Perhaps the most remarkable Neotropical dinosaurs are the pachamacs, gigantic ornithopod dinosaurs that look like mirror-images of the true, saurischian sauropods that were once abundant throughout South America. The origin of the pachamacs is obscure, but genetic studies suggest their closest relatives are the neodryosaurs of Australasia and Madagascar, although they appear to have even split from these sometime in the early Cretaceous. Their most likely origins lie with small, poorly-known South American late Cretaceous ornithopods such as Talenkauen santacrucensis.
A diverse community of macronarian sauropods was thriving in South America up until the Late Eocene after which the group went into rapid decline during the Eocene-Oligocene Extinction Event with just one species surviving to the end of the Oligocene. Taking the titanosaurs' place from the Oligocene to the early Pliocene was a stunning array of endemic ornithopod families, ranging from dainty bipeds to enormous armored ankylosaur-analogues. Within this evolutionary maelstrom, the first pachas appear in the late Oligocene as medium-sized, facultative quadrupeds with fairly short necks. During the Miocene, this lineage became increasingly adapted for high-browsing, developing an ever increasing body-size, longer necks and sturdier limbs.
The Late Pliocene invasion of the North American dinofauna (the Great American Interchange) spelt doom for most of the endemic Neotropical ornithopod groups, the only survivors being the pachas and the small, adaptable viriosaurs. The false-sauropods, in fact, continued to thrive, reaching their apex in the Late Pleistocene in the south. The combination of an ornithopod's chewing mechanism and a sauropod's reach and appetite has led to a truly insatiable and destructive eating machine in the pachamacs.
Great Pachamac (Austrobrontus gigas)
South America is a continent rich with life, supporting rich communities of plant life unlike anywhere else on Earth. The largest of these plants are consumed by an equally large herbivore: the great pachmac.
Over thirteen meters in length, and almost as tall, the great pachamac is the largest living psuedosauropod. Herds of the giants demolish trees across the pampas and open woodlands south of the Amazon Basin. These voracious herbivores are major agents of deforestation, but, paradoxically, are the major vectors for the reproduction of many pampas trees. Pachamacs move about constantly in their search for food, and many trees produce inconspicuous green fruits for the herbivores to take in with their leaves, moving many kilometers before they finally drop the seeds into soil that has now been, conveniently, fertilized.
Monarch Pachamac (Austrobrontus danafacies)
The 10 meter-long monarch pachamac is a slightly smaller and less common species than its cousin, the great pachamac. Robust plant-eaters, monarchs live in the southern portions of South America, in margins of the highland forests of the Andes.
Aquatitan (Aquatitan boothi)The aquatitan is an 11-meter-long, 6-tonne pseudosauropod that dwells in the rivers and swamps of the Amazon Basin. It is well adapted to an amphibious lifestyle with wide, webbed feet and a deep, flexible tail. It spends much of its time submerged in water, feeding on various aquatic plants. When the basin floods, these creatures will often wade through the riparian forests, feeding on inundated land foliage.
A single aquatitan carries a varied community of epiphytes and parasites. Of particular interest are the community of small animals often found within the pseudosauropod's colon, which provide a vital food source for the rectal probe. As a result of this association, the aquatitan has a unique secondary valve positioned 40 centimeters into the colon which denies parasites access to the intestines and reproductive organs.
Viracocha Pachamac (Viracochasaurus gigas)Named after Viracocha, a major Incan god, the long and lean Viracocha pachamac is actually the smallest of the four known pachamacs, but this 8-meter long, 10-ton creature is still the largest dinosaur inhabitant of the Amazon rainforest. These largely secretive creatures spend much of their lives in the denser areas of forest, feeding on whatever's in reach.
-Brian Choo and Daniel Bensen