If *Xiaosaurus* is the sister taxon to Thyreophora, what does this imply about the evolution of armor?

Answer

Armor is a derived feature, evolving after the split from *Xiaosaurus*-like ancestors

The phylogenetic uncertainty surrounding *Xiaosaurus* has a major implication for understanding Ornithischian evolutionary history, particularly concerning the Thyreophora (armored dinosaurs). If *Xiaosaurus* is identified as the sister taxon—meaning it branched off immediately before the Thyreophora lineage—and considering *Xiaosaurus* itself was lightly built and unarmored, this suggests that the defining feature of Thyreophora, heavy armor plating, was a derived characteristic. This implies that the ancestral form of the Thyreophora lineage was lightweight and agile, developing its defensive specialization only after diverging from the *Xiaosaurus*-like stem.

If *Xiaosaurus* is the sister taxon to Thyreophora, what does this imply about the evolution of armor?

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