Back from the Dead?
'Resurrections' of long-missing species lead to revelations
In December 1938, Marjorie Courtney-Latimer, curator of a natural history museum in East London, South Africa, went to the docks to look for interesting specimens among the day’s catch. What she found one day she later described as “the most beautiful fish I had ever seen … a pale mauve blue with iridescent silver markings.” The discovery sent scientists into a frenzy.
The 54-kilogram creature was a lobe-finned fish called a coelacanth. Researchers dubbed it a “living fossil” because the remains of creatures like it had been found only in rocks more than 75 million years old. It seemed that all such fish had died out about 10 million years before the dinosaurs did, yet here was a fresh specimen. And before the century was out, scientists had identified a second living species of coelacanth and had caught or observed the fish in waters from South Africa to Indonesia (SN: 5/5/01, p. 282).
The apparent resurrections of the coelacanth and other long-missing species have led scientists to give such living fossils another name: Lazarus taxa, after the beggar who was raised from the dead in a biblical parable.
In the strictest sense, the modern representative of a Lazarus taxon belongs to the same species that disappeared from the fossil record many years ago. More loosely, researchers apply the term Lazarus taxon to the extremely close kin of ancient apparent extinctions. Coelacanths fall into this category: Although the living species are remarkably similar to some ancient ones, there are no known fossils of today’s coelacanths. The same is true of the Laotian rock rat, a squirrel-size member of a group of rodents previously supposed to have disappeared about 11 million years ago (SN: 4/28/07, p. 260).
How do Lazarus taxa disappear in the first place? Several factors may play a role, scientists suggest. A creature may simply be rare or may live only in an uncommon habitat, or it may live in an environment where remains fossilize poorly or infrequently. In the case of coelacanths, both reasons may apply. Only a few hundred have been found since 1938, and they inhabit deep waters adjoining the steep slopes of volcanic islands, a setting where sediment seldom accumulates quickly enough to bury and preserve a carcass.
To recognize a Lazarus taxon, moreover, scientists must identify its modern representative, and such discoveries may depend greatly on serendipity. Both the Indonesian species of coelacanth (SN: 2/1/97, p. 74). Although a large number of new species evolved following this mass extinction, many of the species that had apparently died out reappeared in the fossil record about 5 million years later, says Fraiser. For example, about 57 percent of the genera of gastropods, or marine snails, found in the fossil record after that 5-million-year gap were Lazarus taxa.
Many scientists contend that the simultaneous reappearance of so many Lazarus taxa indicates that the fossil record from that era can’t be trusted, says Fraiser. Others suggest that the missing creatures simply became so rare that they weren’t captured in the fossil record. Yet others propose that the creatures survived only in small areas and that their fossils haven’t yet been discovered.
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To test these ideas, Fraiser and her colleagues analyzed the fossil record of many types of marine creatures both before and after the Permian extinctions. Groups of species included sponges, shelled invertebrates such as brachiopods and bivalves, and echinoderms such as starfish and sea urchins.
Previously, some scientists had assumed that the fossil records of all of those groups of species would show a large number of Lazarus taxa, says Fraiser. That’s not what her team found, she reported at a meeting of the Geological Society of America last month in Denver. None of the genera of echinoderms or brachiopods found in the fossil record 5 million years after the end-of-the-Permian extinctions represented Lazarus taxa. Only 12 percent or so of the bivalve genera found at that time had been resurrected, she notes. The dearth of resurrections among these groups suggests that the reappearance of the gastropods is genuine, not a sign that the fossil record during that interval is somehow faulty.
Scientists have also suggested that the fossil record immediately after the mass extinctions was poor because the drastic environmental changes that caused the die-offs also affected modes of fossilization. In particular, those scientists have suggested, tissues weren’t being replaced by silica, which produces some of the most durable fossils. However, Fraiser and her colleagues found that about 58 percent of the fossils in the database they studied were formed of silica, which suggests that the fossil record from that era can be trusted. Further analyses will be needed to determine what happened to the Lazarus taxa during the gap in the fossil record immediately after the Permian extinctions, says Fraiser.
“These Lazarus taxa must have been somewhere, maybe in [rocks] that paleontologists haven’t sampled yet,” says Richard J. Twitchett, a paleoecologist at the University of Plymouth in England. “Or maybe their fossils have been misidentified or overlooked.”
Modern discoveries of Lazarus taxa point out the risks of overinterpreting the absence of a creature from the fossil record.
“It’s almost impossible to use the fossil record to define when an animal goes extinct,” Twitchett adds. “Maybe it just became rare or marginalized.”