Smilodon Charles R. Knight On Friday at a National Geographic-sponsored TEDx conference, scientists met in Washington, D.C. to discuss which animals we should bring back from extinction. They also discussed the how, why, and ethics of doing so.

They called it "de-extinction."

There are a few guidelines for which ancient species are considered, and sadly, dinosaurs are so long dead they aren't in the picture. Their DNA has long ago degraded, so researchers are fairly sure that Jurassic Park will never happen.

They chose the animals using the following criteria: Are the species desirable — do they hold an important ecological function or are they beloved by humans? Are the species practical choices — do we have access to tissue that could give us good quality DNA samples or germ cells to reproduce the species? And are they able to be reintroduced to the wild — are the habitats in which they live available and do we know why they went extinct in the first place?

But, this still leaves plenty of other animals on the table. The list of candidates is actually pretty long, considering. The cost of de-extinction varies by species but projects could run into the hundreds of thousands of dollars, if not more. Then there's also the cost of housing the animals once they are created, and re-introducing them into the wild and protecting them from poachers once they are there.

But, if you were the zoo that had that one Woolly mammoth or saber-toothed cat, these costs just might be worth it.

Here are the 24 animals they are hoping to one day resurrect.