At one time many secular paleontologists, for the most part believed that dinosaur eggs were positive evidence that dinosaurs were reptilian .That is not the case now, since most secular scientists of today believe that birds are dinosaurs and therefore there should be a close relationship between bird eggs and dinosaur eggs.
Personally, I do not think they were birds, mammals or reptiles but rather a distinct group by themselves called dinosaurs. Here is some reasons why. The reason dinosaurs are not birds is because bird fossils were found alongside dinosaur fossils. Many kinds of modern bird fossils have been found in the same sedimentary rock layers. Dinosaurs were not mammals either because they did not nurture their young like mammals do. No dinosaur had mammary glands to give milk to their young, because of their sheer size of their bodies, that were far off the ground. But there is one type of animal that could of been a dinosaur but probably were not were the group of animals called reptiles. Dinosaurs are unique from reptiles. even though dinosaurs had scaly skin like reptiles, they were quite different from them because ot their stance. Reptiles have their legs coming out their side of their body and hinging down. Dinosaurs on the other hand, were quite different because of their hip design. Dinosaurs fall into two groups the lizard-hipped (saurischian) and the Bird-hipped (ornithischian). No other animal of any kind except dinosaurs had these two hip design.
Getting back to the legs of modern reptiles such as turtles, lizards, and alligators, they stick out from their body, then angle down to the ground. Their feet are seldom situated directly below their bodies, but instead are placed out on the side. In contrast, dinosaurs had legs orientated nearly straight down to the ground from the hips. The two-legged (bipedal) dinosaurs would have walked somewhat upright, more like humans. Take for example, the Tyrannosaurus rex, was a bipedal dinosaur. His legs were directly beneath him.
Something also unique about dinosaurs were that they had a furcula (wishbone) like a bird but only different. Dinosaurs had a relatively small boomerang shaped bones across the chest. Flying birds on the other hand have a V-shaped furcula similarly located across the chest. Obviously, assistance for flying was not the function of the furcula in therapod dinosaurs.
Dinosaurs are a unique group of created kinds of animals. No other group of created kinds, such as birds, amphibians, reptiles, and mammals can be dinosaurs. Maybe “dragons” would be a better name for dinosaurs. No reptiles living today come close to being dinosaurs.