Antiarchi ("opposite anus") is an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

of heavily armored placoderms. The antiarchs form the second-most successful group of placoderms after the arthrodires in terms of numbers of species and range of environments. The order's name was coined by

Edward Drinker Cope Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontologist, comparative anatomist, herpetologist, and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested ...

, who, when examining some fossils that he thought were armored

tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ca ...

s related to '' Chelysoma'', mistakenly thought that the orbital fenestra (i.e., the hole in the headshield for the eyes, nose and pineal foramen) was the opening for the mouth, or oral siphon, and that the opening for the anal siphon was on the other side of the body, as opposed to having both oral and anal siphons together at one end. The front portions of their bodies were heavily armored, to the point of literally resembling a box with eyes, with the sometimes scaled, sometimes naked rear portions often becoming

sinuous Sinuosity, sinuosity index, or sinuosity coefficient of a continuously differentiable curve having at least one inflection point is the ratio of the curvilinear length (along the curve) and the Euclidean distance (straight line) between the ...

, particularly with later forms. The pair of pectoral fins were modified into a pair of caliper-like, or

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

-like limbs. In primitive forms, such as ''

Yunnanolepis ''Yunnanolepis'' is an extinct genus of primitive antiarch placoderm. The fossils of the various species are found in Early to Middle Devonian strata in Southern China ( Xishancun, Lianhuashan and Xitun Formations). External links ''Yunnanol ...

'', the limbs were thick and short, while in advanced forms, such as '' Bothriolepis'', the limbs were long and had elbow-like joints. The function of the limbs are still not perfectly understood, but, most hypothesize that they helped their owners pull themselves across the substrate, as well as allow their owners to bury themselves into the substrate. Antiarchi, along with Brindabellaspis, form some of the most basal clades of the Placodermi, or Gnathostomata. They are more related to other placoderms and the more derived jawed fish than the

Cephalaspidomorphi Cephalaspidomorphs are a group of jawless fishes named for ''Cephalaspis'' of the osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, because lampreys were once thought ...

Phylogeny

Below is a cladogram from Jia ''et al.'' (2010):

Timeline of genera

ImageSize = width:1000px height:auto barincrement:15px PlotArea = left:10px bottom:50px top:10px right:10px Period = from:-416 till:-350 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:NAM11 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 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 La ...

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 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 th ...

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 b ...

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, whe ...

PlotData= align:left fontsize:M mark:(line,white) width:5 anchor:till align:left color:earlydevonian bar:NAM1 from: -416 till: -411.2 text: Phymolepis color:earlydevonian bar:NAM2 from: -416 till: -411.2 text: Chujinolepis color:earlydevonian bar:NAM3 from: -416 till: -411.2 text:

Zhanjilepis ''Zhanjilepis aspartilis'' is an extinct yunnanolepid placoderm from Pridoli and Lochkovian rocks of Hunan, China. ''Zhanjilepis'' is known only from distinctively ordered plates. Description Plates ''Zhanjilepis'' plates are very similar t ...

color:earlydevonian bar:NAM4 from: -416 till: -407 text:

color:earlydevonian bar:NAM5 from: -411.2 till: -407 text: Liujiangolepis color:middledevonian bar:NAM6 from: -391.8 till: -385.3 text: Grossaspis color:latedevonian bar:NAM7 from: -385.3 till: -374.5 text: Taeniolepis color:latedevonian bar:NAM8 from: -378.1 till: -374.5 text: Lepadolepis color:latedevonian bar:NAM9 from: -374.5 till: -359.2 text: Jiangxilepus color:latedevonian bar:NAM10 from: -374.5 till: -359.2 text: Sinolepis color:latedevonian bar:NAM11 from: -374.5 till: -359.2 text: Xichonolepis 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:

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

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

References

*Long, John A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996. *

External links

Review of Antiarcha at Paleos
{{Taxonbar, from=Q150593 Prehistoric fish orders Articles which contain graphical timelines