Tyrannosaurus rex is without a doubt the most famous dinosaur in the world, and one of the lasting questions people have about this amazing dinosaur is what it was like as a teenager before it was full grown. A paper I co-authored with Bruce Rothschild, a former research affiliate with the University of Kansas Biodiversity Institute, now published online in the journal Cretaceous Research addresses this interesting question.
A nearly complete dinosaur skeleton labeled as BMR P2002.4.1, but more affectionately referred to as 'Jane' in honor of the woman who discovered it, has been the center of a decades long dispute over the validity of a dinosaur called Nanotyrannus lancensis. Nanotyrannus was named by a team led by the famous paleontologist Bob Bakker as a 'pygmy tyrannosaur' from the Late Cretaceous of Montana1. Not all dinosaur paleontologists are convinced of this assessment, and many prominent studies have asserted that Nanotyrannus—specifically 'Jane', the original holotype fossil skull at the Cleveland Museum of Natural History, and a handful of other isolated remains—are instead remanants of immature T. rex. In fact, if you visit wikipedia's page for Tyrannosaurus rex, you will find a proudly displayed image of 'Jane' from the Burpee Museum of Natural History. While paleontologists in this debate have focused on the number of teeth in the jaws2, the overall shape and proportion of the skull3, and whether the texture of the bone is more similar to that of adults of immature individuals4, we observed an isolated character on the skeleton of 'Jane' that shed some additional insight on this debate.
A portion of 'Jane's' lower jaw (called the dentary bone) is marked by a deep groove containing numerous small openings. Bruce Rothschild, who is an expert on ancient diseases and has looked at many jaws from theropod dinosaurs, was unaccustomed to seeing such a feature in a tyrannosaur, and thought this groove was possibly a sign of some disease. It turns out that the other specimens of the embattled genus Nanotyrannus also shared this feature, so it likely wasn't evidence of a disease. After examining additional dinosaur fossils, we found out that, in fact, this groove is found on nearly all theropod dinosaurs outside of the tyrannosauroid group (the group more closely related to T. rex than other meat-eaters like Allosaurus, Spinosaurus, and Coelophysis). Among tyrannosaurs, however, we found an opposite trend: only 7 of 18 tyrannosaurs had this feature, and half of those occurences were found in the group of the earliest tyrannosaurs. We further investigated this question by examining known T. rex material, ranging in age from "baby" all the way to full grown adult, and found that none of these fossils showed the groove we found on 'Jane'!
So what does this mean? It could be that 'Jane' and all the other fossils we call Nanotyrannus really are juvenile T. rex, and they are undergoing a really dramatic bodily transformation during their growth into adults (puberty sure is rough!), but this is unlikely given that none of the undisputed T. rex fossils we investigated have this feature. This groove is a passageway for nerves and blood vessels to move through the bones of the skull, and short of saying that the nerves and veins of the head dramatically changed their placement as the animal grew, if a baby has no groove, a sub-adult has no groove, and a full-grown adult has no groove, one would logically not expect a juvenile to have a groove either. To us (and some of the other scientists arguing in favor of Nanotyrannus), this is evidence that Nanotyrannus is a different dinosaur from T. rex, and they likely preferred different environments and prey even though they lived at the same time.
What does this mean about how Nanotyrannus fits in to the dinosaur family tree? Even though Nanotyrannus has been variously proposed to be a young T. rex or a closely related species, our phylogenetic analysis actually places Nanotyrannus as a close relative of the albertosaurine tyrannosaurs (moderate-sized theropods that lived in what is now Canada). We obtained this result because they are the only group of advanced tyrannosaurs to possesses the groove we studied. This result was interesting, however, because Charles Gilmore, the paleontologist that described the original Nanotyrannus on display at the Cleveland Museum5, thought it was an example of a new species of Gorgosaurus, one of the types of albertosaurines. History seems to have come full circle.
So now what? Well to the fan club of Nanotyrannus, we have some additional evidence that this was in fact a separate dinosaur species. And for now, the hunt is back on for a complete fossil that shows us what the mighty T. rex was like as a teenage terror.
1. Bakker et al., 1988. Nanotyrannus, a new genus of pygmy tyrannosaur, from the Latest Cretaceous of Montana. Hunteria 1:1-28.
2. Larson, P. 2013. The case for Nanotyrannus. Pp. 14-53 in Parish et al. (eds.), Tyrannosaur Paleobiology. Indiana University Press.
