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Why is the early fossil record of chordates incomplete?

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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 early fossil record of chordates is incomplete primarily because early representatives were small and soft-bodied.12

In the study of paleontology, the preservation of an organism typically requires two conditions: rapid burial to prevent decomposition and scavenging, and the possession of hard, resistant skeletal parts. Because early chordates lacked these mineralized structures, they were rarely preserved in the sedimentary rock record.312

While some exceptional deposits—known as Lagerstätten, such as the Burgess Shale and the Chengjiang Biota—have provided rare glimpses of soft-bodied organisms, these occurrences are infrequent. Consequently, most of our understanding of early chordate evolution relies on these few, highly specific sites rather than a continuous or abundant fossil trail.451