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   January 2000 


Living fossils for their time

Recent findings indicate that the number of changes in dinosaur skeletons, and the rate at which they occurred, do not appear to be related to the length of time that a species existed on Earth. Paul Sereno and his team from the University of Chicago recently made the first quantitative assessment of the rates of dinosaur evolution—a project prompted by the discovery of two new dinosaur species.

In 1997, the Sereno team discovered two new species of Cretaceous sauropods in the Sahara Desert of Niger. Two years later, Sereno’s team presented life-sized reconstructions of an adult and juvenile of the new species, Jobaria tiguidensis, at a Nov. 11 press conference at the National Geographic Society in Washington, D.C. Sereno’s research was published in the Nov. 12 Science.

Despite its Cretaceous age, Jobaria tiguidensis was found to be relatively primitive based on several characteristics, including: spoon-shaped teeth, a short and flexible neck comprising only 12 cervical vertebrae, and a relatively small skull compared to its average adult length of 60 feet. Its forelimbs were also less elongate than those of other common Cretaceous-age sauropods, such as Brachiosaurus

.Sereno’s team determined that Jobaria lies outside the neosauropod phylogenic classification, which encompasses all other known Cretaceous sauropods. A lineage of sauropods characterized by a slow rate of evolutionary change gave rise to Jobaria during the Middle Jurassic and the species flourished into the Cretaceous with relatively few skeletal changes for 30 to 40 million years.

      Approximately 135 million years ago, the sauropod
        Jobaria tiguidensis roamed what is now the Sahara
        Desert of Africa.  This new species was discovered in
        Niger by a team of researches led by Paul Sereno of
        the University of Chicago.  Named Jobaria after the 
        legendary beast of Niger, it was an athletic creature,
        like the African elephant, and was probably capable
        of rearing to feed and fight. 
        Michael Skrepnick, National Geographic Society.
Nigersaurus taqueti, another new sauropod, was discovered in a younger Cretaceous formation than that of Jobaria. Nigersaurus is believed to be a more advanced diplocoid. The phylogenetic classification of Nigersaurus is three steps ahead of Jobaria, yet Jobaria walked the Earth merely a few million years before Nigersaurus.

These two finds prompted Sereno to seek a more broad understanding of the rates of dinosaur evolution. The ensuing study used 70 different dinosaur species to compare the total number of skeletal changes per deviation from the evolutionary tree to the minimum elapsed time during which those changes must have come about. Findings indicate considerable variation in the rates of dinosaur evolution.

A flash flood is believed to have wiped out a herd of Jobaria in the region of study, an area of 50 square miles in the Sahara Desert of Niger, leaving the creatures well preserved. The herd behavior of Jobaria and its physical characteristics led Sereno to expand his study to include a comparison of the new species to present-day African elephants. Both the fossil Jobaria and the African elephant have a wide rib cage and narrowly positioned hips and legs, leading Sereno to believe that Jobaria, like the elephant,  was a very agile and athletic creature. This approach opens new doors for understanding how sauropods such as Jobaria were able to move. It is now believed that they were capable of rearing to feed and fight, despite their enormous body mass and weight distribution.

Sereno and his team have already found evidence for another species and they plan to return to the Sahara to pursue further research.

—Laura Wright