By Conrad Mackenzie Wilson
Think of a living vertebrate – in this instance, anything with a backbone will do. If I had to, I would guess that you picked a mammal. They are, after all, our closest kin on the tree of life and our greatest animal companions at home and in the field. Or perhaps, if your interests be biological, you thought of a bird, “reptile,” or amphibian. I would bet that a fish didn’t immediately spring to mind. But if you were picking randomly from a list of all living vertebrate species, it would be a fifty-fifty proposition – more than half of living vertebrates are ray-finned fishes, or “actinopterygians.”
With its abundant modern diversity (and commensurate bulk of data for study), one might think that the evolution of actinopterygians would be well understood. But there are considerable gaps in the narrative of actinopterygian origins. This is, perhaps, less surprising if one considers the history of the study of actinopterygians. Early taxonomists, working within a creationist paradigm, recognized all other major groups of vertebrates, but they did not recognize the actinopterygians. Even as the currency of taxonomy moved from similarity of Divine thought to evolutionary relatedness, the actinopterygians remained problematic. Edward Drinker Cope, most famous for his “Bone Wars” with Othniel Marsh, first proposed the Actinopterygii as a group in 1871, but this grouping was not accepted by his peers, and Cope dropped the idea. It was not until 1891 that the group was resurrected and granted credence.
Although the concept of the actinopterygians as a major vertebrate group is now well-established, this group’s particular pattern of early branching remains controversial.
Although the concept of the actinopterygians as a major vertebrate group is now well-established, this group’s particular pattern of early branching remains controversial. This problem does not stem from inadequacy of workers or methods, but a lack of anatomical information on the earliest actinopterygians – it is impossible to construct evolutionary trees without good information on the animals that populate its branches. Actinopterygians aren’t a particularly glamorous study group, so some fossils—collected in the first half of the 20th Century and earlier—haven’t yet been described. Even those that were haven’t been evaluated using modern methods. The most recent push for better understanding has occurred on two fronts. First, workers have begun re-describing known fossils, garnering a more robust dataset from which to make evolutionary inferences. Second, workers have continued to describe new fossils, again adding to the actinopterygian dataset. These methods have already proven fruitful, but study of vertebrate diversity has suggested where the search for new actinopterygian fossils might be efficiently focused–in some 350-million-year-old rock.
Whereas mass extinctions were previously the domain of pre-Darwinian scientists seeking catastrophic narratives, research in the early 1980s brought the notion into modern scientific consciousness. In their seminal work, J. J. Sepkoski and David Raup proposed a definition for mass extinction: rapid extinction of diverse groups over a large area. Using this definition, they identified five mass extinctions, including one at the end of the Devonian period 350 million years ago. Recently, work with resolution improved by better data has separated this “end-Devonian mass extinction” into two: the Kellwasser, followed shortly by the Hangenberg. The Kellwasser extinction has a negligible effect on vertebrates, as extinction rates are low and community assemblages don’t change. The Hangenberg is difficult to assess, because our knowledge of vertebrate fossils extends only poorly into the early Carboniferous, the period immediately after the Devonian and the Hangenberg extinction. Here, there is a fifteen million-year gap – but the evidence suggests something important is happening. The vertebrate faunas of the Devonian are dominated by animals strange to us – armored fishes without jaws, teeth, or bony internal skeletons abound. By the end of the Devonian, these are gone. The vertebrate faunas of the early Carboniferous are more familiar, with more sharks and tetrapods (the branch that bears all land animals); crucially, the actinopterygians begin a meteoric rise in diversity. The basic pattern of vertebrate diversity, with actinopterygian dominance, seems to have been established in the poorly-known early Carboniferous period. If we are looking for insight into the early interrelationships of actinopterygians, this is where we should be looking.
What we do know of the earliest Carboniferous vertebrate faunas suggests that actinopterygians should be a diverse mix of forms typical of the Devonian and the Carboniferous. Ongoing work on sites in Canada and Europe has yielded a diverse fauna of tetrapods and lungfishes, with Devonian lineages persisting into the Carboniferous before petering out. Several actinopterygian lineages are already known from the few known early Carboniferous localities; as an undergraduate, I described a new actinopterygian from Nova Scotia that displayed affinities with Devonian lineages. As a graduate student, I am excited to continue characterizing the actinopterygians of Nova Scotia, adding to a growing base of new anatomical knowledge from which the long-standing problem of actinopterygian evolution can be approached.
Conrad Mackenzie Wilson has been interested in paleontology for his entire life, beginning with a common childhood fascination with dinosaurs. He stumbled into paleontology as science after encountering Stephen Jay Gould’s Wonderful Life at 14 and has pursued research in this area with vigour since.
Area of Study: M.Sc., Ecology and Evolutionary Biology
Department: Biological Sciences at the University of Calgary
taxonomy: the science of classifying life based on similarity of form
taxonomist: a person who practices taxonomy
Anderson, J.S., Smithson, T., Mansky, C.F., Meyer, T., Clack, J., 2015. A Diverse Tetrapod Fauna at the Base of “Romer’s Gap’. PLoS One 10, e0125446. doi:10.1371/journal.pone.0125446
Mickle, K.E., 2017. The lower actinopterygian fauna from the Lower Carboniferous Albert shale formation of New Brunswick, Canada – a review of previously described taxa and a description of a new genus and species. Foss. Rec. 20, 47–67. doi:10.5194/fr-20-47-2017
Patterson, C., 1982. Morphology and Interrelationships of Primitive Actinopterygian Fishes. Am. Zool. 22, 241–259.
Sallan, L.C., Coates, M.I., 2010. End-Devonian extinction and a bottleneck in the early evolution of modern jawed vertebrates. Proc. Natl. Acad. Sci. 107, 10131–10135. doi:10.1073/pnas.0914000107