The Other “Dwarf” Tyrants

Tyrannosaurus_Rex_Growth_Series

Tyrannosaurus growth series at the Natural History Museum of Los Angeles County. Photo by Maarten Heerlien, from Wikipedia

Wow, it’s been a long time hasn’t it? There are a few reasons for my long hiatus. One was being very busy with school. The other was that I simply didn’t have anything I wanted to write about. But now that I have been hit by a bolt of inspiration, I decided to finally get back to updating this humble little blog. Now, on with the show.

Recently, I’ve gotten into some debates about Nanotyrannus on social media, where I favor the hypothesis that it is a juvenile Tyrannosaurus rex. While such arguments can get rather heated, they have resurrected my interest in tyrannosaur ontogeny. In particular, I would like to talk about two other, lesser known specimens that were once thought to be their own species but are now considered immature T. rex individuals. The specimens in question are the two that form part of the T. rex growth series at the Natural History Museum of Los Angeles County (LACM). Unfortunately, I have yet to see this display, but I think the taxonomic history behind these specimens is a fascinating subject, and I would now like to share it.

Both specimens were originally collected in the 1960’s in the Hell Creek Formation of Montana. The smaller specimen (LACM 28471) consists primarily of the tip of the snout and lower jaw, with a couple other skull and jaw fragments, while the larger specimen (LACM 23845) is a partial skeleton. Unfortunately, they lack nicknames, so I will be referring to them by their catalogue numbers.

The material was first described by Australian paleontologist Ralph Molnar. In 1978, he identified 28471 (also known as the “Jordan Theropod”, after Jordan, Montana) as a possible large dromaeosaur. This was followed by a description of 23845 in 1980, where it was tentatively identified as Albertosaurus lancensis, the name then used for what would later be named Nanotyrannus.

Not much else was done with these specimens until 1988, when Gregory Paul published his now classic book Predatory Dinosaurs of the World. Paul determined that the specimens  represented new species of the tyannosaur genera Aublysodon* and Albertosaurus, with 28471 named Aublysodon molnari, and 23845 named Albertosaurus megagracilis. The following year, Molnar and Kenneth Carpenter agreed with the referral of 28471 to Aublysodon, though they assigned it to the type species, Aublysodon mirandus.

*Aublysodon is a problematic genus based on unserrated premaxillary teeth. The name has pretty much been abandoned, as the lack of serrations may simply be a preservational, rather than taxonomic, feature.

The next major development in the taxonomy of the LACM specimens came in 1995, with an article in the Japanese magazine Dino Frontline by amateur paleontologist George Olshevsky. In this overview of tyrannosaurs, Olshevsky concluded that the species named by Paul in fact represented their own genera. Aublysodon molnari became Stygivenator, and Albertosaurus megagracilis became Dinotyrannus. This new classification scheme meant that, if one also accepted the validity of Nanotyrannus, there were now four genera of tyrannosaur in western North America at the end of the Cretaceous.

Unfortunately for those imagining such a diverse predator assemblage, Thomas Carr came along to ruin their fun. In 1999, he published a detailed paper on the ontogeny of tyrannosaurids, and argued that Nanotyrannus was in fact a juvenile T. rex. This was followed up in 2004 with a paper by Carr and his colleague Thomas Williamson.  They reexamined the LACM material, and concluded that they also represented immature forms of T. rex. Stygivenator, they argued, was a juvenile at a younger growth stage than “Nanotyrannus”, while Dinotyrannus was a subadult. These specimens were then used to construct a growth series of Tyrannosaurus rex, along with “Nanotyrannus” and a couple adult specimens. The assemblage of four Hell Creek tyrannosaurs was reduced to one.

Carr and Williamson 2004 growth series

Growth series of Tyrannosaurus rex, from Carr and Williamson (2004)

Based on this growth series, when the LACM renovated its dinosaur hall, they decided to include these immature specimens in a new exhibit demonstrating the life history of T. rex. Mounts were created which, while somewhat speculative due to the fragmentary nature of these specimens, represent reasonable approximations of how these individuals may have looked. The mounted skeleton of an adult T. rex specimen, nicknamed “Thomas”, completes the sequence. Though Stygivenator and Dinotyrannus are no longer considered valid, their type specimens remain important clues as to how the world’s most famous carnivorous dinosaur grew up.

Sources

Carr, T. D. (1999) Craniofacial Ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology. 19:497-520.

Carr, T. D., and Williamson, T. E. (2004). Diversity of late Maastrichtian Tyrannosauridae (Dinosauria: Theropoda) from western North America. Zoological Journal of the Linnean Society. 142:479-523.

Molnar, R. E. (1978). A New Theropod Dinosaur from the Upper Cretaceous of Central Montana. Journal of Paleontology. 52:73-82.

Molnar, R. E. (1980) An Albertosaur from the Hell Creek Formation of Montana. Journal of Paleontology. 54:102-108.

Molnar, R. E., and Carpenter, K. (1989) The Jordan theropod (Maastrichtian, Montana, U.S.A.) referred to the genus Aublysodon. Geobios. 22:445-454.

Olshevsky, G. (1995). The origin and evolution of the tyrannosaurids. Dino Frontline. 9-10:92-119.

Paul, G. S. (1988) Predatory Dinosaurs of the World: A Complete Illustrated Guide. New York. Simon & Schuster.

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