Over at Evolution News and Views, an article by Dr. Cornelius Hunter titled, Irony Alert: Michael Shermer on “When Facts Fail”, accuses evolutionists (and especially Shermer) of intransigence in the face of awkward facts which spoil their case. Shermer recently authored an article in Scientific American, in which he noted that “people seem to double down on their beliefs in the teeth of overwhelming evidence against them” because revising these beliefs threatens their worldview. Dr. Shermer proposed that the best way to persuade people to revise their erroneous beliefs is to convince them that dropping these beliefs will not require them to change their worldview. When people are reassured that their fundamental worldview is not at stake, they can them examine the evidence dispassionately. Dr. Hunter was not impressed: he maintains that evolutionists are leading offenders, when it comes to refusing to revise their beliefs.
Dr. Hunter points to trilobites as his star example of “facts that fail” to square with the alleged “evidence for evolution.” However, a recurring failing of Dr. Hunter’s criticisms is that they reveal a lack of familiarity with the scientific literature – especially the most recent writings on the issues he raises.
Dr. Hunter cites several sources to back up his contention that trilobites are a stumbling block for evolution. Curiously, though, he never even mentions the leading online resource for information about trilobites: the award-winning Website, www.trilobites.info, which is maintained by Dr. Sam Gon III, a biologist (PhD, Animal Behavior; MA, Zoology [Ecology, Behavior and Evolution]) who is greatly intrigued by the expression of ancient biodiversity that trilobites represent. Dr. Gon is also the Senior Scientist for The Nature Conservancy’s Hawai‘i Field Office in Honolulu.
Instead, Dr. Hunter cites Wikipedia, a 43-year-old article on trilobite eyes, and a 37-year-old article by Niles Eldridge on the absence of fossil transitional forms in the evolution of trilobites. If I were trying to mount a case against evolution, I would want better sources than that.
The evolutionary origin of trilobites
Let’s begin with the Wikipedia article on trilobites, cited by Dr. Hunter:
Early trilobites show all the features of the trilobite group as a whole; transitional or ancestral forms showing or combining the features of trilobites with other groups (e.g. early arthropods) do not seem to exist. (Bolding is Dr. Hunter’s.)
The article lists various alleged transitional forms, declaring them “far from compelling,” but argues that it is “still reasonable to assume… that trilobites share a common ancestor with other arthropods.” It continues:
Evidence suggests that significant diversification had already occurred before trilobites were preserved in the fossil record, easily allowing for the “sudden” appearance of diverse trilobite groups with complex derived characteristics (e.g. eyes)., 
I’ll say more about trilobite eyes in a moment, but first, let’s go back to the original quote. It doesn’t say that there are no fossils of transitional forms; instead, what it says is that there are none “combining the features of trilobites with other groups.” In other words, no mosaics. That’s quite different from saying trilobites have no fossil ancestors.
And now let’s compare what Wikipedia says with Dr. Sam Gon III’s carefully-researched article, Origins of Trilobites. It turns out that a highly plausible sequence of transitional forms leading to trilobites exists in the fossil record:
So where did trilobites come from?
The likely scenario is that trilobites arose from Precambrian bilaterians, arguably arthropods, that gave rise to Cambrian arachnomorphs, among them trilobites. The evidence is neither clear nor unambiguous. The fossil record is spotty, but suggestive, and only some remarkable sites such as Chengjiang, Kaili, and the Burgess Shale reveal the rich diversity of non-calcified arachnomorph arthropods. The fossils of the Precambrian reveal some bilaterian diversity, among them a few species that might be candidates for trilobite ancestors. Perhaps it is the simple, dorsally unsegmented Precambrian fossil, Parvancorina, that offers the most reasonable link to arachnomorphs. Lin et al, 2006 strongly linked Parvancorina to an unambiguously arthropodan Cambrian creature, Skania sundbergi, closely related to Primicaris larvaformis. Similar taxa have been documented in Australia, Chengjiang, Kaili, and the Burgess Shale (see image of Skania fragilis, left). If neither Skania nor the protaspid stage of trilobites were preserved, it would have been difficult, if not impossible, to make the link between Parvancorina and trilobites. As it is, both Parvancorina and Skania/Primicaris can be placed in a relationship that might look something like the sequence below…
There follows a diagram, titled, From Parvancorina to Trilobite in Four Easy Steps. Dr. Gon continues:
The figure above is a series of ontogeny diagrams to demonstrate the sequential steps between Parvancorina and trilobites:
In the ontogeny of Parvancorina (red path on left edge above), ontogeny is simple, and there is little change in the structure of the animal as it grows. The adult animal is a large version of the immature, except with more pairs of legs under a large, undifferentiated dorsal carapace. In a stylized Primicaris (magenta path), there is likewise little change in the animal during ontogeny, but there is incipient dorsal tagmation [segmentation of the back – VJT]; a hint of distinction between the cephalic [head] region and the rest of the body, indicated by a change in the curvature of the lateral margin (and genal spines in Skania), as well as the truncation of lateral (presumed digestive) branches. In Naraoia, (purple pathway) separation of the cephalon from the rest of the body is clear even in early ontogeny (purple arrow), but otherwise the similarity of ontogeny to Primicaris is easily seen. In the Helmetiidae (indigo path), there is a pygidium [tail], and thoracic segments are added to the developing body from the pygidium forward (indigo arrow).
