T. Rex: I Was a Teenage Monster
The news media quickly latched onto a report in Nature1 that Tyrannosaurus rex had a growth spurt in adolescence. Dr. Gregory Erickson of Florida State measured growth lines in leg bones and found faster growth between age 14 and 18 on the famous Rex specimen named Sue, says EurekAlert based on info from Florida State and the Field Museum. (See also National Geographic News, BBC News etc. that figured out this means Sue gained 5 pounds a day as a teen.)
“T. rex is notable for its great size, which is at least 15-fold greater than the largest living terrestrial carnivorous animals today and second only to Giganotosaurus among theropod dinosaurs,” the paper in Nature begins. “How did it attain such great proportions within the Tyrannosauridae?” Because the growth rates of dinosaurs is “a topic of considerable interest in evolutionary biology,” Erickson’s team tried to fit the growth rate to “two competing phylogenetic hypotheses for the Tyrannosauridae,” but no clear winning hypothesis seemed to emerge; T. rex’s “method of attaining gigantism contrasts with that in the largest crocodilians and lizards, where ancestral growth rates were retained and the exponential stages lengthened,” the paper says. Fast growth rate seems to be diagnostic of the Tyrannosauridae, but not its ancestors, according the paper. Also, “A second substantial increase in growth rate optimizes as a physiological autapomorphy of Tyrannosaurus irrespective of phylogenetic hypothesis and optimization criterion.” [Autapomorphy: “a character state that is seen in a single sequence and no other. Sometimes called a uniquely-derived character state.” Source: Molecular Systematics and Evolution glossary.]
From the two competing hypotheses of tyrannosaurid phylogeny it is most parsimonious to conclude that T. rex acquired the majority of its giant proportions after diverging from the common ancestor of itself and D. torosus, a species with an optimized body mass of about 1,800 kg. Direct comparison between the tyrannosaurid growth curves shows that the transition to the exponential and stationary phases of development occurred about 2-4 years later in T. rex (Fig. 2). However, such temporal post-displacement had little to do with the evolution of its gigantism because the exponential stage, during which most body size is accrued, was not extended beyond the ancestral, 4-year condition observed in other tyrannosaurids. Rather, the key developmental modification that propelled T. rex to giant proportions was primarily through evolutionary acceleration in the exponential stage growth rate and the transition zones bounding it. This is reflected in the regions of maximal slope on the growth curves depicted in Fig. 2 and holds true regardless of which evolutionary hypothesis is correct and how the maximum growth rates are optimized….
The actual magnitude of the growth rate change reconstructed at ancestral nodes differs with topology and more drastically with the optimization method. Linear parsimony yields a punctuated pattern with higher changes at individual nodes, whereas squared-changes parsimony forces a ‘smoother’ distribution on the data but also incurs some counterintuitive deceleration in growth for the slower-growing basal taxa.
Since T. rex seems to stand on its own two feet phylogenetically, the study ends on a question instead of a definitive answer: “How other dinosaurs attained gigantism within their respective sub-clades will serve as an interesting line of inquiry in the future. Does the same pattern of acceleratory growth seen here characterize the means by which all or most members of the Dinosauria attained great size?”
The study might help the next Jurassic Park movie. The researchers suggest that T. rex teens might have been able to run. After reaching 1000 kg, they probably were too heavy to chase down a jeep.
1Erickson et al., “Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs,” Nature 430, 772 – 775 (12 August 2004); doi:10.1038/nature02699.
We’ll leave the teenage monster jokes to the imagination of our readers. Notice in passing how the popular-level reports of this paper all talk about how these studies are going to help us understand dinosaur evolution: such as, “With the life history parameters, we can better understand T. rex evolution, biology, biomechanics and population dynamics.” Three out of four, maybe, but what evolution? The paper looked for it and didn’t find it. Give us some evidence, not empty promises. We’re getting frustrated with the ubiquitous unresolved plots in the endless soap opera, Charlie’s Angles.