The Paleozoic Marine Evolutionary Faunas. Shown above are Sheehan’s revision of Boucot’s Ecologic Evolutionary Units. (Drawn by Ashley Dineen).
Every paleontologist, and hopefully everyone who has taken a paleontology course in the past 25 years, recognizes this figure. The main part of it shows one version of the famous Sepkoski curves. By analyzing occurrences of marine invertebrates through the Phanerozoic (past 540 million years or so), Sepkoski noted that the history of marine invertebrates can be divided into three temporally successive, but overlapping “faunas”, named in order the Cambrian, Paleozoic and Modern faunas. Each one is characterized by two features: First, they tend to be dominated by particular types of animals. For example, trilobites are a dominant Cambrian group, brachiopods a dominant Paleozoic group, and bivalved molluscs a dominant Modern group. “Dominant” itself is applied because species of a fauna tend to be present in most of the communities of the fauna, and are often numerically dominant, meaning that they are among the most abundant type of fossil organism in the community. The second feature is the longevity of a fauna; these things persisted for at least tens of millions of years, in the case of the Cambrian Fauna, to hundreds of millions of years in the case of the Paleozoic and Modern! Identification of the faunas, in my opinion, is one of the pillars of the revolution that took place in paleontology between the late 1970’s to early 1980’s, setting paleontology onto its current course, the mainstream of which seeks nothing less than the geobiological processes that explain the history of life on Earth.
In that sense, Sepkoski’s discovery cemented a feature of that history that has been recognized since we started to organize the fossil record back in the days of Cuvier and Lyell, and it is this: Whereas the mechanisms that generate Life’s history, namely variation of the geosphere (climate, tectonics, etc.) and Darwinian evolution, are mechanisms of inexorable change, Life’s history itself is one of fits and starts. By this I mean that at many evolutionary and ecological levels, change simply does not happen for very long spans of time. Steve Gould‘s hypothesis of Punctuated Equilibrium will immediately come to the minds of many readers, and indeed morphological stasis is a real and frequent phenomenon among fossil species. But it goes much further than that. If we examine Sepkoski’s faunas closely, we begin to realize that we can subdivide each one repeatedly into units that are smaller in size (diversity), and shorter in duration, but which also persist relatively unchanged for what are ecologically and evolutionarily long periods of geological time. Look at the figure again, and you will see a scale at the top entitled “EEUs”, or Ecologic Evolutionary Units. These were first described by the paleontologist Art Boucot (later revised by Peter Sheehan), to describe communities that at the genus level remained unchanged for tens of millions of years. EEU’s are not the end, however, and can be subdivided yet again into smaller, shorter units, such as Bambach and Bennington’s “community types” (see below). The bottom line, regardless of scale, is this: In spite of underlying mechanisms of change, ecological communities are organized into a nested hierarchy characterized by non-change.
So, when we combine dominance and the absence of change, one could accuse the fossil record, at those evolutionary and ecological levels, of being rather boring! But of course it isn’t (personal biases aside), and one huge reason for this, whether you’ve recognized it previously or not, is that the non-change of the ecological units serve to emphasize, highlight, “punctuate” the times when things do change! Dramatic changes? The history of life has those in spades: evolution of multicellularity, evolution of photosynthesis, evolution of skeletonized bodies, invasion of the land; all certainly mark the beginning of new ecologic-evolutionary epics. And, of course, things also have endings, and the endings of our units are marked by extinctions. The bigger the extinction, the bigger the unit lost. Or perhaps, the bigger the unit lost the bigger the extinction? The decline and fall of Sepkoski’s first two faunas are marked by mass extinctions, the standout being the Permian-Triassic mass extinction 251 million years ago. To paraphrase Dave Jablonski, mass extinctions remove dominance. Is there a mechanistic relationship between our EEUs and extinction? How are EEUs made, and how do they persist unchanged? Can only extinction bring them to an end? And if so, how are new EEUs made and why are they different?
To me, these are among the most interesting and important questions in paleontology today, and not only because they touch on big pieces of the fossil record. Our modern world is composed of the latest EEUs, which range in age from the end of the last ice age, to a couple million years (when the current ice ages began). Our modern world might also be teetering on the brink of a new, sixth global mass extinction. I therefore believe that answering the preceding questions are very important to the preservation of our modern biosphere, and a sustainable relationship between it and our own complex society. Toward that end, my colleagues and I have been working for several years on the relationship between large fossil ecosystems and mass extinctions, and we recently published a paper in which we present a new hypothesis to explain not only how EEUs arise, but also how they are transformed by mass extinctions. I had a lot of fun writing this paper, but it is probably a bit dry and technical (if I am being honest). So, over the next few weeks I intend to take interested readers through the paper, bit by bit, in a series of posts on this blog. I’ll wrap it up here for now, but will leave you with both a link to a free, unpublished version of the paper, and a figure as a preview (with very little explanation! For now). It’s a “mathematical space” illustrating the relative global stabilities of two alternative type of communities.
Roopnarine, Peter, Kenneth Angielczyk, AllenWeik, and Ashley Dineen. 2018. “Ecological Persistence, Incumbency and Reorganization in the Karoo Basin During the Permian-triassic Transition.” PaleorXiv. November 1. doi:10.1016/j.earscirev.2018.10.014.
An ecological “space” comparing the late Permian Upper Daptocephalus Assemblage Zone of the Karoo Basin, South Africa, to alternative eco-evolutionary histories.
Bambach, Richard K., and J. Bret Bennington. “Do communities evolve? A major question in evolutionary paleoecology.” Evolutionary Paleobiology: University of Chicago Press, Chicago (1996): 123-160.