Echinoderm Morphological Disparity: Methods, Patterns, and Possibilities
Summarized by Whitney Lapic, a Time Scavengers collaborator and graduate student in paleontology. Whitney studies the paleoecology of extinct echinoderms including blastozoans. Outside of research and class time, Whitney is with her cat, Quartz, and can be found tending to her numerous houseplants.
This paper serves as a review of different approaches for and the importance of studying morphological disparity, or varying expressions of physical characteristics across a group of organisms. Since the 1960s, the importance of examining morphological disparity among organisms has become increasingly apparent. Early studies observed disparity at varying taxonomic ranks (e.g., the diversity in a phylum, like Mollusca, the group including snails and clams) while others applied numerical approaches to quantify morphological disparity. Regardless of a quantitative or a taxon–based approach, there is a need for developing some metric to quantify disparity.
What data were used?: While this article does not collect new data, it synthesizes a collection of studies done on echinoderm disparity. Echinoderms, the group including sea stars and sea urchins, offer an opportunity as a model organism for studying morphological disparity. Echinoderms are highly skeletonized and can be abundant and well preserved in the fossil record. Additionally, they present a wide variety of morphologies and are both ecologically and taxonomically diverse. While studying disparity among echinoderm morphologies has significantly helped address some gaps in our knowledge, studying disparity still offers opportunities to explore echinoderm evolution.
Methods: This study reports multiple methodologies and discusses them in depth with their applications, benefits, and caveats. These methodologies include morphometric approaches using landmark-based geometric morphometrics, as well as discrete character-based approaches. Landmark based morphometrics involves the identification of easily recognizable features, such as the point of contact between two plates that can be measured across individual organisms. Landmark based approaches can assist in differentiating species, studying the growth of a species throughout its ontogeny (growth and development), and can help in studying the disparity of a group through time.
Alternatively, character-based methods are often used when fossils are too damaged to do landmark analysis. When continuous measurements of characters cannot be obtained, the expression of a character is divided into categories into which individuals may be placed. This approach presents as a coded matrix in which expressions of a morphological feature would be coded as, for example, 0, 1, 2, etc. as a means of using discrete categories. Realistically, a combination of the two are used in these types of studies. We want to utilize as many approaches as possible. When we obtain comparable results using multiple methods, this is vital in our understanding of and interpretation of potential evolutionary trends.
The variable morphologies and the differences among them can help us explore the morphospace of echinoderms. Morphospace is a graphical representation of all forms of physical characteristics that a particular group can present with. Understanding the morphospace of taxa, and specific regions of a taxon’s morphospace can provide insight into its resiliency and susceptibility to extinction and diversification. For example, we can consider the variable morphologies of echinoderms and how very different morphologies can assist in their survival in different environments.
Why is this study important?: This paper addresses the ways in which echinoderm morphologies and their disparity can be used to further investigate echinoderm evolution. There has been a rich history of utilizing disparity and morphological approaches to study echinoderm evolution, however, there are several opportunities for further study. This paper highlights the need for combining both phylogenetic study and morphologies to gain further insight into evolutionary processes, both those including, and beyond, echinoderms.
The big picture: Understanding disparity is critical to our interpretations of trends in evolution, as well as to the development of methods to test hypotheses regarding the relationship between disparity and extinction events. By quantifying variation in morphologies, we are able to both provide a metric for understanding the degree of change in morphology during the evolution of a lineage and to explore selection towards particular morphologies surrounding extinction events.
Deline, B. (2021). Echinoderm Morphological Disparity: Methods, Patterns, and Possibilities. Elements of Paleontology, Cambridge.
Deline, B., Thompson, J. R., Smith, N. S., Zamora, S., Rahman, I. A., Sheffield, S. L., Ausich, W. I., Kammer, T. W., Sumrall, C. D. (2020). Evolution and Development at the Origin of a Phylum. Current Biology, 30, 1672-1679.