Imagine going back in time to 50 million years ago and taking a dive in a tropical sea. What you see around you would be, in most cases, quite familiar: surgeonfishes, mackerels, wrasses, pipefishes, jacks, flatfishes, barracudas, anglerfishes, and other kinds of fishes that today inhabit tropical environments. And then, alongside these recognizable sights, there would also be saber-toothed anchovies.
In a paper published in Royal Society Open Science, UMMP and Michigan EARTH grad student Alessio Capobianco, together with UMMP director Matt Friedman and an international team of colleagues, reports two species of Eocene fossil fishes characterized by relatively large size, fangs on the lower jaws, and a giant single ‘saber tooth’ coming down from the top of their mouth. Surprisingly, these fishes share a variety of anatomical features with modern anchovies, suggesting that the popular pizza toppings represent their closest living relatives.
Of the two species described in the paper, Clupeopsis straeleni was found in 54-million-year-old deposits of Belgium and was first described in the 1940s. However, only new examination of the holotype through modern, non-invasive techniques such as micro-computed tomography (μCT) revealed its peculiar set of fangs and its affinities with anchovies. Clupeopsis represents the oldest known fossil belonging to the anchovy lineage. The second animal described in the paper is a new species based on a single specimen discovered in 45-million-year-old deposits of Pakistan in 1977, during a joint expedition of the University of Michigan Museum of Paleontology and the Geological Survey of Pakistan. That expedition brought back not only remarkable fossils of some of the earliest whales, bizarre four-legged creatures that lived at the interface between land and water, but also a diverse assortment of fossil fishes. More than 40 years later, one of those specimens has been examined through μCT techniques, revealing several unique similarities with Clupeopsis, including a row of fangs on the lower jaws and a single, laterally-compressed ‘saber tooth’ offset from the midline of its mouth. Named Monosmilus chureloides, this saber-toothed anchovy would have probably reached up to one meter in total length. The extreme dentition and large body size of Clupeopsis and Monosmilus suggest that they were predators, likely preying upon smaller fishes. This stands in stark contrast with the plankton-feeding habits and small sizes of modern anchovies, and reveals previously unrecognized ecological diversity in the early evolutionary history of this fish group.
From a broader perspective, the discovery of saber-toothed anchovies adds an important piece to the puzzle of how fish communities in marine environments evolved after the end-Cretaceous mass extinction (66 million years ago). Models of ecological release predict innovation in surviving lineages after the end-Cretaceous extinction, associated with the invasion of newly opened ecological niches. While most research efforts have been directed towards ‘successful experiments’, like the spiny-rayed fishes that dominate the extant marine biota, the release model also predicts ‘failed experiments’: short-lived groups that did not survive to the modern day with remarkable ecologies that cannot be predicted on the basis of living species. Saber-toothed anchovies are a perfect example of such an outstanding – but ultimately unsuccessful – ecological innovation. These peculiar predatory fishes, along with a host of other ‘failed experiments’ and archaic groups, highlight the post-Cretaceous marine communities as a bizarre mixture of familiar modern lineages and unusual groups lacking extant analogues.
Capobianco, A., Beckett, H. T., Steurbaut, E., Gingerich, P. D., Carnevale, G. and Friedman, M., 2020. Large-bodied sabre-toothed anchovies reveal unanticipated ecological diversity in early Palaeogene teleosts. Royal Society Open Science, 7 (5).