Wednesday, November 25, 2009

Sex was a Costly Affair for Ceratopsian Dinosaurs

In a recent article published in The Anatomical Record several scientists, including Florida State University’s resident dino-osteologist Gregory Erickson, constructed a life table for a population of 80 bird-hipped dinosaurs.

Note, a ‘life table’ is a common tool used by population ecologists/biologists to interpret the birth-to-death maturation cycle of an organism. Essentially, a life table can be thought of as a listing of a population’s members with a corresponding age identified for each individual. Through examination of how the table’s age-ranges are distributed scientists can make inferences regarding the population’s ecology.

Drawing reliable conclusions on the subject of population-level processes can be difficult, particularly when that population happens to be extinct and is only known from the fossil record. Without numerous, quantitatively significant, representatives from a population, discussions of maturation rates, reproductive cycles and mortality rates are all but impossible. However, a mass kill event documented in the Lujiatun Bed of the Lower Cretaceous (Yixian Formation, Liaoning Province of China) provided the Erickson led team with the rare opportunity to do just that, study the demography of an extinct population of dinosaurs – specifically the species Psittacosaurus lujiatunensis.

Compared to the ‘lizard-hipped’ dinosaurs (saurischians) relatively little research has been undertaken in understanding the life history and population dynamics of the Ornithischia (bird-hipped), this makes the case of P. lujiatunensis all the more significant. Through histological analysis of the growth rings found within the fossil bones of the ceratopsians, – analogous to counting the growth rings in a tree – Erickson was able to estimate the age of each population member; the frequency of the age-ranges were then correlated to body size estimates. The result was that, like modern birds and mammals of comparable size, the life history of Psittacosaurus lujiatunensis reflected a pattern in which

“[h]igh attrition in young individuals gives way to lower stabilized values once a threshold size is obtained; however, later in ontogeny mortality rates increase (typically from the effects of senescence) leading to the extinction of the cohort.”

In other words, risk of death was found to be at its greatest when the dinosaurs were young and small – possibly because of vulnerability to predation. Once achieving a certain size and becoming less vulnerable, mortality rates decreased. Mortality risks would then increase again as the dinosaurs got old; through the natural ageing process the senior members of the population would once again become vulnerable to predators, disease, and etcetera.

In addition to the vulnerable young and old members of the Psittacosaurus lujiatunensis population, incidences of increased mortality were also found for those ceratopsians around the age-range associated with reaching sexual maturity. In this case, energy and resources devoted to the pursuit and winning of mates, as well as the rearing of young once a mate was found, likely conspired to cause escalated mortality levels during the reproductive years.

Erickson, G., Makovicky, P., Inouye, B., Zhou, C., & Gao, K (2009). Initial Insights Into Ornithischian Dinosaur Population Biology
The Anatomical Record, 292 (9), 1514-1521

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