Scientists have found the remains of two ancient fish that had fanged teeth on their lower jaws and a huge, single sabre-tooth on the top.
These now-extinct fish are extremely distinct in their appearance, so much so that scientists say there are no other known fish - living or extinct - that looked like them.
The unusual teeth inspired scientists to name one of the fossils, identified as a new species: Monosmilus chureloides, which roughly translates to ‘single knife churel’.
If you’re wondering what a ‘churel’ is, it’s a mythical vampire-like demon creature, with large fangs found in folklore of several South Asian countries, including Pakistan - where this fossil was discovered.
Researchers examined two fossils, a 30-centimetre-long Clupeopsis straeleni fossil embedded in a rock formation near Chièvres, Belgium, that lived during the early Eocene epoch, about 50 million years ago and, a partial fossil Monosmilus chureloides that was discovered in Pakistan’s Punjab province. Monosmilus chureloides was around 1 metre long and lived about 45 million years ago in the shallow seas of what is now Pakistan.
Both fish shared the distinctive feature of a single sabre-tooth on the upper jaw.
The upper jaw of the Monosmilus chureloides included a giant curved fang that study lead researcher Alessio Capobianco jokingly calls the "sabre-tooth."
"When the mouth was closed, it would have extended from the top of the mouth all the way down to the bottom of its chin. Monosmilus chureloides probably used its extraordinary teeth to hunt smaller fishes. However, it is difficult to hypothesise their exact purpose, as no fish today has a similar set of teeth," explained Capbianco, a doctoral student in the Department of Earth and Environmental Sciences at the University of Michigan.
Despite the single Monosmilus chureloides specimen being collected in 1977 by a joint expedition from the University of Michigan and the Geological Survey of Pakistan, it has only recently been examined by Capobianco and the study's senior researcher Matt Friedman, an associate professor of palaeontology at the University of Michigan.
After performing a CT scan on the fossil and establishing the specimen’s strange teeth arrangement, Capobianco was reminded of a similar teeth formation from another fossil (Clupeopsis straeleni), so decided to undertake a comparative study.
The findings of the research is published in the Royal Society Open Science journal and explains that these two fish, despite being found in quite different locations, are both closely related to modern anchovies. Unlike their docile relatives, the Clupeopsis and Monosmilus would have used their fangs to attack their prey, says the report.
"Clupeopsis and Monosmilus are clearly closely related because there is no other known fish (living or extinct) that has the same complement of teeth: a row of large fang-like teeth on the lower jaw, no teeth on the margin of the upper jaw and a single giant 'sabre-tooth' on the top of the mouth," said Capobianco.
When mass extinction events occur, surviving species adapt and change, often resulting in sudden anatomical transformations. It is likely that this happened to these fish during the final Cretaceous extinction.
This unexpected discovery highlights the extraordinary evolutionary changes that followed the end-Cretaceous extinction event, with sabre-toothed anchovies living alongside familiar fish groups that inhabit today’s oceans.
Capobianco, A., et al., ‘Large-bodied sabre-toothed anchovies reveal unanticipated ecological diversity in early Palaeogene teleosts’, Royal Society Open Science, (2020). DOI: 10.1098/rsos.192260