Planet X is terraformed by prehistoric time-traveling aliens, and the dominant form of aquatic and terrestrial fauna/megafauna is theropods.

Surely, this is a captivating concept to explore! Here it goes: Planet Xenorra, located near the fringe of the Pegasus galaxy, was a desolate world about 50 million years ago. This barren globe was visited by an intelligent race we can best describe as 'Time-traveling Aliens' from an alternative future timeline. With advanced technology and extensive knowledge of life forms in the universe, they held a grand vision of transforming Xenorra into a blooming, abundant planet. The aliens began their task by setting up huge terraforming apparatus that manipulated the planet's atmosphere, introducing an optimal mix of oxygen, nitrogen, and other gases conducive to terrestrial and aquatic life. They also engineered an intricate weather system to promote diverse climates across the planet. Using the biological blueprints of prehistoric Earth, they commenced with seeding Xenorra's oceans and lands. They first introduced plankton-like microorganisms to help develop a food chain, and then initiated the proliferation of plants and fungi. Life-rich shallow seas were purposefully created, nurturing an effervescent community of invertebrates, fish, and amphibians. The dry landmasses, meanwhile, began growing lush with vegetation, becoming the homes of a myriad of small reptiles. The climax of this orchestrated evolution was the introduction of various non-avian theropods, embracing species from coelophysoids to unenlagiids. Thousands of small and large theropods dotted the landscapes of Xenorra, eventually diversifying into countless species, from fleet-footed grazers to apex predators. In this utopia devoid of mammalian and avian competitors, theropods excelled, colonizing land and water ecosystems. In the azure oceans of Xenorra, finned spinosaurids became predominant, with modified limbs for paddling, and long snouts for snapping up aquatic prey. On land, this world witnessed the curious evolution of the graviscaels - large, lumbering therizinosaurians mowing through forests with their sickle-like claws, evolving to fill the ecological niche of massive mammalian herbivores on Earth. Flying theropods, also known as pinnatavians, became vital to this planetscape. Branches of coelurids and dromaeosaurids evolved wings and took to the skies, their feathers shimmering in a palette of colors against Xenorra's sunsets. Over millions of years, these theropods morphed into forms not known to Earth's history - herbivorous megalosaurids, chameleonic alvarezsauroids, even nocturnal tyrannosauroids with exaggerated hearing and vision evolved, employing their unique adaptations to survive. Today, Xenorra is a thriving planet full of lifeforms that one could only imagine, a testament to the aliens' magnificent experiment. It stands as an astonishing reflection of what could have been if theropods were the primary inhabitants of our earth. Here, every rustle in the undergrowth, every shadow in the deep, every silhouette in the sky, is shaped by the lingering spirit of Earth's Mesozoic past.