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Of course, they are fossils now. I found a link here which states that herbivors lived far longer than carnivores. The example being a T-Rex being "old" at 29, and a herbivorous Bothriospondylus having not reached half it's adult size at 43 years old.

But, is there any evidence that can indicate their life spans, or life spans of at least one kind of the dinosaurs?

An additional brief question is are their lifespans comparable with modern day descendants?

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    $\begingroup$ @questionhang: The general technique with dinosaurs is to count the rings in bones, although there are limitations to the method. Most notably, the technique gives an estimate of how old an individual was at death, not how long a member of the species could survive per se. Nevertheless, see books.google.com.au/… $\endgroup$ – bshane Dec 14 '15 at 9:58
  • $\begingroup$ The question is unclear. 1) Are you talking about dinosaurs or mammals? 2) What dinosaur are you talking about? 3) Are you interested in estimates of age or description of the methods to find these ages (as suggested in the comments). Note also that you end sentences who are not questions with question marks which is quite confusing. $\endgroup$ – Remi.b Dec 14 '15 at 20:45
  • $\begingroup$ @Remi.b The description is better now? $\endgroup$ – questionhang Dec 15 '15 at 2:01
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    $\begingroup$ I have made some major edits to the question and retracted my close vote. In it's current state the question is very answerable since the question is now asking about a general technique (hinted at by @bshane's comment). $\endgroup$ – James Dec 15 '15 at 3:16
  • $\begingroup$ I also don't find the question particularly unclear in its current form - voting to keep open. $\endgroup$ – fileunderwater Dec 16 '15 at 23:46
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Your first question appears to be answered (tentatively, at least) in the article you linked to...

"Traditional" estimates based on slow, reptilian growth rates, combined with the enormous size of dinosaurs, led scientists to conclude it could be up to several hundred years. However, palaeontologists today believe that dinosaurs grew much more quickly, rather like birds and mammals.

Then...

Dinosaur bones grew like those of other vertebrates, by adding new bone matter to the outside of the bone. Because of annual variation in temperature or the availability of food, periodically bone growth would slow down, and a thin layer of a vascular bone would form a ring or "growth line" in much the same way that tree trunks do. By taking thin slices of bones, these rings can be viewed under a polarised light source. Counting the rings can give an idea of the dinosaur's age at death.

Theoretical ages of some specific species are given in various articles, including Wikipedia and LiveScience.

The infamous T. rex theoretically reached adulthood at the age of twenty (similar to humans) and reached an age of at least twenty-eight years. I think most contemporary animal species mature much more quickly than that...but few approach T. rex in size. The hypothetical ages given for some of the smaller dinosaurs appear to be more in line with modern living species.

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  • $\begingroup$ Not a lot more quickly, a lion takes around five years to reach maturity but can take as long as 8 years. Polar bears take 8-10 years. once you account for the size difference their growth is comparable to mammals and birds. $\endgroup$ – John Nov 25 '16 at 16:54

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