("Spine Chests") were a group of chimaera
who were closely related to the rhenanid
placoderms. Superficially, the acanthoracids resembled scaly chimaeras, lightly armored arthrodires
, or (relatively) heavily armored ptyctodonts
. They were distinguished from chimaeras by the presence of large scales and plates, a pair of large spines that emanate from their chests (thus, the order's name), tooth-like beak plates, and the typical bone-enhanced placoderm eyeball. They were distinguished from other placoderms due to differences in the anatomy of their skulls, and due to patterns on the skull plates and thoracic plates that are unique to this order.
The acanthothoracids were once thought to be closely related to the rhenanids because of similar arrangements of the plate armor, and that the patterns on the former's plates resemble the patterns of the latter's tubercle mosaics.
Fossils of the Acanthothoracids are found in various deposits from the Lower Devonian throughout the world. The Palaeacanthaspids are found primarily in Europe and Canada, while the Weejasperaspids have only been found in the Taemas Wee Jasper
reef, in Southeastern Australia
While the Acanthothoracids' fossils date back to the Lower Devonian, they are believed to have arose during the Silurian, and that Silurian Acanthothoracids either lived in environments wholly unconductive to fossilization, or that their fossils have yet to be found.
From what can be inferred from the mouthplates of fossil specimens, the acanthothoracids were ecologically similar to modern-day chimaeras
, being a clique of shellfish hunters. Competition with their relatives, the ptyctodont placoderms, may have been one of the main reasons for the acanthothoracids' extinction prior to the Mid Devonian extinction event.
Relation to Other Placoderms
Most placoderm experts have reached a consensus that Acanthothoracida is the sister group of the rest of Placodermi
, save for, perhaps, Stensioella
. This is the result of a careful reexamination of the various members of the Acanthothoracid family Palaeacanthaspidae
, in that particular species within that family share various anatomical similarities with other placoderm orders, particularly the anatomies of their braincase
, dermal plate arrangement and bone histology
. As a result, that family is now considered to be paraphyletic
due to the similarities its members have to primitive members of other placoderm orders.
The family Weejasperaspididae, on the other hand, is considered to be monophyletic. Because of the Weejasperaspids' generalized anatomy, and strong similarities with the Palaeacanthaspids, but no overt similarities with any other order, save Brindabellaspida, they are regarded as either basal placoderms or very close to the basal placoderm.
Mikko's Phylogeny Archive
- Janvier, Philippe. Early Vertebrates Oxford, New York: Oxford University Press, 1998. ISBN 0-19-854047-7
- Long, John A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996. ISBN 0-8018-5438-5