March 2023

Dinosaurs come in all sizes. The lumbering Argentinosaurus probably reached 115 feet, the winged Microraptor less than 4 feet. And today, the sole surviving lineage of dinosaurs – modern birds – includes both miniscule hummingbirds and leggy ostriches. (Learn more about why birds are actually a type of dinosaur here.) Scientists have long been interested in how non-bird dinosaurs, which include the largest land-dwelling animals that ever lived, came to have such different body sizes. The answer, of course, is through evolution, but what evolutionary changes were involved? New research helps answer that question.

Where's the evolution?

Scientists have hypothesized that body size evolution occurs through changes in growth rate. To understand it, think about how people grow. We are born, and then for the next 15 or so years grow rapidly, with an especially impressive spurt during puberty. Relatives exclaim over how big we are getting. Parents complain that we outgrow shoes too quickly. And then it abruptly stops. We remain basically the same size the rest of our lives.

One way to end up with a different adult body size is to dial up or down growth rate during that early period of rapid growth. So if a lineage were in an environment where large-bodied individuals had a survival and reproduction advantage, those with faster growth rates during this early period might be favored. Similarly, an environment in which small bodies were advantageous might favor individuals with slower juvenile growth rates.

But that’s not the only way that body size can evolve. If we humans grew at the same rate – but we didn’t stop growing until we reached 25 or so, our final body size would be much larger. Or if we stopped growing at 8 years old, our body size would be much smaller. Evolutionary changes in the timing and length of the growth phase can also affect the body size of a lineage.

Which hypothesis more accurately describes how dinosaurs evolved such a range of body sizes? Was it mainly through changes in growth rate or through changes in the length of the growth period? Recent research investigated this question in theropod dinosaurs, the diverse clade of dinosaurs that includes birds.

To figure it out, the researchers needed two key types of data. First, they had to know how quickly and for how long these dinosaurs grew. It’s easy to figure out how fast a chicken or bean plant grows by measuring it – but long- extinct dinosaurs? Luckily, bones preserve a record of yearly growth much like tree rings do. Each year, new bone growth is preserved as a ring around the outside of the bone. Thick rings represent years where a lot of new bone was produced and correspond to rapid growth, while thin rings indicate slow or minimal growth. Cut a thin slice out of a well-preserved fossil, and these rings can be measured and counted, providing a record of when the animal was growing quickly. The diagram to the right illustrates the general patterns that the researchers were looking for in the dinosaur fossils. Thicker rings mean faster growth – and a higher number of thick rings means a longer period of rapid growth.

Second, the researchers needed to know how the growth had changed in different lineages over time. This meant reconstructing the evolutionary tree, or phylogeny, showing how all theropods are related to one another. By mapping growth and size information onto the tree, the researchers were able to reconstruct the most likely ancestral traits for different lineages – for example, the most likely size and growth pattern of the shared ancestor that gave rise to both T. rex and the small dinosaur Timimus.

The diagram below shows the evolutionary tree of theropods with size and growth information mapped on. Bigger dots indicate larger dinosaurs, and lighter colors indicate faster growth. You can see that Tyrannosaurus were large dinosaurs (big dot) with fast growth (yellow-colored lineage) – and they probably got that way by evolving faster growth rates: along the lineage leading to Tyrannosaurus, the color shifts from purple (slow growth) to yellow. Spinosaurus, on the other hand, are large dinosaurs (big dot) with relatively slow growth (purple color). They did arrive at a large size, but not through the evolution of faster growth rates: the lineage leading up to Spinosaurus goes from light purple to dark purple.

To make sense of all this information and identify the most common patterns of evolution, the researchers looked at the directions of evolutionary change across all the non-bird theropods: body size increase or decrease and whether their reconstruction indicated that the change occurred through a shift in growth rate or through a change in the length of the growth period. All of those data are mapped onto the diagram below. Each line represents a lineage of dinosaurs. Longer lines mean that more evolutionary change occurred on the lineage leading to that dinosaur. And the direction of the line indicates how that evolutionary change occurred – through increases and decreases in growth rate or through increases and decreases in the length of the growth period.

Contrary to the most common hypothesis about body size evolution (that evolutionary change in body size occurs mostly through changes in growth rate), there was no overall pattern to the data. The diagram above looks like a starburst, meaning that there’s no one direction in which evolutionary change was much more likely to occur.  Some theropods got bigger or smaller through evolutionary shifts in growth rate, and others got bigger or smaller through changes in the length of the growth period. So how did big dinos get so big? The answer seems to be, any way they could.

The researchers behind this discovery suspect that other sorts of animals also evolved their body sizes in a range of ways and that this will be clear once we start to look at body size evolution using evolutionary trees. The team is already applying the new approach to look at body sizes among animals on our own branch of the tree of life, mammals – so stay tuned!