Giant Filter Feeding Crocodiles from the Ancient Amazon

Travel back in time to a world ruled by crocodiles. The Amazon jungle during the Middle – Late Miocene (6-10 million years ago) was a terrifying place. 12 metre long caimans such as Purussaurus, weighing more than 5 metric tons, rein supreme.
One of the largest Crocodyliformes that ever existed: Purussasaurus (Purr-russ-a-saur-us). That scale bar is 20 centimetres! This skull is the size of a human being! (Picture via Aguilera et al. 2006)

Extreme niche partitioning of crocodile types occur here, with medium sized generalists (such as Melansosuchus - the modern Black Caiman -) existing along-side crocodiles that specialised on hard shelled organisms such as crabs (Globidentosuchus) and by far the most bizarre that existed within this jungle, was the 12 metre long FILTER FEEDING caiman, the duck faced Mourasuchus. 

Mourasuchus, the crocodile that tried to be a baleen whale. That skull is HUUGE; the scale bar is 20 centimetres! (Picture via Cidade et al. 2017)

The bizarre anatomy of Mourasuchus has been a matter of colossal debate. Filter feeding in crocodile types has been witnessed before, by the hugely controversial taxon Stomatosuchus, a 10 metre long filter feeding Crocodyliform that existed in Egypt 100-95 million years ago. (SIDE STORY: the entire holotype of Stomatosuchus was destroyed during an allied bombing raid in 1944 and only pictures remain of the enigmatic crocodile relative)
The validity of Stomatosuchus has been hotly debated. It would help if we had the original bones. Can we blame the Nazi's for this one? (Image credit: unknown)
For Mourasuchus, its rostrum was broad and dorso-ventrally flattened, with slender mandibles and a short join at the mid line of the jaw (as seen with baleen whales) with small conical teeth that almost seem vestigal. It seems that this weird-ass jaw would be suitable for a “filter-feeding” way of capturing prey, with an envisaged “gular sac” reminiscent of pelicans that would function for this straining technique. 

The jaw of Mourasuchus (20 centimetre scale bar). This animal is one of the most whack things I've ever heard of!(Image via Cidade et al 2017)

Mourasuchus was a dead end experiment in the evolution of crocodiles. The long-term environmental stability of the Amazon would have provided the perfect environment for these bizarre Miocene filter-feeding crocodiles, but monumental change in the earth would soon occur and by the Pliocene, hydrographic changes linked to the Andean uplift would kill off Mourasuchus and its associated gigantic kin.

 References

 Aguilera, Orangel & Riff, Douglas & J, Bocquentin-Villanueva. (2006). A new giant Purussaurus (Crocodyliformes, Alligatoridae) from the Upper Miocene Urumaco Formation, Venezuela. Journal of Systematic Palaeontology. 4. 221-232. 10.1017/S147720190600188X.

 Cidade GM, Solórzano A, Rincón AD, Riff D, Hsiou AS. 2017. A new Mourasuchus (Alligatoroidea, Caimaninae) from the late Miocene of Venezuela, the phylogeny of Caimaninae and considerations on the feeding habits of Mourasuchus. PeerJ 5:e3056 

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