Flightless Flying Theropods
Aves (Sometimes called Neornithes). (Late Cretaceous - Recent) The most recent common ancestor of modern birds and all of its descendants. In broad strokes, the phylogeny of Aves is very clear:
Many of the most significant avian biomechanical issues have been dealt with in other lectures, so in this survey we concentrate on one thing:
- Paleognathae (="old jaw") Recognized by synapomoprhies of the palate and braincase. Includes living:
- Tinamous - South American ground birds superficially resembling northern game birds.
- Ratites - The major radiation of large flightless ground birds, again from the southern continents.
- Neognathae (= "new jaw") Recognized by synapomoprhies of the palate and braincase. Includes two major clades:
- Galloanserae: The proper game birds: pheasants, chickens, quail, etc; and ducks, geese, and screamers.
- Neoaves: Absolutely everything else. The phylogeny of Neoaves is terrifically controversial.
Flightlessness: On many occasions, birds have secondarily lost the ability to fly. How strange is that, given the degree to which their bodies have been transformed for flight? At least as strange as the idea of terrestrial amniotes plopping back into the oceans.
- A few monophyletic groups seem to specialize in flightlessness, E.G. ratites.
- But flightless birds originate from all branches of the avian tree.
- To find them, look on oceanic islands (the smaller the better) and continents that historically lacked large mammalian predators.
For example: Cormorants are foot-propelled diving sea-birds, but also powerful fliers. At some point in the last seven million years, however, a flock got blown to the remote Galápagos Islands. there they encountered a situation in which good fishing could be had right off shore, but once out of range of the islands, the next reliable source of food as 500 miles away on the South American mainland. The paradoxical results:
Thus, growing wings was both a waste of energy and an actual hazard, since they were more likely to get you killed than to save you. The result was the rapid evolution of the flightless Galápagos cormorant, Nannopterus harrisi.
- To fly meant to risk being blown away from the cornu copiae of the waters around the islands, where starvation would probably result.
- The Galápagos had no large land predators from which one needed to fly away.
- To grow powerful wings required lots of metabolic energy
The ecological signal: We distinguish three general settings in which flightlessness evolves:
And yet, there have been special times and places when birds have become established as major parts of the terrestrial biota. Here is a survey. It's broken down taxonomically, but for each, be aware of the ecological setting.
Flying paleognaths: The only extant flying members of this group are the tinamous of South America. Quail - chicken size. They live on the ground and prefer to run but fly clumsily in emergencies.
Tinamous are committed land birds. To the best of our knowledge, there has never been a paleognath, flightless or otherwise, on a small oceanic island.
The remaining paleognaths belong to the ratites - the great adaptive radiation of flightless birds of the southern continents.
Large island ratites:
- The kiwi. (Extant) The oddball ratite of New Zealand: A nocturnal, olfaction-oriented insect-eater, and small - the size of a chicken (or tinamou). Paradoxically, the kiwi lays one of the largest theropod eggs - gigantic in relation to its body. The genus name, Apteryx (="wingless") is almost correct, as only a tiny splint of the humerus remains. The kiwi demonstrates a common two phenomena in flightless birds:
Both derivations are pedomorphic.
- contour feathers quickly lose their coherence and become shaggy or fluffy.
- the sternum (breastbone) loses its keel and becomes smaller.
- The Moa: (Recently extinct) (Polynesian for "fowl") Moas were actually a speciose rediation of ratites on New Zealand, known from copious skeletal and soft-tissue remains (including "mummies"). The largest moa was Dinornis 10 feet tall and weighting 300 kg. The smallest moa - the kiwi, is still clinging to life. The giant moas survived into historical times but archeological evidence suggests that they were driven to extinction by polynesian settlers who hunted the large diurnal ones but apparently left the kiwi alone. Regarding moas:
- Although large, the moas had no terrestrial predator to fear prior to the arrival of humans and their livestock, so they were apparently neither fast nor fierce. Soft-tissue remains indicate, that their coloration was cryptic, however, and it is speculated that they were preyed upon by the extinct Haast's eagle.
- Molecular studies have "sexed" members of the giant moa genus Dinornis, revealing that these were significantly sexually dimorphic, with females being much larger than males. These morphs had previously been taken to represent separate species.
