Placodonts were some of the most resilient groups of Triassic marine reptiles, consistently surviving minor extinction events with minimal diversity loss until the mass extinction.
In one timeline, this did not happen. Instead, their diversity would continue, in some ways replacing the turtles from our timeline.
The Mesozoic reccord of placodonts is relatively stable. Their diversity remains mostly consistent in spite of several other extinction events affecting other marine reptiles (such as thalattosaurs, phytosaurs and pleurosaurs), there being two major spikes of diversity: in the early Jurassic and late Cretaceous.
Soon after the TJ extinction event, Placochelyidae increases significantly in species diversity. They dominate marine habitats from the near shore to the open sea, a few species even occuring in brackish and freshwater habitats like in the Phu Kradung Formation of Thayland. Mesozoic taxa have developed flippers and exploited open sea habitats long before our turtles, the closest analogues in our timeline being some thalassochelydids in the Late Jurassic. Some taxa became cephalopod and jellyfish specialists and lost their teeth, developing long toothless jaws to snatch their prey. Most however remained in the coastoal regions eating hard shelled invertebrates, albeit most genera having a cosmopolitan distribution.
Placochelyids undergo the Jurassic/Cretaceous boundary virtually unaffected unlike other marine reptile groups, but begin to lose steam in the Cenomanian turnovers. They remain relatively important components of marine faunal assemblages, but they are upstaged by other clade of placodonts, the cyamodontids.
Cyamodontids spent most of the Mesozoic up to that point as relatively unspecialised coastoal and freshwater durophagists, like weird reptilian skates. Everything changed in the post-Cenomanian Cretaceous, where a decline in placochelyid diversity allowed them to explode ecologically and become the dominant placodont linage. Most similarly remained coastoal/freshwater taxa, but several took to life in the open seas, developing flippers and either reducing their carapace like leatherback sea turtles (as well as a few placochelyids) or by inversely expanding it into broad lift-generators in order to remain suspended effortlessly, relying on small but powerful flippers for propulsion and steering. Like in placochelyids some became toothless, but instead developed suction feeding like our turtle Ocepechelon.
While cyamodontids became the dominant placodonts, placochelyids still retain a relatively high degree of diversity, and both taxa passed through the KT extinction event with few casualties, aside from the more pelagic taxa. They continued to dominant early Cenozoic oceans until the Eocene, when global cooling events caused their extinction.
This freed the way for the third and last lineage of placodonts, the henodontids. Through most of the Mesozoic these reptiles remained in freshwater and brackish water biomes, but with the onset of the Cenozoic they began to explore marine niches, just before the extinction of their relatives. Surviving the climatic turmoils much better (barring a massive species loss in the Pliocene/Pleistocene cooling), these remaining placodonts are truly the turtles of this world, having lost their teeth in favour of a strong beak. They occur in freshwater and marine habitats, using their broad carapaces to generate lift while the flippers do the work. They feed on all manner of items from crustaceans to algae to fallen fruit to vertebrates they suction feed. They haven’t managed to return to land, the niche of tortoises instead occupied by notosuchians and sphenodonts, but at over 300 species in terms of diversity do they really need to?