Ontogeny, Vision and Life Habits of Sphaerophthalmus alatus (Boeck) from the Furongian of Bornholm  

Abundant specimens of Sphaerophthalmus alatus, which occurs together with Mesoctenopyge tumida, are present in the Furongian of Bornholm.  All the specimens are incomplete and disarticulated.  The described ontogeny is based upon detached cranidia, librigenae (though commonly incomplete), and pygidia, together with some protaspides.  Early meraspid cranidia are remarkably similar in shape to those of full grown adults, being narrow and highly convex.  Each librigena has a well preserved ovoid – subspherical eye.  The smallest eyes are c. 250 µm in length, and about 200 µm in height, with about 70 lenses of less than 30 µm in diameter; the largest are 550 µm long, and 500 µm in height, with about 150 lenses.  The largest lenses are up to 34 µm in diameter; these occur towards the top of the eye, where the hexagonal close packing is fairly regular.  Packing is less well constrained in the centre of the eye, and more regular where the lenses become smaller towards the base of the eye.  The visual field is virtually panoramic, from 0 to 180 degrees in a horizontal plane, and below the equator to 90 degrees above, but the fields of the two eyes do not overlap.The eye-parameter (main visual field) lies at about 5.4 µm rad, which corresponds to recent compound eyes adapted to dim light conditions.  The sensitivity of the eye is comparable to the living Nautilus pinhole eye.  The greatest depth at which the eye could function would be no more than 150 m in clear waters.

Two forms of juvenile pygidia are found in our material; one belonging to S. alatus, the other to M. tumida. The pygidia of the adult M. tumida are unknown, and we have had problems in relating these juvenile forms to the correct adult species.  The form with an initially serrated margin is considered to belong to S. alatus; that with a stout central spine to M. tumida. The body of Sphaerophthalmus is poorly known, but probably resembles that of Ctenopyge (=Eoctenopyge) angusta, which is in many ways intermediate between Ctenopyge and Sphaerophthalmus, and which could have been placed in either genus.

From the studies of the optical systems (eye parameter and visual field), S. alatus appears to be adapted to a benthic life.  This is consistent with previous reconstructions, which show the genal spines arranged horizontally but splayed out laterally.  A more extreme adaptation for a benthic life is that of C. angusta, in which the long genal spines would have acted as sled runners on the mud.  S. alatus, however was probably capable of swimming; the related S. humilis has genal spines which in life pointed downwards so it could not have lived on the sea floor.  The contemporanaeus M. tumida likewise has well preserved eyes which are distinctly kidney shaped, and the two forms are not easily confused.  The ontogeny and optics of this species are still being investigated.