Guest Post: Shark toothed theropods in Asia – introducing Shaochilong

I may be headding off to sunny Inner Mongolia, but have left the keys to the Musings behind in the hands of theropod specialist and memeber of the worldwide Bristol mafia, Steve Brusatte to talk to you about a new paper (and a new taxon) we have described with our colleagues (though I honestly and non-self depreactingly didn’t do much of it apart from coming up with the name). Take it away Steve:

A funny thing happened while I was in Beijing last January. I arranged a two-week visit to examine a bunch of basal coelurosaurs for my PhD thesis, and spent most of my time looking at Dilong and Guanlong. And then my gracious hosts, Dave Hone and Corwin Sullivan, both picked up a nasty flu bug [ed: Corwin got it and gave it to me] that knocked them out of service for an entire week. Suddenly I had the entire office to myself, and freed from Dave’s entertaining interruptions about pterosaurs and football and other passion-inducing topics, I began to work really fast [ed: what’s wrong with pterosaurs eh?]. So fast that I had a couple of free days left over at the end of my second week. Most reasonable people would use those days to act like a tourist and see the sights in Beijing, but I figured I may as well look at other specimens. And that’s how I came across a bizarre theropod called “Chilantaisaurus” maortuensis.

Shaochilong maxilla (top), braincase (middle) and skull roof (bottom) from Brusatte et al., 2009.

This creature, known from a single specimen that includes some very nice skull bones and vertebrae, was originally described by Hu in 1964. Today, over 45 years after it was discovered, “C.” maortuensis remains one of the only large-bodied theropods known from a large gap in the Asian predatory dinosaur fossil record. We know a decent amount about Asian large theropods from the Middle-Late Jurassic, as well as the tyranosaurid-dominated faunas of the Late Cretaceous (Campanian-Maastrichtian). However, the intervening 60 million years is largely a dark period in Asian theropod history. For this reason, “C.” maortuensis remains a critically important animal.

By now, many intrepid readers have probably noticed that I have placed “Chilantaisaurus” in quotation marks. That is because Hu referred this specimen—clearly a new and distinct species—to the genus Chilantaisaurus, which was based on another, much larger theropod specimen from the same mid Cretaceous rock unit in Inner Mongolia where “C.” maortuensis was found. However, there are no overlapping bones between the two specimens, so we really can’t tell if the two species belong to the same genus. This problem has long been noted by many authors, beginning with my colleague Dan Chure when he visited IVPP and examined lots of theropod specimens in the 1990s. Dan had long planned to redescribe “C.” maortuensis and name a new genus, but he was sidetracked by other things.

While in Beijing I figured I should try to see the specimen, as some authors had suggested that it was a tyrannosauroid or a basal coelurosaur of some kind. Either way, it would provide important data for my thesis. But when I opened the drawer and saw the maxilla and braincase staring back at me, I quickly realized something was wrong. “C.” maortuensis is not a coelurosaur, but possesses a number of unique features seen in Carchrodontosaurus, Giganotosaurus, Mapusaurus, and Acrocanthosaurus—colossal theropods closely related to Allosaurus that are grouped within their own subclade, the Carcharodontosauridae (the shark-toothed reptiles, named for obvious reasons).

Carcharodontosaurids are a relative newcomer to the pantheon of large theropods. The first specimens of this group were found by Ernst Stromer in the early 1900s in Egypt, but most were destroyed during World War II (from the same beds as the legendary Spinosaurus). Only in the 1990s were more complete specimens discovered in Early Cretaceous rocks in Africa and South America. For a while people thought that carcharodontosaurids were restricted to the southern continents, but then it became clear that the humongous North American Acrocanthosaurus was also a member of the group. And over the past decade people have been finding more carcharodontosaurids from across the globe, including Neovenator from the Isle of Wight in England. However, one place that seemed to lack carcharodontosaurids was Asia. Some workers interpreted this absence as significant—perhaps it meant that Asia separated from the rest of Pangaea before carcharodontosaurids evolved.

Now we know that carcharodontosaurids lived in Asia as well, thanks to our reinterpretation of “C.” maortuensis. Just the other day our paper on this animal was published, in which we name it a new genus (Shaochilong, meaning “shark teeth dragon” in Chinese, in obvious reference to the carcharodontosaurids), reinterpret it as the first unequivocal Asian carcharodontosaurid known from the fossil record, and discuss its implications for Asian theropod evolution. In a nutshell, Shaochilong is further evidence that carcharodontosaurids were a truly global group, and that the large-bodied theropod faunas of Early-mid Cretaceous Asia were cosmopolitan in nature, not endemic as had been the case during the Middle-Late Jurassic.

Carcharodontosaur phylogeny showing their relationships, ages and continent of origin. In the upper left you can see the appearance of the large bodied tyrannosaurids and how late they appear compared to the carcharodontosaurs. From Brusatte et al., 2009.

Not only that, but the 60 million year gap in the Asian large-theropod record had been a thorny issue. We knew that large basal tetanurans lived in Asia during the Middle-Late Jurassic and that colossal tyrannosaurids dominated ecosystems during the last 10 million years of the Cretaceous. But what large predators lived in Asia during the intervening gap? Were these predators late holdovers from the basal tetanuran lineages, or did large tyrannosaurids actually originate much earlier than observed in the fossil record? Shaochilong is only a single data point, but it suggests that basal tetanurans such as carcharodontosaurids still held sway in Asia deep into the Cretaceous, and that tyrannosaurids experienced a relatively rapid ascent into the top predator role much later in the Cretaceous.

But now the really fun part begins. We need to test these ideas with other fossils, so the race is on to discover more large theropods from the once impenetrable middle Cretaceous gap of Asia. What new discovery lurks within the next outcrop…or within the next museum drawer?

*I would like to acknowledge the help of my co-authors on the project: Roger Benson, Dan Chure, Xu Xing, Corwin Sullivan, and some paleo blogger called Dave Hone [who he? ed]. I also thank Dave, Corwin, and Xu for their hospitality in China, and my travel companions (Mark Norell, my PhD advisor and source of funding, and Gabe Bever and Mick Ellison, my two favorite valentines).
Brusatte, S., Benson, R., Chure, D., Xu, X., Sullivan, C., & Hone, D. (2009). The first definitive carcharodontosaurid (Dinosauria: Theropoda) from Asia and the delayed ascent of tyrannosaurids Naturwissenschaften DOI: 10.1007/s00114-009-0565-2