title: On the sighted ancestry of blindness - exceptionally preserved eyes of Mesozoic polychelidan lobsters.
abstract: BACKGROUND
  Modern representatives of Polychelida (Polychelidae) are considered to be entirely blind and have largely reduced eyes, possibly as an adaptation to deep-sea environments. Fossil species of Polychelida, however, appear to have well-developed compound eyes preserved as anterior bulges with distinct sculpturation.
  
  
  METHODS
  We documented the shapes and sizes of eyes and ommatidia based upon exceptionally preserved fossil polychelidans from Binton (Hettangian, United-Kingdom), Osteno (Sinemurian, Italy), Posidonia Shale (Toarcian, Germany), La Voulte-sur-Rhône (Callovian, France), and Solnhofen-type plattenkalks (Kimmeridgian-Tithonian, Germany). For purposes of comparison, sizes of the eyes of several other polychelidans without preserved ommatidia were documented. Sizes of ommatidia and eyes were statistically compared against carapace length, taxonomic group, and outcrop.
  
  
  RESULTS
  Nine species possess eyes with square facets; Rosenfeldia oppeli (Woodward, 1866), however, displays hexagonal facets. The sizes of eyes and ommatidia are a function of carapace length. No significant differences were discerned between polychelidans from different outcrops; Eryonidae, however, have significantly smaller eyes than other groups.
  
  
  CONCLUSIONS
  Fossil eyes bearing square facets are similar to the reflective superposition eyes found in many extant decapods. As such, they are the earliest example of superposition eyes. As reflective superposition is considered plesiomorphic for Reptantia, this optic type was probably retained in Polychelida. The two smallest specimens, a Palaeopentacheles roettenbacheri (Münster, 1839) and a Hellerocaris falloti (Van Straelen, 1923), are interpreted as juveniles. Both possess square-shaped facets, a typical post-larval feature. The eye morphology of these small specimens, which are far smaller than many extant eryoneicus larvae, suggests that Jurassic polychelidans did not develop via giant eryoneicus larvae. In contrast, another species we examined, Rosenfeldia oppeli (Woodward, 1866), did not possess square-shaped facets, but rather hexagonal ones, which suggests that this species did not possess reflective superposition eyes. The hexagonal facets may indicate either another type of superposition eye (refractive or parabolic superposition), or an apposition eye. As decapod larvae possess apposition eyes with hexagonal facets, it is most parsimonious to consider eyes of R. oppeli as apposition eyes evolved through paedomorphic heterochrony.
  
  
  CONCLUSIONS
  Polychelidan probably originally had reflective superposition. R. oppeli, however, probably gained apposition eyes through paedomorphosis.
authors.affiliation: Université de Rennes 1, EA 7316, 263 Avenue du Général Leclerc CS 74205, 35042 Rennes Cedex, France ; Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements (CR2P, UMR 7207), Muséum national d'Histoire naturelle, Sorbonne Universités, UPMC, CNRS, 8 rue Buffon, F-75005 Paris, France.
authors.forename: Denis
authors.identifier: 0000-0003-3486-3552
authors.initials: D
authors.lastname: Audo
authors.affiliation: Department Biologie II und GeoBio-Center, Ludwig-Maximilians-Universität München, Biozentrum der LMU, Großhaderner Str. 2, 82152 Planegg-Martinsried, Germany.
authors.forename: Joachim T
authors.initials: JT
authors.lastname: Haug
authors.affiliation: Department Biologie II und GeoBio-Center, Ludwig-Maximilians-Universität München, Biozentrum der LMU, Großhaderner Str. 2, 82152 Planegg-Martinsried, Germany.
authors.forename: Carolin
authors.initials: C
authors.lastname: Haug
authors.affiliation: Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements (CR2P, UMR 7207), Muséum national d'Histoire naturelle, Sorbonne Universités, UPMC, CNRS, 8 rue Buffon, F-75005 Paris, France.
authors.forename: Sylvain
authors.initials: S
authors.lastname: Charbonnier
authors.affiliation: Staatliches Museum für Naturkunde Stuttgart, Rosenstein 1, 70191 Stuttgart, Germany.
authors.forename: Günter
authors.initials: G
authors.lastname: Schweigert
authors.affiliation: Zoologisches Institut und Museum, Lehrstuhl Allgemeine und Systematische Zoologie, Ernst-Moritz-Arndt-Universität Greifswald, Anklamer Str. 20, 17487 Greifswald, Germany.
authors.forename: Carsten H G
authors.initials: CH
authors.lastname: Müller
authors.affiliation: Zoologisches Institut und Museum, Lehrstuhl Cytologie und Evolutionsbiologie, Ernst-Moritz-Arndt-Universität Greifswald, Soldmannstr. 23, 17489 Greifswald, Germany.
authors.forename: Steffen
authors.initials: S
authors.lastname: Harzsch
keywords: Deep-sea adaptations; Heterochrony; La Voulte-sur-Rhône; Nusplingen; Osteno; Polychelida; Posidonia Shale; Solnhofen; Superposition eyes