How did freshwater fishes end up separated by oceans? Did they stay in place while the continents split apart (vicariance), or did they find ways to cross marine barriers (dispersal)? Fossils provide a key line of evidence in this debate. (Paleomap adapted from Scotese [2014].)

How and when did living organisms achieve their geographic distribution? Why some groups of organisms are widespread across several continental landmasses, while others have a very restricted geographic range? These are only some of the questions that the field of historical biogeography seeks to answer. Freshwater fishes represent a key study group in this respect, as seas and oceans act as a strong barrier to their dispersal across different continents. Yet, despite this limitation, several groups of freshwater fishes currently display an intercontinental geographic distribution, meaning that their range is divided by one or more marine barriers. Two broad mechanisms have been proposed to explain this observation: the progressive breakup of Pangaea through the Mesozoic causing the fragmentation of a once continuous geographic range (a process called continental vicariance), or long-distance dispersal, either across marine barriers or through transient land bridges. The origin times of fish lineages with intercontinental distributions are crucial to evaluate these different scenarios, as vicariance can be excluded if these origin times postdate the ages of continental breakup estimated from geological data.

In an attempt to understand which process can better explain the widespread distribution seen in some freshwater fishes, I examined the fossil record of freshwater fishes from the last 300 million years in a paper in Biological Reviews co-authored by Matt Friedman. My emphasis was on groups that inhabit southern hemisphere landmasses, which were once united in the supercontinent Gondwana. I estimated the origin times of these fishes (including lungfishes, bony-tongues, killifishes and snakeheads, among others) using the distribution through time of their fossil record and taking in account uneven potential of fossil preservation and recovery. These estimates were combined with paleobiogeographic data to reconstruct the biogeographic histories of these fishes.

The data from the fossil record highlights the complexity of biogeographic histories in several fish groups, suggesting that a combination of vicariance and later intercontinental dispersals shaped their geographic distribution. Additionally, fossils indicate that, in the past, some of these groups were present in areas where they are completely absent nowadays, emphasizing the relevance of extinctions restricted to localized geographical regions and the inadequacy of biogeographic reconstructions based solely on modern geographic distributions. Surprisingly, oceanic dispersal between separate landmasses appears to have been a key process in the biogeographic history of freshwater fishes. While in some cases this can be explained with an invasion of marine environments by a primitively freshwater lineage, as documented by the fossil record of bony-tongues (Osteoglossomorpha), in others there is no evidence for such a shift in ecological habits. Instead, some sort of passive long-distance dispersal – such as current-driven rafting on ‘floating islands’ of vegetation formed during river floods – might have been involved. Peculiar features like air-breathing, drought-resistance and amphibious lifestyle could have helped these freshwater fishes surviving such an improbable journey across the ocean.

Remarkably, the fossil-based origin times we estimated for several fish groups are congruent with evolutionary timescales estimated from molecular data, indicating that fossil record and ‘molecular record’ of freshwater fishes are not as much in conflict as once thought. Thus, probabilistic approaches applied to fossil data can be integrated with molecular approaches to yield a coherent timescale of origin and diversification of living organisms, which can in turn be used as a framework to investigate complex patterns such as biogeographic distributions.

Capobianco, A. & Friedman, M. Early view. Vicariance and dispersal in southern hemisphere freshwater fish clades: a palaeontological perspective. Biological Reviews.