# Antiarchi

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Not to be confused with [Acritarch](/source/Acritarch).

**Antiarchi** ("opposite anus") is an [order](/source/Order_(biology)) of heavily armored [placoderms](/source/Placoderm). The antiarchs form the second-most successful group of placoderms after the [arthrodires](/source/Arthrodire) in terms of numbers of species and range of environments. The order's name was coined by [Edward Drinker Cope](/source/Edward_Drinker_Cope), who, when examining some fossils that he thought were armored [tunicates](/source/Tunicate) related to *[Chelyosoma](/source/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](https://en.wiktionary.org/wiki/sinuous), particularly with later forms. The pair of [pectoral fins](/source/Pectoral_fins) were modified into a pair of [caliper](/source/Caliper)-like, or [arthropod](/source/Arthropod)-like limbs. In primitive forms, such as *[Yunnanolepis](/source/Yunnanolepis)*, the limbs were thick and short, while in advanced forms, such as *[Bothriolepis](/source/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](/source/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](/source/Cephalaspidomorphi).

## Phylogeny

Below is a [cladogram](/source/Cladogram) from Jia *et al.* (2010):[2]

## References

1. Pan, Zhaohui; Niu, Zhibin; Xian, Zumin; Zhu, Min (2023-01-03). ["A novel specimen-based mid-Paleozoic dataset of antiarch placoderms (the most basal jawed vertebrates)"](https://essd.copernicus.org/articles/15/41/2023/). *Earth System Science Data*. **15** (1): 41–51. [Bibcode:2023ESSD...15...41P](https://ui.adsabs.harvard.edu/abs/2023ESSD...15...41P). [doi:10.5194/essd-15-41-2023](https://doi.org/10.5194/essd-15-41-2023). [ISSN 1866-3508](https://www.worldcat.org/issn/1866-3508)

1. Jia, L. T.; Zhu, M.; Zhao, W. J. (2010). "A new antiarch fish from the Upper Devonian Zhongning Formation of Ningxia, China". *[Palaeoworld](/source/Palaeoworld)*. **19** (1–2): 136. [doi:10.1016/j.palwor.2010.02.002](https://doi.org/10.1016/j.palwor.2010.02.002)

- Long, John A. **The Rise of Fishes: 500 Million Years of Evolution** Baltimore: The Johns Hopkins University Press, 1996. ISBN 0-8018-5438-5
- Sepkoski, Jack (2002). ["A compendium of fossil marine animal genera (Placodermi entry)"](https://web.archive.org/web/20041014071343/http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=564&rank=class). *Bulletins of American Paleontology*. **364**: 560. Archived from [the original](http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=564&rank=class) on 2004-10-14. Retrieved 2011-05-20.

## External links

- [Review of Antiarcha at Paleos](http://www.palaeos.com/vertebrates/placodermi/antiarchi.html)

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Adapted from the Wikipedia article [Antiarchi](https://en.wikipedia.org/wiki/Antiarchi) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Antiarchi?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.