A New History of Life (49 page)

Modern toothless birds evolved from the toothed ancestors in the Cretaceous. But this was not so much a replacement as it was an addition, because the earlier primitive birds, toothed and long tailed, continued to thrive and diversify alongside the winged reptiles of the Cretaceous, including the dominant and largest fliers of the last half of the Cretaceous, the pterosaurs. The toothed relics continued to the end of the Cretaceous, but underwent final extinction in the K-T event—at least according to the most recent summation of bird fossils found in the most complete of all latest Cretaceous strata—those in the western interior of the United States, the Hell Creek Formation, home of
Triceratops, T. rex
, and a host of still-primitive birds.

The surviving lineages of birds were the comparatively primitive Paleognathae. These include the large flightless birds such as ostriches, Rheas, cassowaries—and the true giants we have just missed seeing, the enormous moa of New Zealand and elephant birds of Madagascar, wiped out by humans in the last thousand years. Some common birds of today, the aquatic ducks, the terrestrial fowl, and the best fliers of today, the Neoaves, have their roots in the Paleognathae.

The Hell Creek Formation and equivalent rocks in North America have now yielded a total of seventeen species, including seven of the most ancient species of archaic birds of all, including the diving, toothed birds belonging to the hesperornithes group, named for a four-foot-long, stubby diving bird named
Hesperornis
. The recovered assemblage includes both smaller forms and some of the largest fliers known from the Jurassic or Cretaceous, and this tells us quite emphatically that a great deal of avian diversification had happened by the end of the dinosaurs’ reign.

In fact, the “avifauna” from these rocks seem to be highly slanted toward marine birds, which is no surprise because of the nearby inland sea that carved North America into two large subcontinents during the Late Cretaceous. None of these groups are known to survive into the Paleogene, and their presence in the Hell Creek Formation, which included the last 2–3 million years of the Maastrichtian age of the Late Cretaceous, tells us that a mass extinction of archaic birds coinciding with the Chicxulub asteroid impact in fact did take place.
¹⁵
But here is where controversy still exists: while most of the birds found in the North American beds represent “advanced” birds from a morphological sense, none can be categorically placed in the all-important group called the Neornithes. This avifauna is the most diverse known from the Late Cretaceous, although in diversity and disparity (the number of kinds of body plans) it is lower than in modern birds. But this group of fossils is key in helping us understand to what extent the K-T mass extinction affected birds.

If any group of vertebrates could survive the effects of an impact extinction, it surely would be the birds. The burning of most of the world’s forests in the first few days after the collision of the giant rock from space with the Earth; the subsequent acid rain, followed by six months of darkness, and thus starvation and the extirpation of surely every terrestrial ecosystem as well as all of those in the marine and freshwater realms, save for the deep-sea communities—the effects of this impact were immense. Yet even the deep-water ecosystems would eventually have suffered grievously with the slowdown or cessation of the main source of food to the deep sea, the sinking bodies of shallower water plankton and dead animals. On land, the size of animals dictated their survivability, as the larger animals had no chance. But birds are not large animals.

With the ability to disperse rapidly and fly quickly toward less-affected land, it should be expected that birds as a group should show a lower extinction rate than nonflying animals—and nonflying birds. Unfortunately, birds rarely fossilize because they have fragile, hollow bones. Thus bird fossils are rare to begin with. Yet from great diligence in collecting, there is now enough information to make at least
educated guesses about the fate of birds across the Mesozoic-Cenozoic transition, a transition burned into life’s history.

By the Late Cretaceous, birds had already been on Earth longer than the time since the large Chicxulub impact turned a dinosaur-rich world into one with only avian dinosaurs left.

There is another source of information about the timing of modern bird diversification: the use of DNA. In the first decade of the twenty-first century a number of separate studies
¹⁶
proposed new “evolutionary trees” of birds, based on the DNA of extant species (evolving from presumed survivors from the times of archaic birds). This new tree contains several surprises. For example, the closest relatives of common freshwater diving birds known as grebes are—flamingos! Hummingbirds are a specialized form of nighthawk, while falcons are more closely related to songbirds than to other hawks and eagles. And surprising as these new conclusions may be, there were even greater jaw-droppers coming from this study.

For example, the new tree puts an order of flying birds, known as tinamous, on a branch of the tree also shared by the flightless ostriches, emus, and kiwis. The importance of this is that it indicates that flightlessness evolved at least twice in this lineage, or else that the tinamous re-evolved flight from a flightless ancestor. And still more: the new tree demonstrated that the closest relatives of perching birds, or passerines (which are by far the largest and most successful order of birds), are parrots. Yet with all of this new information, the age of the most fundamental split of surviving birds, the fork in the evolutionary road that resulted in the Neognaths and the presumably more primitive Paleognaths remains obscure.

Modern birds are classified in Neornithes, which are only lately known to have evolved into some basic lineages by the end of the Cretaceous, based on the discovery of a bird fossils now named
Vegavis
, known from Vega Island. The Neornithes are split into the Paleognaths (tinamous, ostriches, emus, and kiwis) and Neognaths (all the rest of
the birds). The date for the split of the Neognaths into the familiar birds of today is also poorly understood. The best evidence suggests that a basic split in the Neornithes occurred before the K-T extinction. But how long before, if at all? As noted above, there remains a very convinced group of specialists (such as Alan Feduccia) believing that the modern birds evolved only after the K-T extinction event, as well as those having doubts about whether the radiation of the Neognaths occurred before or after the extinction of the other dinosaurs.

Thus new results from Vega Island in Antarctica are crucial. Vega Island is a small island north of James Ross Island that had previously yielded one of the most significant finds in bird evolution. This discovery offered the first evidence that modern birds existed alongside nonavian dinosaurs at the end of the Cretaceous.

