Surviving Dinosaurs: Austrornithopods

Throughout Oceania and South America live a variety of strange unique ornithischians unrelated to hadrosaurs or thescelosaurs, having evolved isolated since the Late Cretaceous. Unlike the bulky, four-legged, broad or parrot-beaked animals of the north, they retained a relatively generalized form, although modern species have specialized toward certain traits, particularly speed. Oceanian species have been proven to be most closely allied to South American ornithopod species through genetic and morphological studies, suggesting a sister taxon relationship reaching back a very long time, at least eighty million years according to some molecular clock studies. This suggests that they evolved when South America and Australia were still linked via land bridges with Antarctica, a theory further supported by the presence of fragmentary fossils from the Eocene of Antarctica of non-hadrosaurid ornithopods (as either definite Gondwanan ceratopsians or thescelosaurs are unknown from the fossil record, these suggest a likely relationship with the austrornithopods). The austrornithopods are thought to descend from the South American and Antarctic iguanodont ornithopods common during the Late Cretaceous, such as †Notohypsilophodon or †Trinisaura, and are therefore modern-day members of the clade Elasmaria, making up the majority of South America and Oceania's ornithopod diversity (and likely Antarctica's before it froze over).

Austrornithopods are built for speed, with the metatarsals being elongated compared to the shin and thigh, and the bones being fused to produce an arctometatarsal structure similar to some theropods. There is also a reduction in the size of the forelimbs, which in many species are largely vestigial, having no opposable fingers, and the wrist bones being indistinguishable from the bones of the forearm, being barely visible. In a few species they have been lost altogether, and only the more primitive living species retain hands and arms of any use. Heavy crests, armour or other visual ornamentation are usually absent or small, and the tail is long and rod-like to manoeuvre quickly in mid-stride. The skeletal system is among the most pneumatized of any ornithischians, being on par with some saurischian dinosaurs. The upper chest is large and broad to hold an exceptionally large heart and lungs, which are able to supply more oxygen to the body when running, increasing endurance, and unsurprisingly, austrornithopods are among the fastest ornithopods on the planet; some species can exceed ninety kilometres per hour for short distances. This is just as well, since they coexist with very fast predators, such as caenagnathids in South America and abelisaurids in Oceania (and likely abelisaurids in South America as well, before they became extinct), which likely spurred on such an arms-race of lightning fast predator and prey. This said, not all elasmarians are speedy or small, and some are more compactly built or reach substantial proportions.

The history of elasmarians during the Cenozoic begins mysteriously, with their only known presence during the Paleocene being teeth known from Australia, but suddenly during the Eocene, they explode in diversity. Strange new forms suddenly appear from nowhere, armoured tank-like forms from Antarctica and Australia, initially mistaken for ankylosaurs, long-necked browsing forms resembling the long extinct prosauropods and small quadruped grazers with long legs and short bodies known from bone beds of hundreds from Oceania, strange hippo-like tusked beasts from South America, and even an aberrant carnivorous form known from Antarctic deposits. However, during the Neogene, most of this diversity vanished within the stretch of a few million years. Almost all Australian forms suffered following the increasing aridity of the continent, and all forms outside of the austrornithopods were finished off by a bolide event in the Mid Miocene (an event which may have allowed a greater increase in mammalian herbivore size and diversity in their reduction). South American forms outside of austrornithopods were initially believed to have been wiped out during the Great American Interchange, but it now seems competition with newer groups of notosuchians (and possibly hadrosaurs) did them in long before this event. Antarctic forms were inevitably wiped out when the continent froze over as the climate cooled. Austrornithopod survival over all other forms is unclear, although it may simply be they exploited a niche other groups did not or were more adaptable and resourceful than other elasmarians.

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Laetocaudidae

The most speed-specialized group of austrornithopods native to the Americas, the last remnants of a family that was once far more widespread during the ice age, and also far earlier, during the Miocene (when they first appeared), when the savannah and grasslands of South America were once far more widespread. The sole living genus Laetocauda (commonly known as banderabo) is still extremely common and widespread in numerous species throughout the continent, but several extinct genera existed into the Pliocene, before becoming extinct relatively recently as a result of competition with newly arrived North American fauna and the spread of the Amazon rainforest as the ice age ended. They are distinguished by complete loss of the first and fourth toes, and severe reduction of the forelimbs to stubby clawless nubs. Most species are highly social and visual communication is an important aspect of their lifestyle; subtle posturing usually conveys very specific messages that may seem like nothing to an outside viewer. An extremely long tail ending in a colourful flap is present in all species, and this flap can be raised or closed when needed, allowing them to communicate with one another silently. Their primary method of defence is simply fleeing, and they are very good at this, being among the fastest land animals in the Western Hemisphere.

