Sunday, July 20, 2014

Species that don't get enough publicity #8. Dakosaurus, Metriorhynchus and friends

Today I’m going to talk about not a particular species, but a family of animals. I couldn’t narrow it down to just one-collectively perhaps only one or two has been featured in dinosaur books, and only one in my memory has made the headlines.  Remember Pristichampsus? Well, prehistoric crocodiles are fascinating to me so you can expect more. This time, it’s about a niche crocodilians exploited multiple times. Don’t worry, I won’t do them all in one go. The Philodosaurs, Dyrosaurs, and Teleosaurs can wait. Today I will restrict myself to a single but giant family of marine crocodiles. Yes, I said marine crocodiles. These are the Metriorhynchids.

Metriorhynchus’ discovery followed Icthyosaurus only by a few years in 1830, so I suspect it’s second-fiddle status comes from its more prosaic origins and adapations. The three other families of marine crocodiles I mentioned can be used as examples of transitional forms between something like a modern crocodile and Metriorhynchus, while Icthyosaur transitional forms only existed for a brief period of time in the early Triassic and are still unknown in the fossil record.  Metriorhynchus was first described by Christian Von Meyer, who also described Archaeopteryx, Rhamphorhynchus, Plateosaurus, and the previously covered Teratosaurus.

Ironically enough, the earliest  Metriorhynchid found was described in 2011, the genus Neptunidraco from Bajocian of Portomaggiore. At about the size of a modern alligator, it was not the top predator-indeed, a giant pliosaur has been found at the same formation. It was not the lion of the sea, but the leopard or caracal; it ate small fish and cephalopods and was more likely to be the prey for the pliosaurs.

You would think that the success of the pleiosaurs, pliosaurs, and icthyosaurs would deter the crocodiles from flourishing in the sea, but apparently they flooded the middle and late Jurassic.  The Callovian stage had a species of Metriorhynchus, two species of the short-snouted Suchodus, Maledictosuchus from Spain, and one of slender-snouted Gracilneustes in Europe and two of Purranisaurus in Argentina. The Late Jurassic was the apex, however.

In the Oxford Clay, we see a faunal turnover. While Suchodus brachyrhynchus remained from earlier times, it was joined by two other short-snouted sea crocodiles, Aggiosaurus and Tyrannoneustes. Tyrannoneustes is only known from a jaw, but said jaw is two feet long and armed with broad-bladed slashing teeth. This was an animal not evolved to hunt small fish and squid, but large prey.  There’s also a turnover in the more traditional slender-snouts as Gracilineustes acutus must give way to Metriorhynchus superciliosus.  The plesiosaurs Cryptoclidus and Muraenosaurus, the pliosaur Liopleurodon, and Icthyosaurs like Opthalmosaurus rounded out the Oxford cast.

It’s in the Kimmeridge Clay, however, that the sea crocs reach their apex.  The 3 meter, slender snouted, fish-trapping Cricosaurus suevicus and the similar but more generalist Metriorhynchus geoffroyii were joined by their larger relative (5 meters) Torvoneutes, the 6-meter apex macropredator Plesiosuchus and the smaller (4.5 meters) macropredator Dakosaurus.

Let’s break this down. The middle-Jurassic seacrocs have slender snouts and slender teeth. This means they were chasing numerous, fast-moving prey. This is a typical arrangement for ichthyosaur teeth, meaning they were in competition. What allowed them to compete is not sure, but by this time they were fully adapted. They had lost the armor of their ancestors, narrowed their jaws and teeth, and turned their feet into fins. The broad tail was broadened even more and, like other marine reptiles, equipped with a fin.  Icthyosaurs had rounder, shorter bodies with dorsal fins and longer pectoral fins, while the sea-crocs had long, narrow bodies with short fins on both sets of limbs.