Finally, in a typical trilobite (rightmost path), the protaspid stage [an early stage prior to the development of segments – VJT] resembles Parvancorina, the earliest meraspid [segment added] is defined by a single suture between cephalon and pygidium (as in Naraoia), and as growth occurs, segments are added from the pygidium to the thorax, as in helmetiids. What sets trilobites apart are a set of synapomorphies (blue arrow) – such things as calcification of the exoskeleton (from the protaspis onward in advanced trilobites, but only from meraspis forward in Agnostida and Olenellida, as far as is known), dorsal eyes with sutures, specialized hypostomal features, etc.
When examined comparatively, as above, it is fairly easy to see how separation of cephalon from body, and then addition of additional tagmata (segments) during growth are the key developmental character additions that take us from Parvancorina to a trilobite form. Taken in sequence, and with legs added to accentuate their underlying shared arthropod heritage, the links between Parvancorina, Primicaris, Naraoia, Kuamaia (a helmetid), and trilobites seem easier to visualize in the sequence below… (Bolding of phrases is mine – VJT.)
Now, it is quite true that the status of the unsegmented fossil Parvancorina as a trilobite ancestor remains the subject of controversy (but see here for a defense of their place in trilobite evolution). However, a much stronger case can be made for Naraoia, and I would invite skeptical readers to check out Dr. Sam Gon’s Web page on this fossil and make up their own minds. Wikipedia evidently agrees with Dr. Gon, for it declares:
When Harry B. Whittington began dissecting some specimens (Naraoia was among the most populous of the Burgess Shale animals), he discovered that the legs (and gills) of the beasts were very similar, if not identical to those of trilobites, thus the current placement of Naraoia in class Trilobita.
Let me remind readers that if Dr. Hunter wishes to argue that the appearance of trilobites in the fossil record cannot be explained by the modern theory of evolution, then the onus is on him to prove that. In order to undercut Dr. Hunter’s case, all a paleontologist needs to do is to point to a plausible series of fossil ancestors.
Did trilobites evolve?
Dr. Hunter would also have us believe that there are no instances in the fossil record of one species of trilobite evolving into another. To bolster his case, he cites the paleontologist Dr. Niles Eldredge, writing on the difficulties of the standard evolutionary theory:
If this theory were correct, then I should have found evidence of this smooth progression in the vast numbers of Bolivian fossil trilobites I studied. I should have found species gradually changing through time, with smoothly intermediate forms connecting descendant species to their ancestors.
Instead I found most of the various kinds, including some unique and advanced ones, present in the earliest known fossil beds. Species persisted for long periods of time without change. When they were replaced by similar, related (presumably descendant) species, I saw no gradual change in the older species that would have allowed me to predict the anatomical features of its younger relative.
However, Dr. Hunter’s citation from Eldredge is decades old: it dates back to 1980. Dr. Sam Gon’s 2008 online article, Evolutionary Trends in Trilobites, tells quite a different story:
Through the 300 million years that trilobites existed, prior to their extinction in the Permian, there were many opportunities for diversification of form, starting from the presumed primitive morphology exemplified by a species such as Redlichia (left). This typical primitive morphotype had a small pygidium, well developed eye ridges, a simple, lobed glabella, several thoracic segments, and a rather flattened body form. The first trilobites were characterized by this primitive form…
…An analysis of morphological diversity of trilobite forms showed that increasing from the Cambrian, there was a peak in morphological diversity in the Ordovician (which parallels a peak in overall diversity of trilobites families) that decreased only as overall trilobite diversity decreased toward their extinction in the late Permian.
