Rhenanida

Rhenanida ("Rhine (fish)") is an order of scaly placoderms.

These ancient fish had special armor made from many small, unfused scales and bumps. This pattern looks like a mosaic and helps scientists learn about how more advanced placoderm armor might have started from simpler pieces.

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

Evolution

The rhenanids were thought to be simple placoderm fish because their armor was made of many small pieces. Unlike the solid plates of more advanced placoderms like antiarchs and arthrodires, rhenanids had smaller armor pieces. Scientists studied the skulls of Jagorina pandora and found that rhenanids are closely related to antiarchs and their Acanthothoracid relatives.

Presence in the fossil record

The fossil record of Rhenanida is very small. It mostly has tiny pieces like skull parts that look like 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 into small pieces after they died.

Most fossils of these fish are 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. They are Asterosteus stenocephalus, Nefudina qalibahensis, Gemuendina stuertzi, Jagorina pandora, and Bolivosteus chacomensis. All of these belong to a family called Asterosteidae. This family name 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. Its exact place is not certain. The first examples were small bone pieces. These pieces were first thought to belong to a new kind of Asterosteus. Further studies have led some experts to believe these pieces belong to rhenanids. Others think they may be a type of ostracoderm agnathan.