- Moas are classic ecological ghosts, explaining the ubiquity of plants that have evolved complex morphologies such as zigzag divarication to discourage grazers. This pattern has been seen to discourage avian grazers, but not mammals - implying that it evolved under selective pressure from moas.
- The elephant-bird Aepyornis of Madagascar. (Recently extinct) Survived into historical times. Hunted by Malagasy pioneers and reported to early European visitors but never seen. Probably became extinct during the 16th century. Probably the largest paleognath ever: over 10 feet tall and weighting 450 kg. Another superlative - laid the largest known shelled egg (2 gal. or 7.6 liters) - larger than that of any known sauropod!
A digression: Maximum shell size is subject to strict scaling constraints:
Thus, there is an absolute limit on shelled egg size, and Aepyornis seems to be close to it.
- Air gets to the embryo through pores in the eggshell.
- When the egg's linear dimensions increase, total metabolism of the embryo (a function of volume) increases faster than diffusion
- Furthermore, for shell strength to be maintained, shell thickness bust increase with positive allometry
- And yet the chick must be able to break free
Continental Ratities: Although flying and flightless paleognaths are present in the fossil record from the Paleogene onward, they are well represented by living species.
- The rhea of South America is a large (roughly 20 kg.) totally flightless, very cursorial herbivore. The rhea has large wings, with feathers homologous to the flight feathers of flying birds, but they are useless for flight.
- The ostrich of Africa (and in prehistoric times, central Eurasia). The largest living bird (up to 140 kg.), and extremely cursorial, these hold their own in a continent filled with large mammalian predators. Ostriches have lost digit II of the foot, making them effectively two-toed. Like rheas, ostriches retain sizeable wings.
- The emu of the open forests and grasslands of Australia. Up to 35 kg. The emu's wing is greatly reduced.
- The cassowary of the forests of Australia and New Guinea. Up to 60 kg. Unlike the preceding ratites, cassowaries are not very cursorial. Rather, they defend themselves by kicking with the large claw on digit II. Like emus, their wings are greatly reduced.
Neognathae - Galloanserae
Galloanserae are made up of two monophyletic groups:
Of these, only the Anseriformes turn up as flightless birds, but their record is interesting.
- Galliformes - "Game birds" - pheasants, quail, partridges, etc. Ground-nesting land birds
- Anseriformes - Ducks, geese, screamers. Ground or cavity nesting water birds.
Oceanic island Galloanserae:
- The Moa nalos (="lost fowl") of Hawaii. Hawaii had a diverse endemic avifauna that has suffered terribly from habitat destruction and competition with introduced species since its discovery by the Polynesians. Among the more interesting are birds known only from fossils, the moa nalos - three genera of large (4-7 kg.) flightless duck:
- Molecular studies indicate that their closes living relatives are dabbling ducks, probably the Pacific Black Duck. Their lineages diverged within the last five million years.
- Despite the short time span, these animals had quickly lost the power of flight and stomach contents indicate that they become specialized as browsers on ferns. This is reflected in unusual specializations of the beak, including:
- The turtle-like beak of Chelychelynechen
- The serrated beak ofThambetochen.
Continental flightless Galloanserae:
- The Dromornithidae aka Mihirungs (Paleogene to recent (almost)) of Australia. A radiation of large (cassowary-sized ~60 kg.) to gigantic (Aepyornis sized 350-500 kg.) flightless anseriformes. Size estimates are uncertain, but the gigantic Dromornis is the only bird that might have outweighed the ratite Aepyornis. The last ones became extinct around 31,000 years ago. Although coexisting with the ancestors of Australian ratites, they were anatomically very distinct:
Indeed, mihirungs may represent a range of adaptations.
- Their toe-claws were transformed into hooves.
- In contrast to the low flat beaks of ratites, their were extremely tall and vaulted - as if they were reevolving the ancient theropod hatchet-head.
- Their locomotion is controversial. They were certainly not cursorial like emus, but may have moved quickly by dint of brute force and size.
- Likewise with diet. Bone and eggshell chemistry suggest that they were herbivores, but their beak morphology would be consistent with carnivory or specialization on very tough plants.
- But that tall beak also prevented binocular vision - usually an asset to carnviores.