Within this diversification, there were a number of evolutionary trends in morphology that developed in unrelated clades, creating homeomorphy (attainment of similar forms in unrelated groups). These homeomorphic trends, such as effacement, increased spinosity, reduction in body size, streamline shape, and loss of eyes, can not be reliably or consistently used to assess higher systematic relationships. Instead, these features can tell us about selective pressures on trilobites and how similar solutions were derived in parallel by different evolutionary lineages.
And for readers who would like more detail of an actual lineage of trilobites evolving over a period of millions of years, here’s one from Russia that should convince even the most hardened skeptic (see here and here for the family tree):
The trilobite fossil record is among the best of all the animal groups. It is not surprising then that it sometimes reveals sequences of evolutionary transition, recording the adaptation, or descent with modification, to new selective pressures as a matter of survival, or perhaps record failure to adapt, leading to extinction. One great example is the radiation of Baltic asaphid trilobites of family Asaphidae from the region that is now Saint Petersburg, Russia from late in the lower Ordovician and into the middle Ordovician. A hundred years of collecting of these strata has provided tremendous data set for cladistics analysis. The region that has long yielded a tremendous diversity of trilobites is believed to have been an inland sea during the earlier Ordovician that was cut off from the ocean to the west. At some point toward the late Ordovician the inland sea again became connected to the ocean. The resulting flow of sediment (turbidity) clouded the water and settled on the seafloor. Through some 50 foot of limestone encompassing some two million years, some Asaphidae lines of descent developed stalked eyes of ever increasing height, ostensibly to enable them to better spot either prey or predators, or both. The figure shows selected species and lines of descent. Not all lines produced elevated eyes. The figure below depicts the two best known: 1) Asaphus lepidurus to Asaphus expansus, which branches into Asaphus cornutus with moderate stalked eyes, and then to Asaphus kowalewskii, with very high eye stalks that almost seem unnatural. The other branch from Asaphus expanses leads to three species with successively higher eyes, Asaphus kotlukovi, Asaphus punctatus and Asaphus intermedius. (Bolding mine – VJT.)
Selective pressures due to salinity and turbidity led to adaptations of Asaphus genus Russian Ordovician Trilobites (see figure below). Asaphus expansus branched into two Asaphidae lines of descent, one ultimately leading to Asaphus kowalewski with high eyestalks, and the other branching into two lines, one leading to Asaphus plautini with its large, sharp genal spines genal, and the other leading to another sequence of ever higher eye stalks in Asaphus kotlukovi, then Asaphus punctatus, followed by Asaphus intermedius and finally Asaphus convincens. These observed evolutionary trends are supported by a century of cladistics research that document about two million years of adaptive radiation within the Asery horizon that is more than 50 feet thick. (Bolding mine – VJT.)
Once again, I would urge readers to have a look at this family tree. You be the judge: does this look like “no gradual change in the older species”? By the way, Dr. Eldredge is a thoroughgoing evolutionist, who happens to be an outspoken proponent of the theory of punctuated equilibrium. He describes his work on trilobites here.
I think we can fairly conclude that: (i) trilobite evolution is an established fact, and that it occurs gradually on at least some occasions; and (ii) while Dr. Niles Eldredge knows an awful lot about trilobites, he never went digging for them in Russia.
Next, Dr. Hunter argues that trilobite eyes pose a special conundrum for evolution, since these animals’ eyes “were perhaps the most complex ever produced by nature.1” The citation for this assertion is a 1974 paper by Lisa J. Shawver, titled, “Trilobite Eyes: An Impressive Feat of Early Evolution” (Science News, Vol. 105, p. 72), but curiously, no quotation from the paper is given. However, it seems that back in 1976, evolution-skeptic Randy L. Wysong, author of The Creation-Evolution Controversy (Inquiry Press, Midland, Michigan), quoted Shawver (p. 345) as claiming that trilobites “possessed the most sophisticated eyes ever produced by nature” – a quote that has been mined dozens of times since. Trouble is, similar claims have also been made for the eyes of the mantis shrimp – only to be roundly debunked a few months later, when more evidence came to light. An online article at the Virtual Fossil Museum cautions that there is a lot we don’t know about even the finest examples of trilobite eyes: “As incredible as the Phacops crystal eyes that appeared much later in the Ordovician were, we cannot know what the trilobites could see with them.”
Eric Warrant, of Lund University in Sweden, sums up the debate over the pros and cons of insect versus vertebrate eyes: “In some ways we’re better, but in many ways, we’re worse,” he says. “There’s no eye that does it all better.” Among vertebrates, birds of prey appear to have the best eyes, while squid and octopus have arguably the best eyes in the invertebrate world.
In any case, it seems that the compound eyes of the Cambrian predator Anomalocaris, the first animal known to have eyes, were superior even to those of trilobites: they were able to sense night and day, and were 30 times stronger than those of trilobites, making them on a par with most modern insects’ eyes. (I should point out, however, that there is some doubt as to whether some recently discovered fossil eyes that were ascribed to Anomalocaris really belonged to this animal, after all.)
In his article, Dr. Hunter also cites a 1993 text by Professor Riccardo Levi-Setti, declaring trilobite eyes to be “an all-time feat of function optimization.” No page reference is given; apparently the quote is taken from page 29 of the second edition of Professor Levi-Setti’s best-selling book, Trilobites (University of Chicago Press, 1993). Once again, I found out via a quick Google search that this quote is frequently copied and pasted by evolution skeptics.
It might interest Dr. Hunter to know that Dr. Sam Gon, in his online article, The Trilobite Eye , actually cites Professor Levi-Setti’s book in his bibliography. That’s very curious, if (as Dr. Hunter implies) Professor Levi-Setti views the evolution of the trilobite eye as a genuine problem for evolution.
In his highly informative article, Dr. Gon points out that there were actually three different types of trilobite eyes: holochroal, schizochroal (pictured above, for the trilobite Phacops rana) and abathochroal. The most complex of the three was the schizochroal eye, found in just a single suborder of trilobites, which appeared in the early Ordovician period, about 36 million years subsequent to the emergence of trilobites in the Cambrian, 521 million years ago. Funny, that. So much for the claim that Nature’s “most sophisticated eye” appeared in the early Cambrian. Dr. Gon succinctly describes what scientists know about trilobite eyes:
Three types of trilobite eyes
There are three recognized kinds of trilobite eyes: holochroal, schizochroal, and abathochroal. The first two are the major types, with the great majority of trilobites bearing holochroal eyes, and the distinctive schizochroal eye a recognized innovation of the Phacopida. Holochroal eyes are characterized by close packing of biconvex lenses beneath a single corneal layer that covers all of the lenses. These lenses are generally hexagonal in outline and range in number from one to more than 15,000 per eye! Schizochroal eyes on the other hand are made up of a few to more than 700 relatively large, thick lenses, each covered by a separate cornea. Each lens is positioned in a conical or cylindrical mounting and is separated from its neighbors by sclera (cuticular exoskeleton material) that extends deeply, providing an anchor for the corneal membrane, which extends downward into the sclera, where it is called intrascleral membrane.The abathochroal eye is seen in only a few Cambrian trilobites and is somehat similar to the schizochroal eye, but differs in some important respects: the sclera is not thick, and the corneal membrane does not extend downward, but ends at the edge of the lens…
An excellent description of the complexity of schizochroal eyes can be found here. At first sight, the complexity of these eyes looks daunting:
In contrast to the holochroal eye, each lens in the schizochroal eye, which are much larger in size (typically 250 to 500µm) but significantly fewer in number (up to 770 in Dalmanites pratteni Roy, sp. nov.), is separated from its neighbours by a pillar of exoskeletal material known as the interlensar sclera. Lenses are typically packed hexagonally, forming vertical dorso-ventral files and diagonal rows, but square packing is evident in a few rare cases. Each lens is covered by its own individual cornea, appearing as an eye in its own right (Figs. 8 and 9).
To complicate matters further, each lens in the schizochroal eye is formed by a number of components, which differ in composition (Lee et al., 2007). The incorporation of magnesium in the lower part of the lens (intralensar bowl) creates what is known as an ‘aplanatic’ surface, as it differs in refractive index from the upper part of the lens (upper lens unit) – such a surface results in the bending (or refraction) of light helping to bring light rays into focus at some point beneath the lens. This type of lens – known as a ‘doublet’ – was independently designed by mathematicians Descartes (1637) and Huygens (1690), both unaware that they had been beaten by nature by a few hundred million years. (Bolding mine – VJT.)
The schizochroal eye found in some trilobites is so remarkable that it has even inspired valuable work in the field of biomimetics, as scientists attempt to develop ever-thinner video cameras. They’ve now developed one that sees like a trilobite.
Remarkable as it may appear, it seems that the developmental process of paedomorphosis can account for the evolution of the complex schizochroal eye from a simpler, ancestral holochroal eye. To quote from Dr. Gon’s article once again:
How did schizochroal eyes evolve?
