In the Shadow of the Sciurutherines

In Recurocene Australia, the southwestern forests are truly one of the continent's greatest evolutionary laboratories. Cut off from the rest of Australia by a vast expanse of desert, it is virtually an island, and with glaciation meaning that would-be islands like Tasmania and New Guinea are near-constantly joined to the mainland, it is one of the most isolated ecosystems on the continent. The lack of any routes of dispersal from the groups dominating the forests of the north and east, most notably ungulates, has allowed for an entirely different suite of characters to take shape in the southwest, resulting in the evolution some of the most remarkable lineages on the continent. The most famous and successful of these have been the sciurutherines, a bizarre group of terrestrial phalangerids descended from the Common Brushtail Possum that, in the form of the brutes, have since spread across much of arid and monsoonal Australia. The isolation of the southwest was also responsible for providing Australia with the largest and most power mammalian predator the continent has ever known, the Gaiyacyon, which evolved in an evolutionary arms race with the brutes and has since come to dominate almost the entirety of Australia. While these are undoubtedly the most well known and dramatic of the southwest's many examples of "parallel evolution", the region is also home to several smaller radiations that remain endemic to its forests but are nonetheless just as intriguing.

The absence of ungulates, the group that dominates large herbivore guilds in forest regions elsewhere in Australia, and indeed elsewhere in the world, has provided new opportunities for diversification and adaptation among a whole range of different herbivores, not just the sciurutherines. One of the most recent new groups to develop in these southwestern forests, and quickly becoming among the most common, are the Ribbonroos (Notagale). Named after their lowslung build and long flexible tail compared to more typical macropods, they are in fact a giant forest-dwelling form of rock-wallaby, descending specifically from the Black-flanked Rock-wallaby which clung on in the rangelands of southwestern Australia through the age of man. Ribbonroos evolved roughly two million years ago when some rock-wallabies began venturing down into the lowland forests to browse, taking advantage of their greater agility compared to other macropods that they had already developed while adapting to a life amongst boulders and cliff-faces. As time went on they began to grow larger, lost their dependence on rocky environments for shelter, and with their prebuild adaptations for agility they quickly began to push out other macropod species from the forest. Now there are a total of 5 species of ribbonroo spread throughout the forested regions of the southwest, ranging in size from a middling wallaby to a small kangaroo.

Ribbonroos have developed several unique adaptations that have allowed them to become more adept at moving through dense forests than their plains-dwelling relatives, and one of the most obvious of these is reflected by their name. Their tails, like those of the ancestral rock-wallaby, are long and wispy, and have since developed a greater degree of prehensility that gifts them with unparalleled control over their tail movements. This allows them to make sharp twists and turns even while in the air, reducing the chance of colliding with a tree or getting caught in dense vegetation during a chase. However, the most drastic modification has been to their limbs. In contrast to their tails, their hindlegs and feet have shortened, making it easier for them to slip under dense bushes and underbrush. Not only that, but their hindlimbs have much greater capacity for independent movement, almost similar to that of the related tree kangaroos. While this makes their hopping less efficient over long distances, removing the main advantage it gives macropods over ungulates on the plains, it also means they have much greater control as they bound through the forest. These adaptations have allowed the ribbonroos to become the first large macropodines to be specialised for life in dense forests, and with this in mind it's no surprise that they are quickly becoming the most successful herbivores in this part of the continent.

Before the rise of the ribbonroos it was the sciurutherine gerbores that represented the most common herbivores in the forest of south-western Australia, and the rise of the forest-dwelling macropods has put significant pressure not just on them but also on another, very similar looking herbivore. The Cerbara (Elaphomys longipes) is one of the southwest's strangest endemic species, and a relict of a time before sciurutherines or ribbonroos. As can be clearly deduced from its appearance, it is a rodent, but one of immense size - 60 centimetres tall at the shoulder, and taller still when rearing upright to browse. While it has convergently evolved to look quite similar to the maras of South America, it is essentially a giant hopping-mouse, deriving from the Mitchell's Hopping-mouse which occurred in the south-west's drier areas during the Holocene. The largest hopping-mouse species to survive through the age of man, the Mitchell's Hopping-mouse already relied more heavily upon roots and green matter than other hopping-mice, which were mostly seed eaters. The lack of other herbivores besides macropods encouraged them to move further west from the semi-arid woodlands they typically inhabited and into the southwest's wetter forests. Here, they quickly experienced a massive expansion in size in the relatively competition-free environment, and radiated rapidly. By 1.5 million years after man's disappearance, recognisable cerbaras were already thriving in the southwestern forests, with their diversity reaching its peak around 3 million years ago with 6 species. However, the evolution of the gerbores, which are very similar in form, was a significant blow to the diversity of the cerbaras. Compared to the rather derived gerbores, cerbaras were relatively unspecialised and essentially just represented a larger, lankier form of their ancestor, and this put them at a competitive disadvantage. Today only one species remains, the modern Cerbara, which partitions through the fact it is significantly larger than the two forest-dwelling gerbores and because is active at night rather than during the day. Unfortunately, competition has only gotten more intense with the diversification of the ribbonroos, and it is likely that the giant rodents don't have much time left.