Cricosaurus is a continuation of this design, and very wide-spread in both Latin America and Western Europe. The similar but rarer Gracilineustes made it into the Kimmeridgian as well. Equally successful, however, is Metriorhynchus. Slightly older, it’s more successful because the teeth are more akin to a modern crocodile-larger and stronger and more capable of a diverse diet. While small fish and squid-like nautiloids and ammonoids would still make up most of its diet, it would be capable of eating large fish (perhaps even the small but very successful shark Hybodus) and scavenging larger prey like dead plesiosaurs and Leedsicthys (a giant filter-feeding fish that had originally shared the seas with the first sea crocs).

There is a new group of metriorhynchids, however. These are the Geosaurs, with shorter, broader skulls and larger, curved teeth. They ate larger prey, like larger fish and marine reptiles.  These three early genera are huge. First is Torvoneustes; while the body, skull, and teeth are pretty similar to Metriorhynchus, it’s almost twice the size. This suggests a more generalized diet like Metriorhynchus, but with the size to specialize in larger prey. Unfortunately, I’m not well-versed in fish or mollusks enough to identify which.

Second is the reason I began this article. You see, in 1856, Theodor Plieninger.discovered teeth that belonged to what he thought was a new species of Geosaurus  after first thinking it belonged to the dinosaur Megalosaurus. He named it Geosaurus maximus. However, Friedrich August von Quenstedt of the Eberhard Karls Universität Tübingen, a specialist in Jurassic fossils, established it as Dakosaurus “Biting lizard”.

In the past 150 years, we’ve found a lot more of Dakosaurus. While the body is similar to the other sea crocs, the skull is broader and heavily built-the teeth have the same size as shape as a theropod dinosaur, and as mentioned have been frequently mistaken for the teeth of Megalosaurus. These teeth weren’t meant to pierce fish and squid; they were meant to slice flesh with powerful bites.

However, it wasn’t a specialist predator of other marine reptiles. That honor went to its close relative, the even bigger Plesiosuchus. Its teeth are even more mediolaterally compressed and despite the longer snout, the skull mechanics indicate an animal that ate large prey. Metriorhychus and Gracilosuchus might have been prey for it!  Other prey may have been the poorly-known Plesiosaurus Kimmerosaurus, Colymbosaurus, and Bothyspondylus, the very successful teleosaur (more on them eventually) Steneosaurus, and the Icthyosaurs Brachypterygius and the ubiquitous Opthalmosaurus.

More on their cranical mechanics here

However, there was one animal that still overshadowed Plesiosuchus-the giant pliosaur Pliosaurus. Plesiosuchus’ skull may be more than twice the size of Metriorhynchus’, but Pliosaurus’ is twice the size of Plesiosuchus’. Perhaps it was Plesiosuchus’ rise that caused pliosaurs to grow from the 25 foot Liopleurodon to the 40-foot Pliosaurus.  Pliosaurus was the apex predator, and could have easily taken any of the sea crocs as prey.

Finally, we have sea crocs in other places at the end of the Jurassic, the Tithonian age. The main marine formation is the Solnhofen Plattenkalk, a limestone formation of an archipelago in what is now Bavaria. While the legendary pterosaurs Rhamphorhynchus and Pterodactylus and the legendary bird Archaeopteryx flew overhead, the rich sea life continued. Cricosaurus continued its long reign while Metriorhynchus left a descendant, Rhaceosaurus, which had the typical slender build and relatively small size of its ancestors and ate small fish like Gyrodus, Aspidorhynchus, Pholidophorus  and Lepidotes. Geosaurus, which resembled a more-robust headed Metriorhynchus was from a different lineage of sea crocs more like Dakosaurus, probably took over the generalist function of Metriorhynchus and ate larger fish like Hypsocormus. 

