Evolution of Eye Reduction and Loss in Trilobites and Some Related Fossil Arthropods

Abstract

The fossil record of arthropod compound eyes reflects different modes and occasions of eye reduction and blindness. In the best-studied fossil examples, the trilobites [trilobites: extinct arthropods, dominant during the Palaeozoic], which have an excellent geological record, eyes are primary structures, and in all known genera which lack them, eye-loss is always secondary. Once the eyes were lost, they never were never re-established. The most striking examples occur in the Upper Devonian, when two unrelated major groups of trilobites, with different kinds of eyes, underwent eye reduction and even total loss of the eyes over the same time period, undoubtedly due to long-term environmental change. One reason is that a mud blanket spread over a vast area, there was no firm substrate, and many trilobites became small and many became endobenthic, reducing or losing their eyes in the process. Toxic environmental conditions may also have had an effect. Certain coeval forms remained, however, which still possess perfectly good compound eyes. Either they found vacant refuges where they could survive, or alternatively their visual systems were elaborate enough to adapt to the changing conditions. Another inducement for evolving small, reduced compound eyes is to become a tiny organism oneself, with simply not enough space to establish a regular and functional compound eye, and in such minaturised eyes special adaptations for capturing enough photons are necessary. Thus very small compound eyes often establish wide acceptance angles of their ommatidia, collecting light over large angular ranges of space and it is beneficial to have a wide rhabdom provided that it is short, has a wide lens diameter, and perhaps even possess highly sensitive receptor cells. We find such a miniaturised system in the first recorded planktonic trilobite. Another kind of reduction of a compound eye, or parts of it, also occurs, if selective pressure claims for a high specialisation of eyes that results in several facets fusing into a single functional unit. This probably can be found in phacopid trilobites, ~400 million years old. Here the enlarged aperture of a resulting large lens may allow vision under dim light conditions such as at greater depth. The fossil record gives relatively little evidence about parasites, which often have reduced eyes. Agnostida are blind relatives of trilobites which lived during the Cambrian and Ordovician. An early suggestion was that some of these were parasitic, but this was never commonly adopted. Finally penstastomids (Crustacea), worm-like parasitic organisms, already have been blind from the Cambrian (~487Ma).

Hard to get, easy to lose: Evolution of mantle photoreceptor organs in bivalves (Bivalvia, Pteriomorphia)

“…Although we did not obtain statistical support, previous work suggests correlations between eye loss and infaunalization in ark clams and relatives (Arcida) (Audino et al. 2019) and other invertebrate groups, such as trilobites (Schoenemann 2018). We speculate that the broad taxonomic sampling of Pteriomorphia, which included the semi‐infaunal representatives of Pinnidae and Mytilidae, whose ancestors likely never had eyes, might have contributed to mask a possible correlation between loss of eyes and infaunality.…”

Rise and fall of the phacopids: the morphological history of a successful trilobite family

“…2011). This trend was also noticed in other families (Feist & Clarkson 1989; Lerosey‐Aubril & Feist 2012; Schoenemann 2018). As for the Silurian, the existence of blind phacopids was associated with a life in deeper environments (Feist et al .…”

Morphological disparity trends in Devonian trilobites from North Africa

“…6; Table 2). During the Devonian, paedomorphic evolutionary trend s have been observed in Late Devonian taxa showing progressive eye reduction between closely related species that eventually leads to blindness (Feist & Clarkson 1989; Feist 1995; Crônier & Courville 2003; Schoenemann 2018). Such eye reduction implies morphological changes as the enlargement of fixigenae observed among Proetida or the ‘displacement’ of the facial suture towards the outer margins observed among Phacopida (Feist 1995; Crônier & Courville 2003).…”