Acanthothoraci (''spine chests'') is an extinct group of chimaera-like placoderms who were closely related to the

rhenanid Rhenanida ("Rhine (fish)") is an order of scaly placoderms. Unlike most other placoderms, the rhenanids' armor was made up of a mosaic of unfused scales and tubercles. The patterns and components of this "mosaic" correspond to the plates of arm ...

placoderms. Superficially, the acanthoracids resembled scaly chimaeras, or (relatively) heavily armored

ptyctodont The ptyctodontids ("folded-teeth") are placoderms of the order Ptyctodontida, containing the family Ptyctodontidae. With their big heads, big eyes, reduced armor and long bodies, the ptyctodontids bore a superficial resemblance to modern day chi ...

s. 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.

Fossil record

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 Wee Jasper is a hamlet in the Goodradigbee valley at the western foot of the Brindabella Ranges, near Burrinjuck Dam in New South Wales, Australia in Yass Valley Shire. It is located about 90 km north-west of Canberra and 60 km sou ...

reef, in Southeastern Australia.

Ecology

From what can be inferred from the mouthplates of fossil specimens, the acanthothoracids were ecologically similar to modern-day

chimaera Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively. A ...

s, 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.

Families

Three

families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

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 Saudi Arabia, officially the Kingdom of Saudi Arabia (KSA), is a country in Western Asia. It covers the bulk of the Arabian Peninsula, and has a land area of about , making it the List of Asian countries by area, fifth-largest country in Asia ...

, 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

tubercule In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal. In plants A tubercle is generally a wart-like projection, ...

s or scales with stellate patterns. *

Weejasperaspididae Weejasperaspididae ("Shields of Wee Jasper") is a family of three extinct acanthothoracid placoderms indigenous to the Early Devonian of Victoria and New South Wales, Australia. Description The Weejasperaspids are known from median dorsal p ...

is restricted to the

Emsian The Emsian is one of three faunal stages in the Early Devonian Epoch. It lasted from 407.6 ± 2.6 million years ago to 393.3 ± 1.2 million years ago. It was preceded by the Pragian Stage and followed by the Eifelian Stage. It is named after ...

Taemas-Wee Jasper "reef" in what is now

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

, Australia. Compared to the corresponding spines of Palaeacanthaspids, the spines of Weejasperaspids are long and tremendously 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 The Lochkovian is one of three faunal stages in the Early Devonian Epoch. It lasted from 419.2 ± 3.2 million years ago to 410.8 ± 2.8 million years ago. It marked the beginning of the Devonian Period, and was followed by the Pragian Stage. It ...

strata of the Khao Loc formation in Tung Vai, Ha Giang Province of Northern

Vietnam Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making it ...

. 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 placoderm experts have reached a consensus that Acanthothoracida is the sister group of the rest of

Placodermi Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally ' plate-skinned') is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were ...

, save for, perhaps, ''

Stensioella ''Stensioella heintzi'' is an enigmatic placoderm of arcane affinity. It is only known from the Lower Devonian Hunsrück slate of Germany. Anatomy ''Stensioella heintzi'' has an elongated body, a whip-like tail, and long, wing-like pectoral fi ...

'' and

Pseudopetalichthyida Pseudopetalichthyida is an extinct order of lightly armored placoderms known only from rare fossils in Lower Devonian strata in Hunsrück, Germany. Like '' Stensioella heintzi'', and the Rhenanida, the Pseudopetalichthids had armor made up of a ...

. 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 In human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calvaria or skul ...

, dermal plate arrangement and bone

histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vi ...

. In 2011, the genus '' Hagianella'', of the monotypic family Hagianellidae, was reappraised as possibly being the sister-group of

Ptyctodontida The ptyctodontids ("folded-teeth") are placoderms of the order Ptyctodontida, containing the family Ptyctodontidae. With their big heads, big eyes, reduced armor and long bodies, the ptyctodontids bore a superficial resemblance to modern day chi ...

due to similarities of skull anatomies. As a result, Palaeacanthaspidae and Hagianellidae are now considered to be

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

due to the similarities their members have to primitive members of other placoderm orders. The family

, on the other hand, is considered to be

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. 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.

Timeline of genera

ImageSize = width:1000px height:auto barincrement:15px PlotArea = left:10px bottom:50px top:10px right:10px Period = from:-416 till:-359.2 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:5 start:-416 ScaleMinor = unit:year increment:1 start:-416 TimeAxis = orientation:hor AlignBars = 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:

from: -411.2 till: -407 color:earlydevonian text:

Pragian The Pragian is one of three faunal stages in the Early Devonian Epoch. It lasted from 410.8 ± 2.8 million years ago to 407.6 ± 2.8 million years ago. It was preceded by the Lochkovian Stage and followed by the Emsian Stage. The most important ...

from: -407 till: -397.5 color:earlydevonian text:

from: -397.5 till: -391.8 color:middledevonian text:

Eifelian The Eifelian is the first of two faunal stages in the Middle Devonian Epoch. It lasted from 393.3 ± 1.2 million years ago to 387.7 ± 0.8 million years ago. It was preceded by the Emsian Stage and followed by the Givetian Stage. North American ...

from: -391.8 till: -385.3 color:middledevonian text:

Givetian The Givetian is one of two faunal stages in the Middle Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Eifelian Stage and followed by the Frasnian Stage. It is named after the town of Givet in ...

from: -385.3 till: -374.5 color:latedevonian text:

Frasnian The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage. Major reef-building was under way during t ...

from: -374.5 till: -359.2 color:latedevonian text:

Famennian The Famennian is the latter of two faunal stages in the Late Devonian Epoch. The most recent estimate for its duration estimates that it lasted from around 371.1 million years ago to 359.3 million years ago. An earlier 2012 estimate, still used ...

bar:eratop from: -416 till: -359.2 color:devonian text:

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...

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 ''Romundina'' is a small, heavily armored extinct genus of acanthothoracid placoderms which lived in shallow marine environments in the early Devonian (Lochkovian). The name ''Romundina'' honors Canadian geologist and paleontologist Dr. Rómun ...

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 ''Murrindalaspis'' is an extinct genus of acanthothoracid placoderm found in the McLarty Member of the Murrindal Limestone, of the Early Devonian-aged Buchan Group in eastern Victoria, Australia. ''Murrindalaspis'' differs from other acanthotho ...

color:earlydevonian bar:NAM10 from: -407 till: -397.5 text:

Weejasperaspis ''Weejasperaspis'' is an extinct acanthothoracid placoderm found in the Taemas-Weejasper Reef, of the Early Devonian-aged Buchan Group in eastern Victoria, Australia and the type species is ''W. gavini''.White, E. I. (1978). The larger arthrodir ...

PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 bar:period from: -416 till: -411.2 color:earlydevonian text: