Time Scavengers

Evolution and Development at the Origin of a Phylum

by: Bradley Deline, Jeffrey R. Thompson, Nicholas S. Smith, Samuel Zamora, Imran A. Rahman, Sarah L. Sheffield, William I. Ausich, Thomas W. Kammer, and Colin D. Sumrall

Summarized by: Lisette Melendez

What data were used? In this paper, changes in the bodies of early echinoderms (the group that includes marine animals such as starfish and sea urchins!) are tracked in order to understand the trends that separate groups from the rest of the animal kingdom. The main question is: why are all the body plans so different from one another? Figure 1 shows the range of body plans for early echinoderms, but the distinction carries on even today, considering how starfish and sea cucumbers look so different from one another! In order to quantify these changes, the scientists directly studied specimens from various natural history museums, sifted through past echinoderm papers, discussed with experts in organism classification, and consulted the Treatise on Invertebrate Paleontology. Since the scientists were looking specifically at early echinoderms, we are talking about fossils that date back to the Cambrian and Ordovician periods, about 541-444 million years ago! Usually, data from fossils this old is limited because many significant characteristics are worn away with time. However, echinoderms have notable skeletons that retain a great deal of important characteristics, making their skeletons excellent indicators of evolutionary changes through time.

Methods: Once all the data was gathered, the next step was to find a way to accurately portray the changes of early echinoderm bodies through time. A morphospace, or a representation of every possible shape of echinoderms, was created, as shown in Figure 2. Four major echinoderm body plans were revealed in the graph. Three of the groups had radial symmetry (symmetry around a central part), while one was non-radial. Two of the groups were characterized by stem-like stalks that attached the echinoderm to the sea floor, while another group was mobile and free to move around. While Figure 2 shows the overall body plans of early echinoderms, this graph was further broken down into specific time intervals (each about 20 million years long) in order to study how the body plans changed over time. Figure 3 depicts how different the body plans were from one another throughout time.

Results: By studying the graphs, several important evolutionary trends can be picked out. Take, for example, Figure 3. The Cambrian was when the first major echinoderm body plans appeared, but the Ordovician was really where each body plan became more complex and different from one another, pointing to the Great Ordovician Biodiversification Event. Each body plan became more well-defined over time, and the differences between the various body plans are highlighted by the extinction of the transitional forms that connected one body plan to another. Even as evolution continued to progress, sometimes certain species would “readapt” a characteristic that they lost thousands of millions of years previously, showing how flexible evolution can really be.               

Why is this study important?  This study is important in studying the mechanisms behind the nature of the Cambrian explosion: why do all of these major animal groups start appearing and how have the groups changed over time? This study shows how fluid characteristics are throughout time, with the introduction, removal, and possibly even a re-introduction of characteristics to body plans as time progresses (this is called homoplasy). It highlights the various patterns of body types within Echinodermata and the patterns of gaining or losing characteristics over time, indicating the complexity in studying how animals change over time.

The big picture: This study helps us fill in some of the gaps in our knowledge about the Cambrian Explosion, a consequential chapter in the history of living creatures, and how animals have evolved since that point. It shows how evolution has changed the bodies of animals within the same group over time and helps us understand how some animals (like sand dollars and brittle stars) can look so different, yet be closely related to one another. 

 Citation: Deline et al., Evolution and Development at the Origin of a Phylum, Current Biology (2020), https://doi.org/10.1016/ j.cub.2020.02.054