Image formation by bifocal lenses in a trilobite eye?

József Gál, Gábor Horváth, Euan N.K. Clarkson, Ottó Haiman

Research output: Contribution to journalArticlepeer-review

31 Scopus citations


In this work we report on a unique and ancient type of eye, in which the lower surface of the upper calcite lens units possessed an enigmatic central bulge making the dioptric apparatus similar to a bifocal lens. This eye belonged to the trilobite Dalmanitina socialis, which became extinct several hundred million years ago. As far as we know, image formation by bifocal lenses of this kind did/does not occur in any other ancient or modern animal visual system. We suggest that the function of these bifocal lenses may be to enable the trilobite to see simultaneously both very near (e.g. floating food particles and tiny preys) and far (e.g. sea floor, conspecifics, or approaching enemies) in the optical environment through the central and peripheral lens region, respectively. This was the only reasonable function we could find to explain the puzzling lens shape. We admit that it is not clear whether bifocality was necessary for the animal studied. We show that the misleading and accidental resemblance of an erroneous correcting lens surface (designed by Rene DesCartes in 1637 [DesCartes, R. (1637). Oeuvres de DesCartes. La Geometrie. Livre 2. pp. 134. J. Maire, Leyden] to the correcting interface in the compound Dalmanitina lens may be the reason why the earlier students of the Dalmanitina lens did not recognize its possible bifocality. Copyright (C) 2000 Elsevier Science Ltd.

Original languageEnglish
Pages (from-to)843-853
Number of pages11
JournalVision Research
Issue number7
StatePublished - Mar 2000

Bibliographical note

Funding Information:
This work was supported by a Zoltán Magyary postdoctoral fellowship from the Foundation for the Hungarian Higher Education and Research, and by a János Bolyai research scholarship from the Hungarian Academy of Science received by G. Horváth. The grant OTKA F-012858 received from the Hungarian National Science Foundation is gratefully acknowledged. Many thanks are due to Riccardo Levi-Setti (Enrico Fermi Institute, University of Chicago, USA), Hansjürgen Dahmen (Lehrstuhl für Biokybernetik, Universität Tübingen, Germany) and Ronald H.H. Kröger (Anatomisches Institut, Universität Tübingen), who read and commented on earlier versions of the manuscript. Many thanks are for the valuable criticism, suggestions and comments of two anonymous reviewers.


  • Bifocal calcite lenses
  • Image formation
  • Optics
  • Schizochroal trilobite eye
  • Trilobite vision
  • Trilobites

ASJC Scopus subject areas

  • Ophthalmology
  • Sensory Systems


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