Predator Burdles of the Ultimocene

265 million years PE, the world of Serina is changing. Ice caps are spreading over the land, pushing terrestrial ecosystems to the margins. Countless life forms on land have already gone and continue to go extinct, unable to adapt to such rapid changes. Yet in the sea, things are changing in a very different way.

Glaciation and the spread of the polar ice caps has resulted in a global sea level drop of significant proportion, on top of Serina's already shallow global ocean. A cold current circulating around the equator, propelled by the meeting of opposing polar winds, keeps these shallow waters across the world churned up. By constantly disturbing the sediment and releasing its stored nutrients into the water column, the result is a very productive, well-oxygenated and nutrient rich environment highly conducive to aquatic life. It is so that Serina's oceans going into the middle Ultimocene are still much less negatively affected by the cooling climate than the land; though biodiversity is lower than several million years ago as a result of the extinction of tropical reefs, the total biomass of fish and aquatic life in the sea is now higher than it has been since the Cryocene - the last time that it was in a similarly productive state, also caused by cool global temperatures.

Cold and fertile waters support huge quantities of phytoplankton, in turn fed upon by swarms of marine crustaceans and baitfish which themselves provide a source o food for a wide diversity of aquatic birds. Most of these organisms, though highly derived, still have close relatives still eking out a living on the chilling land, including sea mittens (related to the neckbeards and their descendants), and dolfinches (distant relatives of the gravedigger). But one group of surviving aquatic birds no longer has any terrestrial relatives at all. The last mucks - the distant and extraordinarily unique descendants of the serestrider aardgeese of so long ago - are entirely aquatic, their last land-living kin having expired with the freezing of the tropical rainforests in the past ten million years. They are the burdles, and as time goes on they have only become every more aberrant. More than 160 million years ago their tall, tree-browsing ancestors lost their feathered plumage for a scaled pelage, their slowed metabolism and the tropical global climate of that era making insulation redundant. Even as the mucks reduced in size and left the ground to scale the branches they never redeveloped this lost feature, retaining their cold-blooded nature and becoming almost reptilian - a first among birds. Mucks survived the devastating Thermocene-Pangeacene extinction as a single species of semi-aquatic, lizard-like omnivore and then radiated back onto both land and sea in the Pangeacene. Yet their time on land was limited, and secondarily terrestrial species never regained former diversity. The future of this lineage was in the water, where today the aquatic mucks in the form of burdles are still thriving even in a rapidly changing world.

Burdles began as omnivores, sampling a variety of diets and consuming nearly anything organic, a generalist habit which let them eke through the end of the Thermocene. Specialization occurred in the Pangeacene, producing large herbivores that have since died out. Their most impressive forms today are very different: active, fierce, reptilian-looking carnivores which truly push the limits for what can still be considered a bird. Some of the most derived of any to evolve on Serina, the predator burdles will be the last of the mucks. But far from a clade dying out with a whimper, this bookend of evolution contains some of the most fearsome predators ever to evolve.

above: the tortornca, (Tortornca horribilis, a simplified name translating extremely loosely as "horrible, turtle-like bird-orca"), here pictured pursuing a dolfinch, is the largest living burdle. Growing to a length of up to 30 feet, it is a ferocious, solitary, and highly active predator that feeds primarily on other marine birds and molodonts. With a highly streamlined body, it chases its prey down with great agility and over very long distance, relying on endurance rather than a sudden burst of speed, with alternating pulses of its fore and hind flippers, catching exhausted bumblets, sea mittens and other hapless marine creatures in its powerful crushing jaws. No longer tied to the land to breed, it weighs up to six tons.

