Surviving Dinosaurs: Abelisauridae

Here's another entry I contributed to that no K-Pg extinction project that ended up  never coming out, ah the continuous allure of the dinosaurs and their survival for speculative evolution...

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Abelisauridae

After the end of the Mesozoic, the abelisaurs found great success in the worldwide hothouse climate that developed, and in an evolutionary path that mirrored HE's mesonychians and creodonts, enjoyed immense success during the early part of the Cenozoic as small to large-sized predators. Fossils of Eocene abelisaurs are known from every continent, although North American remains are fragmentary and dubious, as during this time, the last tyrannosaurs enjoyed their final hurrah. It seems that as terrestrial crocodyliforms that also began to truly flourish took more heavily-built predatory forms during this period, abelisaurids adapted towards cursoriality, allowing the two lineages to coexist without serious competition. Many species of abelisaurids during this time did not exceed five or six metres (16-20ft) in length, but some gargantuan species did develop by the close of the Eocene; the species †Sarchophoneus ultimus from the Late Eocene to Early Oligocene of Mongolia was believed to be one of the largest land carnivores of all time, perhaps reaching thirteen metres (43 ft) in length. However, also similar to HE's mesonychians and creodonts, their success came to an end as the climate shifted and competition with newer maniraptoran and crocodyliform predators decimated the group and much of its diversity. In South America, competition with caenagnathids arriving from North America during the Pliocene spelled the end for the continent's abelisaurids, while they were wiped out in Africa and Asia by competition with dromaeosaurine dromaeosaurs and uruguaysuchid notosuchians, with the movement of glaciers killing off the last of these in Europe and North America, and all native dinosaurs in Antarctica. The isolated continent of Oceania remains the abelisaurids’ last stronghold in the Holocene.

Once the dominant dinosaurian carnivores of Gondwana and Europe, this group were apex predators on most major landmasses during much of the Late Cretaceous and Paleogene (but with a lineage that dates back to the Mid Jurassic), with fragmentary jaw pieces and teeth known even from Antarctica and North America of the Early Eocene, but soon, a combination of competition with newer hunters and climate change wiped out on most continents. It is perhaps only their extreme range that spared them the same fate as the mesonychians, perhaps the closest HE analogue (a group of predators that greatly prospered during the early part of the Cenozoic before dying out with no descendants), and only a meagre handful of species survives in isolation in Australia and its surrounding islands.

Abelisaurids are very distinct theropods, and even at glance they are easy to distinguish from other carnivorous biped dinosaurs. Their skulls are blunt and sometimes nearly as tall as they are long and are often covered in knobs and pits that support facial ornamentation in the form of horny or scaly crests and spikes. This ranged greatly throughout the differing species, from the famed bull-like horns above the eyes of †Carnotaurus sastrei, to the single frontal horn and wrinkled snout of †Majungasaurus crenatissimus, both of the Late Cretaceous. The arms of abelisaurids are infamously underdeveloped and seem to be mostly if not entirely vestigial; the wrist was nonexistent, the elbow was immobile, and the hand was greatly reduced, with only two clawed fingers. Only some flightless bird species have forelimbs that were similarly underdeveloped, and in modern abelisaurid species, the arms have been lost entirely, although a few nubs may rarely appear on occasion in some individual animals (similar to the back flippers that may appear on some HE cetaceans).

The jaws of abelisaurids are particularly flexible and elastic similar to snakes, allowing them to have a large gape to swallow large amounts of meat in one bite. The teeth are curved and spike-like for stabbing prey, which contributed to the two distinct hunting strategies. One strategy is similar to that of the carnosaurs that they out-competed, to slam down on prey with their jaws like a hatchet to cause tremendous damage; the other strategy is to simply hold and pin struggling prey with their mouth until it is dead, which is similar to the hunting style of HE cats. Both these strategies would have been useful for hunting the large sauropods they generally coexisted with, although the few modern species have only coexistence with one species of sauropod recently arrived from South America, as Australia's own native titanosaurs became extinct approximately fourteen million years ago, as the continent became increasingly arid. Living species are more specialized to hunting animals smaller than they are or around the same size.

