Biology:Theropod growth rates
Theropods are a subclade of saurischian dinosaurs most known for limbs with three toes and hollow bones (including modern avians). Estimations of the growth rates of non-avian theropods are difficult, given their extinct nature and the relative lack of easily accessible data. Growth rates within non-avian theropods vary tremendously, given that they ranged in size from that of modern birds to multi ton species such as Tyrannosaurus rex. Non-avian theropod growth rates were either less than, equal to, or greater than those of precocial birds and mammals depending on the size of the theropod, but all non-avian theropod growth rates exceeded those of reptiles.
Methods of estimating growth rates
In order to estimate the growth rates of theropods, scientists need to calculate both age and body mass. Both of these measures can only be calculated through fossilized bone and tissue, so regression analysis and extant animal growth rates as proxies are used to make predictions. Fossilized bones exhibit growth rings that appear as a result of growth or seasonal changes, which can be used to approximate age at the time of death.[1] However, the amount of rings in a skeleton can vary from bone to bone, and old rings can also be lost at advanced age, so scientists need to properly control these two possibly confounding variables.
Body mass is harder to determine, as bone mass only represents a small proportion of the total body mass of animals. One method is to measure the circumference of the femur, which in non-avian theropod dinosaurs has been shown to be a relatively proportional to quadrupedal mammals,[2] and use this measurement as a function of body weight, as the proportions of long bones like the femur grow proportionately with body mass.[2] The method of using extant animal bone proportion to body mass ratios to make predictions about extinct animals is known as the extant-scaling (ES) approach.[3] A second method, known as the volumetric-density (VD) approach, uses full scale models of skeletons to make inferences about potential mass.[3] The ES approach is better for wide range studies including many specimens and doesn't require as much of a complete skeleton as the VD approach, but the VD approach allows scientists to better answer more physiological questions about the animal, such as locomotion and center of gravity.[3]
Growth rates compared to extant taxa
Non-avian theropods didn't exhibit a group wide growth rate, but instead had varied rates depending on their size. However, all non-avian theropods had faster growth rates than extant reptiles, even when modern reptiles are scaled up to the large size of some non-avian theropods. As body mass increases, the relative growth rate also increases. This trend may be due to the need to reach the size required for reproductive maturity.[4] For example, one of the smallest known theropods was Microraptor zhaoianus, which had a body mass of 200 grams, grew at a rate of approximately .33 grams per day.[5] A comparable reptile of the same size grows at half of this rate.[5] The growth rates of medium sized non-avian theropods (100–1000 kg) approximated those of precocial birds, which are much slower than altricial birds. Large theropods (1500–3500 kg) grew even faster, similar to rates displayed by eutherian mammals.[5] The largest non-avian theropods, like Tyrannosaurus rex had similar growth dynamics to the largest living land animal today, the African elephant, which is characterized by a rapid period of growth until maturity, subsequently followed by slowing growth in adulthood.[6]
References
- ↑ Padian, Kevin; de Ricqlès, Armand J.; Horner, John R. (July 2001). "Dinosaurian growth rates and bird origins" (in en). Nature 412 (6845): 405–408. doi:10.1038/35086500. ISSN 1476-4687. https://www.nature.com/articles/35086500.
- ↑ 2.0 2.1 Anderson, J. F.; Hall‐Martin, A.; Russell, D. A. (1985). "Long-bone circumference and weight in mammals, birds and dinosaurs" (in en). Journal of Zoology 207 (1): 53–61. doi:10.1111/j.1469-7998.1985.tb04915.x. ISSN 1469-7998. https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-7998.1985.tb04915.x.
- ↑ 3.0 3.1 3.2 Campione, Nicolás E.; Evans, David C. (2020). "The accuracy and precision of body mass estimation in non-avian dinosaurs" (in en). Biological Reviews 95 (6): 1759–1797. doi:10.1111/brv.12638. ISSN 1469-185X. https://onlinelibrary.wiley.com/doi/abs/10.1111/brv.12638.
- ↑ Lee, Andrew H.; Werning, Sarah (2008-01-15). "Sexual maturity in growing dinosaurs does not fit reptilian growth models". Proceedings of the National Academy of Sciences of the United States of America 105 (2): 582–587. doi:10.1073/pnas.0708903105. ISSN 1091-6490. PMID 18195356. PMC 2206579. https://pubmed.ncbi.nlm.nih.gov/18195356/.
- ↑ 5.0 5.1 5.2 Erickson, Gregory M.; Rogers, Kristina Curry; Yerby, Scott A. (July 2001). "Dinosaurian growth patterns and rapid avian growth rates" (in en). Nature 412 (6845): 429–433. doi:10.1038/35086558. ISSN 1476-4687. https://www.nature.com/articles/35086558.
- ↑ Horner, John R.; Padian, Kevin (2004-09-22). "Age and growth dynamics of Tyrannosaurus rex". Proceedings. Biological Sciences 271 (1551): 1875–1880. doi:10.1098/rspb.2004.2829. ISSN 0962-8452. PMID 15347508. PMC 1691809. https://pubmed.ncbi.nlm.nih.gov/15347508/.
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