Dinosaurs is the name of a diverse group of reptiles belonging to the Mesozoic Age of Earth and who continue to exist as the dominant life form native to the Land of the Lost. Like their Earth-version counterparts, they are large reptilian-like animals that propagate by laying eggs and forming large single species communities, although a few live in isolated solitary lifestyles. There are many different species of dinosaurs indigenous to the land, including the infamous Tyrannosaurus rex and the Brontosaurus.
Dinosaurs are classified by two separate orders; that of the Saurischia ("lizard-hipped"), such as Tyrannosaurs and Brontosaurs, and the Ornithischia ("bird-hipped"), such as the Triceratops. Several of the species encountered by the Marshalls include a Coelophysis named Spot (Holly also used this name to apply to the others of the species), a Tyrannosaurus rex named Grumpy, an Allosaurus named Big Alice and her son, Junior, a Triceratops named Spike, a female Brontosaurus named Emily and her baby, Dopey.
In addition to the dinosaurs of the land, several non-dinosaur reptiles also existed in the Land of the Losts as the Marshalls also encountered Pteranodons, Dimetrodons and the Sleestak, who might have been descended from a common ancestor of the dinosaurs, but this has yet to be confirmed.
Species in this version included some Parasaurolophus (one baby named Tasha who is currently with the Porters), Tyrannosaurus Rex (one named Scarface), a Triceratops (named Princess), a Stegosaurus (named Spike - not be confused with the Triceratops from the 1970s series), an Apatosaurus and a Pteranodon.
Dinosaurs such as Grumpy the T-Rex, some Pteranodons, a Compsognathus, a female Allosaurus who looked similar to Big Alice, and some form of raptor were encountered. The word Hadrosaur is used to describe an item Will had acquired, but those dinosaurs are not seen. Co-existing with the dinosaurs are amazing creatures like Giant Crabs, Bug's and the Sleestak.
- It is unrevealed if the Sleestak are evolved from intelligent humanoid dinosaurs.