Acanthodes: the spiny fish that reshaped early jawed-vertebrate trees

Acanthodes bronni is one of the best-known acanthodian fishes because flattened fossils preserve more than isolated spines or scales. Specimens from the Lower Permian of Germany retain much of the body outline, fin supports and parts of the skull. Computed tomography has made the braincase especially useful in debates about early jawed vertebrates. The genus belongs among the varied forms in the ancient fish catalogue, but “spiny shark” is a historical nickname, not a statement that it was a modern-type shark.

Quick facts

Why “spiny shark” is not a simple classification

Acanthodians were a diverse assemblage of Palaeozoic jawed fishes. Their fossils often combine fin spines, scales and anatomical features that do not fit neatly into the two living branches of jawed vertebrates. The colloquial name “spiny sharks” can suggest that all were sharks in the familiar sense. It is more useful to treat Acanthodes as an early jawed fish whose relationships are inferred from a changing set of anatomical characters.

Acanthodes bronni matters because it preserves a braincase in unusual detail. Earlier reconstructions gave the skull features that appeared osteichthyan-like. A 2012 redescription of the braincase emphasized similarities to early cartilaginous fishes, while its phylogenetic analysis placed Acanthodes on the osteichthyan stem. A later study of the hyoid arch used computed tomography to locate the hyomandibular attachment below the jugular groove, a condition supporting chondrichthyan affinity for acanthodians. These results should be reported as the conclusions of particular analyses, not merged into a claim that the exact position is beyond debate.

A fish with fewer fin spines

Many early acanthodians carried a spine in front of numerous paired and unpaired fins. The well-known Acanthodes pattern is more reduced: spines occur before the dorsal, anal, pectoral and pelvic fins, without the long additional series between the pectoral and pelvic girdles seen in some other forms. The hard spines supported the leading edge of fins. They may also have discouraged predators, but fossils do not record a defensive display or a particular attack.

The body was elongated and covered in very small scales. The dorsal fin lay far back, and the tail was heterocercal, with the vertebral axis continuing into the longer upper lobe. These features are directly visible in flattened specimens, although burial can compress the body and alter its proportions. A reconstruction can therefore use the overall outline without claiming that the soft fin margins or exact swimming motion are known.

What the skull studies actually changed

In 2012, Davis, Finarelli and Coates described additional external and internal braincase anatomy. Their principal-coordinates comparison found the reconstructed skull more similar to chondrichthyans than to bony fishes, yet their phylogenetic analysis placed the genus on the osteichthyan stem. The distinction between resemblance and a tree result is important: a cluster of similar characters does not automatically settle ancestry.

In 2015, Brazeau and colleagues combined tomography with traditional examination to reassess the hyoid arch. They corrected the inferred length and attachment of the hyomandibula, placing it on a ventrolateral process below the jugular vein groove. The authors interpreted that arrangement as a chondrichthyan-like condition and support for the affinity of acanthodians with the cartilaginous-fish stem. The disagreement reflects how a small number of crucial skull characters can affect early vertebrate trees.

The skull is still a fossil, not a transparent blueprint. Some elements are compressed, and a virtual scan depends on distinguishing overlapping bones and canals. The revised anatomy narrows the range of possible interpretations, but later datasets may still change the exact position of the genus.

Food and size are less securely known

Acanthodes bronni lacked the large tooth rows familiar in many predatory fishes. Its small mouth, delicate jaws and branchial anatomy are compatible with taking small food from the water. Filter feeding has been proposed, but the soft structures that would demonstrate a precise filtering apparatus are incompletely preserved. The available evidence supports a cautious feeding model rather than a measured list of prey.

Published complete or nearly complete individuals were several tens of centimetres long. That estimate applies to the best-known specimens, not to every animal historically assigned to the genus. Older records extend Acanthodes across a broad interval, but many are based on isolated spines or incomplete remains and may belong to other taxa. The genus should not be presented as one unchanged fish spanning that entire interval.

Its fossils do not preserve colour, social behaviour, reproductive structures or a direct record of how it used its spines. A scene showing a small fish in a lake is a reasonable setting for some German specimens; a specific school, hunting event or protective posture would be an artistic choice. What makes Acanthodes valuable is the contrast between a relatively informative braincase and the uncertainty that remains in the wider family tree.

Explore related evidence in the ancient fish catalogue.