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New Mexico State University geologists challenge belief about prehistoric extinction

Two New Mexico State University geology professors and a former student are causing ripples in the world of paleontology by raising questions about a fossil the size of a speck of dust.



A phosphatic microspherule, shown here greatly magnified, is a tiny fossil only two millimeters in diameter. Scientists have identified it as a "pearl" laid down by a prehistoric creature called a conodont, but two New Mexico State University geologists a

Katherine Giles is an associate professor of geology who specializes in the study of carbonate sediments; Nancy McMillan is an associate professor of geology who specializes in geochemistry. Neither is a paleontologist, but they have been invited to publish an article in an upcoming special edition of the international journal Palaeogeography, Palaeoclimatology, Palaeoecology.

In the article, they will argue that some paleontologists who study the late Devonian Era, 380 million years ago, have for decades misidentified fossils called phosphatic microspherules. Giles and McMillan said if their theory is correct it will provide an important clue to a massive extinction in the late Devonian Era that was larger than the event in which the dinosaurs died.

Their argument is based, in part, on research by Brian L. McCarson, a former New Mexico State undergraduate geology student, who did a chemical analysis of the microspherules in 1996.


At the end of the late Devonian Era, 96 percent of the animals living in the shallow seas around the continents of the time disappeared. The extinction opened an ecological niche for modern fish, which at the time represented only a tiny percentage of aquatic life, but scientists are unsure what caused it, Giles said.

Some think the world became too cold, or too hot, at the equator, although these theories are countered by the fact that very few of the land animals of the time became extinct. Others think the ocean around the continents became too hot or that it "turned over" bringing cold, low oxygen, water from the depths to the surface.

Phosphatic microspherules are tiny spheres -- their diameters are only three times the width of a human hair -- composed of calcium phosphate. Since the 1970s, some paleontologists have identified them as "pearls" laid down by an eel-like creature from the Devonian Era called a conodont. But after doing a chemical analysis of the microspherules, conodont fossils and fish fossils from the era, McCarson concluded it is more likely the microspherules are related to the fish than to the conodonts.

Building on McCarson's work, Giles and McMillan state that paleontologists have misidentified the microspherules and that they actually are the inner ear bones -- or otoliths - of fish who survived the extinction.

Giles said recognizing the fossils as otoliths could provide an important clue about the Devonian Era extinction. It also could lead to scientists being able to analyze the chemical composition of Devonian Era sea water, water that disappeared over 300 million years ago. But the identification is still controversial, not only because it overturns long-held paleontological beliefs, but because no modern fish have otoliths composed of calcium phosphate, she said.

Giles and McMillan argue that the microspherules are composed of calcium phosphate -- rather than calcium carbonate like the otoliths of modern fish -- because the fish they belonged to adapted to live in water with a heavy concentration of phosphorus. Such water could only have come from the bottom of the ocean and could only have retained the phosphorus because it was very cold. This provides definitive support for the cold water theory about the extinction, McMillan said.

Furthermore, if the fossils are otoliths, scientists can learn a great deal about the composition of Devonian Era water by analyzing them, the two scientists added. Because fish draw water and the elements suspended in it into their systems, scientists analyzing modern fish can tell much about modern water by analyzing their otoliths, even to the point of being able to identify where the fish were born, Giles said.

Giles said she became interested in the microspherules in the 1990s, when she was looking at Devonian Era rock samples in connection with other research. She noticed that she would find microspherules in samples that did not contain conodonts and, conversely, would find conodonts in samples that did not contain microspherules. She began to question whether the tiny spheres were connected to conodonts. She did not pursue the research, but thought of it when McCarson was looking for a research topic, she said.

When she and McMillan were invited to submit McCarson's conclusions to the paleontological journal, Giles said she felt she should include a theory about what microspherules are. She began to research them and, while leafing through a book on modern fish, came across a photograph of modern fish otoliths.

"The resemblance was so striking that we sent our photographs of microspherules off to an expert on modern fish. He confirmed that they were identical to otoliths, but refused to confirm that they are otoliths, because no fish he knows of have otoliths composed of calcium phosphate," she said.

McMillan, a geochemist, has found that the trace elements in Devonian Era samples of fish fossils and microspherules are the same, while the trace elements in samples of conodonts are different. She also has found that the atomic structures of the samples are the same for fish and microspherules, but are different for conodonts, she said.

It was McMillan who suggested that McCarson, who was interested in geochemistry, could study the chemistry of the Devonian Era samples. McCarson studied the samples by using an electron microprobe at Los Alamos National Labs. The probe focused a highly concentrated beam of electrons on the samples, causing them to give off x-rays. Because each element's X-rays have a unique wavelength, the different X-ray signatures could be studied using a computer to determine which elements made up each sample, McMillan said.

McCarson completed the research as one of the requirements for receiving a Ronald E. McNair scholarship, a U.S. Department of Education program that pays the college expenses of underrepresented or first-generation college students. He completed a bachelor's degree in geology at New Mexico State in 1996. He later completed a master's degree in material science at the University of North Carolina and is working as a materials scientist for the Motorola Production Co.