The ancestors of the terracetes were small dolphins living in the waters around an island landmass roughly equivalent in size and isolation to New Zealand. These cetaceans developed the behavior of driving their aquatic prey up onto the shore, beaching themselves in the process of catching it and then wriggling back into the water – similar to how some modern dolphins are known to intentionally beach themselves when hunting.
They gradually became better and better at maneuvering themselves on the shore, going from ungainly flopping to a more sealion-like bounding using strengthened flexible flippers and their powerful tails. Instead of simply beaching fish they also began outright preying on terrestrial animals near the water’s edge, in a sort of ambush predator ecological niche.
Eventually their tails developed into a tripod pseudo-leg, curling beneath their bodies and being used to propel them forwards to chase terrestrial prey, moving with in an almost frog-like leaping motion. While still semi-aquatic at this point, spending most of their time lurking in the shallows, from here the transitional terracetes gradually started to become more and more terrestrial.
Their melons reduced due to cetacean echolocation working poorly in air, but dorsal fins were retained in some lineages as visual display structures. Terracetes never regained separate digits on their limbs, instead developing tough hoof-like keratinized pads on their flippers and tails, and some species began to sprout sensory whiskers, reactivated from vestigial hair follicles on their snouts.
A lineage known as the BOARPHINS are some of the most successful terracetes, becoming aggressive chunky pig-like animals with large tusks in their lower jaws – a sexually dimorphic feature used mainly for fighting each other. Some members of this group eventually re-aquired body fur and became rather hairy, and also began to develop elaborate bony crests on their heads.
The largest of the horned boarphins are the ENTELODOLPHICORNS, long-limbed fast-moving terracetes somewhat convergent with ancient entelodonts. These opportunistic omnivores preference eating meat, either fresh or carrion, but also supplement their diet with plant matter such as leaves, fruits, tubers, and fungi.
Meanwhile, the fully herbivorous BRONTOLPHINS converge on a chunky long-necked body plan similar to sauropods and giant tortoises.