Dinosaur Die Out Was Gradual, Miserable, Painful
Life went from really bad to worse for dinosaurs in the years preceding the asteroid impact, new research shows.
Dinosaurs gradually began to die out at least 40 million years before the devastating asteroid impact killed the rest in a dramatic way about 66 million years ago, concludes an extensive new study.
The research, outlined in the journal Proceedings of the National Academy of Sciences, is the first to incorporate phylogenetic information - meaning how species are related to one another - when studying speciation and extinction in dinosaurs.
The data provides evidence that things went from really bad to worse for most dinosaurs that did not evolve into birds.
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"Some dinosaurs definitely would have been instantly killed in the impact, those in the vicinity of the impact site for instance; others may have perished in the tsunami caused by the blast," lead author Manabu Sakamoto of the University of Reading's School of Biological Sciences told Discovery News.
He continued, "The majority of the remaining dinosaurs all over the rest of the world would have likely starved to death as vegetation died out owing to the layer of ash that blacked out the sky (nuclear winter)."
A model created by Sakamoto and colleagues Michael Benton and Chris Venditti found that most dinosaur populations were already declining as early as 48–53 million years before the asteroid impact, which left the massive Chicxulub crater in Mexico. They say that the Cretaceous Period was a time of extreme geological and environmental changes.
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"For instance," Sakamoto said, "sea level was fluctuating, climate was shifting from a hot house to a global cooling, there were prolonged volcanic activities, and continents were breaking apart, establishing the main continents we have today."
He explained that the supercontinent breakup would have resulted in less available land for dinosaurs to use for migration, which previously helped to fuel the evolution of new species. Climate change, both then and now, could induce major environmental and ecological shifts.
Sea level rise turned out to be more complex. Initially, it favored the evolution of more dinosaur species, but smaller land areas eventually restricted population growth.
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Our ancestors, the early mammals, could have posed a big threat to dinos as well.
"Another possibility is that mammals, which were small rodent-like creatures at that time, were in some way contributing to the dinosaur's downfall," Benton, a professor of vertebrate paleontology at the University of Bristol, told Discovery News. "Recent studies show evidence that mammals were on the rise prior to the K-Pg (extinction) event, so this scenario would be consistent with our finding."
Venditti, from the University of Reading, added that mammals might have outcompeted dinosaurs for resources, eaten their eggs, spread diseases or caused other problems for the once mighty dinos.
Aside from the dinosaur lineage that evolved to become birds, two other major groups of dinosaurs - the plant-eating hadrosaurs and ceratopsians - remained strong until the asteroid struck. Sakamoto explained that these dinos acquired specialized jaw structures that allowed them to process plant foods efficiently.
Paleontologist Steve Brusatte from the University of Edinburgh's School of GeoSciences said the new study is an impressive take on "a subject that has been studied to death."
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"Cretaceous dinosaurs were evolving into fewer new species, and going extinct at a faster rate, than earlier dinosaurs," said Brusatte. But he does not think that dinosaurs were doomed to extinction before 66 million years ago.
"The asteroid hit at a time when dinosaurs had already been around for a long time, and had already endured their really prolific periods of evolution," Brusatte said. "But the way I see it, the dinosaur extinction still came down to the asteroid. No asteroid, no extinction."
There is an eerie footnote regarding lessons learned from the extinction of dinosaurs, and other animals that bit the dust after the asteroid hit. Any group of animals that is under a prolonged period of decline can be wiped out, should there be catastrophic event.
Sakamoto said, "This means that if some major catastrophe hits, then it is highly possible that whole groups of animals could be completely wiped off the face of the Earth."
