Braun, and Rebecca T. Kimball. “Ratite nonmonophyly: independent evidence from 40 novel loci.” Systematic biology62.1 (2013): 35-49.
Phillips, Matthew J., et al. “Tinamous and moa flock together: mitochondrial genome sequence analysis reveals independent losses of flight among ratites.”Systematic biology 59.1 (2010): 90-107.
Worthy, Trevor H., and Richard N. Holdaway. The lost world of the moa: prehistoric life of New Zealand. Indiana University Press, 2002.
Dyke, Gareth, and Gary Kaiser, eds. Living dinosaurs: the evolutionary history of modern birds. John Wiley & Sons, 2011.
Degrange, Federico J., et al. “Mechanical analysis of feeding behavior in the extinct “terror bird” Andalgalornis steulleti (Gruiformes: Phorusrhacidae).” PloS one 5.8 (2010): e11856.

*India and Arabia are portions of Gondwana that have moved entirely into the northern hemisphere.

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3 Comments so far
Leave a comment

I would love for Cracraft’s old hypothesis of ratites spreading as the continents drifted apart to die the death it deserves and no longer get trotted out. It was always a just-so story with no real evidence to go for it and was never universally accepted, AFAIK. In your article, you probably should have included the existence of Paleogene ratites from Europe as as a further blow to the idea.
As for phorusrhacids, recently it has become likely that these birds were actually present outside the Americas too.*Lavocatavis* from Africa, *Eleutherornis* from Europe and both from the Eocene appear to have been phorusrhacids. There’s also an Eocene phorurshacid from Antarctica. The latter is easily explainable as Antarctica was of course connected to South America back then. The first two are more difficult and to me at least, they seem suggestive of two or three independent losses of flight in basal phorusrhacids. (*Eleutherornis* may have been an African immigrant into Europe.)

Comment by BrianL

Thanks for the comment BrianL. You raise some excellent points – with regard to fossil Paleognaths in the northern hemisphere during the early cenozoic and the occurrence of Phorusrhacids in continents other than.South America.

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