Paleontologists have found what could be the earliest known dinosaurs, a couple of kangaroo-sized plant eaters that roamed Madagascar about 230 million years ago.
The same site also yielded fossils of the reptile lineage that later evolved into mammals.
Researchers from the Field Museum in Chicago, Northern Illinois University, University of California at Santa Barbara, and Universite d’Antananarivo in Madagascar collected the fossils during four separate expeditions between 1996 and 1999.
The findings are reported in the Oct. 21 issue of the journal Science.
The Madagascar fossils will be on display at the Field Museum Oct. 22 through Jan. 2, 2000.
A Revolution in Evolution
“It’s a whole new window in the evolution of animals at a key point,” comments Neil Shubin, a University of Pennsylvania biology professor.
With a relatively sparse fossil record, “Madagascar has always been something of an enigma,” Shubin says. “They’re filling both a gap in time and a gap in space. We see the transition from an archaic fauna of reptiles and amphibians to a more modern fauna. You think about it, these are creatures that were going to be around for millions of years. It’s a revolution in the history of life that happened during this time period.”
Madagascar is an island off east Africa, but 230 million years ago, it and all of Earth’s continents were joined as the supercontinent Pangea.
This period known as the Triassic marked the appearance of many modern groups of animals, including mammals, crocodiles, turtles, frogs and bony fish.
The most noteworthy newcomers were, of course, the dinosaurs, destined to dominate the landscapes for the next 165 million years.
Their beginnings appear to have been humble.
So far, the Madagascar site has yielded only the jaws and partial skull fragments of two early dinosaurs. The shape of the bones indicate the animals were of a group of dinosaurs known as prosauropods.
“This animal is probably the size of a small kangaroo,” says John Flynn, curator and chair of the department of geology at the Field Museum in Chicago and lead author of the Science paper. “They probably would have ambled about on four legs. It’s sort of like the way kangaroos walk around when they’re feeding.”
The region back then was wetter, he says. “It was even more sort of tropical. There were big rivers running through this area. I would imagine it being something like the Mississippi river valley.”
Although small, prosauropods later evolved into behemoth sauropod dinosaurs such as Apatosaurus and Brachiasaurus.
None of the new species has been officially named yet but geologist Andrew Wyss of the University of California at Santa Barbara, who worked on the study, said some of the names would honor local residents who found the fossil bed.
“A boy said that his older brother had found some bones,” Wyss said in a statement. “So we waited around a half day for the brother, Mena, and sure enough, he showed us a hill with a mound of them.”
The Madagascar rocks in which the fossils were found don’t contain minerals that can be dated directly. Rather, Flynn and his colleagues argue their rocks are very old based on what else is and isn’t found there.
Two of the other types of animals — mammal-like reptiles and reptiles with parrot-like beaks — are believed to have died out by 228 million years ago. Also, paleontologists found no aetosaurs — small, armored reptilian plant eaters — that flourished beginning about 228 million years ago.
“The bottom line is, we think we have dinosaurs at least as old as those discovered before,” Flynn says. “It’s not conclusive, but we think there’s a reasonable argument to be made that they may well be the oldest.”
Up to this point, the oldest known dinosaurs are a species known as Herrerasaurus, dug up in Argentina and dated to be just under 228 million years old.
Interestingly, the Argentinean Herrerasaurus was a carnivorous theropod dinosaur, the group that would later include Tyrannosaurus rex and velociraptors. “Some of the major divisions in the dinosaurs actually happened early in their history,” Shubin says and suggests that paleontologists might be “missing more of the record than previously thought.”
Evolving to Mammals Also of interest are the mammal-like reptiles. “This is a group of things that are transitional, going from the big, cold-blooded reptiles to the small warm-blooded true mammals,” Flynn says.
Although fractured and incomplete, the wide range of fossils will help paint a more complete picture of this time.
Says Shubin: “They’re pulling out a giant jigsaw puzzle that they’re going to be figuring out over the next few years.”
Reuters contributed to this report.