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Tyrannosaur Life Tables: An Example of NonAvian
Dinosaur Population Biology
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| Tyrannosaur (Gorgosaurus) in death pose. Juvenile specimens such as this (Royal Tyrrell Museum of Palaeontology Drumheller: TMP 91.36.500) are quite rare, whereas older individuals are quite common. Life table analysis suggests that this pattern reflects low attrition in juvenile tyrannosaurs prior to the attainment of sexual maturity. Photo by Ed Gerken of cast by Black Hills Institute of Geological Research, Inc., Copyright (c) 1995 BHIGR). |
Summary of the Research
Because most species of dinosaur are known from just one or a few specimens, and those that exist in large numbers have not been extensively collected, virtually nothing is known about the population biology of these animals.
The recent development of means to age dinosaurs using growth line counts (See Fig. 1) allows for establishing the age structure of dinosaur populations. Using this information the patterns of survivorship and death that produced the age structure can be inferred (called Life Table Analysis in the field of Ecology).
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Fig. 1. Tyrannosaurus rex growth lines
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We first studied the age structure of a bone bed (an aggregation of skeletons) of the North American tyrannosaur Albertosaurus (a cousin of Tyrannosaurus rex). The assemblage was first discovered and partially excavated by famed dinosaur hunter Barnum Brown (American Museum of Natural History) in 1910 along the Red Deer River in Alberta, Canada. The site was reopened by the Royal Tyrrell Museum of Palaeontology (Drumheller, Alberta) led by co-author Philip Currie (now with the University of Alberta, Edmonton). Currie and the senior author Gregory Erickson worked to establish how many individuals died at the site. They concluded that no less than 22 individuals ranging from 2 to 28 years of age were represented. Notably they found considerably more sub-adult and adult specimens than juveniles.
They then studied and aged individuals of Tyrannosaurus rex and two other North American tyrannosaur species. These were specimens that had been found individually throughout various formations in the US and Canada. The same distribution, i.e. a rarity of juvenile animals, was again revealed.
What caused this pattern in tyrannosaurs? The life table analysis from which a survivorship curves were made (Figs 2 and 3) show that it is best explained by these animals showing exceptional survivorship once they passed the hatchling stage—most deaths occur in neonates in wild populations.
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| Fig. 2. Survivorship curve for the tyrannosaur Albertosaurus compared to curves for populations of living vertebrates. Hypothetical cohort of 1000 based on the Dry Island assemblage where 22 specimens were represented (i.e. the curve shows survival of a 1000 hatchlings throughout development). Following high neonate mortality common to all wild vertebrate populations, the curve shows a period of relatively low mortality rates as juveniles followed by higher rates in association with the progressive entrance of individuals into breeding competition. The shaded backgrounds show hypothetical ecological extremes used to characterize and contrast survivorship patterns. The convex Type I pattern seen in some captive animals and in humans from developed countries, shows relatively low initial mortality followed by massive, senescence-driven die-offs as maximal lifespan is approached. The diagonal, Type II pattern (characteristic of small, short-lived birds, mammals, and lizards occurs in animals whose mortality is relatively constant throughout life. The concave, Type III pattern populations (approached in crocodilians and other large long-lived reptiles experience high, early attrition, with the few survivors that reach threshold sizes standing to experience low mortality and reach maximal lifespan. Long–lived, typically moderate to large birds and mammals and the tyrannosaur show a sigmoidal, pattern with high initial mortality rates, subsequent lower mortality, and increased mortality prior to extinction of the cohort. |
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| Fig. 3. Survivorship curve for the Tyrannosaurus rex compared to curves for populations of living vertebrates. Hypothetical cohort of 1000 based on the individuals from several formations assemblage where 30 specimens were represented (i.e. the curve shows survival of a 1000 hatchlings throughout development). Following high neonate mortality common to all wild vertebrate populations, the curve shows a period of relatively low mortality rates as juveniles followed by higher rates in association with the progressive entrance of individuals into breeding competition. The shaded backgrounds show hypothetical ecological extremes used to characterize and contrast survivorship patterns. The convex Type I pattern seen in some captive animals and in humans from developed countries, shows relatively low initial mortality followed by massive, senescence-driven die-offs as maximal lifespan is approached. The diagonal, Type II pattern (characteristic of small, short-lived birds, mammals, and lizards occurs in animals whose mortality is relatively constant throughout life. The concave, Type III pattern populations (approached in crocodilians and other large long-lived reptiles experience high, early attrition, with the few survivors that reach threshold sizes standing to experience low mortality and reach maximal lifespan. Long–lived, typically moderate to large birds and mammals and the tyrannosaur show a sigmoidal, pattern with high initial mortality rates, subsequent lower mortality, and increased mortality prior to extinction of the cohort. |
Why the increased survivorship as juveniles? In living populations it occurs because they reach threshold sizes whereby predation pressures decrease. By Age 2 most tyrannosaurs were as large or larger than nearly all other predators in their realm.
During mid-life mortality rates in tyrannosaurs increased precipitously. Why? Most likely it is due to the onset of sexual maturity where the physiological demands of oviposition and fasting, increased injuries and stress from agonistic activity in competition for mates, and heightened exposure to predators take their toll. Notably this is when these animal’s growth began to slow as maximal adult size was reached, the size when sexual maturity is reached in most living reptiles.
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