The burdles have always been a very strange group of birds by any standards, and this is a result of undergoing so many distinct adaptations later repurposed for new uses. Still without a single feather, a protective layer of smooth keratin scales covered their entire bodies. A strong beak of serrated pseudoteeth serves to catch slippery prey, and a throat full of additional spines prevents it from slipping back out of the esophagus as it goes down. Four wide flippers beat alternately to propel them through the water in the manner of the extinct Earth pliosaurs; a large wing claw on the flipper - which has not been a functional wing for more than 200 million years by now - was derived as a defensive claw in the ancient serestriders, served until recently in evolutionary time to help pull the creature ashore to lay its eggs in the sand of beaches. But in today's species, all members of one recently diverged clade, this claw has become vestigial, because the modern predator burdle joins the select ranks of just a few other birds - bumblets and metamorphs - in evolving live birth. Earlier burdles evolved to hold a pouch full of eggs until the entire clutch was ready to lay, which required the shells be less ossified to neatly fit together internally without cracking. Evolving due to a similar pressure to that which occurred in the bumblets, this has now resulted in eggs with pliable shells which hatch while still retained in the mother's body. The chicks then swim away and may never see their mother again. The skeletons of the small but entirely independent chicks are flexible and cartilage-based, like those of sharks, until they can accumulate their own calcium to ossify them in their diet as they grow, as a result of the lack of calcium in the shells of their eggs.

To survive in a cooling sea, the predator burdles have become warm-blooded a second time - an evolutionary first. Unlike their early canary ancestors however, they have done so in a unique way. Rather than reverting to a high resting metabolism like ancestral birds, these species instead rely on virtually constant motion of their flapping flippers to create and maintain muscle-derived heat. A large insulating layer of body fat helps to prevent the loss of body heat allows these species to maintain internal body temperatures up to 40 degrees Fahrenheit warmer than the water in which they live. Even in short bouts of sleep they engage in involuntary muscle quivers to maintain their internal warmth. They are often endurance hunters, relying on stamina instead of speed to catch up to and then outmaneuver tired out prey, which they grab and shake to death in the jaws and tear into swallow-able gobbets.

Evolving at sea, not all burdles today live there. One genus several million years ago moved inland, into the cold rivers and wetlands that still flow through southern Serinarcta. These species deal with particularly cold conditions including water that freezes over entirely during winter, blocking access to air, and so are the hardiest of all. They put their ability to drop from an endothermic to an ecothermic metabolism by ceasing muscle movement to their benefit by hibernating. Muckodiles respond to winter cold by digging into the mud and lying in torpor under the ice, shutting their metabolisms down to a level just barely above death, with just a few heart beats an hour, in water just a few degrees above freezing. Their oxygen needs lower to the point they can respire through the vascular tissues of their mouths, and they will spend months every year in this suspended state until spring thaws the ponds and rivers thaw. When the water warms a few degrees and the ice recedes, the muckodile wakes up and begins violently shivering for a period of several hours - kickstarting its metabolism, warming its pectoral muscles and allowing it to reach the surface, take a breath, and further warm itself in the sun near shore.

Through the warm season, these freshwater mucks retain a very different behavior than their oceanic relatives. They are relatively lethargic and slow moving, spending much time at rest under the water. Summer temperatures warm freshwater to temperatures higher than the sea, allowing them to stay active primarily through environmental means, without generating additional body heat by staying in perpetual motion. Only in very cold spells do they need to shiver their pectoral muscles to generate a boost of warmth, and the ability to live life at a more leisurely pace than other burdles has allowed the muckodiles to exploit a novel niche as crocodile-like ambush predators.

above: a riverine muckodile (Stagnumessor miliadon, the "million-toothed swamp reaper", so named for its many false teeth derived from throat spines) ambushes a large antlear species as it comes down to the water of a forest pond to drink. Muckodiles have been able to exploit a crocodile-like niche in the ecosystem even in a very cold climate due to their highly specialized adaptations and the flexibility of their metabolism.

Muckodiles are live-bearing like their relatives, producing eggs in the early summer which only hatch almost an entire year later, the following spring, as a result of their slow development in the cool temperatures while the mother hibernates. As the mother emerges from hibernation, she begins to give birth as soon as she has warmed herself and her young ones emerge and go off on their own, feeding on insects and fish for several seasons before becoming primarily predators of land animals that come to the water to drink. Though they favor large land-lubbing prey, however, even adult muckodiles will opportunistically catch fish and some species may even lie beneath the water with their jaws agape and wiggle their small pink tongues as a lure to draw in prey, exactly in the manner of a snapping turtle, to which the clade as a whole is very similar in many respects save for their much larger size; the largest muckodiles may grow to ten feet long and weigh over a thousand pounds.

All burdles are long-lived, but muckodiles probably live longest due to spending half of their lives in suspended animation. If they survive their first few years, lifespans of more than a century are expected, with two centuries potentially attainable.