Why the lizard-hipped dinosaurs are actually the ancestors of modern birds

Nature · Published 29 September 2026 Swipe more like this →

In short: The term Saurischia translates to lizard-hipped, yet this group includes the ancestors of every bird alive today. While historically defined by a specific pelvic structure, the classification of these ancient giants remains a subject of intense scientific debate, shifting from rigid orders to complex, evolving webs of biological relationships.

In 1888, Harry Seeley proposed dividing dinosaurs into two primary groups based on their pelvic anatomy. He identified Saurischia, or lizard-hipped dinosaurs, by their forward-pointing pubis, a trait shared with modern reptiles. In contrast, he labeled the Ornithischia as bird-hipped, despite birds actually descending from the lizard-hipped lineage. This distinction was intended to categorize the four dinosaur orders identified by Othniel Charles Marsh in 1878: Sauropoda, Theropoda, Ornithopoda, and Stegosauria.

Seeley argued that these two groups were distinct lineages that evolved independently from primitive archosaurs, a view that dominated paleontology for decades. It was not until the 1960s that researchers began to reconsider the idea that saurischians and ornithischians shared a closer common ancestry. Today, most experts accept that dinosaurs form a natural, monophyletic group, though the internal structure of the Saurischia clade is frequently challenged by new cladistic analyses.

Modern research has introduced significant friction to Seeley’s original framework. A 2017 study by a team of three researchers suggested that the traditional definition of Saurischia might be paraphyletic, proposing that theropods should be moved to a new group called Ornithoscelida. While some recent studies continue to support the traditional grouping, others have proposed alternative classifications, such as placing certain early dinosaurs like herrerasaurids outside the group entirely. These shifting models highlight how our understanding of dinosaur evolution remains a dynamic, data-driven process.

The legacy of this classification persists through the clade Eusaurischia, which includes sauropodomorphs and theropods. Although the Cretaceous–Paleogene extinction event wiped out all non-avian saurischians, the lineage survives through birds, which are classified as maniraptoran theropods. The bird-like hip structure seen in some saurischians, such as avialans and therizinosaurians, is now widely viewed as an example of convergent evolution, likely driven by similar dietary adaptations rather than a direct ancestral link to the ornithischians.

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