John Harold Ostrom (February 18, 1928 – July 16, 2005) was an American paleontologist who revolutionized the modern understanding of dinosaurs.

[2][3] Beginning with the discovery of Deinonychus in 1964, Ostrom challenged the widespread belief that dinosaurs were slow-moving lizards (or "saurians").

As a pre-medical undergraduate student at Union College, he originally aimed to prepare for medical school in order to become a physician like his father.

However, an elective course in geology and George Gaylord Simpson's book The Meaning of Evolution inspired him to change his career plans.

Ostrom also worked as a research assistant with Colbert, who was the Curator of Vertebrate Paleontology at the American Museum of Natural History (AMNH), Ostrom earned his doctorate in geology (vertebrate paleontology) in 1960 with a thesis on North American hadrosaurs that was based on the skull collection housed at the AMNH.

As a new professor at Yale, Ostrom was named the assistant curator for vertebrate paleontology at the Peabody Museum of Natural History.

The highway department allowed Ostrom and his team to examine 400 sandstone blocks to find dinosaur fossils.

By examining the olfactory apparatuses of modern reptiles and drawing comparisons via comparative morphology, Ostrom concluded that hadrosaurs likely developed an acute sense of smell by a lengthening of the nasal passages into long chambers that wound around the skull and were protected by bony crests.

[31][32] This hypothesis led Ostrom to further conclude that ecology of hadrosaurs was more likely to be that of dry ground such as conifer forests, rather than swampy, aquatic environments, thought to be the case at the time.

This idea was further justified by a 1922 paper that Ostrom rediscovered in 1964, which described the stomach contents of a mummified specimen of the hadrosaur Anatosaurus, which included conifer needles, twigs, fruit and seeds, plant matter that would be consumed in a terrestrial environment.

Ostrom's reading of fossilized Hadrosaurus trackways led him to the conclusion that these duckbilled dinosaurs were gregarious and traveled in herds.

In subsequent seasons, his team unearthed four specimens of a small bipedal carnivorous theropod, and parts of a larger plant-eating dinosaur.

[22][24] Deinonychus was an active predator that clearly killed its prey by leaping and slashing or stabbing with its "terrible claw", the meaning of the animal's genus name.

[36]: 35 [22] John Ostrom's work on the functional morphology of dinosaurs found that the claws and tendon scars in the tail would indicate a running position.

[39] Ostrom's work on Deinonychus is credited with triggering the "dinosaur renaissance",[2] a term coined in a 1975 issue of Scientific American by Bakker to describe increased interest in paleontology.

Ostrom supported this view by noting the correlation of erect posture and locomotion with high metabolism and body temperature in modern mammals and birds, stating that this relationship cannot be accidental.

[1][2] As a result of subsequent research and comparison with more recently found specimens from the Tiaojushan Formation of China, it was suggested in 2017 that the Haarlem Archaeopteryx actually represents a separate taxon.