Several months ago paleontologists Stephen Brusatte, Roger Benson, Dan Chure, Xu Xing, Corwin Sullivan, and David Hone described the dinosaur Shaochilong, the first representative of the group of large predatory dinosaurs called
The most obvious difference between Shaochilong and its carcharodontosaurid relatives was that it was much smaller in size. While the bones typically used to estimate body size, such as the femur, have not yet been found for Shaochilong, the authors were able to compare the length of the tooth row in the upper jaw to the same part of the skull in related dinosaurs known from more complete remains. What they found was that Shaochilong, while still a large theropod by any standard, was substantially smaller than Allosaurus and even more diminutive when compared to closer relatives such as Carcharodontosaurus. Shaochilong probably measured only between 15 and 20 feet long, though as the authors point out Shaochilong was still larger than all the tyrannosaurs of its time.
But the greatest significance of Shaochilong is that it represents what the authors call a "dark period in large theropod history." Between 140 and 120 million years ago, large relatives of Allosaurus were the dominant large predators in the northern hemisphere, but by 83 million years ago they had been supplanted by the tyrannosaurs. The 40 million years between the dominance of the allosauroids and the rise of the tyrannosaurs is still incompletely known, but the presence of Shaochilong and its close relative Chilantaisaurus (which it lived alongside) in Asia about 92 million years ago suggests that the allosauroids remained dominant for longer than has been previously understood. This might be important to understanding the evolution of the tyrannosaurs, a group that evolved much earlier (around 170 million years ago) but stayed small until the late Cretaceous. Might the dominance of the allosauroids have kept the tyrannosaurs small? At present it is impossible to know, but future discoveries of more theropods from the "dark period" might help explain one of the most dramatic turnovers in dinosaur history.
STEPHEN L. BRUSATTE, DANIEL J. CHURE, ROGER B. J. BENSON, XING XU (2010). The osteology of Shaochilong maortuensis, a carcharodontosaurid (Dinosauria: Theropoda) from the Late Cretaceous of Asia Zootaxa, 2334, 1-46