Despite its size, many of the Cerbara's habits are still similar to the ancestral hopping-mouse, and this includes burrowing. Living in small communities, or mischiefs, of up to six individuals, they dig out interconnected burrow systems with multiple entrances and sleeping chambers that may extend up to four metres below the surface. Usually no more than two cerbaras sleep in the same chamber at once. After wombats, they are the largest burrowing animals in all of Australia. Staying tucked away underground during the day, they emerge at night and largely forage independently, but will join up with other cerbaras to groom one another and socialise. Cerbaras are mostly browsers and can rear up to reach vegetation over a metre off the ground, but will also take a variety of other food - fruit, grasses, tubers, fungus, even insects and small vertebrates. As they tend to be too small and fast for Gaiyacyon, their main predators are Western Dingowolves and Pygmy Woodcats, and they will shoot back to their burrows at the earliest sign of them.

One group of herbivores that have taken great advantage of the lack of competition in the southwest's forests are the emus (Dromaius). Even today, despite the presence of new and more specialised groups like ribbonroos and sciurutherines, the generalised diet of emus has allowed them to thrive alongside mammalian herbivores and remain some of the forest's most prominent and abundant large animals. In most parts of Australia, any particular region or habitat is inhabited by only a single species of emu, with very little overlap in the distribution of species. However, the southwestern forests support a whopping 5 different emu species, all living side by side and differing slightly in regards to size and the way they use the environment. Most of these remain in the ancestral genus Dromaius, as do all other emus, but one species endemic to these forests truly sets itself apart. With large females standing up to 3.2 metres tall and weighing in at 350 kg, the Wertjirung (Ambulornis colossus) is the biggest bird Australia has seen since the extinction of Dromornis in the Pliocene and yet another case of gigantism among the southwest's endemic herbivores.

The Wertjirung parallels Australia's extinct giant birds, the dromornithids, in more than just size. Like them, it is a specialised selective browser - lacking teeth, they cannot process the tough sclerophyllus vegetation as efficiently as the mammalian browsers (most notably the Brachiothere) with which they coexist, and so instead they focus on particular parts of plants that are both more easily digestible and more nutrient rich. This includes fresh shoots, buds, flowers and fruit, although they will also take fungi, insects and small lizards if the opportunity arises. Their large eyes, originally evolved to watch for predators on the open plains, are now utilised to carefully scan the foliage for suitable food and pick out the most succulent vegetation. Indeed, these days they generally have very little to fear when it comes to predation, their huge size making them largely immune to attacks from dingoes or woodcats. They do however fall perfectly into the size range for prey of Gaiyacyon, and their first reaction to encountering one is usually to crash away through the forest as fast as they can. Unfortunately, due to their size they are rather slow, and they usually end up having to face the predator head on. However, they are well armed - not only do their sturdy legs deliver powerful kicks, but they have also evolved a dagger-like claw on the inner toe very similar to that of their relatives, the cassowaries, which they developed specifically in response to predation pressure from Gaiyacyon. A single kick from a Wertjirung could easily inflict a devastating wound, and thus only the most desperate Gaiyacyons will continue harassing an adult once it has assumed defensive positioning.

Unlike other emus, which tend to fluctuate between being solitary, living in pairs or forming mobs depending on the time of year and availability of resources, the Wertjirung forms more fixed social units consisting of a large female, the heavier of the two sexes, and anywhere between 2-7 males. These mobs remain together year round and move nomadically through the forest depending on the availability of food, occasionally joining with other groups to form larger temporary herds. However, during the summer breeding season the head female becomes extremely aggressive towards emus outside her mob and the group adopts a more sedentary lifestyle. The female mates with each male and lays six to twelve dark green eggs, a little larger than those of an ostrich, spread as evenly as possible between one to three nests, depending on the number of males in her mob. The males take turns incubating the eggs, while the female tends to have very little involvement aside from fending off potential nest raiders. The precocial young are ready to set off immediately, and with so many parents protecting them they generally have a lower mortality rate than most other herbivores. Nonetheless, around half of all Wertjirung chicks succumb to predation in their first year, and still more are killed by Gaiyacyons during their early months of independence. Male emus will spend the majority of their lives under the rule of a female, but older individuals that loose their fertility tend to leave the mob and lead a largely solitary existence. However, even these elderly individuals still retain parts of their parental instincts, and as such it is not uncommon so see a so-called "old man emu" with fostered chicks at his feet.