The Evolution Journey of Dinosaurs: Their Precursors and Lineage

The question of how dinosaurs evolved from their prehistoric ancestors is intriguing and continues to engage paleontologists and enthusiasts alike. The evolution of dinosaurs is a story deeply rooted in the archosaur lineage, a group that includes not only dinosaurs but also pterosaurs and crocodilians.

The Precursors to Dinosaurs

Archosauria, the ‘ruling reptiles’, includes two main branches: Avemetatarsalia and Pseudosuchia. Avemetatarsalia, including early dinosaurs, split off from Pseudosuchia (crocodiles and their relatives) around 247 million years ago. Within Avemetatarsalia, the clade Dinosauriformes started to diverge approximately 245 million years ago, with dinosauriformes being the direct precursors leading to the emergence of the first true dinosauria.

Early Aphanosaur Avemetatarsalians

Some of the earliest and most intriguing precursors to dinosaurs date back to the early Triassic period. For example, Teleocrater, one of the earliest aphanosaur avemetatarsalians with fossils dating back to around 232 million years ago, hints at the evolutionary pathway.

The Anisian to Carnian Transition

Dinosaurs evolved within a single lineage of archosaurs between 243 million years ago (Anisian age) to 233 million years ago (Carnian age), marking the latter part of the Middle Triassic period. The well-supported clade Dinosauria is diagnosed by several key features, including the loss of postfrontal bones in the skull and an elongate deltopectoral crest on the humerus.

Defining Dinosauria

The definition of dinosaurs includes the last common ancestor of birds, Saurischia, and ornithischians, along with all of their descendants. This definition excludes Pterosauria and several archosaurian genera that are closely related but do not fit comfortably within the dinosaur family tree. Notable archosaur genera that narrowly missed being classified as dinosaurs include Schleromochlus, Lagerpeton, and Marasuchus.

Early Protodinosaurs

The earliest known dinosaurs were bipedal predators and typically measured around 1 to 2 meters in length. The first confirmed dinosaur fossils include Saurischians such as Nyasasaurus (243 Ma), Saturnalia (225-232 Ma), Herrerasaurus (220-230 Ma), Staurikosaurus (225-230 Ma), Eoraptor (220-230 Ma), and possibly Alwalkeria (220-230 Ma). Among the earliest Ornithischians, Pisanosaurus (220-230 Ma) holds significant importance. Despite Lesothosaurus coming from 195-206 Ma, skeletal features suggest it branched from the main Ornithischia line at the same time or slightly before Pisanosaurus.

Conclusion

The evolution of dinosaurs is a fascinating journey through time, tracing back to the early Triassic period and the lineage of archosaurs. From small bipedal predators to the diverse array of species we explore today, the emergence of the Archosauria clade and the subsequent split into Avemetatarsalia and Pseudosuchia paved the way for the rise of the dinosaurs. Understanding this lineage is crucial to unraveling the complex history of life on Earth.