Final answer:
The dinosaur with very small arms relative to its body size is the Tyrannosaurus Rex (T. rex), a late-evolving and iconic dinosaur known for both its size and relatively tiny arms. The T. rex was more of a brisk walker, not the swift runner depicted in movies like Jurassic Park.
Step-by-step explanation:
The dinosaur described as having very small arms compared to the rest of its body is most likely the Tyrannosaurus Rex (T. rex). This iconic dinosaur is known for its massive head and tiny arms, and it was one of the last dinosaurs to evolve, appearing on Earth approximately 70.6 million years ago. An interesting fact is that humans are closer in time to T. rex than T. rex is to the first dinosaurs that evolved about 230 million years ago.
We also know from recent research that despite its fearsome reputation, the T. rex could not run at great speeds but was more likely a brisk walker, challenging previous beliefs that it could chase at speeds over 60 mph as portrayed in popular culture, like the Jurassic Park series.
In the context of dinosaur classification, the T. rex falls within the Saurischia clade, characterized by a three-pronged pelvis structure. Plus, dinosaurs, in general, were a diverse group of over 1,000 non-avian species with an upright posture not found in lizards, and they even share a common ancestor with birds, as indicated by cladistics studies.