Kid dinosaur was misidentified as a new species
Oops: Tissue confirms Nanotyrannus was not a new species, but merely a juvenile T. rex.
by Jerry Bergman, PhD
Introduction: The Problem of Species Determination
The latest research on one dinosaur called Nanotyrannus (Nano- or dwarf and tyrannus or tyrant) has finally firmed up what it was, a teenage T rex. Most of our information was known primarily from a single proven specimen, CMNH 7541. It was originally believed to be a new distinct genus based on a handful of cranial and postcranial features. Now, additional research on the creature’s bone tissue confirms it was merely a juvenile T. rex. The story of arriving at this conclusion is a good example of a big problem in the science of taxonomy, namely determining what is (or is not) a new species. This case also illustrates the problem of extrapolating conclusions about evolution from the fossil record.
The problem is actually central in documenting evolution. What one may conclude is a transitional form between one species and a more evolved species may well be only a juvenile of a known species, and not a new species. Paleontologists admit that “despite its iconic status as the king of dinosaurs, Tyrannosaurus rex biology is incompletely understood,” even though since its discovery in 1905 the famed King of Dinosaurs “was met with intense scientific interest and public popularity, which persists to the present day.”
The History of the Progression from New Species to Juvenile T. rex
The fossil which later became known as Nanotyrannus was first discovered in 1942 by Cleveland Museum of Natural History paleontologist David Dunkle. He found a slightly compressed skull labelled CMNH 7541. After carefully studying the skull’s features, one of the leading Tyrannosaurus rex experts, Charles W. Gilmore, classified it as a species of Gorgosaurus, one of several smaller relatives of T. rex that is said to have lived during the Cretaceous Period.
One reason for the acceptance of the Gorgosaurus classification, and not the Tyrannosaurus classification, was because Tyrannosaurus and Gorgosaurus have significant differences in the number of teeth. Nanotyrannus had a tooth count close to Tyrannosaurus, but very different than Gorgosaurus. Species differentiation is forced to rely on minutiae such as teeth number because, when all paleontologists have to differentiate one species from another are bone fragments, they are forced to rely on very minor traits. Then, in 1970, a new study of CMNH 7541 concluded it actually belonged to a totally different tyrannosaur genus, Albertosaurus, named after dinosaur deposits in Alberta, Canada. Further studies concluded it belonged to another species.
Many other examples exist of fossil vertebrate species defined almost exclusively by an evaluation of small traits such as dentition or feet or both. These criteria are useful, yet very limited, for classification. As a result, minor differences can be amplified by a researcher into a species-defining character, as the above case illustrates.
Then, in 1988, of one of the leading dinosaur experts, Robert Bakke, and his colleagues, then at the University of Colorado, came to a different conclusion. Based primarily on the creature’s thin face and small sharp teeth, they proposed that CMNH 7541 was an entirely new genus they named Nanotyrannus, literally translated as the “pygmy tyrant.” They considered it a pygmy relative of the famous 13 meter-long Tyrannosaurus rex. One major reason they considered it a new genus was that they judged the skull as belonging to a fully adult individual (skull length: 572 cm). It was considered the most primitive (basalmost) tyrannosaurid from the Hell Creek Formation in Montana.
In 1999, Professor Thomas Carr used the relatively good fossil record of Albertosaurus to determine that the skulls and teeth of tyrannosaurs became more robust over evolutionary time. From this study, Carr concluded Bakker’s notion of skull suture closure was mistaken. In fact, very few skull sutures actually close during tyrannosaurid growth. Furthermore, aside from the internasal and intraparietal sutures, which are closed in other tyrannosaurid juveniles, every other observable suture in the Cleveland skull is actually open.
In 2003, paleontologists at the Illinois Burpee Museum of Natural History discovered a nearly complete fossilized skeleton of Nanotyrannus. It was 20 feet (6 meters) long and 7 feet (2 meters) tall. Often paleontologists discover only bone fragments, or at best a partial skeleton, which makes it difficult to determine the species of dinosaur at hand. For decades, they believed that the find was a Nanotyrannus; thus two species of North American tyrannosaurs were assumed to be alive during the late Cretaceous.
It is now realized that dinosaurs change dramatically, not only in size but also in appearance, as they grow from a grapefruit-sized egg to an adult. One case shows how dramatically paleontologists can come to very different conclusions. Chung Chien Young, called the father of Chinese paleontology, decided from a set of 70 specimens he studied, that he had five new species of prosauropods, including the giant herbivores Diplodocus and Brachiosaurus.
Then Russian paleontologist Anatoly Rozhdestvensky studied the same collection, and his review found only one species, namely Lufengosaurus huenei, that was significantly different enough from the other 70 to call it another species. The other four claimed “species” were just L. huenei at different points in its development from egg to adult. This examples shows how fossils can be misinterpreted as evidence of a brand-new dinosaur species when they are just evidence of a known species at different stages of developmental growth.