All early trilobites (Cambrian), had holochroal eyes and it would seem hard to evolve the distinctive phacopid schizochroal eye from this form. The answer is thought to lie in ontogenetic (developmental) processes on an evolutionary time scale. Paedomorphosis is the retention of ancestral juvenile characteristics into adulthood in the descendent. Paedomorphosis can occur three ways: Progenesis (early sexual maturation in an otherwise juvenile body), Neoteny (reduced rate of morphological development), and Post-displacement (delayed growth of certain structures relative to others). The development of schizochroal eyes in phacopid trilobites is a good example of post-displacement paedomorphosis. The eyes of immature holochroal Cambrian trilobites were basically miniature schizochroal eyes. In Phacopida, these were retained, via delayed growth of these immature structures (post-displacement), into the adult form.
Finally, scientists have powerful evidence for the evolution of animal vision. It turns out that the same master gene — called Pax6 — controls eye development in virtually every creature with eyes. Although advanced eyes evolved independently in different lineages of animals on several occasions, the most basic eye (a simple light-detector) evolved just once. An article by Ed Yong in National Geographic (“Inside the Eye: Nature’s Most Exquisite Creation,” February 2016) handily summarizes the evidence for the common ancestry of animal vision:
We know this because all eyes are constructed from the same building blocks. Nothing that sees does so without proteins called opsins — the molecular basis of all eyes. Opsins work by embracing a chromophore, a molecule that can absorb the energy of an incoming photon. The energy rapidly snaps the chromophore into a different shape, forcing its opsin partner to likewise contort. This transformation sets off a series of chemical reactions that ends with an electrical signal. Think of the chromophore as a car key and the opsin molecule as the ignition switch. They turn, and the engine of sight whirs to life.
There are thousands of different opsins, but they are all related. A few years ago, Megan Porter, now at the University of Hawaii at Manoa, compared the sequences of almost 900 genes, coding for opsin proteins from across the animal kingdom, and confirmed that they all share a single ancestor. They arose once and then diversified into a massive family tree. Porter draws it as a circle, with branches radiating outward from a single point. It looks like a giant eye. (Bolding mine – VJT.)
And now, a team of scientists led by Dr. Roberto Fueda has even figured out when vision originated: around 700 million years ago. Lucas Brouwers explains their reasoning in an article in Scientific American titled, Animal vision evolved 700 million years ago (November 20, 2012):
Opsin is a member of large family of detector proteins, called the ‘G-protein coupled receptors’ (GPCRs)… Most GPCRs detect the presence of certain molecules… But opsin is different. It doesn’t bind molecules physically. Instead, it senses the presence of a more delicate and ephemeral particle: the photon itself, the particles (and waves) that light is made of…
First of all, Feuda confirmed the existence of three distinct opsin types within bilateria (bilaterians are animals with left-right symmetry)…
The first animal to carry three opsins was not the bilaterian ancestor, but the last common ancestor of Bilateria and Cnidaria (jellyfish, anemones, corals and their kin). Feuda found all cnidarian opsins belong to one of three different groups, each of which correspond to the three basic opsin types in Bilateria…
Feuda’s team leapt to another branch of the family tree, and scoured the genomes of two sponges, Oscarella and Amphimedon, for opsin sequences. No dice. Apparently, opsins only evolved after sponges had diverged from other animals, but before the split between Bilateria and Cnidaria… Fortunately for Feuda, there exists one animal lineage in this sweet spot between sponges on one side and cnidarians/bilaterians on the other. Meet the placozoans.
…[T]he placozoan genome harbours two opsins. But here’s the catch: these opsins cannot detect light.
…[O]ur opsins really had two origins. One is the birth of opsin itself, the other is the mutation that turned opsin into a light sensing protein. The opsin lineage itself arose between 755 and 711 million years ago, from the duplication of a single GPCR. The last common ancestor of Bilateria and Cnidaria lived between 711 and 700 million years ago. This leaves a short window of time (evolutionary speaking) in which opsin acquired the light sensing mutation and split into the three opsin families we still carry today.
And if Dr. Hunter were to ask me why I trust these evolutionary trees, I would simply answer: because they make testable predictions, because they explain a lot, and because they fit the known data.
Readers with a strong background in science can find out more about the evolution of opsins here.
We have seen that there is excellent genetic and biochemical evidence for the evolution of all animals’ eyes from a common ancestor, 700 million years ago. We have also seen that there are plausible fossil precursors for trilobites, and several examples of trilobite evolution in the fossil record. Finally, we saw that the most sophisticated trilobite eye, which is confined to a single lineage, did not appear until 36 million years after the first trilobites emerged, and that it seems to have originated from a simpler trilobite eye via the process of paedomorphisis. Whatever one might think of the case for evolution, I do not think that trilobites constitute any special difficulty for the theory.