Sciencespeak: Lazarus taxon

It’s a science fiction staple. An intrepid explorer is walking through the woods when they stumble across an ancient organism not seen for millions of years. Dinosaurs are choice for such appearances, but pterosaurs and other prehistoric critters do just as well. In text and on film, they manage to persist in some isolated pocket where extinction spared them. But such scenarios are not restricted to the realm of fantasy.

In July of 1943, while traveling through eastern China, forestry official Zhan Wang heard a tantalizing rumor. In the town of Moudao, the principal of Xian Agriculture High School told him, there grew a tree that no one could identify. That was enough for Zhan. He altered his travel route across Hubei Province to find the mystery tree, and, sure enough, he found it. With a few snips Zhan collected some branches and cones according to standard botanical protocol and was on his way.

Once he had a chance to fully examine his sample, though, Zhan wasn’t sure what the tree was. The plant’s anatomy resembled that of the Chinese swamp cypress – a tree known for decades – but small details of the leaves, branches, and cones were all wrong. Not wanting to go out on a limb, Zhan classified the tree as Glyptostrobus pensilis?, the question mark a reminder that the species might not be the swamp cypress, after all.

The next summer botanist Zhong-Lun Wu was looking through the herbarium collections at the National Central University at Chongqing when Zhan’s mystery cypress caught his eye. It looked like something new. This sparked a flurry of comparison and discussion among China’s botanists and dendrologists that ultimately arrived at a startling conclusion. The tree was not new to science. Astonishingly, it was a living species of Metasequoia – the “dawn redwood” that had been named from fossils just a few years before.

Some called Metasequoia a “living fossil“. Whether the term fits or not depends on what you think about how much the tree has changed in the last five million years or so. But there’s another term that definitely applies to discoveries like this. Metasequoia is a Lazarus taxon.

For those who are little shaky on their New Testament stories, Lazarus is the fellow that Jesus is said to have raised from the dead. And while the miracle of finding the Metasequoia was one of science, rather than religion, paleontologists Karl Flessa and David Jablonski coined the term Lazarus taxon for organisms that reappear after their presumed extinction.

There are plenty of other examples of Lazarus taxa. The most famous is the coelacanth – an ancient form of fish thought to have gone extinct over 66 million years ago only to turn up in a South African fish market. A genus of ant first found in amber, a midwife toad, and a whole group of marine invertebrates called monoplacophorans fit the bill, too, though the term isn’t restricted to living species.

“Lazarus taxon” was originally coined for organisms – from a single species up to an entire group – that seem to disappear during one of Earth’s “Big Five” mass extinctions only to pop up again in the fossil record. That’s because “fossilization lows” seem to immediately follow mass extinctions wherein, for one reason or another, not as many organisms wind up locked in stone. And applied more widely to the fossil record, the extensive list of Lazarus taxa includes a lineage of weasel-like protomammals called diademodontids that reappear in the Triassic rock of South Africa after an absence of 21 million years and a slew of odd invertebrates that were thought to have gone extinct by 501 million years ago before turning up in rocks 488-472 million years old.

So why do some creatures seem to blink out of the fossil record only to be revived? There’s more than one reason. The simplest is that the fossil record is not only incomplete, but incompletely-studied. There are fossil-bearing strata that have yet to feel the boots of curious paleontologists, and there are always significant specimens that get overlooked. Not to mention that recognizing living Lazarus taxa is an interdisciplinary effort that requires paleontologists and field biologists to be aware of what the other group is doing. There may be living species that count as Lazarus taxa but haven’t been recognized as such just yet.

Then there’s the nature of the fossil record itself. A species or lineage might go extinct in a given area but persist elsewhere. This geographic problem may be why we don’t have a good fossil record for the living coelacanth, for example. The fish may have clung to existence in deep sea haunts that either didn’t fossilize or have not been discovered yet. And in the case of Lazarus taxa that pop up after mass extinction, it may be that populations temporarily fell too low to allow for a good chance of fossilization. The fossil record is a wonderful window to view ancient life, but we need to be aware of the cracks and smudges while gazing into prehistory.

[Note:  I started this feature as “Science Word of the Day”, but it’s not daily and I want to include phrases that are more than one word. So “Sciencespeak”, it is.]

References:

Abdala, F., Damiani, R. Yates, A., Neveling, J. 2007. A non-mammaliaform cynodont from the Upper Triassic of South Africa: a therapsid Lazarus taxon? Palaeontologia Africana. 42: 17-23

Fara, E. 2001. What are Lazarus taxa? Geological Journal. 36: 291-303

Ma, J. 2002. The history of the discovery and initial seed dissemination of Metasequoia glyptostroboides, a “living fossil”The history of the discovery and initial seed dissemination of Metasequoia glyptostroboides, a “living fossil”The history of the discovery and initial seed dissemination of Metasequoia glyptostroboides, a “living fossil”. Aliso. 21 (2): 65-75.

Ma, J. 2003. The chronology of the “living fossil” Metasequoia glyptostroboides (Taxodiaceae): A review (1943-2003)The chronology of the “living fossil” Metasequoia glyptostroboides (Taxodiaceae): A review (1943-2003)The chronology of the “living fossil” Metasequoia glyptostroboides (Taxodiaceae): A review (1943-2003). Harvard Papers in Botany. 8 (1): 9-18

Shao, G., Liu, W., Chen, J., Ma, J., Tan, Z. 2000. Zhan Wang (1911-2000). Taxon. 49 (3): 593-601

Van Roy, P., Orr, P., Botting, J., Muir, L., Vinther, J., Lefebvre, B., Hariri, K., Briggs, D. 2010. Ordovician faunas of Burgess Shale type. Nature. 465: 215-218.