Acanthothoraci

Acanthothoraci, also called spine chests, were an ancient group of fish-like creatures known as placoderms. They lived a very long time ago and are now extinct, meaning they no longer exist today. These creatures looked a bit like a mix between modern chimaeras and other armored placoderms, but they had some special features.

One of the most noticeable things about Acanthothoraci were the large spines that came out from their chests, which is why they were called "spine chests." They also had big scales and plates covering their bodies, along with strong, tooth-like beak plates in their mouths. Their eyes had extra bone around them for protection.

Acanthothoraci were different from other placoderms because of how their skulls were built and because of special patterns on their skull and chest plates. Scientists study these ancient creatures to learn more about how fish and other early vertebrates evolved and lived in the ancient oceans.

Fossil record

Fossils of these ancient fish-like creatures, called acanthothoracids, have been found in many places around the world from the early Devonian time period. Fossils of one group, called Palaeacanthaspids, are found in places like Eurasia and Canada. Another group, the Weejasperaspids, has only been found in a special reef area called Taemas Wee Jasper, located in southeastern Australia.

Ecology

The acanthothoracids were similar to modern-day chimaeras. They probably ate small sea creatures. They went extinct before a big event called the Mid Devonian, maybe because other fish-like creatures were around.

Families

Three families have been recognized:

• Palaeacanthaspidae is the most common of the three families. Fossils of palaeacanthaspids are found in rocks from early times in Europe, Canada, Saudi Arabia, and East Asia. Palaeacanthaspids have short rostrums and large nostrils on the top of their heads, right between their eyes. They also had strong spines coming from the back of their shoulder-girdles and a spine on plates in front of their chest fins. Their skin plates were decorated with either tubercules or special scale patterns.
• Weejasperaspididae is found only in the Emsian Taemas-Wee Jasper "reef" in what is now New South Wales, Australia. The spines of weejasperaspids are longer and thicker than those of palaeacanthaspids. The armor of weejasperaspids had small tubercules on it.
• Hagiangellidae is a monogeneric family, with only one type, Hagiangella goujeti. It is found only in rocks from the Lochkovian time in Tung Vai, Ha Giang Province of Northern Vietnam. From the pieces of fossils found, H. goujeti had a rounded head, unlike the more pointed heads of other acanthothoracids, and had small points on the spines in front of its chest fins.

Relation to other placoderms

Most experts think that Acanthothoracida is closely related to other groups of Placodermi, except maybe Stensioella and Pseudopetalichthyida. This idea comes from studying the Acanthothoracid family Palaeacanthaspidae. These creatures share many features with other placoderm orders, especially in their braincase, how their skin plates are arranged, and the structure of their bones histology.

In 2011, scientists studied the genus Hagianella, from the family Hagianellidae, and think it might be related to Ptyctodontida because of how their skulls look. Because of this, Palaeacanthaspidae and Hagianellidae are now seen as paraphyletic, meaning they share traits with earlier members of other placoderm groups.

The family Weejasperaspididae is considered monophyletic, meaning all its members come from one common ancestor. Weejasperaspids have simple body shapes and look a lot like palaeacanthaspids, but they don’t resemble any other group except Brindabellaspida.

Timeline of genera