The more modern ones would have longer legs and necks. But the fossil record doesn't show that. The fossil record doesn't show a gradual change from any creature into Tyrannosaurus Rex, or any other dinosaur. He reasoned that a small change in a gene might cause a large change in a mutant offspring. Of course it is true that sometimes five-legged frogs are found living in polluted streams.
Mutant creatures can be radically different from their parents.
Maybe a reptile did once lay an egg, and a bird hatched out of it. But it isn't very likely. It has often been observed that mutations are harmful and rarely, if ever, beneficial. If Goldschmidt's Hopeful Monster theory is true, then there had to be thousands of mutants that each developed into a different species. It is a hard theory to take seriously.