Several other studies have concluded all of the differences that were claimed to support the separate Nanotyrannus species lie within the normal variation of the T. rex species. They have turned out to be individually, or ontogenetically, normal variable traits, or the result of bone distortion due to fossilization contingencies.
Line of arrested growth, or LAG to Determine an Animal’s Age
Based on the fact that their growth slows greatly in the winter, like the rings of a tree, fossil bones contain a record of the annual rhythms of their individual lives. To complete this evaluation requires a cross-section slice of the bone in order to count the growth rings, a destructive testing method that paleontologists are very hesitant to undertake, especially if only one specimen of a particular species exists. The fibula (the accessory shinbone to the weight-bearing tibia) of CMNH 7541 was cut, polished, and examined under a microscopic. They estimated the age at death was about 12 years old.
Bone Osteo-Histology Research
Bone histology (tissue) has been used in the past 15 years to investigate several aspects of the T. rex life history which were inaccessible from gross examinations of bone and skeletal morphology. Bone histology is used to help answer questions concerning a dinosaur’s ontogenetic age, growth rate, skeletal age, and sexual maturity. Woodward et al. examined femur and tibia bone “microstructure of two tyrannosaur skeletons of controversial taxonomic status recovered from the HCF: BMRP (Burpee Museum of Natural History).” Their estimated age at death was about 13 years, very close to the line of arrested growth (LAG) technique discussed above. The conclusion of this study, related to our present concern, was that “rather than two sympatric tyrannosaurid taxa … only one valid tyrannosaur species—T. rex—is currently recognized.” The interpretation of CMNH 7541 changed, and now it is considered an awkward youngster of a T. rex – meaning that genus Nanotyrannus never existed.
Why Classification (Taxonomy) is So Difficult
The focus of this review is to show that classification of life is very problematic. Paleontologists who group a large variety of animals into one species are called “lumpers” and paleontologists who tend to form many new species are called “splitters.” Carol Yoon in 2009 said that scientists increasingly come up with
more and more, new and (some say) improved species definitions; yet each also brought its own new complications. Like sailors in a leaky dinghy, taxonomists and biologists have found their boat forever sinking, as each tried endlessly to patch his or her own or someone else’s deflating definition, to what everyone would agree has been absolutely no good end … Taxonomists … could not agree on how the world was ordered, how particular groups had evolved and come to be, how to decide to order or name life. They couldn’t even agree on what a species was.
The same problem exists in human and ape fossil taxonomy. The former senior editor of Nature, Henry Gee, said that “almost every time someone claims to have found a new species of hominin, someone else refutes it. The species is said to be either a member of Homo sapiens, but pathological, or an ape.” This paper briefly reviews why no one definition of the many definitions offered has, as of yet, “satisfied all naturalists.” Confusion is built into taxonomy, because
every naturalist knows vaguely what he means when he speaks of a species. The amount of difference considered necessary to give to two forms the rank of species is quite indefinite. One text on evolution reviews seven statements of what the mythic word might mean and—as does every attempt to impose order on the chaos of life—fails.
The Darwin Connection
In the over 150 years since Darwin published his On the Origin of Species, his work
had shaken the foundations of science, philosophy, religion, and politics; he had changed humanities view of its place in the universe. But somehow, even after a full century, his work had not made any significant dent in the workings of taxonomy, a field to which his discoveries were fundamental.
Clearly, the species concept used to classify all life is an imperfect and problematic science. As Yale-trained taxonomist Carol Yoon wrote, the species classification was the “gray area of the field” and, more problematically, “it was a moving target.” The late Harvard professor Ernst Mayr concluded for this reason that, “There is probably no other concept in biology that has remained so consistently controversial as the species concept.” And Biologist Professor Beeland wrote the “species problem … refers to the centuries old struggle of biologists to define exactly what species are. No current definition works when applied across all taxonomic kingdoms.”
 Woodward, Holly; et al., 2020. Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy “Nanotyrannus” and supports ontogenetic niche partitioning in juvenile Tyrannosaurus. Science Advances, 6(1): eaax6250; doi: 10.1126/sciadv.aax6250, January 1.
 Tsuihiji, T.; et al., 2011. “Cranial osteology of a juvenile specimen of Tarbosaurus bataar from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia.” Journal of Vertebrate Paleontology, 31(3): 497–517. doi:10.1080/02724634.2011.557116, May 9.
 Riley, Alex. 2016. Meet Nanotyrannus, the dinosaur that never really existed. BBC Science. http://www.bbc.com/earth/story/20160816-meet-nanotyrannus-the-tyrannosaur-that-never-really-existed, August 17.
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 Carr, T.D. 1999. Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology, 19(3): 497-520, January 5.
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 Jones, Steve. 2000. Darwin’s Ghost: The Origin of Species Updated. New York, NY: Random House, p. 53.
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 Beeland, T. DeLene. 2013. The Secret World of Red Wolves: The Fight to Save North America’s Other Wolf, Chapel Hill, NC: The University of North Carolina Press, p. 26.
Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,500 college libraries in 27 countries. So far over 80,000 copies of the 40 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.