Arctolepis

What is Arctolepis?

Arctolepis is an extinct genus of placoderms, or fishlike animals, that lived during the early Devonian Period approximately 416 million to 360 million years ago.¹

As a member of the arthrodires, or jointed-neck fishes, Arctolepis possessed a bony head and trunk shield, while the remainder of its body was unarmoured and tapered to a pointed tail. The creature was small, measuring less than 30 cm (12 inches) in length, and featured a tail fin developed only on the lower margin. Notably, Arctolepis had significantly developed pectoral spines, which likely assisted with swimming and served as a defensive deterrent against predators.¹

What are placoderms?

Placoderms (class Placodermi) were a group of primitive, extinct jawed fishes known exclusively from fossil records. Flourishing during the Devonian Period (approximately 416 million to 359 million years ago), they were the dominant group in marine and freshwater environments across the globe, though only two species survived into the Carboniferous Period.²³

The name “placoderm” translates to “plate skin,” referring to their characteristic dermal bone armour that covered the head and trunk, often connected by a joint in the neck region. Key features included:²³

• Jaws: They were among the earliest jawed vertebrates, though their jaw structures were highly specialized and likely not ancestral to modern bony fishes.²³
• Size: While most were small (under 30 cm), some arthrodires reached lengths of 10 metres (33 feet), such as the apex predator Dunkleosteus.²³
• Diversity: Early forms were bottom-dwellers, while later species adapted for active swimming.²

Their evolutionary origins remain unknown, and they eventually went extinct as other, more adaptable fish groups emerged.²³

What defines the Devonian Period?

The Devonian Period, spanning from approximately 419.2 million to 359 million years ago, is widely known as the “Age of Fishes” due to the remarkable diversification and abundance of fish species that inhabited its seas.⁴

Key Characteristics

• Biological Milestones: The period saw the emergence of forests and the first appearance of ammonites. By the late Devonian, the first four-legged amphibians appeared, marking the colonization of land by vertebrates.⁴
• Geological Setting: Early in the period, the continents of North America, Greenland, and Europe collided to form the supercontinent Laurussia (or Euramerica), a process marked by the Caledonian orogeny mountain-building event⁷. Much of the globe remained covered by oceans.⁴
• Climate: The Earth was generally warmer and more equitable than it is today, with widespread desert conditions and carbonate reefs.⁵
• Extinction Events: The period was punctuated by significant extinction episodes, such as the Kellwasser Event, which caused massive losses among marine life, including corals, trilobites, and brachiopods. These events were often linked to environmental stresses like low oxygen levels in the seas and fluctuating sea levels.⁴⁶⁷

How did arthrodires get their name?

The term arthrodire is derived from New Latin, combining the Greek roots arthr- (joint) and -dira, which originates from the Greek word deirē, meaning “neck.”⁹

This name accurately reflects a defining anatomical feature of these placoderms: in many later arthrodires, the bony head shield and the body shield were connected by a joint on each side of the neck region. This specialized articulation allowed for free movement of the head, a significant adaptation for these predatory fishes.¹⁰