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, 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 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 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 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 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.
Period = from:-416 till:-359.2TimeAxis = orientation:horizontalScaleMajor = unit:year increment:5 start:-416ScaleMinor = unit:year increment:1 start:-416TimeAxis = orientation:horAlignBars = justify
Colors = #legends id:CAR value:claret id:ANK value:rgb(0.4,0.3,0.196) id:HER value:teal id:HAD value:green id:OMN value:blue id:black value:black id:white value:white id:paleozoic value:rgb(0.6,0.75,0.55) id:cambrian value:rgb(0.49,0.63,0.33) id:ordovician value:rgb(0,0.57,0.44) id:silurian value:rgb(0.70,0.88,0.71) id:devonian value:rgb(0.8,0.55,0.22) id:earlydevonian value:rgb(0.90,0.71,0.43) id:middledevonian value:rgb(0.96,0.81,0.51) id:latedevonian value:rgb(0.96,0.89,0.71) id:carboniferous value:rgb(0.4,0.65,0.6) id:mississippian value:rgb(0.4,0.56,0.4) id:pennsylvanian value:rgb(0.8,0.77,0.53) id:permian value:rgb(0.94,0.25,0.24) id:mesozoic value:rgb(0.38,0.77,0.79) id:triassic value:rgb(0.51,0.17,0.57) id:jurassic value:rgb(0.2,0.7,0.79) id:cretaceous value:rgb(0.5,0.78,0.31) id:cenozoic value:rgb(0.95,0.98,0.11) id:paleogene value:rgb(0.99,0.6,0.32) id:neogene value:rgb(0.999999,0.9,0.1) id:quaternary value:rgb(0.98,0.98,0.50) BarData= bar:eratop bar:space bar:periodtop bar:space bar:NAM1 bar:NAM2 bar:NAM3 bar:NAM4 bar:NAM5 bar:NAM6 bar:NAM7 bar:NAM8 bar:NAM9 bar:NAM10
bar:space bar:period bar:space bar:era
PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 shift:(7,-4) bar:periodtop from: -416 till: -411.2 color:earlydevonian text:Lochkovian from: -411.2 till: -407 color:earlydevonian text:Pragian from: -407 till: -397.5 color:earlydevonian text:Emsian from: -397.5 till: -391.8 color:middledevonian text:Eifelian from: -391.8 till: -385.3 color:middledevonian text:Givetian from: -385.3 till: -374.5 color:latedevonian text:Frasnian from: -374.5 till: -359.2 color:latedevonian text:Famennian
bar:eratop from: -416 till: -359.2 color:devonian text:Devonian
PlotData= align:left fontsize:M mark:(line,white) width:5 anchor:till align:left
color:earlydevonian bar:NAM1 from: -416 till: -414.4 text: Kimaspis color:earlydevonian bar:NAM2 from: -416 till: -409.8 text: Kosoraspis color:earlydevonian bar:NAM3 from: -416 till: -407 text: Radotina color:earlydevonian bar:NAM4 from: -412.8 till: -411.2 text: Dobrowlania color:earlydevonian bar:NAM5 from: -412.8 till: -411.2 text: Palaeacanthaspis color:earlydevonian bar:NAM6 from: -412.8 till: -411.2 text: Romundina color:earlydevonian bar:NAM7 from: -411.2 till: -407 text: Breizosteus color:earlydevonian bar:NAM8 from: -407 till: -397.5 text: Kolymaspis color:earlydevonian bar:NAM9 from: -407 till: -397.5 text: Murrindalaspis color:earlydevonian bar:NAM10 from: -407 till: -397.5 text: Weejasperaspis
PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 bar:period from: -416 till: -411.2 color:earlydevonian text:Lochkovian from: -411.2 till: -407 color:earlydevonian text:Pragian from: -407 till: -397.5 color:earlydevonian text:Emsian from: -397.5 till: -391.8 color:middledevonian text:Eifelian from: -391.8 till: -385.3 color:middledevonian text:Givetian from: -385.3 till: -374.5 color:latedevonian text:Frasnian from: -374.5 till: -359.2 color:latedevonian text:Famennian
bar:era from: -416 till: -359.2 color:devonian text:Devonian