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We assume the following for this scenario:

  • There is a kind of surviving non-avian dinosaur.
  • They live primarily in Congo Basin.
  • They evolved from a smaller, omnivorous kind.
  • They fill multiple niches in a small area deep in the Congo.
  • One of their kind evolved to be a carnivore, thus adopting a raptor-like body plan.
  • One evolved to live on an elephant-like diet, thus adopting a long neck and gigantosaurus-like body plan.
  • They mostly inhabit a small area in the Congo Basin, but there is one of them, known as Mokele-mbembe, whose habitat was more widespread, thus his existence was known by (some) humans.
  • Their height is no taller than 2 meters at most.

How plausible is this ?

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Dinosaurs survived in caves, sheltered from the great impact

Your dinosaur species would have to be true survivors indeed. Not only they survived the nuclear winter following the famous asteroid impact disaster on Yucatán peninsula, they will have had to survive the period before, and after.

Before the asteroid impact

You'll need a healthy species to start with. Choose your dinosaurs with care. In any case, you'd have to take into account many species of dinosaurs already did not survive during the 10 million years preceding the asteroid..

We investigate the influence of ecological and physical factors, and find that the decline of dinosaurs was likely driven by global climate cooling and herbivorous diversity drop. The latter is likely due to hadrosaurs outcompeting other herbivores. We also estimate that extinction risk is related to species age during the decline, suggesting a lack of evolutionary novelty or adaptation to changing environments. These results support an environmentally driven decline of non-avian dinosaurs well before the asteroid impact.

https://www.nature.com/articles/s41467-021-23754-0

Lack of evolutionary novelty or adaptation will impair a species, when it has to survive anyway. Your species must have a versatile genome, allowing for adaptations.

Exceptional circumstances

The nuclear winter following the impact affected the climate world wide. All land bound dinosaurs died..

Yours is the exception.

Caves

Suppose these allowed for maintaining a comfortable temperature and survival of the nuclear winter following the asteroid. Water (warm?) allowed the crocodile to survive, caves provided shelter to your dinosaurs.

Time since

The asteroid is not the only hurdle. An important question that would have to be answered, suppose dinosaurs did survive, could they have survived 66 million years of changes ?

Did the food e.g. persist in the shelter, a very long lived, isolated population would be a result.. or did your dinosaurs go outside and compete with the mammals ? For instance, your max. 2 meter height would not be an advantage against elephants and giraffes. In the 66 million years to the present, your dinosaurs will have developed as well.

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  • $\begingroup$ "Time since" geographic isolation might might help perhaps, like Australia but smaller, remember all those old movies with a bunch of them on a high plateau? $\endgroup$
    – Pelinore
    Commented Mar 19, 2022 at 15:46
  • $\begingroup$ 66 million years is a lot of time, even geologically. Outside the caves, they would have to survive the mammals. But if one of them is raptor, it may have gotten specialized to hunt primates. As humans originate (about) from the same region, it could have caused Australopithecus to go extinct. Prepocalypse ? $\endgroup$
    – Goodies
    Commented Mar 19, 2022 at 15:58
  • $\begingroup$ That's the point of "geographic isolation", they don't have to compete, it means that somehow by chance a breeding population finds itself somewhere there just happen to be no mammals that other animals from elsewhere can't reach, this is why Australia's fauna is the way it is and so different, you'd need to start this isolation further back of course for dinosaurs 🤔 but yes, 66 million years is an implausibly long time 🤗 then again, implausible isn't impossible 😁 $\endgroup$
    – Pelinore
    Commented Mar 19, 2022 at 16:48
  • $\begingroup$ @Pelinore Ostrich may be the surviving land dinosaur in Australia? Although descending from dinosaurs elsewhere, they may have survived as ostriches in Australia. $\endgroup$
    – Goodies
    Commented Mar 19, 2022 at 17:10

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