Diatryma (Paleogene) of Eurasia and North America. A large biped, usually interpreted as carnivore in the spirit of the old-fashioned theropods, but lacking arms and teeth. Diatryma was a rough contemporary of the hoofed crocodylian Pristichampsus, and if it was a carnivore, was also attempting to take over the terrestrial top predator niche.:
Neognathae - Neoaves
- A minority opinion holds that Diatryma was a herbivore.
- Like mihirungs, Diatryma had a tall, "hatchet-head" beak. Unlike them, the beak of Diatryma was hooked - a common feature in carnivorous birds.
- Long held to be a gruiform (i.e. crane), recent phylogenetic analyses place it among Anseriformes with the ducks, geese, and mihirungs.
- Their locomotion is controversial as in mihirungs. Their legs were robust and probably not cursorial like emus, but may have moved quickly by dint of brute force and size.
Oceanic flightless Neoaves: Flightless Neoaves are almost common on oceanic islands, with many Pacific islands boasting their own species of flightless rail (small ground running marsh birds, including the Phillipine rail and the takahe mentioned before). Other neoavian groups have contributed flightless ground birds, including the pigeons, notatbly:
- The Dodo Raphus cucullatus of Mauritius in the Indian Ocean. A large (16 kg.) herbivorous non-cursorial ground dove driven to extinction in the 17th century by European sailors and introduced mammals.
- The Solitaire Pezophaps solitaria of Rodriguez in the Indian Ocean. A smaller and slenderer version (13 kg.) of the dodo - a herbivorous non-cursorial ground dove driven to extinction in the 17th century by European sailors and introduced mammals.
- Psst! The closest living relative of the dodo and solitaire is believed to be the attractive Nicobar pigeon.
Continental flightless Neoaves: One neoavian group has joined the ranks of large continental flightless birds, the Phorusrachidae of Neogene South America:
- A speciose radiation (14 genera) ranging from a height of 70 cm to over 2 m.
- Members of Gruiformes (cranes and rails) thought to be closely related to the extant flying Seriema.
- Occupied the dominant pursuit predator niche in South America for much of the Neogene. Indeed, there is no ambiguity about their ecology. Although larger forms are stockier, al seem to have been somewhat cursorial.
- After the Great American Exchange, some members came noth, including Titanis. This taxon, at least, had reacquired claws at the ends of digits I and II of the hand.
- Known phorusrachids, did not challenge the mihirungs or Aepyornis in size, however a newly discovered skull, representing a new taxon, might possibly be from a 500 kg. animal.
The birds most likely to evolve flightlessness:
- Live and nest near the ground or the shores of fresh water. Hence, many rails, pigeons, and ducks, but no songbirds, woodpeckers, or other arboreal birds.
- Are likely to fly over water and get blown off-course. Hence many ducks and geese, but no pheasants.
The places in which flightlessness is most likely to evolve:
- Places with no scary predators.
- Places where food can be obtained from the ground.
- Places where flight might be a liability.
So how do we explain the exceptional cases of continental flightless birds:
- The early Paleogene was a time in which there were no well-adapted large terrestrial predators. That niche ahd been occupied by non-avian theropods who had just been wiped out.
- At that time, the southern continents were either still touching or in close proximity.
- The ratite radiation (herbivores) took advantage of the absence of large predators. Its members could still walk through most of their Gondwanan range.
- By the time powerful mammalian predators emerged, some ratites were safe on small land masses that had rifted away (New Zealand and Madagascar) and had no need of defensive adaptations.
- In continents where mammalian predators evolved, ratites evolved cursoriality and a degree of nastiness.
- In the northern continents, Diatryma also took advantage of the lack of large mammalian predators to become established as a large predator itself (maybe).
- The island continents of Australia and South America lacked large mammlain pursuit predators for much of their history, allowing flightless birds a degree of freedom lacking elsewhere. Thus, ratites and mihirungs prospered in Australia and Phorusrachids in South America.
- Many many flightless birds were extinguished by human settlers, their livestock, and animals they accidentally introduce (moas, Aepyornis, moa nalos, dodos, etc.)
- Some succumbed to the evolution of powerful mammalian predators (Diatryma).
- Some were extinguished through competition with natural immmigrants (Phorsurachids)