Acanthodes was an extinct fish genus from Earth’s ancient past, often called a “spiny shark.” While this nickname highlights its superficial resemblance to modern sharks, Acanthodes was not a true shark. This creature offers a glimpse into diverse prehistoric aquatic life. Studying Acanthodes helps scientists understand early vertebrate evolution.
Introducing Acanthodes: The Ancient ‘Spiny Shark’
Acanthodes inhabited aquatic environments from the Early Carboniferous to the Early Permian periods (approximately 359 to 245 million years ago). These ancient fish were modest in size, typically reaching lengths of around 30 centimeters, though some species grew up to 80 centimeters. Its streamlined body, paired fins, and upturned tail gave it a shark-like appearance, contributing to its “spiny shark” moniker.
Acanthodes were jawed fishes that primarily lived in freshwater and estuarine environments. Their fossils have been discovered across Europe, North America, and Australia, indicating widespread distribution.
Unique Anatomy: Spines, Scales, and Filter Feeding
A distinguishing feature of Acanthodes was the prominent spines that supported its fins. Unlike modern fish with flexible fin rays, Acanthodes possessed stout, largely immovable bony spines positioned in front of each fin, excluding the tail. These spines, typically six in total—one each for the pectoral, pelvic, anal, and dorsal fins—offered both support and a degree of defense against potential predators.
The body of Acanthodes was covered in a unique type of scale. These were tiny, rhomboid platelets with a bulbous base, a neck, and a flat or slightly curved diamond-shaped crown. These scales differ significantly from those found on modern bony fish and even from the denticles of true sharks. The delicate nature of their cartilaginous skeletons means that often only these durable spines and scales remain in the fossil record.
Acanthodes lacked teeth, which strongly suggests a filter-feeding lifestyle. This dietary adaptation meant they likely consumed plankton and possibly small invertebrates by straining them from the water. The absence of teeth, coupled with their relatively few defensive spines, has led some researchers to hypothesize that Acanthodes might have formed groups, relying on safety in numbers for protection.
Acanthodes in the Tree of Life: Unraveling Its Evolutionary Past
Acanthodes belongs to the extinct class Acanthodii. This classification has been a subject of extensive scientific debate, as acanthodians exhibit a mix of features found in both cartilaginous fish (like sharks and rays) and bony fish. Initially, some paleontologists considered them a transitional group between these two major fish lineages.
More recent scientific understanding places acanthodians, including Acanthodes, as a paraphyletic group basal to extant Chondrichthyes, which encompasses living sharks, rays, and chimaeras. This means they are considered “stem-group Chondrichthyes,” representing early branches in the evolutionary tree that led to modern cartilaginous fish. This updated classification highlights their closer relationship to sharks than to bony fish.
Studying Acanthodes and other acanthodians is highly significant for understanding the early evolution of jawed vertebrates. They are among the earliest known jawed fishes, appearing millions of years before the first true sharks. Their distinctive features provide valuable clues about the divergence of major fish groups and the development of key anatomical traits in early fish evolution.