Dinosaurs and the Red Queen

Hadrosaurs are often called the "cows of the Cretaceous." They were common, had few defenses compared to their armored ornithischian kin, and were a favorite prey for predatory dinosaurs. Natural selection appears to have applied sufficient pressure for at least one genus of hadrosaur, Hypacrosaurus, to change, however. It did not develop horns or spikes or a club, but instead ontogenetically outpaced their predators.

The key to determining how theropods and Hypacrosaurus grew can be found inside their bones, and a new study in the Proceedings of the Royal Society B using histological evidence to compare growth rates. Inside the femur and tibia of an adult Hypacrosaurus, for example, there are up to eight rings called "lines of arrested growth" (or LAGs), which denote the boundaries of annual growth. (The LAG actually marks where the periosteum, a membrane that covers most of the surface of bone, was during a pause in growth. It is a marker that indicates "This is where the surface of the bone used to be.") The trick is that LAGs don't show up regularly until these dinosaurs are about half grown and earlier LAGs are obscured by changes in the bone. This means that bones often present a missing chunk of the early life of the animal, and the authors propose several ways to detect the "lost time" (from detecting overlapping LAGs to measuring the differences between defined LAGs) without which age can't be accurately projected.

The overall picture of hadrosaur growth presented by the authors of the paper is one of rapid size increase to adult size. The question is "How fast is fast?" Determining definite dates in years can be tricky based upon one adult specimen (the age range found based on the femur and tibia ranged from 11 to 38 years, the best supported trajectories putting the specimen at anywhere from 11 to 20 years old, 13 being the accepted result in the paper) but looking inside the bones of other dinosaurs from about the same place and time can provide something to compare the Hypacrosaurus data to. The team used already existing data for Albertosaurus and Tyrannosaurus (as a proxy for Daspletosaurus) for comparison and found that by the time Hypacrosaurus was fully grown at 13 Albertosaurus was only half grown. (Other recent research has shown sexual maturity likely preceded maximum body size, as well.) Both Hypacrosaurus and the tyrannosaurs grew quickly but it seems that the prey did so much faster than that of the predators.

Life would have been very dangerous for young Hypacrosaurus. Not only were there both fleet-footed (Albertosaurus, Gorgosaurus) and robust (Daspletosaurus) tyrannosaurs around but the smaller predator Troodon may have also presented a threat. As is commonly seen today, predators often target the old, the infirm, and the young, young hadrosaurs being especially vulnerable as their small size meant that they would not be able to run as quickly and would be easier to catch. The faster Hypacrosaurus reached full size, the better. Quick, early growth was not something restricted to Hypacrosaurus, however. Although variable, it seems that dinosaur growth rates in general fell in between those of living birds and reptiles (Erickson, 2005; Lehman and Woodward, 2008), meaning that quick growth in Hypacrosaurus was not a novel evolutionary trait but something that was present in earlier ancestral ornithischians (as well as other groups of dinosaurs, like theropods) and perhaps intensified by predation pressure. Indeed, the evolution of relatively high growth rates (compared to living reptiles) might have occurred early and been spread amongst many groups of archosauriforms and archosauromorphs (Ricqles et al., 2008).

Although paleontology is often thought of as a field science, researchers being primarily occupied with digging up bones in some desolate place, many of the discoveries that are changing what we think about dinosaurs are being made in the lab. It is increasingly common for dinosaurs to be brought beneath the microscope, such minute attention opening up new areas of research. Ancient bones are indeed fascinating but they are much more than curiosities. The fossil record is all that remains of creatures that lived many millions of years ago, and what they left behind in death is allowing us striking new insights into their lives.

[Hat-tip to Not Exactly Rocket Science]

References;

Cooper, L.N., Lee, A.H., Taper, M.L., Horner, J.R. (2008). Relative growth rates of predator and prey dinosaurs reflect effects of predation. Proceedings of the Royal Society B: Biological Sciences, -1(-1), -1--1. DOI: 10.1098/rspb.2008.0912

Erickson, G.M. (2005) "Assessing dinosaur growth patterns: a microscopic revolution." Trends in Ecology and Evolution Vol. 20 (12), pp. 677-684

Lehman, T.A.; Woodward, H.N. (2008) "Modeling growth rates for sauropod dinosaurs." Paleobiology, Volume 34 (2), pp. 264-281

Ricqles, A.; Padian, K.; Knoll, F.; Horner, J.R. (2008) "On the origin of high growth rates in archosaurs and their ancient relatives: Complementary histological studies on Triassic archosauriforms and the problem of a "phylogenetic signal" in bone histology." Annales de Paléontologie, Volume 94 (2), pp. 57-76