What happened during evolution that led to the appearance of jaws and skulls in chordates?

The appearance of jaws and skulls in chordates is attributed to evolutionary advancements in early vertebrates, specifically the development of cartilaginous structures that provided support and protection. These features emerged from modifications of the pharyngeal arches, which initially served a respiratory function. Over time, natural selection favored individuals with stronger, more effective jaws for improved feeding strategies, leading to the diversification of vertebrate species. This evolutionary transition marked a significant step in the complexity and adaptability of chordates.