The ontogeny of the Ordovician trilobite Ovalocephalus and its bearing on the affinity and evolution of the genus

Conference item

Ontogenetic material of Ovalocephalus primitivus has been recovered from the Ordovician Dawan Formation (Arenig Series) of Anhui, South China. The metaprotaspis has a forwardly arched anterior margin, anteriorly expanded glabella, and short anterior and midfixigenal spines. The Hammatocnemidae, in which Ovalocephalus has hitherto been placed, is therefore now better regarded as a separate subfamily within the Pliomeridae. Ontogenetic changes include the successive abaxial and forward expansion of the glabella, posterior shift of the palpebral lobe and related shortening of the posterior fixigenal field, abaxial shift of the posterior branch of the facial suture, reduction of the fixigenal spines, widening of the axis and the terminal area, and reduction in the number of pygidial axial rings and ribs. Ovalocephalus contains 11 species ranging from Arenig to Ashgill in age. Adults of stratigraphically later species have a more anteriorly situated palpebral lobe, a longer posterior fixigenal area, and a narrower basal part of the glabella (all immature features of the ancestral form) suggesting that paedomorphosis plays an important role in the evolution of the genus.

Authors: Yuan, W; Zhou, Z; Siveter, D; Zhou, Z

• Publication status: Published
• Host title: TRIBOLITES AND THEIR RELATIVES
• Volume: 70
• Issue: 70
• Pages: 259-269
• Publication date: 2003-01-01
• ISSN: 0038-6804
• ISBN: 0901702811
• Keywords: evolution; Pliomeridae; trilobite; Ovalocephalus; Hammatocnemidae; ontogeny; Ordovician