The issue, though, can be making certain the species actually is extinct. There are ongoing efforts to find relic populations of a number of presumed-extinct species (two of the best known are the ivory-billed woodpecker and the thylacine). Naysayers have criticized the efforts to find these species as nothing more than wishful thinking, but it bears keeping in mind that there is a long list of organisms thought to be extinct that have turned out to be very much alive.
They're called Lazarus taxa, after the biblical character Jesus raised from the dead. Some of them are astonishing. The one that always comes to mind for most people is the coelacanth, a crossopterygian fish that was only known from fossils preceding the Cretaceous Extinction sixty-six million years ago, which was discovered living in the Indian Ocean in 1938. But that's only one of many. Here's a sampler of Lazarus taxa:
- The South American bush dog (now split into three separate species in the genus Speothos) was only known from some Pleistocene-age bones found in a Brazilian cave, but is now known to have a range from southern Central America all the way to northern Paraguay. Its reclusive habits and rarity still make it the least-studied canid in the world.
- The nightcap oak (Eidothea hardeniana and E. zoexylocarya), which aren't oaks at all but a member of the Protea family (Proteaceae), were known only from fifteen-million-year-old fossils -- and then a stand of them were discovered growing in a remote part of Australia. The Royal Botanical Gardens in Sydney has a cultivation program for the two species, which are threatened because the seeds are frequently eaten by introduced mice.
Eidothea hardeniana [Image is in the Public Domain]
- The monito del monte, or colocolo opossum (Dromiciops gliroides), was not only thought to have gone extinct eleven million years ago, it was believed that its entire order (Microbiotheria) was gone as well. It was found -- alive -- in the temperate bamboo forests of the southern Andes Mountains in 1894, and has no near relatives anywhere in the world. (The closest are the Australian marsupials, but even those are very distant cousins.)
[Image licensed under the Creative Commons José Luis Bartheld from Valdivia, Chile, Monito del Monte ps6, CC BY 2.0]
- In 1898 a fish was discovered that was a near perfect match to Oligocene-age fossils on the order of twenty-eight million years old. It's Lignobrycon myersi, and is only known from the Rio Braço and Rio Contas in east-central Brazil. Somehow, it alone of its genus survived through all of those years and made it down to the present day.
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[Image licensed under the Creative Commons Alexandre dos Santos Rodrigues et. al., Lignobrycon myersi specimens (9382613) (cropped), CC BY 4.0]
- The monoplacophorans were a group of mollusks common during the Silurian and Devonian Periods, but were last seen in the fossil record in the mid- to late-Devonian, around 375 million years ago. After that -- nothing. Reasonably, biologists thought they'd gone extinct, until live monoplacophorans were discovered in deep water off the west coast of Costa Rica. Further surveys have found no fewer than thirty-seven different species in deep water across the Pacific.
A live specimen of Neopilina filmed off the coast of Samoa by the 2017 Okeanos Explorer mission [Image is in the Public Domain courtesy of NOAA]
- Even the monoplacophorans don't hold the survival record, though. That honor goes to Rhabdopleura, which is a graptolite -- a (very) distant relative of chordates known mainly through Cambrian-age fossils. The last Rhabodopleura was thought to have gone extinct in the mid-Cambrian, five hundred million years ago (and the rest of the group didn't make it past the mid-Carboniferous). In 1869 they were discovered living in the deep water of the Pacific, and since that time nine living species have been identified.
A drawing of Rhabdopleura normani [Image is in the Public Domain]
While the general rule still applies -- extinction is forever -- it's worth keeping in mind that sometimes we find ourselves in a situation a little like Mark Twain did, resulting in his quip, "Rumors of my death were great exaggerations." The Earth is a big place, and there are still plenty of poorly-explored regions where we might well have lots of surprises in store.
All of which should be encouraging to the folks out there chasing the ivory-billed woodpecker and thylacine. Don't give up hope. If Rhabdopleura could survive for five hundred million years unobserved, surely these two could manage a century or so.
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