Switching Up the Dinosaur Family Tree

A new hypothesis of dinosaur relationships and early dinosaur evolution
M.G. Baron, D.B. Norman, and P.M. Barret
Summarized by Time Scavengers contributor, Maggie Limbeck

What data was used? This study looked at a wide range of dinosaurs and dinosauromorphs (dinosaur-like animals) including those from around the world from the Middle Triassic to Cretaceous. The data were focused on the early dinosaurs, from which there previously wasn’t a lot of focus on evolutionary studies. By studying the fossils of all of these different dinosaurs the researchers were able to find similarities and differences in their morphologies (structure, shape, function) to create a character list to be used to create a new phylogenetic hypothesis (evolutionary hypothesis) for dinosaurs.

Methods: The character list that was created by studying the fossils of the dinosaurs in question was scored for each dinosaur. This means that every dinosaur had the same questions asked about it and answered as a yes/no question. This data set was then run through TNT 1.5-beta, a phylogenetics software that generates a phylogenetic tree based on those characters. After a new tree was created based on this data it was tested for support using Bremer support (which calculates the difference between the most parsimonious tree and the next most parsimonious tree that is missing a particular clade (grouping of organisms)) and constraint trees.

Results: The major result of this study is the reorganization of the dinosaur phylogenetic tree that changes the relationships that were thought to be true since 1887. Since that time the theropods (e.g., T. rex) and sauropods (e.g., Apatosaurus and Brachiosaurus) were thought to form a group because their hips have a classic “reptilian-hip” structure while ornithischians (Triceratops) have a “bird-hip” structure. After completing this study, it was found that contrary to this belief that the groups Ornithischia and Theropoda are more closely related to each other than Saurischians and Theropoda are. Additionally, this new hypothesis of the evolutionary relationships between the major dinosaur groups helps to provide an explanation for morphological features that were previously thought to be examples of convergent evolution (similar traits shared by organisms that are not closely related) between theropods and ornithischians.

The newly hypothesized phylogenetic tree for dinosaurs. B shows the reorganization of Ornithischia (bird hipped dinos) to be most closely related to Theropoda (T. Rex style dinos). This is different than the relationship that was believed since 1887 that Theropoda and Saurischia were most closely related.

Why is this study important? This study is important because it represents a critical shift in the way that we think about and study dinosaurs. A change in the evolutionary relationship between major groups of dinosaurs will require current studies to evaluate how this change may affect their results. However, this could aid research that had unanswered questions or odd data points that may now be explained by these relationships.

Big Picture: The big picture with this study is that even things that we as scientists and science enthusiasts have thought to be true for years can be redefined. We see here that the dinosaur “family tree” has changed dramatically with just this one study. However, this paper has sparked a lot of internal fact-checking and conversation that is an integral part of science that is often forgotten or hidden in the background. Many scientists have run their own phylogenetic analyses and have used different methods to decide if what these authors are claiming is, in fact, correct. All phylogenies are just hypotheses, especially with dinosaurs since we only have fossil data to use. Fossils are not always in the best shape to help us learn from them, and especially fossils from the early Triassic are lacking. By having the scientific community be so shaken by this news and running their own analyses, they are helping to strengthen the validity of this science and making it more powerful. So while yes, this is groundbreaking science, it is also a good reminder that scientific hypotheses are still extensively tested and retested parts of science, not just a guess or a hunch.

Citation:Baron, M.G., Norman, D.B., Barret, P.M., 2017, A new hypothesis of dinosaur relationships and early dinosaur evolution: Nature, p. 501-506.

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