Ascent of the Antlear

It has now been 27 million years since the evolution of the first circuagodonts (or wheeljaws), a new clade of cursorial tribbetheres notable for their highly specialized wedge-shaped jaws which operate like tree-trimmers in front and as grinders in back, allowing them to bite and chew in a single forward and back motion of the jaw. They evolved from the smaller, seed-eating molodonts, which appeared a few tens of million years before, in response to the proliferation of razorgrasses - vigorous plants that protected themselves with sharp blade-like leaves. Initially adapted to crop these harsh, silica-rich plants and masticate them thoroughly before swallowing, this novel jaw form gave them a strong competitive advantage over the serezelles - lanky quadruped birds - that made up the first wave of grazing animals at the beginning of the Pangeacene. While the circuagodont jaw initially evolved to feed on razorgrass, however, their jaws were highly adaptable to different food sources; in just a few million years the clade was already widespread across the planet, having dispersed across most of its fairly homogeneous continents in the many millennia before they began to break apart, and branching off into new diets and new habitats. Carnivores appeared early on, switching their attention from grass to carrion and then to live prey and utilizing their beaks as bone saws to dismember carcasses, but herbivory remained a successful niche in which to persist and diversify. Browsing was the inevitable next step for the plant-eaters to take; their jaws were practically purpose-built to clip hard plant tissues and clipping woody stems was no difficulty. As the wheeljaws followed the retreat of the serezelles from the plains into the woodlands, feeding on branches and bark, they pressured these birds ever further into more limited niches.

For a few million years more the serezelles found a niche with which they found little competition: browsers of the high canopy in Serina's forests. With the long and flexible neck that birds are so known for, they accessed food sources that were simply out of reach of their tribbethere competitors; the circuagodonts were restricted by a quirk of their ancestry from evolving truly long necks to reach food more than a few meters off the ground by a stiff, inflexible neck supported by only four vertebrate. The ancestors of circuagodonts were short, stocky animals, and this arrangement was of no concern for them. It was only with the wheeljaws, which needed to graze the ground while fleeing from their own enemies on long legs, that this anatomical constraint gained relevance as their necks grew longer, and in turn increasingly stiff and inflexible, to facilitate this lifestyle that no tribbet had done before. The rather elongate necks of the first circuagodont species, necessary to reach the ground to graze, were indeed already near the limit that their anatomy could support. Constructed of four very long bones, they were still just barely flexible enough to allow the creature to rotate its head around sufficiently to turn and scan their surroundings for danger. Any longer, and the neck would be too immobile to let the animal turn its head or to lower it to the ground to drink.

And so for the past 28 million years the circuagodonts have been unable to infiltrate tall browsing niches, and the remainder of the serezelles have survived in one of their last strongholds therein as browsers. Now, however, even this niche may no longer be out of the wheeljaw's reach for long, as a new and rapidly diversifying clade has appeared. We will refer to these newcomers as "antlears," for reasons that will soon be clarified, and they threaten to at last access this final food source for themselves through some of the most novel adaptations of any tribbethere so far evolved.

Basal antlears are largely conservative in their anatomy, and they retain the long, thin legs and wedge-like jaw profile of the earliest circuagodonts. They are vegetarians, feeding on woody plants in forest settings, as their kin have done for millions of years. The antlears, though, have not all settled for feeding on the low-growing bushes and shrubs that their ancestors were forced to. By adapting an assuming part of their bodies into a pair of manipulatory appendages, their necks no longer prevent them from reaching high into the trees for find food. So which body part they have so specialized? Their ears.

The ears of the antlears are indeed their defining trait, having adapted from purely hearing structures into two-to-four jointed, antler-like mobile limbs that are now, in some species, able to reach up and hook branches, pulling them in reach for the beak to cut them down. Superficially this innovation appears extreme and perhaps even absurd, for tribbetheres much resemble earth mammals in profile, and our own ears are little more than fleshy structures of weak cartilage and no skeletal support. Yet we must recall that the ears of tribbetheres, including the circuagodonts, evolved from the bony gill plates of their ancestors and so remain not only mobile but supported by an underlying bony matrix. A joint at the base of the ear allowed it to swivel, and in the antlears secondary joints have formed higher up the structure from formerly loosely connected bones that now allow the animals to rotate and close their ears inward and together to grasp. Grip is improved by prominent bony spikes along the ridge of the ear which function as claws to secure a stronger hold.

The earliest antlears however did not evolve their specialized ears to browse, but rather to fight. Some ten million years ago, the males of one species (above, top left) evolved small, hard osteoderms - plates of bone - along the edges of their ears and head-butted one another in competition for mates, using these reinforced structures to align their skulls together when battling head-to-head. Over time the complexity of these structures rose, with small osteoderms giving rise to large bony spikes and the ear itself becoming both larger and more reinforced as well as flexible, to rotate and reach forward to grab hold of the opponent's own rack (top, right) in combat. Though the highly specialized ears were initially a sexually-dimorphic trait, absent in females - and remain so in some extant species - in others it was only a matter of a few simple chromosomal mutations that transferred the trait to both sexes, which likely first occurred in a population highly subject to predation, where isolated antlered females would be substantially better able to defend their young from predators than mothers without these specialized weapons. Once so established in both sexes of a species some forms subsequently adapted additional uses of the new structures, most notably to facilitate feeding.

As it stands today the antlears have only just begun to reach for the sky and venture toward the high-browsing lifestyle. Their ears, though highly specialized, still make clumsy hands, and there is very little flexion at their tips where joints are only semi-developed and there is very limited range of motion. Yet in these species, still rapidly diversifying and building upon their prior adaptations to come up with new and innovative solutions to the problem of survival, there is likely to be great evolutionary success in the near future, and for some time to come.

And as the cirguadonts at last access the high-browsing niche, the serezelles, a group of such rapid but short-lived success, will soon be but another fossil memory as time marches on.