3. Carr, T. 1999. Craniofacial anatomy in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology 19:497-520.
4. Currie, P. 2003. Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada. Acta Palaeontologica Polonica 48:191-226.
5. Gilmore, C. 1946. A new carnivorous dinosaur from the Lance Formation of Montana. Smithsonian Miscellaneous Collections 106:1-19.
A fundamental part of being a scientist is publishing your research. Scientists ask questions, formulate hypotheses, rigorously test these hypotheses, and publish their research and their results. Other people can then read these results and build off of these studies, either to question or refute the findings, or to use the findings to ask other questions. It is how science grows and evolves.
What almost all scientific publications lack, however, is the flair, the backstory, and general behind-the-scenes action that is part of everyday research. Scientific publications are whittled down to the most concentrated version, filled with the jargon of the discipline, and stripped of any extraneous behind-the-scenes anecdotes. So while any given scientific paper can be exciting to a scientist who wants to learn more about the organism or the methods addressed, they can be a bit unfriendly to a general reader.
So for fun, I have decided to tell some behind-the-scenes stories of the research I do, in the context of my published papers. Hopefully I give you a sense of what it is really like to be a paleontologist, and the work that is involved.
I’ll begin with my two solo-authored papers that I published in 2013. The papers can be found here and here, and if you cannot access those journals, please contact me at firstname.lastname@example.org and I will send you a PDF.
These two papers establish a new genus and two new species of fishes within a group called semionotiforms. Semionotiforms are an extinct group of fishes, but are closely related to living gar, and like gar, their bodies were covered with thick enamel scales (ganoid scales). Semionotiforms are found in geologic deposits worldwide, and range in age from Middle Triassic (~237 million years ago) to Early Cretaceous (~145 million years ago). A lot of variety occurs in semionotiforms in the shape of the body, the characteristics of the skull, the teeth, etc., and part of my research is to figure out what makes these particular fishes different from other species that have been described in the literature by other scientists. So you could say that my hypothesis for these studies is that these fishes represent new species, and I am testing that hypothesis by comparing the anatomy and morphology of these fishes to other semionotiform fishes to see if my hypothesis is correct or incorrect.
Some of the fossil specimens I work on are from museum collections, such as the American Museum of Natural History (AMNH) and the Smithsonian and were collected in the 1950s and 1960s, yet remained in these collections unstudied and undescribed for decades. I began working on these fishes in 2006, when I worked at the St. George Dinosaur Discovery Site (SGDS) as an undergraduate student intern and later as the prep lab and collections manager. The crew of staff and volunteers from SGDS had just gone out to a site in southeastern Utah and collected hundreds of fossils (outlined in Milner et al., 2006), but most of these fishes were not identified. So as I started cleaning the fossils (fossil prep—to be discussed in a later blog!), I started looking for characteristics that defined them as either new or belonging to a described species of semionotiform fish. While I worked on the new specimens, I looked at older literature, in particular a (1967) paper by an AMNH paleontologist Bobb Schaeffer, who mentioned collecting many semionotiforms from the same area but didn't describe them or give them names. So, in 2008, I went to the collections of the AMNH to look at those old specimens collected decades before and reexamined them, seeing which of them could be the same species as the new specimens the SGDS crew had just collected. I identified at least two different species, though there are likely more than that.
Now, identifying a new species is more than just a “Eureka!” moment. A scientist cannot know what is new unless he/she knows what already exists, and so scientists have to be very familiar with other scientists’ work in the field. An inordinate amount of any scientist’s time is spent reading books and papers, and I spent months pouring over scientific literature, some as old as 1820, to find the characteristics of other semionotiforms. As I looked at each bone on the fossil fishes from the AMNH and those newly collected from SGDS, I compared it to the same bones in other semionotiform fishes, and I had to look for similarities and differences. Eventually, I found a suite of anatomical and morphological characters that distinguished these fishes from all other semionotiform fishes, and I had enough to publish two papers on two distinct species. In these papers, I had to give an exhaustively detailed description of every single bone, and I mean EVERY bone (these fishes have hundreds of bones, dozens in their skull alone!) that I could see on the specimens, because other scientists, when trying to identify new species of their own, may turn to my work for comparison, and so my papers have to be provide as much anatomical detail as possible!
Next time….naming a new species!!
Milner, A.R.C., Mickelson, D.L., Kirkland, J.I., and Harris, J.D. 2006. Reinvestigation of Late Triassic fish sites in the Chinle Group, San Juan County, Utah: new discoveries. In: A Century of Research at Petrified Forest National Park: Geology and Paleontology (Eds. Parker, W.G., Ash, S.R., and Irmis, R.B.). Museum of Northern Arizona Bulletin 62: 163–165.