More than 400 million years ago, long before the first forests covered the land and long before jaws had even evolved, a small armored fish was living a fast-paced life.
A new study of fossils of Protaspis, a jawless fish that inhabited warm, shallow waters in what is now Wyoming during the Early Devonian period, has upended a long-held assumption in paleontology that the earliest vertebrates were slow-growing, long-lived creatures, researchers report in a preprint published in bioRxiv. The findings, they say, instead suggest that rapid growth and high population turnover were already well-established strategies hundreds of millions of years ago.
The findings “challenge the assumption that extant armored fish, represented by ‘living fossils,’ are appropriate ecological models for early agnathans,” researchers write in the paper.
Nanako Okabe at the Okinawa Institute of Science and Technology and her colleagues studied more than 450 specimens of Protaspis housed at the Field Museum in Chicago and the University of Kansas. Because the fossils appear to represent a mass mortality event, a near-simultaneous snapshot of a living population, they preserve individuals ranging from tiny juveniles to full adults, something exceptionally rare among ancient jawless fish.
To reconstruct how Protaspis grew, they used ELEFAN — a quantitative tool developed in fisheries science and ordinarily used to estimate growth rates in living fish populations. The analysis revealed a growth coefficient (K) of approximately 0.95 per year, a figure that places Protaspis firmly among today’s small, fast-growing fish species rather than among slow-growing “living fossils” such as sturgeons or paddlefish.
Researchers say, in the paper, that these characteristics are in sharp contrast to the long-held assumption that early vertebrates were uniformly slow-growing.
The specimens fell into two distinct peaks — one cluster around 15 millimetres in dorsal plate length, another around 90 millimetres — suggesting at least two age groups coexisted in the same habitat, with reproduction occurring once a year in a well-defined season. Juveniles outnumbered adults considerably. This pattern is consistent with high early mortality and rapid cohort turnover, much like what is seen in many short-lived modern fish.
One of the most striking details was the relationship between growth and armor. Protaspis was encased in an elaborate series of bony dermal plates — a formidable defensive structure. Yet the analysis showed that during its first year of life, when the fish was growing most rapidly, these plates were loosely integrated and incomplete. It was only in the second year that the armor fused and hardened. The animal, it seems, prioritized speed of growth over the security of a full defensive shell. Such trade-offs are well-documented in living armored fish.
Journal Reference: BioRxiv. DOI: 10.64898/2026.04.26.720944
