Image Credit: sulla55
Most people know that Trilobites had compound eyes, if only because creationists use them as an example of how evolution simply must be false. The thing that a lot of people don’t know is that there are different types of Trilobite eyes, and scientific debates over them.
Trilobites do provide some of the earliest examples of complex visual systems, but eyes didn’t evolve in Trilobites. Richard Fortey suggests that eyes developed up to 250 million years before the first known trilobites appeared.1 Eyeless species certainly existed, but besides the visually impaired Agnostids, eyeless Trilobites were the exception, not the rule. Eyeless species seem to have lost their vision over time, likely because they didn’t need it in the ecological niche they inhabited (for instance, if they lived in a very dark environment).
There are three types of Trilobite eyes: Holochroal, Schizochroal and Abathochroal. Holochroal eyes were the most common, and can be found in most Trilobite orders. Schizochroal eyes are rarer, seeming to be an evolutionary development unique to portions of the order Phacopida. Abathochroal eyes are also uncommon, present only in the Agnostid suborder Eodiscina during the Cambrian. 2
Trilobite eyes are extremely unique in the material they are made of, calcite crystal. Calcite (aka. Calcium Carbonate) is very common, with limestone and chalk being two examples. In its purest form, calcite is clear. According to Richard Fortey, Trilobites are unique among animals in having these “crystal eyes,” as he refers to them.3
Holochroal eyes are composed of a large number of hexagonal (six-sided) calcite crystal prisms. By the nature of the crystal’s structure, light can only pass unhindered through it along the crystalline axis (C-Axis). If light hits front any other angle, refraction will occur within the crystal, and if the crystal is long enough, the only light that can make it from one end to the other is along the c-axis. The picture above shows the c-axis of the cystal, as well as how they are “stacked” to form a Holochroal compound eye. Each of these crystal lenses has a cluster of photosensitive cells at the bottom which transmit the light information into the neural system of the Trilobite.
What all that means is that in Holochroal eyes, each lens can only “see” light that comes in at an angle almost exactly perpendicular to the surface of the lens. In other words, each lens can only see things that are basically straight in front of it. This is essentially how a modern compound eye works too, and it results in low resolution images, a large mosaic of very narrow images. To create a wide field of vision, the lenses are placed on a curved surface, as shown below.
Humans and many other creatures are able to focus their eyes by changing the shape of the lens (accomodation, in scientific parlance). With their crystal eyes, Trilobites couldn’t do this (and neither can modern compound eyes). But this type of eye does have some benefits. Compound eyes sense movement really well, something that was probably more important than image resolution for a Trilobite that needed to enroll (roll up into a ball) before a predator snapped it up. Simple eyes (like those of humans) are also quite limited in their field of vision. By stacking thousands of lenses on a curved surface, some Trilobites had basically a 360 degree view of their environment.
Holochroal eyes, although unique in some ways, are just normal compound eyes in the end. But this was not the case for the Schizochroal eyes of order Phacopida. I can’t lie to my readers. I didn’t write this post to talk about Holochroal eyes. I wanted to talk about Schizochroal ones. Unfortunately, this post is pretty long already, so I’m leaving it at this, and tomorrow I’ll talk about schizochroal eyes, which are unique to Trilobites (and only Phacopids at that) in many ways.
As a postscript, Craig Dylke (his blog is Weapon of Mass Imagination) dug up this video, which has a good visualization of a compound eye (although in this case it’s the compound eye of an Anomalocarid). Consider the Arabic subtitles a special bonus, as well! The compound eye-view segment occurs about 1:26 into the video:
Don’t forget to check back tomorrow for Part 2 of this post, which will take an in-depth look at the SUPER COOL eyes of Phacopid Trilobites! If you find anything inaccurate in this post, please comment! A fair bit of research went into it, but we all make mistakes!