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Wurguru (Notodiabolus velox)

Among the numerous baurusuchid and metatherian predators of Oceania is one which may not seem out of place elsewhere, but in the upside-down world of Australia, it stands out from the crocodyliform and mammalian predators that dominate the outback. The wurguru is the last of the abelisaurid ceratosaurs that once ranged far and wide, with the continent's isolation having protected them from extinction. Wurguru are carnotaurines, and likely crossed from South America when Australia was connected through Antarctica during the Cretaceous and Paleogene. The genus is known from fossil remains up to the Pliocene, with the species appearing just at the end of this epoch. The species takes the cursorial adaptations characteristic of the carnotaurines to an extreme; the tarsus is nearly twice the length of the tibia, while enormous, densely-packed bundles of muscles in the thigh and base of the tail power the legs. Much of the dermal armour common in extinct ceratosaurs, as well as the remnants of the vestigial arms, have been lost as weight reducing adaptations, while the lungs and heart are enlarged, the largest in comparison to body-size of any living abelisaur species. These together create an unparalleled speed demon of the Australian outback, a predator capable of reaching speeds of nearly a hundred kilometres per hour (62 mph) over very short distances or keep up a pace of fifty kilometres an hour (31 mph) for up to seven minutes.

This extreme specialization towards speed is undoubtedly to hunt the elasmarian austrornithopods that they coexist with, which are able to match the wurguru in a predator-prey arms race. Wurguru are medium-sized predators, reaching between four and five metres (13-16 ft) in length, half of which is a long, broad tail. Lifetime monogamous animals, they usually hunt in pairs, greatly increasing their chances of capturing the wily little ornithopods. A common attack strategy is for one animal to lead the prey into a trap, where its mate lies in ambush; should this fail, the initial chaser continues to run down the prey, falling back to allow its trailing partner to take up the lead and continue the chase, switching back and forth whenever either gets tire until the prey animal begins to exhaust itself. This strategy is very effective in the open savanna, but less so in the forest undergrowth. If possible, prey will often attempt to seek cover in scrubland or among rocks where they can more easily outmanoeuvre the abelisaurs. Once the prey animal comes within reach, the wurguru attempts to trip it up by ramming it or pulling on its tail and go for the throat once the prey is down. Smaller animals will be slashed into pieces with their feet or flailing with the mouth and quickly consumed on the spot in large chunks, while larger kills are dragged to a secluded den to be eaten at relative leisure, with leftovers being buried or covered with branches; both are strategies to minimize theft by the larger baurusuchians they coexist with, which will bully the theropods from their catch if they can find it.

Wurguru are probably Oceania's widest ranging large predator, found throughout the Australian continent, as well as Tasmania and parts of southern New Guinea (although the New Guinea form has been proposed to be a separate species by some, elevating it from subspecies level), although scarce in the harshest interior desert and densest coastal jungles. Aside from austrornithopods, they prey on marsupials, squamates, small mammals, and coelurosaurs. Occasionally they patrol the coastlines to sea if the sea has washed up any carcasses or wade for swimming prey such as crocodiles and monotees. The largest prey animal they are known to hunt are adult ursotaurs, although these are rare, as the bearcows' thick skin and long claws make them formidable adversaries that are difficult to kill, and it is more difficult to hide such large carcasses from baurusuchian kleptoparasites. Large prey must be brought down through endurance, wearing the animal down through repeated chase and injury until it becomes too weak to injure them and then going for the throat. Their armoured head is covered in thick, bony scales for defence against prey biting and scratching at their face and is often too tough for the stings and fangs of venomous animals to penetrate. However, this creates an unusual chimeric animal with a bulky, armoured head combined with its lithe, slender body. Their robust skull allows them to bludgeon small prey to death by headbutting them at high speeds, their thickened cranium and neck muscles allowing them to withstand the impact without injury.

The wurguru breeds year-round, nesting in forested regions out of sight; they construct large mounds of soil and branches to incubate their eggs. The female usually watches the mound during the day, while the male watches during the night. The young have a heavily mottled patterning to act as camouflage, and after hatching, they abandon the nest to follow the parents as they forage. While nesting and rearing young, the parents usually take smaller prey than they normally do, as one adult will almost always remain behind to guard the eggs and chicks; occasionally, an adult offspring from a previous clutch may assist its parents in rearing the young, allowing the parents to hunt normally. Care of young extends for a year; by eight or nine months, the parents begin taking the young hunting with them, and this is the unfortunate of times for their austrornithopod prey, as the normal two abelisaurs may suddenly be four or five in number. Although the young are technically left to fend for themselves after a year, their parents will tolerate them in their territory for a further few months, even leaving out carrion for them to find. Young wurguru are more opportunistic and live in the forest rather than out in the open, but as they gain experience, will begin hunting the elasmarians as their parents do in earnest; they reach sexual maturity at four years, although usually do not breed for another two or three years.