Paleontologists have just assembled the most comprehensive family tree of meat-eating dinosaurs. Published in the journal Current Biology, the family tree reveals how diverse carnivorous dinosaurs were and how birds eventually evolved from them. Tyrannosaurs, including
Tyrannosaurus rex
, are one key group on the meat-loving dino family tree. Lead author Steve Brusatte of the University of Edinburgh's School of GeoSciences told Discovery News, "The most iconic dinosaurs of all, tyrannosaurs were more than just the 13-meter-long (nearly 43 feet long), 5-ton monster predator
T. rex
." "Tyrannosaurs were an ancient group that originated more than 100 million years before
T. rex
, and for almost all of their evolutionary history they were small carnivores not much bigger than a human in size."
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"Some of the rarest theropods (two-legged carnivorous dinos) of all, compsognathids are represented by about half a dozen species," Brusatte said. "They were small, sleek meat-eaters which ate small prey like lizards." One of the more recent finds, Juravenator from Germany, is known from a nearly complete fossil.
Ornithomimosaurs were theropods called "ostrich mimic" dinosaurs -- for a reason. "Like living ostriches, they could run fast on their long legs and used their sharp, toothless beaks to eat a varied diet of small prey, plants, and perhaps even small shrimps in the water just like living flamingos," Brusatte explained. "A recent find in Canada showed that not only were ornithomimosaurs feathered, but they also had complex feathers on their arms that would have formed something of a wing, although they couldn't fly."
Brusatte describes therizinosaurs as "perhaps the weirdest theropods of all." "These were big, bulky, cumbersome dinosaurs that ate plants. They had fat barrel-shaped chests, stocky legs, and big claws on their arms," Brusatte said. For many years paleontologists argued about which group this dinosaur belonged to, only recently settling on theropods. This means they were fairly closely related to birds, despite their weird anatomy.
Alvarezsaurs were among the smallest dinosaurs of all, measuring just a few feet long and weighing less than 5 kilograms (10 pounds). "Some of them had only a single functional finger on their hand, which they probably used to prod deep into the nests of bugs, which were one of their main food sources," Brusatte said.
Oviraptorosaurs, were a group of small omnivores that were lightweight, lacked teeth and had tall, hollow crests on their skulls. The recently discovered Anzu -- the so-called "
" -- came by its nickname honestly. It towered more than five feet tall, weighed more than 400 pounds, and was covered in a coat of feathers.
"Troodontids were probably the smartest dinosaurs of all, as they had the largest brains relative to their body size of any dinosaur group," Brusatte said. "Most troodontids were small, fast-running dinosaurs that probably ate both meat and plants." Among the most recently discovered of this group are the small, feathered Anchiornis and Xiaotingia, which lived in China about 160 million years ago. "They look eerily similar to birds, so much so that some researchers think they could be primitive birds rather than troodontids with wings and feathers," Brusatte said.
Dromaeosaurids were "raptor dinosaurs" that include Velociraptor from "Jurassic Park" fame. These dinosaurs were pack hunters who wielded a sharp, hyper-extendable "killer claw" on their second toe. "One of the most recently discovered dromaeosaurids is Balaur, a poodle-sized terror from Romania which had not one, but two 'killer claws' on each foot."
"The oldest birds, like Archaeopteryx that lived 150 million years ago in Germany, are very hard to distinguish from their closest dinosaurian relatives," Brusatte said. "Unlike living birds, they had teeth, sharp claws on their wings, and long tails." "Over the past two decades," he continued, "over 50 new species of Mesozoic birds have been discovered in northeastern China, in the same rock units as the famous 'feathered dinosaurs.' So many birds are preserved here because entire ecosystems were buried by volcanic eruptions, turning animals to stone like a dinosaur version of Pompeii."
"The 10,000 species of birds that live today -- from hummingbirds to ostriches -- are modern dinosaurs," Brusatte said. "They are dinosaurs in the same way that humans are mammals. The classic body plan of living birds -- feathers, wings, wishbones, air sacs extending into hollow bones -- did not evolve suddenly but was gradually assembled over tens of millions of years of evolution. But, when this body plan finally came together completely, it unlocked great evolutionary potential that allowed birds to evolve at a super-charged rate." "They underwent a burst of evolution early in their history, which eventually led to the 10,000 species alive today -- more than twice the number of mammals."