Holocene On Land

Land during the Holocene. A, a Tyrannosaurus about to hunt, B, a small herd of Triceratops, as, C, a lone elephant ignores the attacking giant carnivorous reptile. D, A Brontosaurus watches the fight while, E, a lion hunts, F, a horse who was wandering peacefully. G, a dylanus looking for small prey items.

During the Holocene, mammals weren't the only rulers of Earth, as Earth was also ruled by reptiles such as dinosaurs. Paleontologists have currently discovered more than 2,500 species of dinosaurs, 937 species of pterosaurs (aka pterodactyls), and 510 species of marine reptiles (excluding sea turtles which still live today). Just like mammal reconstructions, paleontologists also had reconstructed dinosaurs as reptilians related to crocodilians and/or lizards. Theropods like T-Rex may have been upright hunters that dragged their tails and, like all dinosaurs, could have likely been cold-blooded. Theropods in general could have all jumped to pounce onto larger prey such as elephants and sauropods, due to their highly specialized ankles. Sauropods likely had bendy necks and, like all dinosaurs, dragged their tails like modern lizards, and due to their large size, they spend most of their times in the water, only to come onto land to find new food sources and/or lay eggs, elephants could have done the same thing as sauropods.
Holocene Seas

Life during the Holocene, A, an ichthyosaur swimming near, B, a pod of dolphins hunting for small fish. C, a giraffe hunting, D, an extinct species of fish that left no descendants alive today as, E, a ratfish, the same species alive today attempts to steal food from a giraffe.

Cerotopsids like Triceratops could have used their horns for defense and may have used their frills to fight back and/or defense against carnivorous theropods and mammal predators (including lions, dylanusids, etc). Stegosaurs had small brains and therefore were likely dumb, so they probably had second brains in their hip bones. Ankylosaurs were most likely a group of Stegosaurs which gained tail clubs in place of spikes and armored backs instead of plates. Hadrosaurs and other large Ornithopods were upright tail-draggers that most likely swimming reptiles that escaped danger by going into the water. Small Ornithopods like Hypsilophodonts and Heterodontosaurs most likely were tree-dwellers that climbed up to have escaped from ground. Pterosaurs were likely dinosaurs or birds that developed wing membranes and ruled the skies alongside more advanced birds such as eagles, vultures, etc. Marine reptiles (including dolphins, ichthyosaurs, plesiosaurs, pliosaurs, mosasaurs, etc) were likely dinosaur descendants that ruled the seas. No one knows why all dinosaurs and most mammals became extinct in the Holocene, but it may be due to intense volcanic activity spelled their doom.
Community content is available under CC-BY-SA unless otherwise noted.