Australia: The Land Where Time Began

A biography of the Australian continent 

Anaspida "no shield"

Anaspids were simple agnathans that had laterally compressed and an appearance somewhat like an eel. According to Long their bodies may or may not have been covered by elongated, thin scales. Most were small, less than 15 cm in length. Some might have possibly have grown much larger. Long has described some anaspid scales found in Early Silurian deposits in Canada as being huge, though he didn't mention their size. A lot of information of the anatomy of anaspids has been provided by Alex Ritchie (Ritchie, 1964, 1980), a group that flourished from the Silurian to the Early Devonian. Simple fins developed along the dorsal and ventral ridges of their bodies, and forms such as Jamoytius and Pharyngolepis also had radials supporting lateral fins that were well developed. A form found in the Middle Silurian of Scotland, Cowielepis, has demonstrated the possibility that there may have been a long ventrolateral fin along both sides of the body of all anaspids (Blom, 2008). The tail was supported by the body axis directed downward and a thin dorsal (epichordal) lobe was present.

There were elaborate dorsal scale arrangements along the body ridge on forms such as Birkenia and Lasanius. There was a single nasopharyngeal opening on the top of the head, as is present in lampreys and osteostracans. Gills took the form of a row of holes along the side, from 6-15 pairs. Anaspid fossils are known only from the Euramerican continent and Scotland, Norway, Estonia and Canada. Anaspids have been suggested by Philippe Janvier and Marius Arsenault to be the fossils closest to the ancestral line on lampreys, based on Endeiolepis and Euphanerops, anaspid-like animals of Late Devonian age that were recently discovered at Scaumenac Bay in Canada. Endeiolepis is suspected by Janvier to possibly be the same as Euphanerops, since it has been found that ventrolateral scales of Endeiolepis are now known to be the internal cast of the branchial basket of Euphanerops. Both these fossil forms from Canada have long rows of gill arches possibly stretching almost to the tail, possibly being as many as 30 gill pairs in Euphanerops.

In 2007 Henning Blom completed a major revision of Birkeniid anaspids from the Northern Hemisphere. He described 15 new special, 10 new genera, and 2 new families of anaspids, providing a detailed phylogenetic framework for the group.

Long suggests that anaspids probably had a lifestyle very similar to lampreys, either feeding on live fish as parasites or on detritus. Apart from the Scottish species that could have come from freshwater deposits, their remains have mostly been found in marine sediments. He also suggests they may have spent part of their lives in fresh water and the remainder in the oceans as lampreys do.

Sources & Further reading

  1. Long, John A., 2011, The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 2011

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 31/10/2011 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading