However, Erickson et al. On the presumption that dinosaurs, like modern birds, were endothermic, they deduced that dinosaur egg incubation times were at least twice as long as those of birds, being up to d, which is comparable with those of eggs of poikilothermic modern reptiles. Using this tooth growth-line technique, Varricchio et al. Even if female dinosaurs had sufficient hydroxycholecalciferol in their bodies to produce the hormone 1,dihydroxycholecalciferol to allow egg shell formation in the oviduct, unless they had a daily input of cholecalciferol to incorporate into the yolk being laid down in ovarian follicles, the eggs would have contained insufficient cholecalciferol to meet the needs of embryos during their prolonged period of incubation.
The cholecalciferol-deficient dinosaur embryos, like those of the modern birds would either die during development, or else just before hatching. Although environmental changes leading to failure of food supply would certainly threaten the survival, over years or decades, of the wide variety of dinosaur species, the most effective mechanism of extinguishing the dinosaur family altogether would be one that prevented reproduction.
One could speculate that if the modern bird descendants of theropod dinosaurs were today unable to obtain cholecalciferol from their environment, then over a short period of time they too would become extinct from reproductive failure. Of course, if reproductive failure because of cholecalciferol deficiency was the cause of all dinosaur species becoming extinct, why then did other terrestrial vertebrates survive and evolve into the animals we know today?
It is apparent that modern birds do not have a storage mechanism for cholecalciferol. Could other animals, such as mammals, have developed a means of conserving cholecalciferol to allow its many functions to continue when the environmental supply ceased?
Present-day nocturnal mammals which seldom are exposed to solar UVB radiation seem to be able to maintain the functions of cholecalciferol without any apparent input from its formation in skin. 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 , — The end of the Cretaceous Period saw one of the most dramatic mass extinctions Earth has ever seen.
Find out what brought about the end of the dinosaurs and many other animals too. The fossil record shows that for the first million years of their existence, dinosaurs took on a huge variety of forms as the environment changed and new species evolved that were suited to these new conditions.
Dinosaurs that failed to adapt went extinct. But then 66 million years ago, over a relatively short time, dinosaurs disappeared completely except for birds. Many other animals also died out, including pterosaurs, large marine reptiles, and ammonites.
Although the number of dinosaur species was already declining, this suggests a sudden catastrophic event sealed their fate - something that caused unfavourable changes to the environment to occur more quickly than dinosaurs and other creatures could adapt.
The exact nature of this catastrophic event is still open to scientific debate. Evidence suggests an asteroid impact was the main culprit. Volcanic eruptions that caused large-scale climate change may also have been involved, together with more gradual changes to Earth's climate that happened over millions of years.
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