Revising echinoderm relationships based on new fossil interpretations

A re-interpretation of the ambulacral system of Eumorphocystis (Blastozoa, Echinodermata) and its bearing on the evolution of early crinoids

by: Sarah L. Sheffield and Colin D. Sumrall
Summarized by Sarah Sheffield

What data were used? New echinoderm fossils found in Oklahoma, USA, along with other fossil species of echinoderms. The new fossils had unusual features preserved.

Methods: This study used an evolutionary (phylogenetic) analysis of a range of echinoderm species, to determine evolutionary relationships of large groups of echinoderms.

The arms of Eumorphocystis. A. This is an up close image of the arms that branch off the body. B. The arms of Eumorphocystis have three separate pieces comprising them: these three pieces are highlighted in yellow, blue, and green. This arm structure is nearly identical to early crinoid arms, indicating that crinoids might be more closely related to creatures like Eumorphocystis than we previously thought.
Results: Eumorphocystis is a fossil echinoderm (the group that contains sea stars) that belongs to the Blastozoa group within Echinodermata. However, it has unusual features that make it unlike any other known blastozoan: it has arms that extend off of the body, which is something we see in another group of echinoderms, called crinoids. Further, these arms have a very similar type of arrangement to the crinoids: the arms have three distinct pieces to them (see figure). Researchers placed data concerning the features of these arms, and the rest of the fossils’ features, into computer programs and determined likely evolutionary relationships from the data. The results indicate that Eumorphocystis is closely related to crinoids and could indicate that crinoids share common ancestry with blastozoans.

Why is this study important? This study indicates that our understanding of the big relationships within Echinodermata need to be revised. Without an accurate understanding of these evolutionary relationships, we can’t begin to understand how these organisms actually changed through time-what patterns they showed moving across the world, how these organisms responded to climate change through time, or even why these organisms eventually went extinct.

The big picture: This study shows that crinoids could actually belong within Blastozoa, which could change a lot of what we currently understand about the echinoderm tree of life. Overall, this study could help us understand how different body plan evolved in Echinodermata and how these large groups within Echinodermata are actually related to one another. Data from this study can be used in the future to start to understand evolutionary trends in echinoderms.

Citation: Sheffield, S.L., Sumrall, C.D., 2018, A re-interpretation of the ambulacral system of Eumorphocystis (Blastozoa, Echinodermata) and its bearing on the evolution of early crinoids: Palaeontology, p. 1-11. https://doi.org/10.1111/pala.12396

To read more about Diploporitans please click here to read a recent post by Sarah on Palaeontology[online].

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