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When does the extensive vertebrate fossil record begin?

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A translucent X-ray fish (Pristella maxillaris) swims in an aquarium, showing its visible backbone and internal structures through its body. — Chordate X-ray fish (*Pristella maxillaris*), is an example of a chordate with a visible backbone.

The extensive vertebrate fossil record begins approximately 400 million years ago.1

While earlier vertebrate fossils exist—such as fragments of jawless fishes from the Ordovician Period dating back about 450 million years—these remains are often fragmentary or limited to specific, specialized forms. The Silurian Period, which preceded the 400-million-year mark, contains evidence of both jawed and jawless vertebrates, but the broader fossil record for these groups remains incomplete due to the erosion of early freshwater deposits and the lack of preservation for soft-bodied ancestors.23