Is there any evidence for secondarily poikilothermic vertebrate?
What do you mean by cold-blooded?

There's an issue with what you mean when saying "cold blooded". The correct words you may want to use are homeotherm, poikilothermic, ectotherm, and endotherm. In short&hellip;

Source of heat 


endo = inside
exo = outside


Variance in warmth


Poikilo = varies
homeo = does not vary


Any combination of these two axes exist. For example: If the temperature in the environment never varies you can be homeotherm without needing to be endotherm. A fun example also are the large dinosaurs that are thought to be homeotherms because their metabolism produce some heat and they are so large that they remain warm thanks to this heat source. However, they were probably not able to regulate actively their temperature. Therefore, I would tend to qualify dinosaurs as homeo-exo-therm but I wouldn't be surprised if someone prefers to call large dinosaur home-endo-therm individuals. Hope that makes sense to you.

Note that the terminology might be a bit more complicated in reality as the diversity of mechanisms of body temperature regulation is important.

What did you mean in your question when talking about "cold blooded" species?

Transition from endothermy to ectotherm

Thanks to @tchrist. naked mole rat is ectotherm while its ancestors where endotherm.