This article was originally posted at Science 2.0 on 21 April 2010 as a follow up to my article What Do Ardi, Raptorex, and Komodo Dragons Have in Common?. In light of recent debate* about Raptorex’s identity I thought this was worth a second look.
Normally, being wrong sucks. It’s all -10 points and you don’t get into Harvard. Sadness. But, not in science. One of the best things about scientific method is that it makes being wrong fun. That does not mean that scientists always like to hear they are wrong. We are after all sinful, prideful beasts like the rest of you – just smarter – just kidding.
A while ago, I discussed some relatively recent, amazing contributions of paleontology in order to illustrate that, while DNA may trump fossils for reconstructing evolutionary histories and the relationships between organisms, paleontology provides information on physiology and geographical location that can only be inferred by other disciplines. One of the discoveries discussed was of a 125 million year old, man-sized Tyrannosaurus rex ancestor, Raptorex, reported in Science on 17 September 2009.
The existence of a microscale version of the peculiar, but iconic, T. rex body type – long legs, large head cantilevered over the legs, balancing tail, massive biting musculature, and diminutive forelimbs – challenges some of the general thinking in paleontology and evolution. The group of T. rex-like dinosaurs, the tyrannosaurids, consists almost exclusively of large bodied animals with estimated weights over 2.5 tons. A variety of small bodied tyrannosaurid ancestors known as tyrannoraptors had been found, but these ancestors did not share the stereotypical tyrannosaur body plan. Rather, tyrannoraptors had elongated forelimbs and lacked over-sized heads. The discovery of Raptorex moves the observation of the tyrannosaurid body type much earlier in evolutionary history than previously known.
Which is interesting on its own, but is less important than its broader implications. The concept that a body type would scale proportionately over such dramatic size differences is surprising. As one moves from tyrannoraptors to tyrannosaurids, the heads become larger and the forelimbs are reduced. The size of different body parts changes at different rates in evolution. This is what we expect to see. Why do we expect this?
When we look at normal development, we see different body parts growing at different rates. When a human is born, the head is quite large relative to total body size and the limbs are quite short. As humans age, the head and limbs grow, but not at the same rate, such that an adult has a head that is physically larger, but proportionately smaller than in their infancy, with the inverse occurring for their limbs. This implies that the regulation of growth of different body parts is controlled by different mechanisms, which would lead us to conclude that the proportional size of different body parts should evolve differently too. Raptorex suggests that this logic is not a generalizable principle.
The conclusion that Raptorex was an ancestor of T. rex was based on four lines of evidence. First, Raptorex clearly shares tyrannosaurid physiological characteristics, like a large skull and diminutive forelimbs. Second, the specimen appears to have characteristics that are not observed in later, larger bodied tyrannosaurids, such as some specific characteristics about the skull. Third, the specimen was identified as a young adult of 5-6 years that should have achieved a substantial fraction of its final size. Fourth, the specimen was reported to have come from a site that was identified as being approximately 130 million years old, substantially earlier than any previously known tyrannosaurids.
At the time, I did not have the opportunity to follow up on this story. And, if one simply stuck to watching further Raptorex information, there would be no follow up. There have, however, been new ideas in paleontology and evolution put forward in the peer-reviewed literature that we can use to re-evaluate the Raptorex story.
The expression of those new ideas has centered on the work of paleontologist Jack Horner and his proteges. On 14 October 2009, the Skeptics Guide to the Universe interviewed Horner. Technically, Horner was on the program to promote his book, How to Build a Dinosaur: Extinction Doesn’t Have to Be Forever, which talks about how one might turn chickens back into dinosaurs. At the same time, Horner was making minor waves by suggesting that we have over-identified the number of dinosaur species (i.e., specimens of the same species are mistakenly identified as different species). In this context, Horner also spoke about his own interpretation of the newest addition to the collection of dinosaur species: Raptorex.
Why does Horner think that we have over-estimated the number of dinosaur species? In short, Horner argues that we do not understand dinosaur development very well, in part because the fossil record is sparse. Accordingly, Horner’s group at the Museum of the Rockies has been working very hard to build what they call “ontogenic series” for a variety of dinosaurs. An “ontogenic series” is simply a set of specimens that represent a variety of developmental stages – essentially different ages – for an organism. If a butterfly collector showed you a box containing a caterpillar, cocoon, and butterfly from the same species, you would be looking at an “ontogenic series” for that butterfly species.
Using these collections of specimens of different “ages”, Horner’s group has argued that dinosaurs do not grow by hatching as a scale miniature of the adult and getting bigger. Instead, dinosaurs grow like us. Different body parts grow at different rates, leading to dramatic changes as the dinosaurs develop from juveniles to adults. They have published on changes in skull shape in both triceratops and pachycephalosaurs. In the case of pachycephalosaurs, they argue that the changes are so dramatic that different development stages of one species have been misclassified as three separate species.
So, what did Horner think about Raptorex? Not surprisingly, he was skeptical that it was a new species, but thought that it was a juvenile of one of the known tyrannosaurid species, thus shooting down all those nice things I told you above about how interesting Raptorex is for evolutionary biology. But what about those lines of evidence for Raptorex being a paradigm-breaking novel species?
No matter who is correct, we expect Raptorex to show tyrannosaurid characteristics. It is interesting to note, that according to the Science paper size is the only physical characteristic that distinguishes Raptorex from classical tyrannosaurids.
Without the knowledge to conduct a detailed critique of the analysis provided in the literature – which I do not have and will not pretend to have, I can only suggest that any apparently distinguishing characters could indicate species differences or developmental differences.
The Raptorex discoverers conclude that the specimen was from an individual nearing full maturity. Horner’s conclusion requires that either the estimate of the specimen maturity is wrong or that our understanding of dinosaur maturation is lacking.
The reported site of discovery of the Raptorex specimen set the age of the specimen at about 125 million years. That age alone would place Raptorex outside the known tyrannosaurid species. This appears to be definitive, until one learns that there is a chain of possession issue. The researchers never actually observed the specimen in the location it was reported to be from. Instead, the specimen had a dubious and rather illegal history – while not discussed in the paper, you can read and hear about it in several interviews.
The tiny dino had been excavated in Mongolia illegally and sold to a private collector. He contacted Sereno to evaluate the preserved tyrannosaurid. Sereno agreed to do so if the dinosaur was donated to science, and eventually returned to China.
Although the sediment associated with the specimen seems consistent with its reported source, this issue raises doubts about the strongest piece of evidence for Raptorex‘s specialness.
So, is Raptorex a game changing T. rex ancestor or just a your standard ne’er-do-well tyrannosaur teenager?
We have dueling assumptions. On one hand, Paul Sereno and colleagues assume that the reported source location for Raptorex is accurate. On the other hand, Jack Horner assumes that the maturity estimate is wrong. Which assumption is simpler and more reasonable?
Paging William of Occam.
*Hat tip to Brian Switek.