Pterichthyodes
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Extinct (fossil)
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Pterichthyodes is a genus of placoderm fishes from the Devonian period, now all extinct. They were one of the first species recognized for what they were, as their fossils are common in the Old Red Sandstone formation studied by geologists in the early 1800s. Due to their extreme divergence from modern-day fish, they were a puzzle unsolved until Charles Darwin brought forward his theories on evolution.
Pterichthyodes had heavily armored heads and front bodies, while their tail ends were uncovered. As placoderms, they were members of one of the first group of animals to possess jaws, though they had grinding plates rather than teeth. The Pterichthyodes are distinguished easily from other placoderms by their odd wing-like appendage where fins would be found on a modern fish ("pterichthys" is Ancient Greek for "wing-fish") -- strictly speaking, these are not fins as we normally think of them, which evolved in another group of fish, the Actinopterygii.
