Cardassian Evolutionary Theory: a Mesothermic Semi-Aquatic Wetland Species
So, here are my headcanons regarding Cardassian biology and their evolution.
While Cardassia might be mostly desert now, that probably wasn't always the case. I think proto-Cardassians spent a lot of time in fresh water, like lakes and rivers, as ambush predators. They would have been mostly terrestrial (their hands and feet are better suited to terrestrial living) but they've got a number of adaptations that would be beneficial to spending large chunks of time in the water.
Cardassian neck ridges and eye ridges would give them an advantage. Their neck ridges could stabilize the head, allowing them to swim fast and allow them to explode powerfully from the water. The eye ridges shield eyes suited to dim light (or murky water) from the sun as well as keep rain and mud out of their eyes.
Considering the hole in Dukat's skull we see in Picard looks similar to what we see in a parietal eye in reptiles, the spoon could be a parietal eye that would be beneficial to Cardassians that are underwater in dark, murky depths. It being sensitive to light means they'll always be able to tell where the surface is.
The lack of body hair in favor of scales is also beneficial in a species that spends a lot of time in water. If they spent much of their time lying in wait in the water with just their heads peeking out, the hair on top of the heads would help protect the head from the heat and sun.
Cardassians prefer their environment to be roughly 32 C (90 F). A mesotherm (great white sharks, leatherback sea turtles, echidnas, likely some dinosaurs) sits between ectotherms and endotherms. They do not have to dedicate large amounts of energy to maintain an internal temperature like warm-blooded animals do, but neither do they need to conserve energy because they derive their temperature solely from the environment like cold-blooded animals. They can get their heat from their environment, allowing them to take the energy they would have spent on regulating their body temperature into moving or ambushing prey, but they are also able to keeps moving in cold temperatures. They are gas/electric hybrid car, capable of both.
They likely need to give birth on land, so depending on your reproductive headcanon that means either a head-first orientation during live birth or an egg with an harder shell. I personally think they have live births and that they do not possess a true cloaca but rather a cloaca-like structure called a reproductive vestibule with a separate urethra and anus. This configuration means there is a lot less hazard of bacterial infection since when everything is combined like in a true cloaca waste can easily cross and cause contamination.