Finally, Dakosaurus and Plesiosuchus managed to make it into the Solnhofen as well, feeding on the larger fish and on the smaller sea reptiles. In this case, Plesiosaurus and the ichthyosaur Aegirosaurus would have been well within their ranges. However, as far as I can tell, no Pliosaurs have been found from this region yet. Perhaps Pliosaurus was here as well, or perhaps the waters proved too shallow for the giant reptile and so the Meerckrodilers had their chance to rule.  This kind of arrangement is described by Andrade and Young in their paper High Diversity of thalatosuchian crocodylians and niche partition in the Solnhofen sea, explaining the respective niches of Kimmeridgian sea crocodiles.

There’s one more fauna I’d like to spotlight. In the New World, particularly Mexico and Argentina, more marine fossils have been found. A giant Mexican pliosaur, named the Monster of Aranberri, of unknown species was the largest animal on the continent, but Dakosaurus was the runner up. The Argentinan species Dakosaurus andiniensis was nicknamed Godzilla when it was found, and was no doubt the terror of the South seas. Dakosaurus maximus had a short, high skull, powerful jaws, and large cutting teeth, but D. andinensis had an even boxier skull and with more robust jaws and teeth. Perhaps the Mexican pliosaurs never came far enough south to challenge Dakosaurus. Purranisaurus potens continued the Middle Jurassic genus, and had many anatomical traits of the basal metriorhynchids. It ate small squid and fish, and may have been prey itself for Dakosaurus.  Finally, Cricosaurus, the very successful metriorhynchid and possible descendant of Purranisaurus, rounded out the cast. Plesiosaur species cannot be identified to the specific level, but Cuba has the cryptoclidid Vinialesaurus and the pliosaur Gallardosaurus and their descendants may have been present in the Americans. Icthyosaurs are represented by the large Caypullisaurus.

The Jurassic extinction took a heavy toll on marine life. Cryptocleidids went extinct. Pliosaurs never reached the scale of Pliosaurus again. Icthyosaurus declined to one family and fell. The Metriorhynchids joined them. The last to survive were Geosaurus lapparenti in Valanginian France, Dakosaurus andiniensis in Berrasian Argentina, and the still-strong Cricosaurus with its Valangian species C. saltillensis and C. macrospondylus. None made it past the Valanginan about 135 million years ago.

What happened? One possibility is that these swift, powerful large ichthyosaurs, the Platyptergiinae, displaced the Metriorhynchids and ruled the oceans for another 30 million years. Another is a new group of marine crocodiles, the Tethysuchids, replaced them. They probably evolved to fill the niche of the extinct Teleosaurs, and had very similar anatomy. Finally, a new shark order, the advanced Lamniformes (which were since only overshadowed by mosasaurs and cetaceans) evolved and may have replaced both them and the ichthyosaurs.

In pop culture they’ve played second fiddle, appearing mostly in books than in any other media. Metriorhynchus was omitted from the first Oxford Clay episode of Walking With Dinosaurs, but in the followup Sea Monsters was shown joining Hybodus and Liopleurodon in devouring a Leedsicthys. In the film Ice Age 2: the Meltdown, a Metriorhynchus and a Globidens (separated by the same amount of geologic time than Tyrannosaurus and humans)  were the voiceless, personality-less antagonists killed by falling boulder.   In art, sea crocs have been ignored in favor of plesiosaurs and mosasaurs; the late, great Dan Varner was a master of mosasaurs,  but I have only found one depiction of Metriorhynchus. In terms of toys, Safari Ltd’s toobs of small animal toys have provided two examples. In their excellent Prehistoric Sea Life toob, each major marine group is given a representative, and Metriorhynchus naturally represents the sea crocs. Likewise, in their also excellent Prehistoric Crocodiles toob, the only marine archosaur among them is Dakosaurus.  I do hope large versions of all these animals will appear in future toy lines.

I would also like to add to more papers, but some particular science journals like to keep their ivory tower shut to poor people like me.

So anyway, Hollywood, give this crew a break. Museums, put them on display. Toy companies, here’s a new fresh face. Authors, think about the crap you’re writing and how this company could perk up the place. And remember, shop Metriorhynchidae co. where you work or play.

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