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Saurodontidae

October 31, 2012

Saurodon leanus by NTamura.

Bony fish simply don’t get the respect they deserve. This is especially sad when Mesozoic seas were filled with wonderful ancient ray-finned and sarcopterygian clades that competed with sharks and marine reptiles, from the giant filter feeding pachycormids to the raptorial ichthyodectids.

Among these long gone fish clades were the saurodontids, which gained new found interest after Gwawinapterus turned out to be one of them rather than an istiodactylid pterosaur. As their name indicates, these were raptorial fish, specialised in feeding on proportionally large prey, much like modern pikes and barracudas.

Except some just got plain huge.

What exactly are saurodontids?

Ichthyodectiforme cladogram

Saurodontids are generally considered to be related to the iconic ichthyodectids like the famous Xiphactinus, and indeed sometimes are classified within this clade as Saurodontinae. However, some have proposed that these fish diverged quite early, probably as far back as the mid-Jurassic, and should be classified as an independent clade. For the sake of this article, we’re treating these clades independently. Within this clade, Prosaurodon is generally considered the more “primitive” member, though this honour might now belong to Gwawinapterus. Saurodon and Saurocephalus are usually treated as sister taxa, if not actually within the same genus.

Ichthyodectiformes, the clade that includes both Ichthyodectidae and Saurodontidae, were as a whole large, raptorial fish that occured in both marine and freshwater deposits. Their earliest known forms date to the late middle Jurassic, though their origins might go further back in time, and they part of the vast radiation of teleost fish that took place in the mid-Mesozoic, replacing niches previously held by Triassic sharks, sarcopterygians and non-teleost actinopterygiians (though these remained very common until the KT event).

They are generally considered to be Osteoglossiformes, and perhaps quite basal ones at that, though their exact relationships with living taxa aren’t sure. They occupied multiple ecological niches, from filter feeders to mollusc eaters, but the most iconic forms are obviously the large oceanic predators often depicted swallowing marine reptiles whole.

Palaeobiology

Saurodontids, for the most part, occupy the latter category. They aren’t called “lizard teeth” for nothing. Compared to their relatives, they also had longer rostrums, unlike the short “bulldog snouts” of ichthyodectids, and in fact some species could easily be mistaken for barracudes if they swam in modern oceans.

The most well known genus is Saurodon proper.

Saurodon leanus

Saurodon exemplifies the general body plan of the saurodontids. With a long, sinous body, this was a pelagic predator, adapted to wander in open waters, and likely was also very fast. It’s teeth are serrated like those of a shark, adapted to slice flesh. At 2.5 meters of length, it was far from being an apex predator, but it was far more formidable than our pelagic perciformes.

Most importantly, Saurodon has a very weird lower jaw. Longer than the upper jaw and quite robust, it bears a spear-like “beak” formed from fused teeth. It most likely used this strange jaw as means to strike at large shoals, just like modern swordfish and marlins, though it’s serrated teeth might suggest it was also used to rip off pieces of flesh from larger swimmers, spearing them and them removing chunks of meat.

Prosaurodon and Saurocephalus also bear similar structures, though it at least Saurocephalus it is shorter and blunter than in Saurodon, perhaps indicating that the animal did not engage in such behaviours as frequently. Gwawinapterus appearently lacks this structure, though it may simply not had been preserved. Prosaurodon also has blunter, more widely spaced teeth focused on the posterior area of the jaw, indicating a role in just grabbing rather than slicing.

Saurodontids were found nearly all over the globe, clearly indicating their pelagic, cosmopolitian ranges. As obvious, most of the best preserved fossils come from North America, from the fossil sites that were once the sea bottom of the Western Interior Seaway, though the best preserved fossils come from the Maastrichtian of Jordan. Saurodon is the earliest member of the clade, occuring from the Coniacian to the Santonian, while Saurocephalus is the most recent genus, occuring from the mid-Campanian to the KT-event; it is easy to construct Saurocephalus as being derived from Saurodon, thus rendering the latter polyphyletic. Gwawinapterus and Prosaurodon are both known from the Campanian, though they most certainly are ghost lineages dating at least to the Coniacian

Saurodontidae phylogenetic tree

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Not much is known from the biology of these fish. Most likely, they laid eggs in the open ocean, like modern large pelagic teleosts. The larger Saurocephalus, Gwawinapterus and Saurodon were almost certainly raptorial, predating not just on small fish, but seemingly on larger prey like sharks or marine reptiles if their dentition serves of any indication, while Prosaurodon more likely targetted smaller prey.

They were almost certainly specialised animals, their extinction alongside the dinosaurs being the result of the catastrophic destruction of pelagic ecosystems.

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