Evolution by Stephen Baxter

scrub, their unused weapons in their hands, their hunt thwarted. But it wasn’t all bad news. When the allos were done feeding there might be meat to be scavenged from the fallen diplo.
    Then came that last whiplash. The huge diplo’s tail landed clean across Stego’s back, laying his skin open to the bone. He screamed and fell, tumbling out into the open, his mouth agape. The slit pupils in his eyes pulsed as he gazed up at Listener.
    And one of the allosaurs, not far away, turned with glassy interest. Listener stood stock still, shocked.
    With a single bound the allo reached Stego. Stego screamed and scrabbled at the mud. The allo poked him curiously, almost gently, with her muzzle.
    Then, with astonishing speed, the allo’s head shot forward and delivered a single clean bite, all but severing Stego’s neck. She grabbed him by the shoulder, lifting him high. His head dangled by a few threads of skin, but his body twitched still. She carried him to the edge of the forest, away from the herd, where she began to feed. The process was efficient. The allo had joints within her jaw and skull, so that like a python she could open her mouth wide and position her teeth, the better to consume her prey.
    Listener found herself staring stupidly at an allosaur track, a three-toed crater firmly planted in the trampled mud. A hunter without her mate is like a herd without its matriarch : The ornith proverb sounded in her head, over and over.
    The big matriarch diplo swung her head around to stare directly at Listener. Listener understood. The orniths’ antics had given the allos their chance to attack. So, with her whiplash, the matriarch had exposed Stego. She had given him to the allos. It had been revenge.
    The matriarch turned away, lowing, as if contented.
    Something hardened, a dark core, in Listener’s mind.
    She knew she would spend the rest of her life with this herd. And she knew that the matriarch was its most important individual; providing protection to the rest with her sheer bulk, leading them with her wisdom acquired over long years. Without her the herd would be much less well coordinated, much more under threat. In a way, this matriarch was the most important individual creature in Listener’s life.
    In that moment, she swore vengeance of her own.

    • • •
    Each night the orniths retreated to their ancestral forest, where once they had hunted mammals, insects, and the nests of diplodocus. They scattered in little pockets, and surrounded the area with heavily armed sentries. That evening, the mourning was extensive. This ornith nation was only a few hundred strong, and could ill afford to lose a strong, intelligent young male like Stego.
    Even as the cold of night drew in, Listener found it hard to rest.
    She gazed up at a sky across which auroras flapped, steep three-dimensional sculptures of light, green and purple. In this age Earth’s magnetic field was three times the strength it would be in the human era, and, as it trapped the wind streaming from the sun, the shining auroras would sometimes blanket the planet from pole to pole. But the lights in the sky meant nothing to Listener, and brought no comfort or distraction.
    She sought refuge in memories of happier, simpler times when she and Stego, emulating their distant ancestors, had hunted for diplo eggs. The trick was to seek out a patch of forest floor, not too far from the edge, that looked apparently lifeless, strewn with leaves and dirt. If you put your sensitive ear to the ground you could hear, if you were lucky, the telltale scratching of diplo chicks in their eggs. Listener had always preferred to wait, to guard “her” nest from others, until the diplo chicks began to break out of their eggs and stick their tiny heads out of the scattered dirt.
    For an inventive mind like Listener’s, there was no end to the games you could play.
    You could try to guess which chick would come up next. You could see how quickly you could kill a new emergent,