Antiarchi

Not to be confused with acritarch.

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.

Antiarchi Temporal range: Ludfordian[1]- Famennian 425.6–358.9 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Bothriolepis canadensis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	†Placodermi
Order:	†Antiarchi Cope, 1885

Fossil fish slab, cast. Late Devonian, Mandagery Sandstone, Canowindra, New South Wales (Australia). Preserves 114 fish individuals, which died when their freshwater pond dried up. Most of the individuals in the slab are the antiarch placoderms Remigolepis walkeri and Bothriolepis yeungae. One sarcopterygian individual is present, Canowindra grossi, the largest fish in the slab. Two small and inconspicuous juvenile Groenlandaspis are also preserved.^[2]

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.

Phylogeny

Below is a cladogram from Jia et al. (2010):^[3]

Antiarchi

Parayunnanolepis Dayaoshania Grenfellaspis Euantiarcha Bothriolepidoidei Luquanolepis Nawagiaspis Jiangxilepididae Jiangxilepis Ningxialepis Dianolepis Bothriolepididae Bothriolepis Grossilepis Monarolepis Briagalepis Microbrachiidae Wudinolepis Hohsienolepis Microbrachius Asterolepidoidei Byssacanthus Hunanolepis Kirgisolepis Pterichthyodes Gerdalepis Sherbonaspis Stegolepis Asterolepis Remigolepis Pambulaspis

Timeline of genera

Chuchinolepis, Vanchienolepis and Yunnanolepis

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)". Earth System Science Data. 15 (1): 41–51. Bibcode:2023ESSD...15...41P. doi:10.5194/essd-15-41-2023. ISSN 1866-3508.
2. Gennatas, Lou-Andrea (2022-03-28). Geochronology and provenance of the Late Devonian Canowindra fish bed, Lachlan Orogen (thesis thesis). Macquarie University. doi:10.25949/19432913.v1.
3. Jia, L. T.; Zhu, M.; Zhao, W. J. (2010). "A new antiarch fish from the Upper Devonian Zhongning Formation of Ningxia, China". Palaeoworld. 19 (1–2): 136. doi: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)". Bulletins of American Paleontology. 364: 560. Archived from the original on 2004-10-14. Retrieved 2011-05-20.

External links

• Review of Antiarcha at Paleos