How could freshwater streamflow have influenced the evolution of dermal armour in early vertebrates?

While the origin of early vertebrates in freshwater environments remains a subject of debate among paleontologists, it is hypothesized that the development of dermal armour may have been an evolutionary response to the physical and biological pressures of freshwater streamflow.1

In such environments, the mobility of the body and the protection afforded by heavy dermal armour could have provided a significant survival advantage. Specifically, this armour may have served two primary purposes:1

• Physical Protection: The armour provided a defense against the mechanical stresses of navigating stream currents.1
• Biological Defense: It offered protection against predation by clawed invertebrate eurypterids, which inhabited the same freshwater waters.1

While the dermis is known to give rise to membrane bones—including the bony scales of fishes—the fossil record for these early stages is limited, as many ancestral remains in upland freshwater regions have likely been lost to erosion. Consequently, while the theory links armour to freshwater adaptation, the marine distribution of surviving primitive chordates leads many experts to remain cautious about confirming a strictly freshwater origin for these early vertebrates.12