CONES AND PRISMS: POSSIBLE SEXUAL DIMORPHISM IN THE HOLOCHROAL EYE STRUCTURE OF ASAPHID TRILOBITES

Holochroal eyes in trilobites have received comparatively little study in comparison to the schizochroal eyes of phacopid trilobites. Holochroal eyes are regarded as apposition compound type arthropod eyes and are distinguished by their often-elongated, prism-like lenses, as seen in Asaphus raniceps. We sectioned and studied the eyes of three species of Asaphus (A. cornutus, A. kowalewskii, and A. punctatus) from the Middle Ordovician near St. Petersburg, Russia using SEM, EDS, and EBDS techniques. The three studied species are easily distinguished by the length of their eyestalks, which are shortest in A. cornutus and extremely elongated in A. kowalewskii.

The lenses of all three species consist of single crystals of calcite, with microcrystals of calcite forming the cornea. A. kowalewskii displays prismatic lenses, like those observed in A. raniceps, although shorter along the prism length. A. cornutus and A. punctatus specimens display both prismatic and conical lenses. This dimorphism in eye structure could be explained by sexual dimorphism, which is also observed in modern insects and related to the different ability of males and females to detect light.

The structure of trilobite eyes has played a prominent role recently in debates over the affinities of the clade—either being related to basal chelicerates or basal mandibulates. Crystalline cones are present in mandibulate eyes whereas chelicerates, like Limulus, have prism-like lenses. Published accounts of holochroal trilobite eyes clearly show the spherical or prism-like lenses. Yet, recently, there has also been a claim of crystalline cones in two species of trilobites—a claim that is debated in relation to diagenesis and preservation of original structures. This study clearly demonstrates the presence of crystalline cones in holochroal trilobite eyes. The dimorphism of these lenses urges caution in using this characteristic to evaluate chelicerate vs. mandibulate affinities of trilobites, while it can help clarify aspects of ecological adaptation of Asaphus trilobites.