Acanthodii

• 1. Introduction

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  Acanthodii were the first jawed fishes to swim on Earth, and for roughly 200 million years they ruled the waters of every continent. They looked, at first glance, like sharks. A streamlined body, paired fins, a strongly upturned tail. But they were something stranger and older than any shark that ever lived. The earliest unequivocal fossils of these creatures date from the beginning of the Silurian Period, some 50 million years before the first true shark appeared. So who were they, really? How did they fit into the tree of life? And why, after such an extraordinary run, did they finally disappear? The answers turn out to involve a detective story about scales, spines, cartilage, and the very origin of the jaw itself.

• 2. The Spiny Shark Problem

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  The nickname "spiny sharks" has stuck to acanthodians for a long time, and it is easy to see why. Their body plan echoed the shark blueprint almost point for point. Yet calling them sharks would be like calling a bat a bird. Acanthodians had a cartilaginous internal skeleton, as sharks do, but they also carried bony structures that sharks never developed. Their fins had a wide, bony base and were reinforced along the front edge with a stout dentine spine. All fins except the tail were braced this way. Some groups also carried rows of smaller "intermediate" or "prepelvic" spines between the pectoral and pelvic fins. Those spines are what make fossilised acanthodians so recognisable. Because soft cartilage rarely survives burial, the isolated spines and tiny scales are often all that remains of these fishes in ancient sedimentary rocks. Their epidermis was not covered in the typical shark-skin texture but in tiny rhomboid platelets, more like the scales seen in gars and bowfins today. A creature that resembled a shark, moved like a shark, but was built from an entirely different toolkit.

• 3. Reading Scales Like a Clock

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  Each order of Acanthodii left behind a distinctive scale ornament. That peculiarity turned their tiny scales into a geological tool. Palaeontologists can use scale characteristics to determine the relative age of sedimentary rock layers, reading the fossil record the way a geologist reads strata. The scales themselves follow a consistent design: a bulbous base, a neck, and a flat or slightly curved diamond-shaped crown. Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower shoulder girdle. Others developed a bony flap over the gill openings, a structure analogous to the operculum that would later appear in bony fishes. That bonelike material in their skins once led many palaeontologists to classify acanthodians as close ancestors of the bony fishes. The interpretation shifted once researchers understood that most of these features trace back to a common placoderm ancestor and appear in basal cartilaginous fish as well. The scales were not a sign of kinship with bony fish; they were an inheritance from an even older lineage.

• 4. Four Orders, One Grade

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  Acanthodians have been divided into four distinct orders, each occupying a different ecological niche. Climatiiformes were early forms with robust bony shoulder girdles and the rows of small sharp spines between their pectoral and pelvic fins. Within that group, the Diplacanthida were eventually elevated to their own order, Diplacanthiformes. Ischnacanthiformes took a predatory route, carrying tooth plates fused directly to their jaws. Acanthodiformes went in the opposite direction: filter feeders with toothless jaws, long gill rakers, and a single dorsal fin. This last group was the most specialised of the traditional acanthodians. They were also the most persistent, surviving all the way until the Permian period, long after their relatives had vanished. The group as a whole is now understood to be paraphyletic, meaning "Acanthodii" does not describe a single natural clade but rather a grade of fish lineages, some more closely related to cartilaginous fish, others positioned differently in the tree. The oldest remains attributed to acanthodian-grade chondrichthyans are Fanjingshania and Qianodus, from the Early Silurian of China, dating to around 439 million years ago.

• 5. Where They Fit in the Tree

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  For much of the twentieth century, acanthodians were treated as a unified group sitting somewhere near the base of the bony fish lineage. A 2012 study by Davis and colleagues changed that picture dramatically. Their analysis found acanthodians split between the two major clades of jawed vertebrates: Osteichthyes and Chondrichthyes. The well-known acanthodian Acanthodes was placed within Osteichthyes, despite displaying many chondrichthyan features in its braincase. Then a newly described Silurian placoderm named Entelognathus, whose jaw anatomy is shared with bony fish and tetrapods, prompted another revision. Acanthodians were repositioned as a paraphyletic assemblage leading toward cartilaginous fish, while bony fish were seen as evolving from placoderm ancestors. A 2016 study by Burrow and colleagues went further, finding chondrichthyans nested within Acanthodii, most closely related to the genera Doliodus and Tamiobatis. A 2017 study of Doliodus morphology reinforced the point: this genus appears to display a mosaic of shark and acanthodian features, marking it as a transitional fossil between the two groups. The jaw itself, acanthodians' most consequential feature, is thought to have evolved from the first gill arch of some ancestral jawless fish whose gill skeleton was made of jointed cartilage.

• 6. Rise, Dominance, and Extinction

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  The earliest acanthodians lived in the sea. During the Devonian, freshwater species became predominant, thriving in rivers and lakes. The group colonised coal swamps during the Carboniferous as well. Compared to other fish of their time, acanthodians were relatively conservative in their body forms and ecological roles. Their diversity rose during the Late Silurian, reaching its apex during the Lochkovian stage of the Early Devonian. A decline followed during the Pragian, then a recovery during the Emsian, then another decline through the middle and Late Devonian. Diversity held low but stable through the Carboniferous and edged slightly downward into the Permian. By that stage, bony fishes were demonstrating the capacity to dominate aquatic environments, and their competition proved overwhelming. The spiny sharks died out in Permian times, approximately 250 million years ago. The youngest known records of the group are isolated scales and fin spines from Middle-Late Permian strata in the Paraná Basin of Brazil, a faint last trace of a lineage that had once been the most successful jawed fish on the planet.