The intermittent survival of a Madagascan plant-eating notosuchian crocodylomorph during the KPG extinction event might have led to the evolution of birds and mammals.

Scenario: If Simosuchus or its close relative had survived the KPG extinction event 66 million years ago, their evolution would highly depend on the environmental conditions, availability of food, competition, and various other factors. After the KPG extinction, our version of Earth sees a simosuchus lineage that takes advantage of new niches left open by the extinction of large dinosaurs. Next few million years comprise warm and dry periods ideal for reptilian life. The reduced competition allows them to expand into different ecosystems. They evolve based on the principles of natural selection and adaptations specific to their environments. In areas with dense vegetation, these crocodylomorphs evolve to occupy a similar niche as present-day herbivores like capybaras, evolving denser bone structure and size adaptation for browsing on mid-level foliage. Others would further develop their plant-eating tendencies, their jaws evolving to masticate tougher plant matter as grasslands spread. Some grow armor-like outer coverings for added protection against predators, not unlike armadillos, evolving alongside their mammalian counterparts. In the coastal regions, different evolutionary pressures exist. Here, some develop a semi-aquatic lifestyle, evolving elongated bodies and limbs for better swimming capabilities - almost akin to otters but retaining their reptilian characteristics. In the avian context, some birds would become their primary predators or competitors, influencing the evolution of these crocodylomorphs to become quicker, more alert, and in some cases, even develop rudimentary camouflage or stealth tactics for protection and survival. There could be a unique development where some of these evolved Simosuchus species share a symbiotic relationship with certain bird species, acting as their "cleaners" by eating parasites off their skin. This alternate Earth would have complex ecosystems, where different species of reptile-herbivores, from small grazing critters to large, slow browsers, roam the forests and grasslands. Meanwhile, mammals diversify into their niches, small insectivores to large predators and grazers. All in all, if Simosuchus had survived the KPG extinction and continued evolving, it would have created a unique and diverse world filled with both living fossil crocodylomorphs and advanced mammalian species.