Dinosaur Evolution: A Journey from Tetrapod to Bird

Okay before I get in to direct Dinosaur evolution I must first weave the tale of how these guys came along, okay lets start with the Fish that crawled out of the water otherwise it would be a drag explaining the evolution of fish.

Okay along time ago back around the Devonian time, a group of Lobe-fin fishes gradually evolved lungs and slowly crawled out of the water on to land to possibly escape from hungry predators.

These fish became the first Tetrapods or Amphibians, millions of years later and millions of generations later prototreptile amniotes then evolve from Reptiliomorph amphibians during the Carboniferous peroid, officially around the Pennsylvanian Epoch but it is possible they could of appear in the Mississippian epoch.

Animotes later diverged in to two clades: Synapsids (Mammal-like Reptiles and later Mammals) and Sauropsida (Classic Reptiles).

Now since we are going in to later dinosaur evolution we can mark out the Synapsids from the discussion since that would be a topic for a later time.

Sauropsida is then divided into Parareptilia (at the side of reptiles) and Eureptilia (True Reptiles)

Eureptilia later gives rise to several lizard-like early reptiles and then the clade Romeriida evolves which will give rise to Diapsid reptiles (Modern Reptiles and their extinct relatives)

Diapsid Reptiles would evolve in to several primitive forms before two notable clades would eventually win out so to speak. Lepidosauromorphs (Snakes, lizards, Tuatara, and their extinct relatives like mosasurus) and Archosauromorphs (Ruing Reptiles). Pantestudines in my opinion belongs with Archosauromorph due to DNA and Fossil record backing. This ends the Permian peroid.

Archosauromorphs, now where getting closer. After several basal clades we finally get Archosauriformes. Most Archosauriformes roughly resembled crocodiles if more lightly armored to barely armored but lets flash forward to the clade of Archosauria; seeing as the other Archosauriformes are no long relevant to this topic. Which happens mostly in the Early Triassic.

Archosauria can be divided in to two super clades: Avemetatarsalia/Ornithodira and Pseudosuchia (Crocodilans and their extinct relatives like the Rauisuchians)

Avemetatarsalia also known as Ornithodira or Pan-Aves is the clade that holds the Pterosauromorphs and the Dinosauromorphs.

Dinosauromorphs clade contains all Dinosaur clades including modern birds and their extinct relatives. Dinosauromorphs that were closer related to true Dinosaurs were Dinosauriformes while Non-dinosaurian dinosaurmorphs belong to Lagerpetids.

Dinosauriformes housed several dinosaur-like forerunners but the most developed group besides the dinosaurs themselves were the Silesaurids. These creatures appeared roughly in the Middle Triassic.

And now we finally arrived to the clade Dinosauria which itself is broken up in to two major clades: Ornithischia (Bird-hip Dinosaurs) and Saurischia (Lizard-hip Dinosaurs) both arrived in the Late Triassic.

Ornithischia can be splitted up in to three primary clades: Ornithopods (basically beaked bipeds and duckbills), Thyreophora (Plated and armored dinosaurs with attack tails), and Marginocephalia (Head Armored Dinosaurs)

Saurischia is simply divided in to two clades: Sauropodomorphs (Long Necked herbivores) and Theropods (Bipedal Carnivores, later some beaked long necked bipedal herbivores, and birds)

The earliest members of Saurischia were fairly theropod-like but we do know the earliest members of each clade. Eoraptor long suspected to be a primitive theropod or even a basal Saurischia is now thought to be a primitive sauropodomorph along with the similar Guaibasaurids while Eodromaeus snags the spot of the Eoraptors previous position as an early theropod along side the slightly more advanced Herrerasaurids.