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Pampas Banderabo (Laetocauda imorufus)

The lush prairies and savannahs of South American’s southern lowlands are the home of the continent’s numerous grazing animals, from its old-type endemic hadrosaurs and sauropods, to newer arrivals such as the ceratopsians and thescelosaurs. One of the most common dinosaurian grazers of the grasslands is the long-tailed pampas banderabo, and hundreds of thousands, if not millions of them populate the open lowlands, and great herds of them roam the lands, often only visible by their bright red tail-tips as they pluck at the soft grasses. Although reaching nearly three metres (ten feet) in length, they weigh less than forty kilograms (88 lbs), as more than two-thirds of this length is their long tail.

Their long tails are used for signalling and communication, allowing the animals to keep track of one another even when they are camouflaged in the tall grass. They also communicate with soft chirps, particularly between parents and their chicks, and when signalling danger to the herd. When threatened, the entire group of banderabo moves as one, hundreds of dinosaurs sprinting across the grasslands. True to austrornithopod form, they will soon leave any predators in the dust, as they can reach speeds of over seventy kilometres an hour (43 mph). Even chicks a few days old are capable of running, although they may have difficulty reaching such speeds as well as the adults. They are monogamous, and build secluded nests in the tall grass, laying between two and eight dark-coloured eggs. Young are born with a dark striped down that is shed after a few weeks.

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Andean Banderabo (Laetocauda maculatus)

A larger species of banderabo native to the western highlands, it reaches up to four metres (thirteen feet) in length, and seventy kilograms (154 lbs) in weight. Groups of several dozen are commonly seen roaming the Bolivian Plateau and Patagonia, feeding on the low-growing vegetation, often following the herds of larger hadrosaurs for protection, and occasionally taking small animals disturbed in their wake for extra protein. They time their nesting season to coincide with the hadrosaurs, allowing them the greatest chance of success in hatching and rearing their offspring. Females lay up to twenty eggs, building them at the edge of the hadrosaur nesting colonies, and in turn give the hadrosaurs a warning system against potential predators.

Being a larger banderabo species, they will sometimes mob or lash out at smaller predators with strong kicks, slashing them with their long claws. On more than one occasion, a vacu was seen limping away with the tell-tale injuries from such an encounter. This behaviour is particularly evident during nesting season, when parents become aggressive in defence of their eggs and chicks. Smaller banderabo species simply attempt to bait and lure away predators from their nests, and then fleeing when they’ve judged the threat to be far enough away.

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Amazon Banderabo (Laetocauda minor)

One of the most cryptic and little-known species of banderabo, living mostly in the interior depths of the darkest Amazon Basin. Its dark plumage blends in exceptionally well with the blackened undergrowth, making them often impossible to spot when they don’t want to be found. Often the only sightings consist of a quick white flash of face and tail-tip as they flee through the dense foliage. A small species, the Amazon banderabo reaches only about five or six feet in length, and maybe around fifteen kilograms (33 lbs) in weight. They live in much smaller groups than other banderabo species which live in the open, usually consisting of a dominant pair, their offspring and occasionally their siblings as well, and usually do not exceed one or two dozen in number. They are nomadic and move through the jungle continuously feeding and resting; in the understory there is little difference between night and day, and they are believed to be equally active day and night.

Occasionally they may come out onto the open marshes of the Pantanal at night to feed on seasonal-growing vegetation, but such occasions and fleeting and by the sunrise, they will have vanished back into the forest. They communicate vocally using chirps like other banderabo species but will also let out a shrill whistle to startle predators or communicate with other groups of banderabo from long distances; this call is a much more often heard of the species than the species itself seen. Their nesting habits are unknown, but care of eggs and young is likely like that of other banderabo. Their remains have frequently been found in scat belonging to the yagu and the river bauru, suggesting these may be two of its major predators.

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Guiana Banderabo (Laetocauda flaveolus)

Commonly seen grazers of the northcentral plateaus and mesas of South America’s Guiana Shield, they emerge from the lowlands every night to feed upon the highlands each day, slipping back down the slopes as the sun sinks below the horizon and it becomes more difficult to spot predators. They are able to clamber up and down the cliffs with surprising skill, partly due to their ability to jump great distances and sense of balance which were already inherent skills necessary for such quick and agile runners. On the tepuis, there are usually few terrestrial carnivores large enough to be a significant threat to them, baring azhdarchids and predatory birds, but these can usually be spotted coming unless a fog shrouds the plateaus, which they may use as cover to hide their approach.

The Guiana banderabo reaches about seven feet in length and around twenty-five to thirty kilograms (55-66 lbs) in weight, making it one of the larger herbivores found on the tepuis, although it sleeps, nests, and occasionally feeds in the lowlands surrounding them; they will often move between plateaus to avoid overgrazing specific peaks. The banderabo travel in herds of around thirty to fifty, although groups exceeding eighty are not uncommon. They breed year-round, forming hidden nests of vegetation in which parents take turns brooding. Young are cared for in the lowlands as they are unable to make the daily climbs up into the tepuis until a few weeks old, when they become more sure-footed. Both adults and young are well-camouflaged in the vegetation, and it can be easy to walk through a herd of sleeping banderabo and not see a single animal.

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(I forget what the context for the accompanying sketch was, it's some sort of rauisuchian-like sebecosuchian apex predator, I think it was for the Australian radiation of austrornithopods that never got made).