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Thunder-Thighed Dinosaurs Arose much Quickly from Predecessors

Thunder-Thighed Dinosaurs Arose much Quickly from Predecessors
Dinosaurs evolved from their predecessors much faster than was previously thought, according to a new study.
The analysis puts the time gap between the dinosaurs and these predecessors, called dinosauromorphs, at an estimated 5 million years instead of 10 million.
Dinosauromorphs look rather like dinosaurs but were relatively small, often no more than a meter (yard) in length, and had various skeletal differences including more uniform backbones divided into fewer segments. They mostly walked on two legs.
Uncertainty clouds dinosaurs’ early history, in large part because it’s often hard to precisely measure where and when fossils come from. 
In the new work, Randall B. Irmis of the Natural History Museum of Utah and colleagues focused on a geologic formation in Argentina where fossils of early dinosaur relatives appear in layers of rock, corresponding to different ages.
They also estimated those ages using a newly developed form of radioisotopic dating, a method that is based on the rate at which concentrations of certain radioactive elements change over time. It turned out “these early dinosaur relatives were geologically much younger than previously thought,” said Irmis, which “was totally unexpected.”
These ages, combined with other chronologies from the same formation, reveal that the first clear evidence of dinosaurs appears just 5 million years after dinosauromorphs, the researchers added.
Despite the rapid emergence, the finding suggests that dinosaurs rose to prominence in a fairly smooth and gradual way, without causing a fundamental shift in the balance of the ecosystem, the investigators said. The findings may challenge several theories about dinosaur origins based on less rigorously dated fossil evidence, they added.
The findings are published in this week’s issue of the research journal Proceedings of the National Academy of Sciences.
Irmis and colleagues studied the Chañares Formation, a roughly 75-meter-thick-(250-foot) formation made of sediments deposited by rivers, streams and lakes during the Triassic Period in present-day La Rioja Province, northwestern Argentina.
The team found that the formation, and therefore the fossils in it, is 234 to 236 million years old, or 5-10 million years younger than previous estimate of a Middle Triassic age.

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