There is no definite answer but any opinions will be approved of. Popular beliefs are: asteroid/comet collision, climate change, eruption of volcanoes, disease, etc. Or maybe it is a combination of a few?
I think it's disease/parasite because all of the species were wiped out, and diseases spread quickly.
Excuse the long comment (below) but I couldn’t resist! Are you sure you meant what you said that “any opinions will be approved of”?
Are you absolutely certain that dinosaurs are extinct (think coelacanth)? There is evidence that some were still around in comparatively recent times. For example, the discovery in recent years of dinosaur bones containing remnants of soft tissue; which could never have survived more than a few thousand years.
There are also legends of dragons and sea monsters that resemble nothing known to be alive today. The oldest (and presumably least exaggerated) of which resemble resemble fossil animals such as dinosaurs. So they may not be as mythological as most people think. The Biblical description of "behemoth" in the book of Job resembles a sauropod dinosaur (such as Apatosaurus). Some have suggested “Behemoth” was a hippo or elephant, but it says that his tail “sways like a cedar” (someone joked it must have been a bonsai cedar to have been one of those!)
And what about the ancient tale of Beowulf? The description of the monster Grendel, and how Beowulf tore off one of its “arms”, (which must have been comparatively tiny, even allowing for exaggeration of the hero's strength), suggest it could have been a Tyrannosaur of some kind. There is also an old account of creatures that sound like pterosaurs near a village in Mid Wales. They were said to have “glowed like jewels” as they flew. Unfortunately the villagers killed them because they were taking their chickens!
Then there are carvings drawings etc in various parts of the world that closely resemble dinosaurs. The tomb of a 15th century bishop in Carlisle has a brass surround with figures of various animals, two of which look exactly like sauropod dinosaurs. Their necks are twined together as if they are either courting or fighting in a similar way to modern giraffes. Their tails are held up rather than dragging on the ground; even though modern palaeontologists have only realised recently that this is the correct way.
Here is a link to a blog about Bishop Bell’s tomb.
I find parts of it highly amusing (emphasis added). Hopefully the "sceptic) author said it tongue-in-cheek:
“Along one of the brass strips, if you look carefully, you can see two animals. The one on the left is obscured by the one on the right that looks suspiciously pre-historic. It clearly has a long tail, an even longer neck and head. With a neck like that we might have been tempted to think giraffe if it were not for the four thick-set legs and large rounded body. It is for us, of course, a classic image that every school child would immediately recognize. However the one animal that it cannot possibly be is a dinosaur. That of course is quite impossible.
“Dinosaurs as we all know were not discovered until 1840 and this tomb was constructed in 1496. No one even knew that there were dinosaurs 500 years ago, never mind what they looked like. So it can not be a dinosaur. QED.”
“the one animal that it cannot possibly be is a dinosaur. That of course is quite impossible.”
I think that, if they are extinct, many of the last dinosaurs were killed by people! The only reason we think that is “impossible” is because we have been told so often that they died out millions of years ago. The evidence suggests otherwise.
"For example, the discovery in recent years of dinosaur bones containing remnants of soft tissue; which could never have survived more than a few thousand years."
The scientists that made this discovery are apparently tying themselves in mental knots trying to understand how the soft tissue could have been preserved for over 60 million years. It doesn't seem to occur to them that the dating could be wrong!
Wow! I knew nothing about that before! This has certainly made me see dinosaurs in a different light! I can't quite imagine dinosaurs actually being alive at the same time as humans! I had always imagined the monster in Beowolf to be a mythical, made-up being, originating from a creative mind. However, now, I'm not too sure... Thank you for the fascinating and truly enlightening reply! I have learnt a lot!
Thank you for this reply, which I found encouraging because most people are so fixed in the millions of years mindset that they seem unable to consider any other possibility even when the evidence is there before them.
To update, I just read about coloured dinosaur skin that has been found, (which apparently has remnants of the colour cells - one specimen was clearly black).
I'm sure we are unlikely to face a Jurassic Park kind of situation any time soon :-), but the possibility that maybe the skin isn't millions of years old after all still doesn't seem to have occurred to anyone doing the research. Yet, to me, that is what science should be about, questioning, examining various possibilities, and being willing to let go even of our deeply held "pet theories" if the evidence doesn't support them.
In case you want to check it out, here are some references from the article I found:
Kretzel, L., Scientists use U of S synchrotron to study dinosaur skin, Saskatoon News—News Talk 650 CKOM, www.ckom.com, 29 April 2013.
Scientists study rare dinosaur skin fossil to determine skin colour for first time, phys.org, 29 April 2013.
Foley, J., What color were dinosaurs? Test of ancient skin sample will reveal final answer, Nature World News, natureworldnews.com, 29 April 2013.
In the skin of a … hadrosaur?, Scientific Canadian, http://tylerirving.ca/?p=1101, 6 May 2013
The way I see it is dinosaurs were large, specialist animals that lived on an ecological knife edge.
Back then things were much warmer and the oxygen level was considerably higher than it is now.
A meteor or comet possibly struck, but also there was possibly increased volcanic activity, which means higher CO2 output, temperature rise, and generally less clean air. Such a push, even a small one, could tip such an ecosystem into chaos, and could lead to extinction.