A last question has tantalized paleontologists for years. In the mid-Cenozoic, birds tried to become giant carnivorous dinosaurs once again. The most famous of these were the “terror birds,” with a number shown in the illustration here. Clearly there must have been severe competition with the then-rising modern carnivores, ancestral to all the major terrestrial carnivorous mammals of today (dogs, cats, bears, weasel groups).

The evolution of the large, flightless birds (ratite) still found today—such as the iconic ostrich, cassowary, Rhea, and others—has always seemed like a return to the bipedal dinosaur body plan. But because these giant birds cannot jump from island to island, or cross vast continents in the kinds of migrations so prevalent in the flying birds of the present and past, it has long been assumed that each of the major flightless groups evolved independently through the formation of isolated species. As most of these kinds of birds are found in the modern-day southern continents, which were in the Mesozoic all combined in one large landmass, the implication is that the ostriches of Africa, rheas of South America, and cassowaries of Australia were products of the breakup of the ancient landmass Gondwanaland. But a major surprise of the new DNA work is that these flightless birds actually evolved into their groups not
after
they lost the ability to fly, but
before
.
¹⁷

Because Africa and Madagascar were among the first large hunks of landmasses splitting apart from the Gondwanaland supercontinent, it was predicted that the early isolation of Africa and Madagascar would have allowed evolutionary forces to create the oldest of the ratites, the ostrich of Africa and the and even larger elephant bird of ancient Madagascar. But because of the closeness of Madagascar to Africa, ostriches and elephant birds should have been closely related to each other but quite distinct from the other flightless birds, including those of South America and New Zealand, where the ancient moa (now extinct) and still-living kiwi evolved in their own isolation—at least supposedly. Yet when DNA became available surprises were present.

The DNA work showed that the Madagascar elephant birds were more closely related to the New Zealand birds than to the nearby African ostriches. This unexpected result strongly supports the conclusion that these groups became evolutionarily distinct before they lost the ability to fly.

The living ratites are—and were not—the only large, dinosaur-like birds of the past. The largest land birds, now extinct, showed an evolutionary return to the body plan of the bipedal Mesozoic carnivorous dinosaurs. Known scientifically as phorusrhacids, the terror birds evolved in South America about 60 million years ago, and lasted until about 2 million years ago, the time the great ice sheets were spreading across the globe in the first Pleistocene ice advances. Some, at least, made it into North America as well, and for most of the Cenozoic these were the top carnivores of South America. There are nothing like the terror birds living today, which might be a blessing.

New research in 2010, using CT scanning technology, has given us a new understanding of how these behemoths lived and died. The scans revealed that the gigantic beaks of these monster predators were hollow, which was a surprise. This must have made the beak weak and vulnerable to breaking when moved from side to side. Instead they may have used the beak like an ax, and also used their powerful, talon-equipped legs for help in killing prey.

Like most flightless birds, terror birds had stubby little wings but long, powerful legs that ended in large, taloned feet. The muscular
legs produced great ground speed; it has been estimated that some species of terror bird could reach speeds of nearly seventy miles per hour over flat terrain, and on the vast South American pampas, there was plenty of room to run. In this they were probably comparable to a cheetah. The combination of running, a monstrous beak, and the deadly talons on powerful legs surely made the terror birds very effective predators.

They had very big heads and the largest brains of any bird. This leads to some uncomfortable realizations. Recent work on the intelligence of African gray parrots has led neuroscientists and psychologists alike to realize that we have vastly underestimated the level of bird intelligence. While primatologists try to link various primates to higher cognitive function, birds in general—and perhaps terror birds in particular—may have been among the most intelligent species to ever walk the Earth.

Some decades ago, several books came out suggesting that the world might be entering into a new mass extinction (including two books by coauthor Ward,
The End of Evolution
, and an update of that book retitled
Rivers in Time
).
¹
One of these books, Richard Leakey’s
The Sixth Extinction
,
²
was overtly referring to the big five mass extinctions we have profiled in this book, those with more than 50 percent species loss: the events of the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous. Here we advocate that there have actually been
ten major
mass extinctions so large that they merit differentiating from the more minor events, such as the PETM of a previous chapter, and several smaller events in the Jurassic and Cretaceous periods. These top ten are as follows:

  
1.
   
The Great Oxidation Extinction
. This may have been the most catastrophic of all extinctions, if judged on the percentage of species and individuals killed off. Oxygen would have been a deadly poison to virtually all microbes at the time. Combined with the nearly contemporaneous first snowball Earth, this might have been the worst of them all, and also the first out of the gate. Imagine that you walk outside and there is no longer breathable air. Air, all right, but different. So it was to those aquatic organisms that constituted life on Earth. The seas were filled with poison gas: oxygen.

  
2.
   
The Cryogenian Extinctions
. The combined snowball Earth episodes of the late Proterozoic. Thick dirty ice covers oceans and land. Photosynthesis slows and mainly stops. A rich, diverse assemblage of life on land and in the sea (far, far richer in the sea) dies out. Not just diversity, but biomass tumbles.

  
3.
   
The late Vendian-Ediacaran Extinction
. This included stromatolites, microbial mats and especially Ediacarans at the Proterozoic-Paleozoic Boundary. That supposed garden of Ediacara, invaded by voracious—and more important—active, moving animals, eating everything in their path and ravishing the slow-moving, microbially draped oceans and land.

  
4.
   
The late Cambrian SPICE Extinction
. Extinction of most trilobites, many “weird wonders” of the Burgess Shale, and so much else. Most important, there was a wholesale change of trilobites away from having primitive segmentation and eyes, incapable of enrolling for defense, little defensive ornament—probably due to an increase of predators as much as anything else. The first really large, mobile, and armored carnivores, the nautiloid cephalopods, were involved in this extinction, as were chemical changes.