10-444: Weejasperaspididae † Hagiangellidae † Palaeacanthaspidae † Acanthothoraci ( spine chests ) is an extinct group of chimaera-like placoderms closely related to the rhenanid placoderms. Superficially, the acanthoracids resembled scaly chimaeras and (relatively) heavily armored ptyctodonts . They were distinguished from chimaeras by their large scales and plates, a pair of large spines that emanate from their chests (thus,
20-681: The Taemas-Wee Jasper reef in Australia . When it was first discovered in 1980, it was originally regarded as a Weejasperaspid acanthothoracid due to anatomical similarities with the other species found at the reef. According to Philippe Janvier, anatomical similarities of B. stensioi' s brain and braincase with those of jawless fish, such as the Osteostraci and the Galeaspida , strongly suggest that B. stensioi , and also
30-609: The Early Devonian of Victoria and New South Wales , Australia . The Weejasperaspids are known from median dorsal plates with distinctive, blade-like crests in the median-posterior portion, and ossified eye capsules. The main reasons why the weejasperaspids are not considered to be closely related to other non-acanthothoracid placoderms, as opposed to the palaeacanthaspids , are that their skull anatomies and plate histologies are generalized, and do not bear any similarities to any specific non-acanthothoracid group, and that
40-632: The Mid Devonian extinction event. Three families have been recognized: Most placoderm experts have reached a consensus that Acanthothoracida is the sister group of the rest of Placodermi , save for, perhaps, Stensioella and Pseudopetalichthyida . 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
50-637: The Weejasperaspids have only been found in the Taemas Wee Jasper reef, in Southeastern Australia . 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
60-483: The anatomies of their braincase , dermal plate arrangement and bone histology . In 2011 the genus Hagianella , of the monotypic family Hagianellidae, was reappraised as possibly being the sister-group of Ptyctodontida in light of similarities of skull anatomies. Accordingly, Palaeacanthaspidae and Hagianellidae are now considered paraphyletic in view of similarities of their members to primitive members of other placoderm orders. The family Weejasperaspididae , on
70-560: The internal anatomy of the braincase resembled more like those of osteostracans or galeaspids than those of other placoderms. Mikko's Phylogeny Archive [1] Archived 27 September 2006 at the Wayback Machine Brindabellaspis stensioi Brindabellaspis stensioi (" Erik Stensiö 's Brindabella Ranges Shield") is a placoderm with a flat, platypus-like snout from the Early Devonian of
80-627: The order's name), tooth-like beak plates, and the typical bone-enhanced placoderm eyeball. They were distinguished from other placoderms by differences in skull anatomy and by patterns on the skull plates and thoracic plates that are unique to this order. Fossils of the acanthothoracids are found in various deposits from the Lower Devonian throughout the world. Fossils of the Palaeacanthaspids are found in Eurasia and Canada, while
90-468: The other hand, is considered 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. Weejasperaspididae Weejasperaspididae ("Shields of Wee Jasper ") is a family of three extinct acanthothoracid placoderms indigenous to
100-474: The patterns of ornamentation on their dermal plates are unique to this family. The placoderm Brindabellaspis stensioi was once regarded as a weejasperaspid because of the similarities between its dermal plates to the other weejasperaspids. Even before it was split off into its own order, it stood out from the others because of its long, pointed snout. French paleontologist Phillipe Janvier placed B. stensioi in its own order, Brindabellaspida , because of how
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