Dead Fish Tell Evolutionary Tales

by Phil Berardelli

Stages. Researchers found that the soft tissues of lampreys and hagfish (inset) decay in a consistent, predictable sequence.

Credit: Robert Sansom; (inset) Mark Purnell and Sarah Gabbott

Talk about suffering for a noble cause. A group of paleontologists at the University of Leicester in the United Kingdom spent almost every day for more than 6 months watching fish rot in an attempt to find patterns in their decay. The results show that a body's soft tissues deteriorate in a consistent and predictable way. The findings, experts say, could greatly improve our understanding of evolution by helping researchers better interpret the fossil record.
We know what we know about extinct vertebrates almost entirely because of the mineralization of their bones. Over thousands of years, the calcium phosphate in an animal's skeleton combines with other minerals in the ground, forming fossils that can survive for hundreds of millions of years. But bones tell only a small part of the story, because the body of a vertebrate consists mostly of soft tissues—skin, muscles, nerves, and the like—that decay quickly.
In some cases, researchers can deduce specifics of an extinct animal's soft tissues by comparing them with those of living creatures. Recently, for example, researchers studied the bone joints of dinosaurs and those of modern-day birds and reptiles and concluded that some dinosaurs must have sported much thicker-than-expected pads of cartilage in their joints. Therefore, some dinosaurs must have stood a little taller than they've been depicted in museum exhibits. Such anatomical details are often lost to paleontology.
Soft tissues are even more important in the most primitive vertebrates because they show the subtle and complex changes that occurred as this group emerged, says lead author Robert Sansom, who collected lamprey specimens for the study from streams in the United Kingdom and hagfish from fjords in Sweden. He says the research team chose the two species because they are among the most primitive forms of fish, so they can provide more clues about the bodies of their long-dead ancestors.
Sansom and colleagues Sarah Gabbott and Mark Purnell spent 200 days observing the decomposition of the fish carcasses and documenting which body parts disappeared in what order. Despite ventilating fans, industrial deodorizers, and breathing masks, he says, "the stench was pretty pungent." And although Sansom say he has grown accustomed to the environment, "I do not get many visitors to the lab."
The hardship seems to have been worth it. From their observations, the researchers were able to detail what they call a largely unknown "decay bias." That is, Sansom explains, the later-evolved parts of the body are lost soonest. In lampreys, for example, certain parts of the brain and the mouth that distinguish the animals from earlier relatives begin a rapid decay within 24 hours. Those missing structures, says Sansom, can cause researchers to misinterpret fossil organisms "as more primitive than they would have been in life," something that potentially skews evolutionary history.
With the new results, researchers can compare the presence or absence of features preserved in fossils with the decay patterns in the present-day specimens. In fossils that preserve only characteristics known to be decay resistant, it's possible that some evolutionary features "could be missing due to decay," Purnell says. If so, scientists could "lack the information to interpret that fossil correctly." The team will publish its aromatic results online tomorrow in the Proceedings of the Royal Society B.
The study provides "important constraints on the interpretation of rare fossils, including some of our earliest ancestors," writes paleobiologist Derek Briggs of Yale University's Peabody Museum of Natural History in an e-mail. The results with decaying fish, he adds, "can be applied to the interpretation of soft-bodied fossils in general, and particularly to determining their true place in the tree of life."