Australia: The Land Where Time Began

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Galeaspida (Helmet shield)

The Galeaspida are a group of extinct agnathans found only in southern China and northwestern Vietnam. Long suggests that the evolution by the galeaspids of a style of armour that differed from that of other agnathans was probably the result of isolation for long periods of time on the ancient continental blocks of southern China and northern Vietnam. According to Long some of the galeaspids had some of the most bizarre appearing armour of any known fish. There were up to 45 gill pair openings in forms such as Dongfangaspis. There was a single shield of bone, not separated into plates, apart from the ventral side of the head, as occurred in such forms as osteostracans and pituriaspids. In galeaspid armour the unique feature is a median dorsal fenestra, a large hole in front of the paired holes of the eyes, opening directly below into the nasal cavities that are paired. It has been discovered as a result of phylogenetic analysis that was carried out recently that 2 lineages independently evolved this fenestra, once in each of the polybranchiaspids and huananaspidiforms (Zhu & Zhikun, 2006).

The more than 80 known species of galeaspids are a diverse group. The tubes of laminated bone surrounding the soft tissue of the head is well preserved providing much information about the soft anatomy of these fish. The brain was complex and the inner ear consisted of 2 vertical semicircular canals. By the start of the Silurian the group had appeared in the fossil record, reaching their peak of diversity in the Early Devonian, most becoming extinct by Middle Devonian times, 1 species known from the Emsian and 1 from the Eifelian stages (Zhu, 2000). From Ningxia, northern China, of latest Devonian age, a single intermediate galeaspid form is known, the group finally becoming extinct at the end of the Devonian.

Polybranchiaspis, representing the average galeaspid, had radiating lines of sensory grooves on the dorsal surface of a simple ovoid shield. Tridensaspis represents the galeaspids that had triangular shields. Hanyangaspis that is now thought to be one of the basal forms (Zhu & Zhikun, 2006) represents the galeaspids with broad, almost semicircular shields. Huananaspis and Lungmenshanaspis are genera representing forms with more extreme shape development, having long, drawn-out bony processes in front and at the sides of the armour. Forms such as Sanchaspis also had a tube projecting forward, but had a bulbous enlargement at the end. Found in the north of Vietnam and throughout China, Polybranchiaspis was one of the first galeaspids to be named.

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 01/11/2011

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