Therizinosaurs possessed large abdomens for processing plant food, and small heads with beaks and leaf-shaped teeth. First thought to be prosauropods, these enigmatic dinosaurs were later proven to be highly specialized, herbivorous theropods. The first confirmed non-carnivorous fossil theropods found were the therizinosaurs, originally known as 'segnosaurs'. For example, a Compsognathus longipes fossil was found with a lizard in its stomach, and a Velociraptor mongoliensis specimen was found locked in combat with a Protoceratops andrewsi (a type of ornithischian dinosaur). Fossilized specimens of early theropods known to scientists in the 19th and early 20th centuries all possessed sharp teeth with serrated edges for cutting flesh, and some specimens even showed direct evidence of predatory behavior. All early finds of theropod fossils showed them to be primarily carnivorous. However, discoveries in the late 20th and early 21st centuries showed that a variety of diets existed even in more basal lineages. Strict carnivory has always been considered the ancestral diet for theropods as a group, and a wider variety of diets was historically considered a characteristic exclusive to the avian theropods (birds). Theropods exhibit a wide range of diets, from insectivores to herbivores and carnivores. Specimen of the troodontid Jinfengopteryx elegans, with seeds preserved in the stomach region

In the Jurassic, birds evolved from small specialized coelurosaurian theropods, and are today represented by about 10,500 living species. Theropods first appeared during the Carnian age of the late Triassic period 231.4 million years ago ( Ma) and included all the large terrestrial carnivores from the Early Jurassic until at least the close of the Cretaceous, about 66 Ma. They were ancestrally carnivorous, although a number of theropod groups evolved to become herbivores, omnivores, piscivores, and insectivores. Theropods are generally classed as a group of saurischian dinosaurs. Theropoda ( / θ ɪəˈr ɒ p ə d ə/ from Ancient Greek θηρίον ( thēríon) 'wild beast', and πούς, ποδός ( poús, podós) 'foot'), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three-toes and claws on each limb. Third row: Allosaurus sp., Ceratosaurus sp., and Dromaius novaehollandiae. Second row: Alaskan saurornitholestine, and Serikornis sungei. First row: Spinosaurus aegyptiacus, Carcharodontosaurus saharicus, and Saltriovenator zanellai.

Late bird dinosaur