Order Asaphida
The Order Asaphida is a large and diverse group
of trilobites, including approximately a fifth of the entire class.
They probably originated in the middle Cambrian from the Anomocaracea
and, except for one genus, Rhaphiophorus, which lasted into the later Silurian, all died out in the mass extinction event at the end of the Ordovician (see below). Why this may be was discussed by Fortey and Chatterton (1988),
who suggested the planktonic asaphoid larva may have been affected by a
change in oceanic circulation; the planktonic graptolites were also
badly affected by this extinction. There are various features that characterise the Asaphida, such as the ventral median suture in primitive members, the common asaphoid protaspis and a glabellar tubercle present in advanced members of the group.
This website offers a brief introduction to the group, some of its major sub-groups, morphological features and life habits. A glossary is also provided for reference.
Acknowledgements. Thanks are
given to the Palaeontological Association for permission to use the
diagram of evolutionary relationships given below.
Evolutionary relationships of the Asaphida. From Fortey and Chatterton (1988).
Author: Laurence Dale
Last updated: 19/11/06
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