Revised Tyrannosaurus Rex Growth and Lifespan

A new study, published in PeerJ, indicates that Tyrannosaurus rex experienced a more prolonged growth phase than previously understood, reaching its full adult size over four decades instead of the prior estimate of 25 years. Previous research suggested T. rex generally ceased growing around 25 years of age and had a lifespan extending up to 30 years.

Methodology

The research, led by Holly Woodward of Oklahoma State University with statistical analysis by Nathan Myhrvold and contributions from Jack Horner, involved analyzing slices of fossilized leg bones from 17 T. rex specimens. Researchers utilized advanced statistical algorithms and circularly polarized and cross-polarized light to identify previously undetected growth rings, providing a more comprehensive growth history.

Because dinosaur growth rings typically record only the last 10 to 20 years of an individual's life, a new statistical method was employed to synthesize growth records from specimens spanning early juvenile to adult stages, constructing a full growth timeline. This study represents the largest dataset ever compiled for Tyrannosaurus rex.

Key Revisions and Implications

The new analysis suggests a slower growth rate for T. rex, indicating that the species spent a considerable portion of its life in a mid-body size range. The study also noted variations in growth ring spacing within individuals, suggesting that growth was flexible and potentially influenced by factors such as resource availability and environmental conditions.

This extended growth phase may have allowed younger tyrannosaurs to occupy diverse ecological roles, which researchers suggest contributed to their dominance as apex carnivores during the late Cretaceous Period by reducing competition among different age groups.

Species Identification and Future Research

The observed differences in growth curves among specimens contribute to ongoing scientific discussions regarding whether Tyrannosaurus rex represents a single species or a complex encompassing multiple species or subspecies. The study notes that some specimens previously identified as T. rex, such as "Jane" and "Petey," exhibit growth curves that may be incompatible with T. rex, supporting independent analyses that suggest they could belong to other species, such as Nanotyrannus.

While the study's growth rates do not definitively establish separate species, researchers indicate the evidence points to this as a possibility. Steve Brusatte, a professor of paleontology and evolution not involved in the study, commented that these findings align with recent research concerning Nanotyrannus and suggest greater variation among T. rex specimens.

The identification of new types of dinosaur growth rings under polarized light may necessitate revisions to standard growth study protocols for other dinosaur species.

Discovery of an Ancient North American Tyrannosaur

A separate analysis of a shinbone has identified what may be the oldest known giant tyrannosaur in North America, suggesting that Tyrannosaurus rex had a large ancestor.

Fossil Details and Dating

Discovered in the 1980s at the Kirtland Foundation in New Mexico, US, the fossil is approximately 74 million years old, placing the tyrannosaur in the Late Campanian period. This timeframe predates most T. rex fossils, which typically date to the late Maastrichtian, although some Campanian overlap exists in the fossil record.

The bone's age was determined using argon isotopes found in volcanic ash within a layer named the Hunter Wash, with radiometric dating of layers above and below confirming the presence of large tyrannosaurids in Laramidia during the late Campanian.

Implications for T. rex Ancestry

Paleontologist Nicholas Longrich of the University of Bath and his colleagues conducted a detailed analysis of the shinbone's physical characteristics, suggesting it represents a close relative of Tyrannosaurus. The shinbone measures 84 percent of the length and 78 percent of the width of the shinbone of "Sue," the largest known T. rex specimen.

Based on these dimensions, the team estimates the tyrannosaur's weight at approximately 4,700 kilograms (5.2 tons), making it the largest tyrannosaur identified from this era.

The discovery, published in Scientific Reports, supports the hypothesis that T. rex originated in southern North America, specifically from a landmass known as Laramidia. Researchers state that this specimen represents the oldest known giant tyrannosaur from North America and potentially the oldest known member of the Tyrannosaurini.

The precise position of this new dinosaur on the family tree remains partially unclear due to the availability of only one bone. However, the bone's robust nature, straight shaft, and broad, triangular shape at the foot end are features consistent with a Tyrannosaurus tibia, distinguishing it from Bistahieversor, another confirmed tyrannosaurid found in the same prehistoric rock layer, which was significantly smaller.

The unusual size of the Hunter Wash tyrannosaur indicates an earlier appearance of large tyrannosaurids in the late Campanian than previously understood. Further remains are required to more accurately define the size and relationships of this specimen.