Acanthothoraci

Acanthothoraci, also known as spine chests, were an ancient group of fish-like creatures called 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 that made them unique.

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. Another interesting feature was their eyes, which had extra bone around them for protection.

Acanthothoraci were different from other placoderms because of the way 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 discovered in many places around the world from the early part of the 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 likely hunted small sea creatures. They went extinct before a big event called the Mid Devonian, possibly because of competition with other fish-like creatures called ptyctodont placoderms.

Families

Three families have been recognized:

• Palaeacanthaspidae is the most widespread of the three families. Fossils of palaeacanthaspids are found in Lower Devonian strata of Europe, Canada, Saudi Arabia, and East Asia. Palaeacanthaspids have short rostrums and large nostrils situated dorsally on the forehead, almost directly between the eyes. They also had stout spines emanating from the back of the median dorsal plates of their shoulder-girdles and a spine emanating on plates directly in front of the pectoral fins. Dermal plates were decorated with either tubercules or scales with stellate patterns.
• Weejasperaspididae is restricted to the Emsian Taemas-Wee Jasper "reef" in what is now New South Wales, Australia. Compared to the corresponding spines of palaeacanthaspids, the spines of weejasperaspids are long and markedly stout. The armor of weejasperaspids were decorated with small tubercules.
• Hagiangellidae is a monogeneric family, currently represented by Hagiangella goujeti, which is restricted to Lochkovian strata of the Khao Loc formation in Tung Vai, Ha Giang Province of Northern Vietnam. From what can be discerned from the various scrappy fossils, H. goujeti had a relatively high-domed head, especially when compared to the bun-shaped heads of other acanthothoracids, and had serrations on the spines in front of its pectoral fins.

Relation to other placoderms

Most experts agree 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 looked again at the genus Hagianella, from the family Hagianellidae, and think it might be closely 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. This makes them either very basic placoderms or very close to the most basic placoderms.

Timeline of genera