Purussaurs: monster caimans of the Miocene

While googling for astrapothere images recently I came across the image used here: wow! This is a life-sized reconstruction of the gigantic Miocene alligatoroid Purussaurus, first named in 1892 for P. brasiliensis from the Upper Miocene Solimões Formation of Brazil. Most of the salient features that are diagnostic for Purussaurus can be seen in the reconstruction: the snout is incredibly deep, wide, rounded at its tip, and decorated with bumps and ridges, the dorsal surface of the snout is strongly concave, the external bony nostril opening was proportionally huge and anteroposteriorly elongate, and the nasal bones were shortened and reduced. And, of course, the animal was huge. Complete skeletons are unknown (purussaurs are known primarily from cranial material: vertebrae and ribs are also known), but - with a skull 1.5 m long - it's estimated that P. brasiliensis reached 12 m (some sources say as much as 15 m, but that might be pushing it)...

Two species are known in addition to P. brasiliensis. P. neivensis - originally described as Dinosuchus neivensis (not to be confused with another giant alligatoroid, Deinosuchus) - is from the famous Middle Miocene La Venta fauna of Colombia. P. mirandai, from the Upper Miocene Urumaco Formation of Venezuela, was named in 2006. It has a particularly flattened skull compared to the other species and has a wider, bigger nostril opening. Why purussaurs had this immense bony nostril is unknown. In life it was surely roofed over by soft tissue, but it must have had some function. A large concave area that surrounds the nostrils is seen in some other, closely related alligatoroids.
purussaurus-skull.jpg (12.36 KiB) Exibido 2567 vezes
In his large and nicely illustrated monograph on the crocodilians of La Venta, Langston (1965) regarded Purussaurus as a species of Caiman and the name Purussaurus sort of fell out of favour after that. However, it's been back in use since the late 1980s and new material has substantially increased our knowledge of these animals. Is Purussaurus really a 'giant caiman' as Langston thought? Recent phylogenetic analyses of Alligatoroidea indicate that yes, it is. Within Caimaninae, it appears to be particularly close to Orthogenysuchus from Lower Eocene Wyoming and Mourasuchus from La Venta, with the clade that includes extant Caiman and Melanosuchus also closely related (the dwarf caimans, Paleosuchus, are the sister-taxon to this Purussaurus + Caiman clade) (Brochu 1999, Aguilera et al. 2006).
purususaurus-with-lady.jpg (24.29 KiB) Exibido 2561 vezes
Unfortunately we essentially know nothing about the lifestyle and biology of Purussaurus. Its massive size and broad skull suggest that it was strongly aquatic, and its fossils occur in faunas where diverse aquatic, amphibious and terrestrial vertebrates occur. Purussaur teeth are subcircular at their bases but somewhat flattened at the crown, and those of P. neivensis have been described as curving backwards and slightly inwards (Langston 1965). These features all suggest that purassaurs were predators of vertebrates, perhaps grabbing large mammals, but also eating turtles, smaller crocodilians, fish and other prey. The closely related, even more bizarre 'nettosuchid' caimans (which include Mourasuchus) have been suggested to be part-time herbivores, and it's even possible that Purussaurus might have eaten plants on occasion given that modern caimans will do this (Brito et al. 2002).

More astrapotheres next...

UPDATE (added 7-2-2008): I've since learnt that the model used at the top can be seen at the Museo de Historia Natural Javier Prado de la Universidad Mayor de San Marcos. The gentleman standing next to the model is Rodolfo Salas Gismondi. For more information go here... and among other things you'll see a giant turtle shell apparently bearing bite marks made by a purussaur. Despite losing a leg, its tail, and a chunk of its shell, the turtle survived.

Refs - -

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

Brito, S. P., Andrade, D. V. & Abe, A. S. 2002. Do caimans eat fruit? Herpetological Natural History 9, 95-96.

Brochu, C. A. 1999. Phylogenetics, taxonomy, and historical biogeography of Alligatoroidea. Journal of Vertebrate Paleontology 11, Supplement to Number 4, Memoir 6, 9-100.

Langston, W. 1965. Fossil crocodilians from Colombia and the Cenozoic history of the Crocodilia in South America. University of California Publications in Geological Sciences 52, 1-169.

link: http://scienceblogs.com/tetrapodzoology ... aimans.php
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