Antiarchi ("opposite anus") is an order 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, who, when examining some fossils that he thought were armored tunicates related to Chelyosoma, 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, particularly with later forms. The pair of pectoral fins were modified into a pair of caliper-like, or arthropod-like limbs. In primitive forms, such as Yunnanolepis, 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.
Below is a cladogram from Jia et al. (2010):[1]
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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
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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 color:earlydevonian bar:NAM4 from: -416 till: -407 text: Yunnanolepis 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: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