Fossil Find Linked to Mammal Evolution
The discovery of a fossil of a mouse-size critter that walked like a lizard and ate insects is filling a gap in scientists’ understanding of how humans evolved from ancient mammals.
Paleontologists knew of three types of prehistoric mammals, and they guessed from bone fragments that a fourth once existed, scurrying below the towering Tyrannosaurus rex and other dinosaurs of the late Jurassic Period.
But the discovery of a nearly complete skeleton in China confirmed clues that another type of mammal developed after egg-laying mammals like the duck-billed platypus but before animals like kangaroos, which incubate their young in pouches. The most advanced mammals that produce live births, such as cats, cows and humans, came last.
The discovery of the 5-inch skeleton--Zhangheotherium quinquecuspidens--was reported recently in the journal Nature.
“This is an important treasure,” said Michael Novacek, a mammal expert at the American Museum of Natural History in New York. “It gives us a step of evolution that lies between one of the major mammalian branches and the other major mammalian branch.” The fossil is named for amateur collector Zhang He, who found it in the village of Jian-Shan-Gou in China’s Liaoning province in 1992.
A cast of the skeleton was sent to Pittsburgh in early 1996 for analysis at the Carnegie Museum of Natural History, which has long collaborated with Beijing’s Institute of Vertebrate Paleontology and Paleoanthropology.
“I was literally awestruck at how beautiful it is,” said Carnegie paleontologist Zhexi Luo. “What I immediately zeroed in on was the ear region.”
Luo was coauthor of the Nature article, along with Yaoming Hu, Yuanqing Wang and Chuankui Li of the Beijing Institute.
In his lab filled with old oak bookcases, dinosaur models and mammalian skeletons, Luo focused a microscope on the tiny fossil cast to show a visitor what had so excited him. “The little beast,” as he calls it, had a one-hinged jaw and a single bone protecting its inner ear or cochlea, a structure that distinguished it from reptiles, which have several bones around their inner ear.
Eureka! It must be a mammal, Luo said. He pointed to its sharp, triangular-shaped teeth as another hint of its warm-blooded nature. “It is our ancient cousin. It shares the same basic design of teeth of animals that give birth to live young, such as humans,” Luo said.
Also exciting was that it was a fairly complete skeleton of a mammal that had yielded only fragments--and guesses--as to its evolutionary place 120 million to 140 million years ago.
Because “the little beast” lacked a spiral structure in the inner ear that gives advanced mammals high-pitched hearing, Luo said it must have preceded marsupials, which lived about 100 million years ago and had developed advanced ears.
Its link to earlier egg-laying mammals is a tiny spur on its rear feet that ejects a poisonous venom. Such telltale spurs are found only in egg-laying mammals--known as monotremes--such as the duck-billed platypus and a spiny anteater called an echidna. Both reproduced with eggs, but were hairy and fed their young with milk--other mammalian attributes.
