Morgo the Mighty was clearly inspired by both Tarzan and At the Earth’s Core. It features a feral white man who is the master of his jungle environment and that jungle environment is located in caverns beneath the earth. There are monstrous creatures that he must battle to survive.
Contrary to online descriptions of the novel (and illustrations that accompanied it when it was serialized in The Popular Magazine) there are no dinosaurs or other prehistoric creatures in Surrilana. The beasts in the caverns are evolved (and often gigantic) rodents, bats, insects and birds. It’s a more realistic scenario than a land somehow populated by dinosaurs. I’m not saying it’s a better scenario. I love dinosaurs.
Birds are the descendants of the dinosaurs. So, perhaps, the chicken fiends of Surrilana can be considered dinosaur stand-ins.