In total, this mass extinction event claimed three quarters of life on Earth. For now, two leading ideas are battling it out within the scientific community: Were dinosaurs victims of interplanetary violence, or more Earthly woes? One of the most well-known theories for the death of the dinosaurs is the Alvarez hypothesis, named after the father-and-son duo Luis and Walter Alvarez.
In , these two scientists proposed the notion that a meteor the size of a mountain slammed into Earth 66 million years ago, filling the atmosphere with gas, dust, and debris that drastically altered the climate. Iridium is relatively rare in Earth's crust but is more abundant in stony meteorites, which led the Alvarezs to conclude that the mass extinction was caused by an extraterrestrial object.
At about 93 miles wide, the Chicxulub crater seems to be the right size and age to account for the dino die-off. In , scientists drilled a rock core inside the underwater part of Chicxulub , pulling up a sample stretching deep beneath the seabed. This rare peek inside the guts of the crater showed that the impact would have been powerful enough to send deadly amounts of vaporized rock and gases into the atmosphere, and that the effects would have persisted for years.
And in , paleontologists digging in North Dakota found a treasure trove of fossils extremely close to the K-Pg boundary , essentially capturing the remains of an entire ecosystem that existed shortly before the mass extinction.
Tellingly, the fossil-bearing layers contain loads of tiny glass bits called tektites—likely blobs of melted rock kicked up by the impact that solidified in the atmosphere and then rained down over Earth. However, other scientists maintain that the evidence for a massive meteor impact event is inconclusive, and that the more likely culprit may be Earth itself.
Ancient lava flows in India known as the Deccan Traps also seem to match nicely in time with the end of the Cretaceous, with massive outpourings of lava spewing forth between 60 and 65 million years ago.
Today, the resulting volcanic rock covers nearly , square miles in layers that are in places more than 6, feet thick. Proponents of this theory point to multiple clues that suggest volcanism is a better fit. Other research has found evidence for mass die-offs much earlier than 66 million years ago, with some signs that dinosaurs in particular were already in a slow decline in the late Cretaceous.
This all makes sense, supporters say, if ongoing volcanic eruptions were the root cause of the world-wide K-Pg extinctions. Increasingly, scientists trying to unravel this prehistoric mystery are seeing room for a combination of these ideas.
This nearly whole, deep-black skull belongs to the most complete specimen of Tyrannosaurus rex on display in Europe, an individual nicknamed Tristan Otto. But that notion depends a lot on more precise dating of the Deccan Traps and the Chicxulub crater.
This debate may rage for years, as scientists dig up new clues and develop new techniques for understanding the past. There is evidence that the ancestors of modern mammals at the time of the dinosaurs had a nocturnal lifestyle , By being both viviparous and nocturnal they may have avoided the reproductive failure of egg-laying dinosaurs. It is also possible that some dinosaur species survived the catastrophic events that were associated with most dinosaur extinctions.
A few species that lived within the Antarctic Circle had anatomical features that suggested they were able to see in the dim light of winter and may have died out long after other Dinosauria had become extinct , Apart from being able to see in the darkness of the Antarctic winter and accommodate to a cooling climate from the asteroid impact and volcanic eruptions, these polar dinosaurs may have also been able to conserve sufficient cholecalciferol, produced during the Antarctic summer, to maintain their capability for oviparous reproduction.
Would it be possible to test a cholecalciferol-deficiency hypothesis by further study of fossilised dinosaur eggs and embryos? Evidence could come from determining whether fossilised dinosaur embryos had bone pathology comparable with that, for example, of cholecalciferol-deficient turkey embryos that had died before hatching.
Likewise, this theoretical cause of complete elimination of the dinosaur family would be supported if fossilised dinosaur egg shells, dated more exactly to the era of extinction, had the defective structure of egg shells produced by cholecalciferol-deficient birds.
National Center for Biotechnology Information , U. Journal List J Nutr Sci v. J Nutr Sci. Published online Mar Author information Article notes Copyright and License information Disclaimer. Fraser, email ua. Abstract Palaeontological deductions from the fossil remnants of extinct dinosaurs tell us much about their classification into species as well as about their physiological and behavioural characteristics.
Abbreviation: Mya, million years ago. Acknowledgements There are no acknowledgements. There was no financial support. The author has no conflicts of interest. References 1. Science , — Ganapathy R A major meteorite impact on the Earth 65 million years ago. Evidence from the Cretaceous—Tertiary boundary clay. Nature , — Can J Earth Sci 38 , — Proc Nat Acad Sci , — Biol Rev Camb Philos Soc 90 , — J Geophys Res , 28,—28, Sci Rep 7 , Sci Rep 6 , J Geophys Res , 21,—21, J Geophys Res , — J Biosci 34 , — GSA Bull , — J Geophys Res , B Atmos Ocean 46 , — Geophys Res Lett 25 , — Atmos Ocean 24 , — Poult Sci 11 , — Poult Sci 63 , — Calcif Tissue Res 40 , — Biochem J , — A Look at Dinosaur Reproduction.
Paleobiology 5 , — Poult Sci 66 , — Lee SA Incubation times of dinosaur eggs via embryonic metabolism. Phys Rev E 94 , Sci Rep 8 , Nat Ecol Evol 1 , — As far as Saskatchewan goes, the ecological community at the time leading up to the extinction was like a big game of Jenga. The constantly shifting ecological stability made major upsets—like an asteroid striking at the wrong place, at the wrong time—especially disastrous. Species that survived the impact were typically small, semi-aquatic or made burrows, and able to subsist on a variety of foods, but there are some key contradictions.
There were some small non-avian dinosaurs that had these advantages and still went extinct, and many reptiles, birds and mammals died out despite belonging to broader groups that persisted.
Generally speaking, smaller dinosaurs and other animals should have had better chances at survival than their larger relatives, but this was not always the case.
Pat Holroyd of the University of California Museum of Paleontology likens these investigations to what happens in the wake of airline accidents. Holroyd estimates that much of the relevant research about which species survived the impact has only been published or uploaded to the Paleobiology Database in the last decade. This new information allowed Holroyd and colleagues to study patterns of turnover—how long species persisted on land and in associated freshwater habitats—long before and after the asteroid impact.
Some of the patterns were familiar. Fish, turtles, amphibians and crocodylians all generally fared better than strictly terrestrial organisms.
But the resilience of waterbound species had never been quantified in detail before, and the new analysis is revealing that the solution to the extinction pattern puzzle may have been right in front of us all along. Species living on land, particularly large species, tend not to persist as long as those living in freshwater environments.
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