Heterochrony in Silurian phacopid trilobites as suggested by the ontogeny of Acernaspis

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Abstract
New material of early growth stages of the Silurian (Llandovery) trilobite Acernaspis is described. Pre-adult ontogenetic stages of this genus closely resemble adults of the post-Llandovery genus Ananaspis. A heterochromic descent of Ananaspis from Acernaspis is proposed. Ananaspis is interpreted as paedomorphic, having arisen largely through neoteny. Neotenic changes already appear in the lineage in the last Acernaspis species, and Ananaspis then underwent continuous neotenic change throughout its known Silurian history.