A new fossil find shows that the ancestors of mammals grew up more slowly in the Jurassic Era than they do now.
Introduction
Have you already lost your baby teeth? Do you ever wonder why you even have baby teeth at all? It turns out most other mammals have them too! From tiny mice to large rhinos, modern mammals have an amazing variety of shapes, sizes, and behaviors. Despite the differences, all mammals have a few things in common. We all need to breathe air, we all have hair or fur (though some mammals don’t have very much of it), and we are warm-blooded. We even have similarly shaped hearts!
These things may seem like a coincidence. But to a biologist, it’s a big clue that we share a common ancestor! We share these traits because all mammals descended from the same ancient animals. We adapted and changed over millions and millions of years.
Studying the evolution of species is tricky. Paleontologists look for fossils to learn more about the story of life on earth. Fossils are rare! For an organism to turn into a fossil when it dies, a lot of things have to happen just right. Usually, only the hard parts of an organism can turn into fossils, like bones, teeth, and claws.
We studied two amazing fossils of a mammal-like animal called Krusatodon kirtlingtonensis. Krusatodon lived in the Jurassic Period. They are part of a group of animals called mammaliaforms, which includes all mammals and their closest ancient ancestors. Our fossil finds were from a juvenile and an adult. We asked ourselves, could these fossils help us answer questions about how mammal life cycles evolved?
Methods
We used a technique called computed tomography to analyze the fossils. This technique uses X-rays from multiple angles to make a 3D picture of the fossils – without having to scrape away all the rock surrounding it!
We made a digital model of the fossils from the 3D scans (see Figure 1). Then we looked for clues that would tell us about the age and size of the animal. We carefully labeled and measured each bone and tooth. In the juvenile, we also looked at the order the adult teeth were coming in.
Another important clue comes from the dental cementum. This is the hard material covering the roots of your teeth. Your body is always making more layers of cementum. The pattern of dark and light layers in cementum tracks how many years old the tooth is, like the rings in a tree. So, we counted the layers of cementum in each specimen’s teeth to estimate their age.
Results
We estimate that the juvenile Krusatodon was between 7 months and 2 years old, and the adult was at least 7 years old. About half of the juvenile’s adult teeth had already come in. Using the sizes of the leg bones, we estimated that the adult’s mass was between 54 and 156 grams. The juvenile was between 32 and 80 grams. So, we think that the juvenile was at least half its adult size. That means it was probably already past weaning age.
We compared the juvenile Krusatodon’s growth stage to modern animals of similar size (Figure 2). Most small mammals today would already be at 100% of their adult weight by their first birthday. But Krusatodon was only a little past halfway there! That means that Krusatodon must have grown up at a much slower rate than mammals do today.
Discussion
It’s lucky to find a fossil of even one bone. It’s luckier to find a nearly complete skeleton. But to study the life cycle, you need to find fossils from animals of different ages. That’s why we were excited to find Krusatodon fossils from both a juvenile and an adult!
When we find fossils with the same traits as an animal alive today, it’s a good sign that that trait evolved even earlier. When a dentist looks at your teeth, they look to see if your teeth are growing in the right order. The juvenile Krusatodon’s adult teeth were emerging in a similar way to modern mammals, so we can tell that this pattern appeared much earlier. But when we looked at Krusatodon’s teeth compared to its age, we could tell that it was growing up at a much slower rate. We also know animals like Krusatodon kept growing when they were adults.
Little clues like this help us piece together the big picture of evolution. Our study tells us that there must have been a big change in the life cycle of mammals sometime after Krusatodon to make them grow up more quickly – and then stop growing when they were adults.
Conclusion
Humans and other mammals go through major changes as we grow up. Think about the differences between babies, elementary school kids, teens, and adults. What do you think are the biggest milestones along the way? It might seem strange, but you share many of these milestones with other animals! We are more closely related than it sometimes appears. If you can, visit a zoo, natural history museum, or wildlife preserve in your area. What similarities can you find between yourself and the animals you see?
