This is the group that includes the worlds largest trilobite, Isotelus rex (Rudkin et al. 2003). Evit (1961) studied growth series of members of the Asaphidae from the Ordovician
of Virginia. He found there was a gap in the fossil record between late
larval growth stages and typical growth stages of post-larval animals,
the reasons for which are unclear. He noted the similarity of the
asaphid larvae with those of remopleuridids, the 'asaphid-remopleuridid
type' (later the asaphoid protaspis of Fortey and Chatterton 1988) and suggested that Asaphidae and Remopleurididae were closely related. However, Fortey and Chatterton (1988) put Ceratopygidae as the sister group to the Asaphidae, the Remopleurididae being placed within the Remopleuridacea.
Ogygiocarella debuchii, Family Asaphidae (photo by author)
