Could my Dog-Turtles exist?

Question

Dog-turtles are a species of Sauropsida from the Archelosauria clade, and from the Testudines class. They are a Koopa pastiche (Koopas are from Nintendo's Mario) (Koopas are themselves a Kappa pastiche from Japanese mythology). They are my goblins. Some basic characteristic include:

1. Are sequential omnivores (they start life as carnivores, and they are herbivores as adults) (when they are juveniles, they eat insects, earthworms, fish, amphibians, and carrion from other amniotes) (when they are adults, they eat fruits, leaves, seeds, grasses, barks, and flowers) (their trademark favourite food as juveniles is carrion from other amniotes, and as adults, they prefer plants from the Rosales order like apples, pears, strawberries, roses, plums, peaches, apricots, cherries, almonds, figs, and even cannabis);
2. They are bipedal with a Tyrannosaurus rex-like gait;
3. They have dog-level intelligence;
4. Adult females are on average 1.25 meters tall and weigh 55 kilograms, and are 20 % bigger than adult males;
5. They have human-level strength (and I do not mean humans who suffer from muscular dystrophy, I mean large human athletes like sumo wrestlers);
6. They mostly live in freshwater, but they often get out of it for eating (like hippopotami, except they are better swimmers) (hippopotami only walk in the water);
7. They age 2.4 times faster than humans, and they can start reproducing at the age of 5 years old (comparable to a 12 years old human), their digestive system (of herbivorous nature) is fully adult at the age of 7 years old and a half (comparable to a 18 years old human), and their brain development is complete at the age of 10 years old and two fifths (comparable to a 25 years old human), and they are adolescents all their life in a sense that they never stop growing;
8. They are solitary, but they almost never kill each other (like orangutans, and basking sharks);
9. Compared to the average tetrapod species, they have a high risk of cancer, but a low risk of noninfectious heart disease;
10. They are mesothermic (lukewarm blooded) (like tunas, great white sharks, and naked mole-rats);
11. They have an excellent sense of smell, touch, and hearing, but a relatively poor sense of eyesight (they see more colours than humans, but they are myopic) and taste (they cannot taste umami when their digestive system becomes fully adult);
12. The place on Earth that can be called their cradle is Colombia (especially Pacific region), but they can be found everywhere in continental South America including France (French Guiana), Brazil, Uruguay, and even Tierra del Fuego (some humans from around the world even have them as pets).

Given these characteristics, what species could they have evolved from and what evolutionary pressures would lead to them?

Answer 1

You aren't positing any novel biology, even by real-world Earth standards. You aren't positing any novel or exotic biophysics (it's not 210 meters tall, etc.).

This is a plausible organism, and it might have evolved on Earth had things been different. It doesn't even necessarily require distinct evolutionary pressures... a small population of proto-koopas just fell into an unexploited niche 40 million years ago and went with it.