Evolution has not only produced land predators with giant teeth like the saber-toothed cats – there were also apparently animals with large fangs in the water, as fish fossils discovered in Belgium and Pakistan now prove. These relatives of modern herrings and anchovies, around 50 million years old, had a series of sharp fangs in the lower jaw and a single, long saber tooth in the upper jaw. It was so big that it protruded from the closed mouth. The paleontologists see these bizarre predatory fish as confirmation that nature has created new, “experimental” creatures after the extinction of the masses at the end of the Cretaceous Period.
Around 66 million years ago, earthly nature experienced an enormous cut: The mass extinction at the transition to the paleogene not only ended the era of dinosaurs, pterosaurs and many other animal groups, it also created new opportunities for the survivors. Because they were now able to develop new variants that occupied the ecological niches that had become free. This also applied to the bonefish, which today include 95 percent of all fish species. “One of the most striking features of the change in marine fish on the border from the Cretaceous to the Paleogen is the disappearance, decimation, and displacement of many previously dominant groups of large predatory fish,” said Alessio Capobianco of the University of Michigan at Ann Arbor and his colleagues. This freed up many niches for such predators and created new, sometimes bizarre and short-lived “experiments in nature”.
Big and snappy
Capobianco and his team have now identified two of these “experimental” prehistoric fish species using fossils from Belgium and Pakistan. The species baptized Clupeopsis straeleni is around 54 million years old, the very similar fossil of Monosmilus chureloides is around 45 million years old. Microtomography studies revealed that both types of fish resembled today’s anchovies in their bone structure. The paleontologists therefore assign them to this family and to the order of the herring-like species (Clupeiformes), which today include herring, anchovy and many other mostly small, plankton-eating fish species. In contrast to these harmless schooling fish, the two prehistoric fish species were significantly larger and more defensive than their modern relatives, as the researchers report. Clupeopsis was probably around half a meter long, monosmilus even one meter. “This makes them both very large compared to recent anchovies and other types of herring,” said Capobianco and his colleagues.
The most striking feature, however, is the teeth of the two primeval fish: “Both species are characterized by remarkable teeth: they had a number of enlarged teeth on the lower jaw, combined with a single, extremely large saber tooth in the upper jaw,” report Capobianco and his team. The single saber tooth was saber-shaped and bent back so long that it protruded from the closed mouth. Also unusual: Both prehistoric fish each had only one of these fangs and in both cases it was not in the middle, but slightly to the side of the middle. On the other hand, the tooth pit in the ploughshare bone (Vomer), the middle bone of the upper jaw, was empty, as the micro-CT examinations showed. “Extended fangs in the Vomer are very rare in bony fish and, to our knowledge, there is no group that shows such an asymmetrical tooth pattern as these two species from the Eocene,” say the researchers.
Predatory and “experimental”
According to the paleontologists, these fossils demonstrate two things: First, the size and teeth of these fish suggest that they were predatory and likely to hunt other fish. This means they are in contrast to many herring species living today, which were previously considered to be primarily plankton-eating. According to current theory, individual predatory species are said to have developed independently of one another afterwards. However, the discovery of the two new fossils, which are among the oldest known representatives of the anchovies, could now speak against this view. As Capobianco and his team explain, there are several modern fish groups, of which a transition from an originally predatory to a plankton-eating lifestyle is known. Therefore, some researchers suspect that it could have been the same with the herring-like species. “Clupeopsis and Monosmilus could now support this course of trophic evolution,” said Capobianco and his colleagues.
In addition, however, the two fossils confirm that evolution brought about some unusual animal forms after the Cretaceous mass extinction. “Our discovery underscores the extraordinary evolutionary trial and error after mass extinction,” says Capobianco. “Back then, anchovies with saber teeth and other failed experiments in nature still lived in the ocean with familiar fish groups.” While saber-tooth anchovies and other bizarre contemporaries died out relatively quickly, the forerunners of today’s fish species prevailed and developed further.
Source: Alessio Capobianco (University of Michigan, Ann Arbor) et al., Royal Society Open Science; doi: 10.1098 / rsos.192260