Did the earliest vertebrates evolve in freshwater or marine habitats?

Whether the earliest vertebrates evolved in freshwater or marine habitats remains a subject of significant debate among paleontologists.12

The earliest known fossils, which belong to the superclass Agnatha (jawless fishes), are typically found in shallow marine or marginal marine deposits, such as estuaries and lagoons. Because many primitive chordates are marine, some researchers doubt a freshwater origin.12

Conversely, other scientists hypothesize that these early vertebrates may have originated in upland freshwater environments. This theory suggests that features like dermal armor and body mobility evolved as adaptations to streamflow and as protection against predatory invertebrates found in those waters. Under this view, the fossils found in marine beds are considered to be the remains of freshwater animals that were washed downstream into coastal deposits. Because early freshwater fossil beds were likely eroded away, definitive evidence for this origin may never be recovered.1