Acanthodians were an extinct group of fish-like vertebrates. They had a distinctive appearance, with prominent spines preceding their fins, earning them the nickname “spiny sharks.” Their unique traits offer insights into the early diversification of jawed vertebrates. Their fossil record reveals how life evolved and adapted in prehistoric waters.
Ancient Spiny Fish: An Overview
Acanthodians are early jawed vertebrates, or gnathostomes, a group that includes all modern fish and land vertebrates. Their fossils appear in the Lower Silurian period and persist until their extinction in the Permian period, a span of approximately 150 million years. Though never dominant, their fossils are found globally, indicating wide distribution in ancient oceans and freshwaters.
Their body plan resembled modern sharks, with a streamlined shape, paired fins, and an upturned tail. Unlike true sharks, acanthodians had unique characteristics, especially their scales and the presence of bone. Early forms were marine, with later species inhabiting freshwater, showing adaptability.
Distinctive Physical Traits
Their most striking feature was robust, bony spines positioned in front of all fins except the tail, from which their name (Greek for “spine”) originates. These spines provided support and defense. Their skin was covered with tiny, rhomboid dermal scales, often with an onion-like growth pattern. These scales differed from those of many modern bony fish.
Acanthodians had a cartilaginous skeleton, like modern sharks, but their fins had a broad, bony base reinforced with a dentine spine. This combination of cartilaginous skeleton with bony elements made fossilization challenging, often leaving only spines and scales preserved. Many also displayed multiple paired fins beyond the typical pectoral and pelvic fins, an unusual arrangement.
Life in Ancient Waters
Acanthodians occupied a variety of aquatic environments, with early forms being exclusively marine and later species predominantly found in freshwater. Their streamlined bodies and fin structures indicate they were active swimmers. The large, forward-positioned eyes of some species imply a strong reliance on vision for navigating their surroundings and locating food.
Their diet likely varied, with some acanthodians possessing numerous small teeth suitable for a predatory lifestyle, probably preying on small invertebrates. Other species, however, lacked teeth and instead developed modified gill rakers, suggesting they were filter feeders, consuming plankton or other suspended particles from the water column while swimming. The prominent spines they possessed would have served as a deterrent against larger predators in their ancient habitats.
Evolutionary Legacy and Extinction
Acanthodians are a transitional group in evolutionary history. They bridge the evolutionary gap between early jawless fish and the two main lineages of modern jawed fish: cartilaginous fish (Chondrichthyes, including sharks and rays) and bony fish (Osteichthyes). They exhibit a mosaic of features shared with both groups, such as shark-like body shapes and scales, alongside bony elements reminiscent of early bony fish.
Despite their evolutionary importance, acanthodians experienced a decline in diversity after the Devonian period and ultimately became extinct by the end of the Permian period, approximately 250 million years ago. This extinction may have been influenced by significant environmental changes during that time, or increased competition from more advanced and diverse groups of bony fish that were emerging and thriving.