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Step-by-step Evolution

Mining the Gaps: Transitional fossils are the hardest to find, but sometimes tell the best stories

By Sid Perkins -January 2009 issue; Vol.175 #3 (p. 30) - Science News

When Charles Darwin proposed the idea of evolution in On the Origin of Species, he wrote “if my theory be true, numberless intermediate varieties, linking most closely all the species of the same group together, must assuredly have existed.” At the same time, he bemoaned the dearth of such transitional fossils as perhaps “the most obvious and gravest objection which can be urged against my theory.”

Surely it was serendipity when, just two years later, quarriers unearthed fossils of Archaeopteryx. This creature, now recognized by many scientists as the first known bird, has a mosaic of features that links it with the disparate groups of species on either side of it in the fossil record: While its teeth, tail and overall body shape are distinctly reptilian, its feathers have the same complex structure as the lift-generating feathers of modern birds. In other words, it is just one of the “numberless intermediate varieties” that Darwin predicted must have existed.

“It was the right discovery at the right time,” says Richard Fortey, a paleontologist at the Natural History Museum in London.

Darwin blamed the lack of transitional fossils in part on the poorness of the paleontological record. It’s a rare accumulation of fortuitous events when a creature is fossilized, its remains are preserved over millions of years, and then those remains are discovered.

In many cases, that critique still holds true: Researchers have yet to discover fossils of a creature that fits in the gap between bats, which seem to appear suddenly in the fossil record about 54 million years ago, and their mammalian predecessors. The gap in the fossil record between Archaeopteryx and its reptilian ancestors also remains unoccupied, although several discoveries of feathered dinosaurs in China have given researchers clues about what these still undiscovered intermediate creatures may have looked like.

Many of the gaps in the fossil record that remained unfilled in Darwin’s time now throng with creatures, such as the ones used to chronicle the 48-million–year series of evolutionary changes between whales and their predecessors . And particular biomarkers — chemical fossils, if you will — in rocks more than 240 million years old have provided clues about the evolution of flowering plants.

Paleontologists still randomly stumble across transitional fossils these days, such as a creature found in Texas that falls in a 50-million–year gap in amphibian evolution and helps pin down when the groups that include salamanders and frogs arose.

As often as not, however, transitional fossils are found when researchers head into the field with a specific target in mind: By focusing on rocks deposited during an interval where gaps in the fossil record exist, scientists can boost the chances of making a critical discovery. That’s how researchers unearthed Tiktaalik, a 2.7-meter–long beast that plopped into a 9-million–year gap in the chronicle of vertebrates’ transition from water to land.

Techniques such as CT scanning, used to reinvestigate fossils collected decades ago, have revealed new insights about the anatomy of semiaquatic creatures that preceded Tiktaalik. Even genetic analyses of living creatures can provide insight into the fossil record: The evolutionary changes observed in fossil fish deposited over a time period of 20,000 years in an ancient lake can be linked to a particular gene often studied in that species’ modern-day kin.

Amphibian enigma

Gaps in the fossil record can be large in terms of time — sometimes many millions of years — and in the extent of the evolutionary changes seen when comparing creatures before and after the gap. When Archaeopteryx was discovered, for instance, the fossil record was sparse and the disparity between known fossil reptiles and birds was vast.

Until recently, the gap in the fossil record separating frogs and salamanders from their amphibian ancestors was similarly huge. About 290 million years ago, a diverse assemblage of primitive amphibians walked the land, says Jason Anderson, a vertebrate paleontologist at the University of Calgary in Canada.

But in rocks documenting the 50 million years or so that followed, amphibian fossils are few and far between. Only in rocks deposited after 240 million years ago do such fossils — and specifically, those of frogs and salamanders — appear. These two groups of creatures are distinct both from each other and from their ancestors, and they apparently evolved during an interval for which few fossils have been discovered.

Recently, however, Anderson and his colleagues unearthed Gerobatrachus hottoni, a species whose genus name means “elder frog.” The single specimen unearthed so far is about 11 centimeters long, the size of most modern-day salamanders. It was found in a two-foot-thick knob of 290-million-year-old, fine-grained siltstone in north-central Texas. Even though the fossil was found in rocks deposited just before the start of the lengthy gap in the fossil record, the remains have features characteristic of the frogs and salamanders that presumably descended from it or others like it, Anderson says.

A main clue is that some of the bones in the first and second innermost toes on each of Gerobatrachus’ feet are fused together, a trait characteristic of salamanders but rarely found in other creatures. Because some of the other bones in the fossil aren’t fully developed, Anderson and his colleagues suggest that the creature was a juvenile, indicating the fusion of the toe bones occurred even before adulthood — a stronger sign that it betrays an evolutionary link to salamanders.

But like frogs, Gerobatrachus has a broad skull and a shortened tail, the researchers reported last May in Nature. The shape and configuration of bones in the creature’s skull, and particularly those in its palate, are very froglike. Therefore, “this fossil seals the gap” between primitive amphibians and the frogs and salamanders that evolved later, Anderson says.

On the amphibian family tree, Gerobatrachus and its kin are ancestors to salamanders and frogs, the researchers contend, and the evolutionary split between those two groups probably occurred between 260 million and 270 million years ago.

Gerobatrachus was “quite advanced” compared with other amphibians of its era, he adds. Another way to look at it, he notes, is to consider the amphibians appearing 290 million years ago to be evolutionary holdovers best representing species that first evolved long before.

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