Introduction: A Turning Point in the Biology of the "Tyrant Lizard"
Published in the journal PeerJ on January 14, 2026, the paper by Holly N. Woodward, Nathan P. Myhrvold, and John R. Horner, titled "Prolonged growth and extended subadult development in the Tyrannosaurus rex species complex revealed by expanded histological sampling and statistical modeling," has become a study that overturns established theories of the past 20 years in dinosaur paleontology, particularly in the ontogeny and taxonomy of theropods.
We explain the new biological paradigm presented by this paper, the discussions it has sparked in the academic world, and its impact on the long-standing controversy regarding the validity of "Nanotyrannus."
Using the largest-ever histological sample of 17 individuals, this study not only revised the growth rate of Tyrannosaurus but also demonstrated with statistical rigor that "certain specimens do not fit the growth curve of Tyrannosaurus."
Background: Departure from the 2004 Paradigm
Review of "Rapid Growth and Early Death"
"Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs.", 2004.
by Erickson, Gregory M.; Makovicky, Peter J.; Currie, Philip J.; Norell, Mark A.; Yerby, Scott A.; Brochu, Christopher A. Nature.
Prior to this study, understanding of Tyrannosaurus growth dynamics relied primarily on the model established by Erickson et al. (2004). This model depicted Tyrannosaurus as undergoing explosive "exponential growth" during its teenage years, increasing in weight from a few kilograms to over 8,000 kg in about 20 years.
This life history model, described as "Live fast, die young," has been widely accepted as evidence that dinosaurs had high metabolic rates, but early models had methodological limitations (such as mixed-element sampling that did not account for differences in remodeling rates by bone type).
Methodological Innovation: The Frontiers of Bone Histology
The reliability of Woodward et al.'s study is underpinned by thorough sampling that surpasses conventional methods and the adoption of advanced visualization techniques.
Discovery of "Hidden Time" via Polarized Microscopy
The greatest technical breakthrough of this study lies in the identification and verification of overlooked growth marks using Polarized Light observation.
It was demonstrated that layers called "annuli," which were unclear under normal transmitted light, can be clearly visualized using cross-polarized light (XPL) or circularly polarized light (CPL). This revealed that areas previously determined to have "no growth lines" by conventional methods actually recorded several years of time.
Results: The Rewritten Life of Tyrannosaurus
Late-Maturing Giant: Maturity at 35-40 Years
The analysis revealed that the T. rex species complex underwent much slower and more prolonged growth than in previous models. While previous models suggested growth stopped around age 20, the new data indicates that adult size was reached at 35 to 40 years of age.
Extended Growth Period
The delay in maturity implies a dramatic extension of the "subadult stage." Tyrannosaurus would have continued growing for the majority of the time from hatching to becoming a giant adult (probably over 20 years).
| Feature | Erickson et al. (2004) | Woodward et al. (2026) (Model A) |
|---|---|---|
| Sampling Method | Mixed elements (ribs, fibula, gastralia, etc.) | Weight-bearing bones only (femur/tibia) |
| Growth Line Count | Normal transmitted light (PPL) only | PPL + Cross-polarized light (XPL) |
| Growth Cessation Age (Asymptotic) | Approx. 20 years | 35-40 years |
| Sexual Maturity | Linked to inflection point (around 14-16 years) | Unrelated to inflection point |
| Treatment of "Jane" | T. rex juvenile | Statistical Incompatibility (Suggests Nanotyrannus) |
Impact on the Nanotyrannus Debate
What is the Nanotyrannus Debate:
A long-standing debate over whether the small tyrannosaurid "Nanotyrannus" found in the Hell Creek Formation and elsewhere is a distinct species or a young individual (juvenile) of T. rex.
Of the 17 individuals included in the study, the statistical model delivered a shocking verdict on two immature specimens: BMRP 2002.4.1 (aka "Jane") and BMRP 2006.4.4 (aka "Petey"). The growth data for these specimens was deemed "statistically incompatible" with the growth series of other T. rex specimens.
Specifically, when attempting to force these specimens onto the T. rex growth curve, they emerged as outliers, indicating they did not follow the same growth rules as other individuals.
This result overturns the conclusion of a 2020 paper that stated "Jane and Petey are T. rex juveniles," strongly supporting the possibility that Nanotyrannus is a distinct species with different growth rules than T. rex.
| Specimen ID (Nickname) | Feature | Fit to Growth Model |
|---|---|---|
| BDM 050 (Bertha) | One of the largest individuals | Fits (Part of T. rex species complex) |
| BMRP 2002.4.1 (Jane) | Small individual from Hell Creek Formation | Does not fit (Outlier) |
| BMRP 2006.4.4 (Petey) | Small individual similar to Jane | Does not fit (Outlier) |
| MOR 555 (Wankel Rex) | Standard specimen with 49% skeletal preservation | Fits |
Discussion and Conclusion
Filling the Absence of Medium-Sized Carnivores
The "extended subadult phase" revealed by this study is key to solving a major mystery about the Late Cretaceous North American ecosystem. It is known that there were extremely few species of medium-sized carnivorous dinosaurs in the Hell Creek Formation and elsewhere, but it is highly likely that "Tyrannosaurs in their teens and twenties" filled this niche.
Unlike adults, they spent decades as smart and agile predators, suggesting they played a role in the ecosystem essentially "connecting a food chain different from that of adults."
Conclusion: The Late-Blooming King
The study by Woodward et al. (2026) has redefined T. rex from a "precocious monster" to a "late-blooming, long-lived predator." Furthermore, using the objective metric of statistical incompatibility, it has given extremely strong support to the "distinct species theory" in the Nanotyrannus debate.
This study is an attempt to extract "time" from the silent evidence of fossils using the power of light and statistics, and may serve as a catalyst for us to learn more diverse aspects of Tyrannosaurus.