Ancient fish catalogue A to Z

More than one branch of fish evolution

The catalogue now spans several cartilaginous-fish radiations. Hybodontiforms form an extinct branch outside living sharks and rays. Xenacanthus belonged to another extinct freshwater lineage. Macrorhizodus and Abdounia belong to orders that include living sharks.

A streamlined outline therefore does not make every card the same kind of animal. Teeth, fin spines, articulated skeletons and classification reveal separate histories.

Why teeth dominate the record

Cartilage usually decays, whereas mineralised teeth and fin spines resist destruction. Each fish also replaced teeth repeatedly. A locality may therefore yield hundreds of crowns without one articulated body.

Teeth still preserve useful evidence. Crown height, cusps, ridges, roots, wear and position reveal loading and help diagnose species. Their limit is that similar diets can produce convergent shapes.

Grasping, cutting and crushing

Hybodus divided work between pointed front teeth and broader rear teeth. Acrodus emphasised a reinforced crushing battery. Asteracanthus carried grasping teeth, contrary to the crushing dentition historically assigned to it.

Lonchidion, Lissodus and disputed Polyacrodus show why an entire tooth row matters. The smooth blades of Macrorhizodus and heterodont row of Abdounia add two later solutions for capturing and cutting prey.

Body size needs a body

The articulated Solnhofen Asteracanthus exceeded two metres, and associated Hybodus skeletons support roughly metre-scale animals. Xenacanthus also preserves bodies that constrain length. Tooth-only taxa do not inherit those measurements.

Scaling a crown requires secure identification, jaw position and a comparable skeleton. Without them, exact length and mass create precision that the fossils do not contain.

Habitats changed through time

The profiles range from freshwater Carboniferous and Permian basins to Mesozoic seas and Paleogene shelves. Some hybodontiform genera include both marine and continental records, while the best-known Xenacanthus fossils are freshwater.

Transport and reworking can move small teeth. Sedimentology and associated organisms are therefore part of the habitat evidence.

Limits of reconstruction

Colour, social behaviour and exact feeding scenes are usually artistic. Teeth, jaws, spines and associated skeletons define the firmer scientific core.