In a future where the American hippopotamus survived the Miocene and continued to adapt into the modern ecosystem, they would fill the niches similar to true hippos of the Old World.

Scenario Title: The Return of Amynodonts - American Hippos In this intriguing twist on geological history, the North American family of Rhinoceros, Amynodontidae, managed to weather the hostile forces of evolution and survive into the modern day. Decimated in the late Miocene, these large, semi-aquatic mammals, affectionately known as the American Hippopotamus, algorithmically embraced their altered fate and adapted to the changing climates and the challenging demands of their evolving ecosystem. Through the Pleistocene epoch, the American Hippopotamus evolved extensive bodily adaptations to deal with the cooling climates. Their small ears and skinny, tapering tails enlarged to improve thermoregulation - trapping heat in the cool era and expelling heat in the warmer periods. Their physique became bulkier, and the skin thickened, turning into a protective layer preventing heat loss, effectively equipped to thrive in the rigorous winters that would come and go. Simultaneously, as the North American continent bore witness to the waxing and waning of the ice ages, inland freshwater bodies appeared and disappeared. The American Hippopotamus evolved to become semi-aquatic, developing powerful lower bodies that allowed them to swiftly swim through large bodies of water in search of food and escape predators. Their nostrils moved up to the top of their snouts, enabling them to submerge into the water for extended periods, with just the top part of their heads visible, much akin to their African counterparts. In this alternate timeline, the vegetation of North America changed drastically, with coniferous trees giving way to expansive grassy plains and savannas. The American hippopotamus underwent a concurrent transformation, their diet shifting from ferns and conifers to grass, turning them into ruthless grazers. Their teeth grew longer and sharper, aiding them in tearing through tough grass and warding off predators. As these giant mammals adapted and became integral parts of the ecosystem, they occupied niches similar to the true hippos in the Old World. They became key contributors in shaping their local environments, acting as ecosystem engineers by reshaping the landscape through their feeding and movement habits and stimulating biodiversity in their habitats, evident from the myriad of species that thrived in their vicinity. In this carefully constructed alternate timeline, the primordial landscapes of North America harbor an awe-inspiring spectacle, a glimpse of what could have been, as the American Hippopotami, true survivors from a bygone era, grace the waterways and plains, a testament to nature's endless marvels.