By DeLene Beeland
Evidence from a study led by the Florida Museum of Natural History confirms that the carnivorous, seven-foot-tall “terror bird” likely arrived in North America from South America several million years before a land bridge connected the two continents. Previously, scientists assumed the 330-pound, flightless bird must have walked north from South America, but the new study—led by Bruce MacFadden, a vertebrate paleontologist with the Florida Museum—revised the age of Titanis walleri to 5 million years old in Texas and 2 million years old in Florida.
Scientists were uncertain of Titanis’s age because its fossils had been uncovered in rivers in north central Florida and Texas where 10,000-year-old stratigraphic layers had mixed with two- and five-million-year-old layers.
As rivers age, flowing water incises down into underlying sediment or rock. When fossils are buried in sediment below a riverbed, the water erodes down through the sediment layers—eventually uncovering and moving the fossils around and mixing stratigraphic layers.
To pinpoint Titanis‘s exact age, MacFadden and his team used an established geochemical technique that analyzes rare earth elements. Rare earth elements are a group of naturally occurring metallic elements that share similar chemical and physical properties.
When an animal dies, its porous bones absorb groundwater as they fossilize. As the local groundwater conditions change, the rare earth elements’ concentrations change, resulting in a unique chemical signature.
“We used rare earth elements because they’re highly specific to certain time periods and different groundwater conditions,” MacFadden said. “This is the first time that the uptake of rare earth elements during the early stage of fossilization has been used to determine the age of fossils in North America.”
It was previously thought that Titanis immigrated to Texas across the Panamanian land bridge that formed about 3 million years ago connecting North and South America.
“But the rare earth element analysis of a fossil Titanis bone from Texas determines its age to be 5 million years old, MacFadden said. “This shows that the bird arrived 2 million years before the land bridge formed, probably swimming between islands that formed what today is the Isthmus of Panama.”
The rare earth elements signature in the Florida fossils of Titanis indicated the specimens were 2 million years old, and not 10,000 years old—the age of other fossils it was found with.
Geologists have used the rare earth elements technique to study igneous and metamorphic rocks, but only one other researcher worldwide has applied this technique to date the age of fossils: professor Clive Trueman from the University of Southampton in England.
“It is very difficult to assess the age of fossil bones directly as they are too old to be carbon dated,” Trueman wrote in an e-mail. “Bones can also be moved after death, further confusing their true age. MacFadden’s approach compares bones of disputed age with those of known age. If the chemistry matches, the bones are of the same age irrespective of their final resting place.”
David Grandstaff, a professor and chairman of the geology department at Temple University, said the technique is timely and important.
“If a fossil gets moved or reworked from its place of formation, it will have a fingerprint that is different from the others nearby,” Grandstaff said. “Who knew that all these fossils essentially have a tag that says ‘hey, I’m from over here!’ ”
MacFadden’s study was published Jan. 23, 2007 in the online version of the journal Geology and featured in its February print edition. Co-authors of the study include Richard Hulbert Jr. of the Florida Museum of Natural History; Joann Labs-Hochstein, who at the time of the study was a postdoctoral student of MacFadden’s; and Jon Baskin of Texas A&M University.
Original publication date: January 23, 2007.