In 2001, a paleontology field crew from Burpee Museum of Natural History (Rockford, IL) were prospecting for dinosaur fossils near Ekalaka, Montana, when they discovered bones of a half-grown T. rex weathering out from exposures of the Hell Creek Formation. "Jane", as she was later named, turned out to be the most complete adolescent T. rex ever discovered, filling a critical gap between juvenile and adult that had caused decades of scientific debate.
Prior to Jane's discovery, a small lightly built tyrannosaur skull collected near Ekalaka in 1942 had been at the center of a controversy over how much T. rex changed during growth. The skull had spent an uneventful half century on display at Cleveland Museum of Natural History, Ohio, when in 1988, famed paleontologist Dr. Robert Bakker redescribed the fossil as a new species, Nanotyrannus lancensis, proposing that it represented a smaller, more sleek cousin of T. rex.
This interpretation has been controversial since 1999 when Dr. Thomas Carr showed that the differences between "Nanotyrannus" and those of adult T. rex are also seen during growth in other species of tyrannosaurids. This suggested that the Cleveland skull was from a juvenile T. rex rather than being a separate species in its own right. However, this hypothesis met with surprising resistance. Could a dinosaur really change that much during growth?
"The extreme changes from the sleek skull of juveniles to the robust skull of adults were too much for some people to believe; for example, they didn't like to hear that T. rex lost tooth positions as it grew from a juvenile with many teeth, to an adult with fewer teeth. Regardless, the search was on for a transitional specimen that could test the hypothesis."
Enter Jane. Her fine skull and skeleton was intermediate in size and shape between the Cleveland skull and fully adult T. rex. Carr's research team presented a detailed study of Jane at the Society of Vertebrate Paleontology 2015 annual meeting in Dallas.
"Jane shows us that the gap is in fact bridgeable because many features seen in her are more similar to adult T. rex than to the Cleveland skull. The features are exactly what we'd predict are necessary to make the change to a full adult." said Carr.
Another important dimension of the "Jane" story is that she was discovered on public lands, then collected and mounted for display by a public museum. "Dinosaur fossils such as this emphasize the importance of accredited institutions collecting on public lands, which makes the specimens on them available for scientific study", asserts Dr Carr.
In a world where commercially collected dinosaurs demand ever upwardly spiralling prices, Jane is a world-class dinosaur that didn't come with a million dollar price tag. Burpee Museum director of science and exhibits, Scott Williams, summed up:
"Jane is simply the best preserved and most complete example of a publicly accessible, subadult Tyrannosaurus rex in the world. For the last 10 years she has been available to qualified researchers as well as exhibited to the general public. The quality of the specimen and its availability will undoubtedly provide researchers decades of important data regarding the ontogeny of the most recognized dinosaur species in the world."
Regardless of Jane's completeness and growth stage, she doesn't close the book on T. rex growth and evolution; there is still a gap for yet undiscovered fossils to fill between her sleek form and the deep, imposing skulls of adults.
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CORRESPONDING AUTHOR
THOMAS CARR
Carthage College
Kenosha, WI, USA
tcarr@carthage.edu
A SUBADULT TYRANNOSAURUS REX AND ITS BEARING ON THE NANOTYRANNUS HYPOTHESIS
CARR, Thomas D., Carthage College, Kenosha, WI, USA, 53140
HENDERSON, Michael, Northern Illinois University, DeKalb, IL, USA
ERICKSON, Gregory, Florida State University, Tallahassee, FL, USA
PETERSON, Joe, University of Wisconsin - Oshkosh, Oshkosh, WI, USA
WILLIAMS, Scott, Burpee Museum of Natural History, Rockford, IL, USA
CURRIE, Philip, University of Alberta, Edmonton, AB, Canada
SCHERER, Reed, Northern Illinois University, DeKalb, IL, USA
HARRISON, Bill, Northern Illinois University, DeKalb, IL, USA
In 2002, a rare, surprisingly complete, and partially articulated skull and skeleton of a subadult tyrannosaurid (BMRP 2002.4.1) was collected by the Burpee Museum (Rockford, IL) from the Hell Creek Formation in southeastern Montana. We refer the skeleton to Tyrannosaurus rex based on the presence of several autapomorphies of that taxon: a long caudolateral process of the nasal, extensive contact between the antorbital fossa and the nasal, and a narrow snout and wide temporal region that orient the orbital fenestrae forward.
The ~20 foot (6 m) -long skeleton gives an unprecedented view of an early growth stage of a dinosaur whose fossil record is dominated by adult skeletons. Evidence for its subadult growth stage comes from several lines of evidence, including size, relative development of hundreds of osteological features, and bone histology, which gives a chronological age of ~11 years. Based on a cladistic analysis of ontogenetic characters, BMRP 2002.4.1 was found to occupy a new growth stage, between a smaller juvenile (CMNH 7541) and a larger subadult (LACM 23845). With the addition of the new specimen, the early part of the growth series is more completely understood.
In this context, BMRP 2002.4.1 is important in testing recent claims of the validity of Nanotyrannus lancensis; the specimen is similar in many ways to the holotype of N. lancensis (CMNH 7541), which has been critically assessed and identified as a juvenile T. rex. The sequential position of these specimens in the growth series suggests that the similarities are the result of their relative immaturity, not autapomorphies of a novel taxon.
In a larger context, the growth changes seen early in T. rex ontogeny are also seen in the growth of all derived tyrannosauroids (Bistahieversor + Tyrannosauridae). This shows that ontogeny in this clade is highly conserved, specifically the gross differences between subadult and adult specimens.
Among other changes, our BMRP 2002.4.1 data suggest that in T. rex, maxillary and dentary tooth counts increase early in ontogeny before decreasing through adulthood; the lacrimal has a prominent horn at this growth stage, which is later lost to inflation in adults; and caudal neurocentral suture closure follows an anteriorward sequence. The specimen reveals a distinct feature of T. rex, where the humerus of BMRP 2002.4.1 is relatively long in contrast to subadult tyrannosaurids of similar growth stage. Finally, despite the discovery of this important specimen, a substantial gap in the growth series is seen between the gracile subadults and the robust adults.