Free link to recent paper in Science (D’Emic et al. 2023) here: https://www.science.org/stoken/author-tokens/ST-1041/full
New Ways of Understanding Dinosaur Growth
Fortunately for paleontologists, the bones of many animals, including dinosaurs, slowed or paused their growth every year, leaving marks akin to tree rings. Counting these marks gives the age of the animal, and their spacing indicates how fast it was growing.
In the broken dinosaur shin bone to the left, the bone fractured along annual growth rings, which are very closely spaced. This animal grew slowly for many years.
Do bigger animals grow faster than smaller ones?
Examining animals alive today, one would see that bigger animals grow more quickly (see for example the diagram from the classic paper to the right). That would suggest that animals evolve larger sizes by growing faster than their ancestors. But like everything in evolutionary biology, looking at patterns without thinking about how the species are related can be misleading.
Or do bigger animals grow for longer than smaller ones?
Connecting the species on the graph with a hypothetical family tree (known as a phylogenetic tree) shows that the opposite of our intuition could be true: it could be more common that species get larger by growing more slowly, but for a longer period of time (notice there are many more branches that slope downward to the right rather than upward).
So what is most common in evolution — changes in growth rate or duration?
My co-authors (including two former Adelphi University Master’s students, and one former Adelphi undergraduate) and I set out to answer this question by comparing growth rates across a wide variety of dinosaurs. We looked inside bones using CT scanners, which were powerful enough to penetrate some of the dense fossils, allowing us to see growth rings (as in the image to the upper left). In most cases, however, to see growth rings we had to make lots of big microscope slides (cutting and polishing bones [see lower left] to make microscope slides). This involved painstakingly, evenly sanding fossil bones down so that they would be thin enough for light to shine through, revealing their rings. In the end, we measured about 500 growth rings in a few dozen of the two-legged, mostly meat-eating species of dinosaurs that are closely related to birds—a group called Theropoda, which includes familiar dinosaurs like T. rex and Velociraptor.
Mapping growth rates onto a tree
Here you can see growth rates mapped onto a tree. Older species are towards the left, and younger towards the right (there’s a timescale at the bottom, in millions of years [abbreviated ‘Ma’] for reference). On the tree, more orange/yellow colors indicate faster growth (like some mammals and birds today), and purples indicate slower growth (like most reptiles today). The dots show the adult body mass of each species. Notice that the bigger dots don’t correlate with any specific color. That means that bigger species didn’t necessarily grow faster, unlike living dinosaurs today —the birds.
A tree for context
To take our analysis further, we created a diagram called a phylomorphospace plot. This distorts an evolutionary tree onto a simple graph of body mass (horizontal axis) versus growth rate (vertical axis), as shown in this animation. The lines still show the relatedness of the species, but where they appear on the graph depends on how big they were and how fast they grew.
This phylomorphospace plot illustrates just how many different ways dinosaurs grew—big and fast, small and slow—and everything in between.
A drawing of a theropod thigh bone (femur) is shown, with a drawing of a typical cross section. The concentric circles at each corner of the graph are hypothetical cross sections of thigh bones, showing examples of what big, small, fast-growing, and slow-growing theropods would look like. The shaded polygons highlight different groups of theropods—even within families, growth patterns and rates were diverse.
Coming full circle
To get an answer to our question—do animals mostly change size by changing growth rate or duration—we took all of the lines of the phylomorphospace above and moved their origins to the center of a circle. Each line represents an evolutionary lineage, now with the ancestor at the center of the circle and the descendent pointing outward like spokes of a wheel. What do we see? A nearly even number of spokes in the upper right + lower left (changes in rate) as there are in the upper left + lower right (changes in duration). It seems that there’s no preferred way that dinosaurs evolved to be bigger or smaller, contrary to our predictions from animals today!
Do our results extend to other groups of animals? Why or why not? These are questions that require a lot more data and time to answer, and we hope our work inspires similar studies in other groups.
Note on some of the images on this page: "This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science, (2023-02-24), doi: 10.1126/science.adc8714".