Australia: The Land Where Time Began

A biography of the Australian continent 

Trilobites

Trilobites of the Soom Shale, South Africa

Mucronaspis, a genus of trilobite, that has a widespread distribution, has been found in the Soom Shale about 100 km (60 miles) from the main Keurbos Locality. The Soom Shale has been dated based on this trilobite and the brachiopods. Soomaspis splendida, a strange animal, has been described (Fortey & Theron, 1995). As with an agnostid trilobite, it has no eyes, 3 thoracic segments, a cephalon and a pygidium about the same size (isopygous). Because it has an uncalcified cuticle it is placed among the naraoiids (Naraoia from the Burgess shale). Re-examination of the relationship between the naraoiids and all other trilobites by Fortey & Theron was prompted by the discovery of  Soomaspis.

Agnostid and Naraoiid trilobites resemble young stages of trilobites that are more typical, at least superficially, as they have a reduced number of thoracic segments, a feature of the young of other trilobites that begin life with no segments, adding them 1 at a time through ontogeny. The small size and reduced number od appendages of the agnostids suggests they evolved by the process of progenesis, an immature stage of their ancestral trilobite reaching maturity before completing the rest of the growth stages of the ancestral animal. The result of this early maturation allowed them to avoid a long existence as benthic animals, remaining within the plankton and have a rapid turnover of generations.

Naraoiids were larger trilobites with few if any thoracic segments, and as can be seen in Naraoia from the Burgess Shale, they had uncalcified shields with many appendages beneath. During the development of naraoiids the thoracic segments are the only ones to be lost, while they grew at normal or enhanced growth rates, maturation occurring as usual when adult size was reached, the process involved being hypermorphosis. Based on the small size and calcified exoskeleton of agnostids, as well as the different development route followed, it has been argued that naraoiids and agnostids were not related (Fortey & Theron, 1995), though it is still possible they could both be members of the Trilobita.

The authors¹ suggest that agnostids can be derived from normal Cambrian trilobites, their loss of eyes, small size, etc. are secondary developments. It is believed naraoiids are primitive trilobites that didn't develop dorsal eyes, a cuticle that was calcified, or the many segments that were normal in typical trilobites.

Sources & Further reading

1. Selden, Paul & Nudds, John, 2004, Evolution of Fossil Ecosystems, Manson Publishing.

Links

1. A guide to the orders of trilobites
2. Introduction to Trilobites
3. Images of Trilobites