Some dinosaurs walked on two legs, some on all fours, some could do both, but an apparently fairly select band seemed to shift from one to the other as they grew. A few dinosaurs have previously been identified as apparently going from quadrupeds as juveniles to bipeds as adults, and the reverse, but a new paper documents detailed changes to the bones of a small herbivore called Psittcosaurus (the "parrot-lizard", named for its beak) and demonstrates just how it may have undergone the shift.

By comparing the simple proportions of various individuals, the researchers were able to show that the smaller animals tend to have arms that were nearly as long as their legs, and thus were suitable for walking quadrupedally, whereas larger animals had rather longer legs than arms, limiting the use of the latter and implying they were bipedal. This is no big shock – several researchers have examined similar apparent shifts and adult Psittacosaurus have long been thought to be primarily bipedal, not just because of their long legs, but also some other anatomical features that suggest they preferred this posture.

What is more interesting and novel is that this basic analysis is integrated with sections of the bones of the animals that looked at their microscopic structure, which allows palaeontologists to determine both the age of the animals, and which of their bones were growing. Rather like the rings of trees, many animals (including dinosaurs) have rings in their bones that vary in appearance according to how fast the animal was growing at the time, and also tend to put down dense lines each year as their growth slows in the poor annual conditions (typically winter). By counting the rings you can tell the age of the individual (since there's only one winter per year), and here it has been shown that those with the longest arms were not only the smallest, but also the youngest. Older animals (closer to three years of age than one or even less than a year) were both larger, and had longer legs.

These growth patterns also revealed how the proportions shifted for Psittacosaurus. Essentially the long bones of the arm slowed right down, while the bones of the legs accelerated their growth, leading to a shift from proportionally long, to proportionally short forelimbs.

Speaking about the paper, author Corwin Sullivan said: "This was a very interesting study to be involved in because of the way the histological data and the actual measurements of bones of different sizes provided complementary records of growth, giving us a much more complete picture than we could have got from either line of evidence alone". This elegant integration of different types of data really helps bolster the case for the shift and being able to include (relatively) exact ages to the transition point between the two shows both how and when they changed. For growing Psittacosaurus at least, four legs were good, but apparently two were better.

Histology and postural change during the growth of the ceratopsian dinosaur Psittacosaurus lujiatunensis. Zhao, Q., Benton, M.J., Sullivan, C., Sander, P.M., and Xu, X., 2013. in Nature Communications