Triceratops horns evolved over time
A new study reveals that small-scale evolutionary changes can be tracked in dinosaur fossils

The skull of an adult Triceratops showing the three horns for which it is named. Photograph: Wikimedia Commons
 
Aligning the results of classical biological studies and those in palaeontology is rarely easy. The fossil record is great for demonstrating many things about past life on Earth over large timescales and that involve big changes such as the origins of major groups of organisms. Biology is generally the reverse, with even long-term studies on fast-breeding species unlikely to get through too many generations and observe the major changes that are routine in studies of fossils that span millions of years. However, a new study is able to take the biological approach to the fossil record with one of the most famous and popular dinosaurs, the three-horned herbivore Triceratops.

Normally such approaches are limited by the available information, and there’s not enough of it, or it has problems with it. To look at changes over a relatively short period of time you need to have several things all together – a large number of specimens to measure, that are all from the same place, and are from slightly different times. Critically, you also need to know exactly how old each one is relative to the others. Getting all of this together, especially for larger vertebrate fossils (which are generally rare), has therefore been tricky.

Collections of very large numbers of species often occur in single layers of rock which likely represent a very limited timespan, or are scattered across huge areas where local differences are as viable an explanation as change over time. Some species are known from large numbers of specimens but their exact origins were never recorded accurately, so which came when isn’t known. The only real way to solve this may be to start from scratch and collect everything yourself so you know what it is and where it has come from and this is essentially the approach taken by a team from the Museum of the Rockies in Montana.

Sampling in the famous Hell Creek Formation of the end Cretaceous (around 66 million years old) over many years they have collected a huge number of Triceratops specimens and carefully documented the exact location of each. The layers they have excavated span between one and two million years, a tiny slice of time in the scale of the fossil record, but exactly the length we might expect to see some long-term changes take hold in an evolving population. Measuring a series of key features on the skulls of the various finds shows some consistent patterns.

Over time, we see the small horn on the tip of the snout increase in length, while the big brow horns are initially quite variable in size but then stabilise to a relatively small size later on, and there are some shifts in other proportions and subtle features of the skull. Naturally there is some variation and inconsistency in places (after all, not all individuals of a population look exactly the same) but the overall trends seem strong.

Although a great many species of extinct animals are represented by only one or a handful of specimens, this shows the power of traditional palaeontological research methods – collecting a great deal of information over a long time and using it as the basis for a major analysis. Of course it comes as no great surprise that animals like dinosaurs evolved every bit as much as other species and it shows that the fossil record can be used to demonstrate shorter-term evolutionary trends, even for large terrestrial animals living tens of millions of years ago.

The study has been welcomed by dinosaur researchers, with Dr Andy Farke of the Raymond M. Alf Museum of Paleontology in California noting that, "It's pretty interesting to track the change of skull shape in Triceratops through time – people have tried (without success) for some other species of dinosaurs. Maybe the Triceratops population was doing something unique? Or maybe it's just a particularly good test case. Either way, the study is a great contribution."

These results sadly do not tell us what might have prompted such changes to occur. The functions of the frill and horns of Triceratops are rather controversial, though there is good evidence that the horns were involved in fighting and that the frill likely functioned as some kind of signal. Still, this lays the foundation for future studies to start applying our understanding of such features to this dataset and there is a huge amount of potential here for expanding our understanding of dinosaurian evolution in the Mesozoic and how populations might evolve in response to local or even global changes in the distant past.