Gravebear

The gravebear was a rare hybrid of the dire bumblebear and the savage gravedigger, two species only distantly related, which is only known from a window of around 3,000 years at the boundary of the ocean age and the hothouse era, and which became most common only in the last few centuries of that time span. Highly intermediate in most characteristics between its two parent species, it was a formidable apex predator second only to its dire parent as the largest land animal of its era. Weighing up to 1,200 lbs, it was capable of hunting and killing the nimicorn thorngrazer, the biggest herbivore in its environment, and would likely have fed primarily on this species (though scavenging may have been easier than hunting much of the time, especially when this prey animal became extremely numerous.) Long legged, strong-jawed, fast, powerful, and capable of long distance travel, it exhibited hybrid vigor, and was in many ways fitter than both of its parent species, and better adapted than either to follow herds of thorngrazers and prey upon them. Though some aspects of its hybrid anatomy did not fit together precisely (its beak, with especially sharp fang cusps, often failed to close together completely, and it inherited primitive featherless hind legs despite neither parent having them), it was a successful animal on an individual level. But the gravebear had one major fault as a lineage, which prevented it from surviving long-term: it was sterile. Gravebears could not reproduce, not with each other nor either parent species, for an odd chromosome number prevented successful reproduction in both sexes. This meant they were an evolutionary dead-end, and no amount of success on the scale of the individual hybrids could change that.

Gravebears only appeared at the end of the dire bumblebear's own history, when their populations became too small, fragmented, and low in genetic diversity to reproduce at a level that matched their death rate. Though this species - the top land predator of the ocean age - did survive into the hothouse, their numbers were depleted from the wide scale destruction of the land ecosystem by fires and volcanic eruptions, and as few as five individuals may have survived this cataclysm at their lowest. This would have allowed an initial population resurgence, perhaps to several hundred or even more than a thousand animals, only to ultimately result in a weakened and unhealthy population as harmful recessive genes spread widely through the related animals. It would have taken only one outbreak of disease, to which none had immunity, to spell the end of their lineage. The fewer bumblebears existed, the harder it became for them to locate mates, and the more likely lonely and widely wandering males were to settle for a less preferred partner. Hybrid pairings were occasional, but remained very uncommon for over two thousand years after the ocean age ended, for they occurred only when a bumblebear could not find a more suitable mate, and where bumblebear populations were still large the pure species would likely kill the hybrids if they came across them.

But the gravebear suddenly becomes more common than dire bumblebears by several times in the last few centuries of that species' existence, which must indicate that by then so few bumblebears still existed that the vast majority of breeding which occurred was with the next closest species the bears could find, the savage gravedigger. And this may have then hastened the bumblebear's demise, for once hybrid individuals outnumbered bumblebears they would likely have gained an upper hand in competing against them for food and territory, for they were more effective hunters. They were faster runners to chase prey, while lighter and less densely feathered, and so less encumbered by the increasing temperatures in the newly warming world. But this would, of course, be a short-lived win. For when the last bumblebear died, so too went one of the parents of the more adaptable hybrid. Though many gravebears outlived dire bumblebears by a lifetime, they were unable to perpetuate their own numbers. The extinction of one parent meant they, too, were destined to slowly burn out and become extinguished as the last of the crosses aged and died. In less than a century from the last bumblebear's death, all its descendants would too have perished. Only the smaller and more generalistic gravediggers, who had survived in better numbers and were not subject to inbreeding depression, would go on to see the new era that was coming. And they walked ahead into the brave new world ahead on their own.