Bony Beginnings: The Rise of Vertebrate Innovation in the Devonian

The Devonian period, spanning around 60.3 million years and representing just 1.3 percent of Earth’s geological history, was a time of profound evolutionary change that fundamentally shaped the course of life on our planet. Throughout this period, the first forests emerged, and the oldest known fossils of insects and spiders appeared—marking a major turning point in terrestrial ecosystems. Toward the end of the Devonian, a 375-million-year-old “fishapod” was making the monumental leap from aquatic to terrestrial life. Despite these significant milestones, the Devonian is referred to as the “Age of Fishes” because of the exceptional diversification of fish species that characterized the oceans during this time.    

Today, fish are the most numerous vertebrates, or animals with backbones, on Earth. Remarkably, around 400 million years ago, fish already had great diversity. The Devonian seas teemed with the ancestors of modern sharks, ray-finned and lobe-finned fish— taxonomic groups that continue to populate the world’s oceans. Yet, many fish from this period were characterized by bony plates of protective armor. These are called the Osteostracans and the Placoderms. While the two could be discussed in tandem because of this shared characteristic, they exhibited noticeable differences in their anatomical structure and physical features.  

The Osteostraci, or osteostracans, first appeared in the Early Silurian around 430 million years ago. This group consisted of around 200 species and belonged to a group of vertebrates called the agnathans, meaning ‘without jaws.’ Most osteostracans were less than a foot (30 cm) in total length, were characterized by a horseshoe-shaped head shield, and had a simple mouth on the lower half of their head. Due to the absence of jaws, these armored fish were constrained in the size and variety of prey they could consume. Consequently, most osteostracans obtained their food by sucking up small benthic organisms or by filter-feeding on suspended organic material. 

Although agnathans dominated as the primary fish group during the Silurian, a dramatic shift occurred with a key evolutionary milestone—the development of jaws. By the start of the Devonian, all major groups of jawed fish had emerged, and their diversity and abundance only continued to grow throughout the period. Placoderms, the earliest type of jawed fish, capitalized on this evolutionary advantage. Armed with powerful jaws, they became very abundant and led to the decline of armored jawless fish and more broadly, jawless fish species overall. The advent of jaws marked a turning point for placoderms, enabling them to hunt and feed on larger prey.  

The most diverse group within the placoderm family tree was the order Arthrodira (jointed necks), which includes the closest relatives of all modern jawed fish. Comprising approximately 60 percent of known placoderm species, these armored fishes were predominately small in size. However, certain lineages, such as Titanichthys and Dunkleosteus, grew to impressive sizes, showcasing the remarkable diversity within the group.  

The most renowned species of Dunkleosteus, Dunkleosteus terrelli, was a formidable predator. Its jaws were formed by sharp, bony plates capable of biting, slicing and shearing through prey with remarkable efficiency. Additionally, D. terrelli possessed the ability to rapidly open its mouth, while simultaneously delivering extremely high pressure as it closed its jaws. This predatory arthrodire boasted one of the most powerful bite forces in vertebrate history, surpassing that of all other Devonian fish species. This combination of speed and power in D. terrelli likely allowed it to dominate the Devonian seas, consuming virtually all other aquatic species and earning its place as one of the first true apex predators in the vertebrate fossil record.

Although Dunkleosteus is perhaps the most famous of the armored fish due to its menacing appearance, many placoderms resembled the smaller, less intimidating osteostracans such as the genus, Bothriolepis. Belonging to the second-most successful order of placoderms, Antiarchi, these ancient, armored fish are considered the most successful placoderms of all time. They were the first vertebrates on Earth to radiate into dozens of species, ultimately inhabiting marine, brackish, and freshwater environments. It is even hypothesized that species of Bothriolepis may have used their bony pectoral appendages to “walk” out of the water, utilizing lung-like organs to breathe as they crawled into new aquatic habitats free from predators and rich in organic matter.

Despite their early success and diversity, placoderms lived for only about 70 million years—a relatively brief period compared to sharks, a lineage that emerged around the same time and has lasted to modern day.  

So, what purpose did this bony exoskeleton serve, and why aren’t our modern aquatic ecosystems teeming with armored fish? The most obvious reason is for protection, especially when considering the diversity of life found within the Devonian’s crowded seas. Specimens preserved with bite marks on armored bone and remains of armored fish in fecal matter are a common find in the fossil record. But another less obvious reason for the development of this unique outer layer has to do with the storage of vital minerals like calcium and phosphorus. In modern animals, bone is used as a repository for these minerals as they are necessary for building bone, muscle activity, and metabolism. Without calcium and phosphorus armored fish would have been unable to function properly.

Using bony armor as not only protection, but also as a mineral store likely played a critical role in the widespread distribution of armored fish across marine, freshwater, and estuarine habitats during the Devonian period.  

Either way, though fish like Dunkleosteus dominated the seas during this time, their armor couldn’t protect them from everything. The conclusion of the “Age of Fishes” was marked by a catastrophic mass extinction event, one that eradicated vast amounts of life on Earth. Unfortunately, this included the armored fish species that had exploded into an astonishingly diverse array of forms during the Devonian. In their absence, the surviving sharks and bony fish filled their place—a dynamic that persists to this day, over 360 million years later.