Growth

Hypothesized growth sequences in theropods, from hatchlings to adults, are well supported in a few species abundantly preserved in the geologic record. For example, the best represented dinosaur species is the ceratosaur Coelophysis bauri, of which hundreds of individual specimens are known from the western USA. Because abundant remains of Coelophysis are in the same stratum and presumed to be more or less contemporaneous, a reasonable picture of their population structure can be reconstructed through biometry. These data are based specifically on the size distribution of limb and skull lengths. Another ceratosaur, Syntarsus of southern Africa, has provided sufficient specimens to use as another example of a growth series in a theropod species. Abundantly represented species such as these can show gradations in sizes that presumably reflect intermediate and end members of their growth.

The tetanuran Allosaurus fragilis, although not as abundant as Coelophysis or Syntarsus, nevertheless also has a good body fossil record in comparison to most dinosaurs and is particularly abundant in the Late Jurassic Morrison Formation from the Cleveland-Lloyd Quarry of Utah (Chapter 3). Growth series based on femur length are readily discernible from Morrison specimens of Allosaurus in this region. Nevertheless, whether such assemblages are representative of a contemporaneous population of Allosaurus is questionable and is based on a paleoenvironmental interpretation that was not originally examined in great detail.

The long-standing paleoenvironmental interpretation of the Cleveland-Lloyd deposit is that it was a "predator trap." In this scenario, a muddy area, such as a watering hole, mired a few hapless prey dinosaurs, such as the sauropod Apatosaurus. These hapless animals then attracted numerous predators or scavengers that likewise became stuck and died at nearly the same time. A recent taphonomic analysis in the Cleveland-Lloyd Quarry tested this hypothesis by plotting bone orientations to determine the energy level in the environment. For example, a low-energy environment, such as a watering hole, may have caused a random orientation of bones. In contrast, a higher-energy environment, such as a river, should show preferred orientations of the long bones, such as limb bones (Chapter 7). The result of the analysis was that the bones show a weak orientation along a preferred direction, which is evidence supporting some current orientation. These data thus may mean that at least some allosaurid body parts in the Cleveland-Lloyd deposit are allochthonous and represent an assemblage that was averaged over time and crossed multiple generations of allosaurids.

Bone histology has aided in the study of theropod growth, providing an approximate measure of how long theropods lived and how quickly some of them grew (Chapter 8). Based on growth lines in any given individual, a minimum age can be estimated. However, some growth lines can be absorbed during the lifetime of a vertebrate, especially early in its life. Nevertheless, a study of growth lines in Troodon and Syntarsus bones shows that they may have lived a minimum of 5 and 7 years, respectively. Recent recognition of abundant Tyrannosaurus bones has allowed calculations of its life history; examined specimens indicate ages of 2 to 28 years old. Having an adult age to work with, paleontologists can then calculate relative rates of growth by using the size distribution of a species in conjunction with an adult's life expectancy. For Tyrannosaurus, rapid rates of growth were needed for it to reach its full, mature size (at least 5 tonnes) in about 20 years.

Growth-rate calculations reveal that some theropods grew rapidly in comparison to other dinosaurs and modern crocodilians, and at rates comparable to those of extant large, flightless birds. Interpreted theropod growth rates pertain to whether they were precocial when young, leaving their nests and parents at an early age, or altricial, staying at home and depending on their parents for support. If juvenile theropods were able to "hit the ground running," then they may not have needed parental care as much as dinosaurs that were not developed enough to leave the nest for a few years. Future research should test these ideas, derived from the currently limited database, to see whether theropods in general grew up sooner than dinosaurs of other clades, or whether this trait was limited to only a few theropods.

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  • hessu
    How is dinosaur growth interpreted from fossils?
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