Rhenanida

Rhenanida ("Rhine (fish)") is an order of scaly placoderms. These ancient fish had a special kind of armor made from many small, unfused scales and bumps, forming a mosaic pattern. This pattern helps scientists understand how the armor of more advanced placoderms might have developed from simpler, unfused pieces.

All rhenanids were flat, ray-like fish that lived on the ocean floor. They were hunters, using their unique body shape to catch food in marine environments. These fascinating creatures give us clues about the early evolution of armored fish.

Evolution

The rhenanids were thought to be among the simplest placoderm fish because their armor was made of many small pieces, unlike the solid plates of more advanced placoderms like antiarchs and arthrodires. But by studying the skulls of Jagorina pandora and comparing them to antiarchs, scientists now believe rhenanids are closely related to antiarchs and their Acanthothoracid relatives.

Presence in the fossil record

The fossil record of Rhenanida is very limited, mostly containing small pieces like skull fragments that look similar to Gemuendina stuertzi, the most famous rhenanid. We know this fish mostly from fossils found in the Hunsruck slates. Even though rhenanids lived all over the world, we don’t find many fossils because their armor broke apart into small pieces after they died.

Most fossils of these fish come from the early part of the Devonian period, especially in the United States and Germany. Recently, a new type called Nefudina qalibahensis was found in northeastern Saudi Arabia. Another species, Asterosteus stenocephalus, comes from the middle Devonian in Ohio. There is also Bolivosteus chacomensis, found in what is now Bolivia, in South America. The latest known rhenanid, Jagorina pandora, lived in the late Devonian in Germany.

Taxonomy

There are five known types of rhenanids, each in its own group: Asterosteus stenocephalus, Nefudina qalibahensis, Gemuendina stuertzi, Jagorina pandora, and Bolivosteus chacomensis. All of these belong to a family named Asterosteidae, which was created by Woodward in 1891. Other family names for Rhenanida are now thought to be the same as Asterosteidae.

There is also a sixth group, Ohioaspis, but its exact place is uncertain. The first examples were small bone pieces that were first thought to belong to a new kind of Asterosteus. Further studies have led some experts to believe these pieces belong to rhenanids, while others think they may be a type of ostracoderm agnathan.