Now is the winter of our fish content

once again the dreaded lurgy has struck our family this winter with Anwen needing a stay in hospital. So this post and the next few to follow were supposed to be posted a month ago.

Teleost fish outnumber all other modern vertebrates two to one. Despite this staggering diversity it is accurate to call fish palaeontology the poor cousin of amniote palaeontology, particularly when it comes to grabbing publicity. Nevertheless with such a huge diversity it is not surprising that the clade has thrown up more than a few subgroups that do grab public attention. What is surprising is that the austral winter’s edition of Journal of Vertebrate Paleontology (volume 29, number 2) carries articles on no less than three of these attention grabbing teleost groups.
One is the giant ocean-going sunfish (Molidae). These giant jellyfish-suckers include the largest living bony fish but due to their poorly developed skeletons and pelagic habits have left a scrappy fossil record. Thus the finding of three articulated skeletons that exceed modern sunfish in size (one skeleton reaches 4 metres from fin tip to fin tip) is a big deal.
Another curious fossil fish reported in this issue is a deep-sea anglerfish (Ceratoidea). These bizarre fish are well known for their disproportionately large mouths lined with needle-like fangs and the ability attract prey with a luminescent lure. Finding a fossil of one of these is also quite unusual for the ceratoid clade is not all that old by geological standards. There simply aren’t that many places where such recent sediments have been laid down at great depth but have since been brought to a position above sea-level where someone might find any fossils that they might contain.
Both of these discoveries are blogworthy finds, however it is the fossil piranha that I want to highlight in this post (Cione et al. 2009). Piranhas are, of course, the fabled freshwater fish of South America that are said to be able to skeletonise a cow in a matter of minutes when a school is whipped up into a feeding frenzy.
The fossil piranha was found late Miocene (about 5 to 11 million years old) river sediments in northeastern Argentina. It has a concave dorsal margin of the premaxilla and tall, sharp triangular teeth that indicate that its affinities lie with the piranhas among the serrasalmids. Aptly named Megapiranha the fish is immediately striking for its great size. Known from a single jaw bone (the premaxilla) and some isolated teeth it is about two and a half times larger than the premaxilla of an average modern piranha. Assuming similar proportions to a modern piranha may have approached a meter in length.

A modern piranha against a silhouette scaled up to fit the size of the Megapiranha premaxilla.

Now the thought of a school of those getting into a feeding frenzy is worthy of any Hollywood B-grade creature feature. However it is far from certain that Megapiranha would have indulged in such hypercarnivorous behavior. It should be noted that piranhas form a clade of closely related species amongst a broader family of fish known as Serrasalmidae. Most serrasalmids (for example pacus) and even some piranhas are vegetarian, indicating that the herbivory is the ancestral diet of serrasalmids. Given the primitive position of Megapiranha (the sister group of all other known piranhas) it is quite likely that Megapiranha was at least partly vegetarian.
What makes Megapiranha interesting, other than its size, is that it gives us some idea how the piranhas evolved their famous dentition. Primitive herbivorous serrasalmids have seven rounded, flat-topped teeth arranged in two rows. In contrast piranhas have a single row of six double-cusped, blade-like teeth. One would expect that the single rowed condition evolved from the double rowed condition by simple suppression of one of the rows, most likely the inner row which contains just two teeth. An alternative, proposed by Gosline (1951) is that the two rows integrated to become one. Megapiranha provides evidence that supports Gosline’s hypothesis, for it shows just a single tooth row but with the teeth placed in a staggered arrangement as if two rows were merging.

The premaxillae of a pacu (top), Megapiranha (middle), and a modern piranha (bottom)in lateral (left) and ventral (right) views. Scale bars equal 1 cm. Images from Cione et al. 2009.

The teeth themselves are intermediate as well for although they bear tall, sharp-edged triangular cusps like modern piranhas, there is no secondary cusp and the bases are broad, perhaps supporting the idea that Megapiranha was not a hypercarnivore. It is a pity really, I find the idea of a giant ground sloth or an astrapothere being stripped to its bones by a school of meter-long piranhas somehow appealing.

References

Cione, A.L., Dahdul, W.M., Lundberg, J.G. & Machado-Allison, A.(2009) Megapiranha paranensis, a new genus and species of Serrasalmidae (Characiformes, Teleostei) from the upper Miocene of Argentina. Journal of Vertebrate Paleontology 29: 350-358.

Gosline, W. (1951) Notes on the Characid fishes of the subfamily Serrasalminae.
Proceedings of the California Academy of Sciences 27: 17–64.