Trilobites are the arch-iconic Cambrian animal, having burst onto the scene without known ancestors along with the other Cambrian phyla, equipped and ready to explore the world with those sophisticated compound eyes that “evolution” generously invented.
The mystery of how animals “burst onto the scene” in such a marvelous fashion is the question with which Stephen Meyer begins his investigation in Darwin’s Doubt. As Dr. Meyer argues, sudden appearances and explosive events in the fossil record are an enigma that bedeviled Darwin in his day, still disturbs the sleep of evolutionists, and forms an initial observation in the case for intelligent design.
What explains the special iconic status of the trilobite in particular? One thing might be its resemblance to a familiar still-living arthropod, the horseshoe crab, a “living fossil” according to Darwin’s coinage that has been around seemingly unchanged for almost as long, 450 million years. (By the way, see Casey Luskin’s wonderful post, “What Do ‘Living Fossils’ Mean for Evolution?“)
We now know that like the horseshoe crab, trilobites reproduced by seeding gametes from a pore in the head. We know that because fossilized trilobite eggs have been found in situ in Upstate New York, the results having been published in the journal Geology. These trilobite eggs are from the Ordovician, which followed the Cambrian. Their discovery is a first.
The Abstract reads:
Despite a plethora of exceptionally preserved trilobites, trilobite reproduction has remained a mystery. No previously described trilobite has unambiguous eggs or genitalia preserved. This study reports the first occurrence of in situ preserved eggs belonging to Triarthrus eatoni (Hall, 1838) trilobites from the Lorraine Group in upstate New York, USA. Like other exceptionally preserved trilobites from the Lorraine Group, the complete exoskeletons are replaced with pyrite. The eggs are spherical to elliptical in shape, nearly 200 µm in size, and are clustered in the genal area of the cephalon. The fact that the eggs are smaller than the earliest-known trilobite ontogenetic (protaspis) stage suggests that trilobites may have had an unmineralized preliminary stage in their ontogeny, and that the protaspis shield formed only after hatching. The eggs are only visible ventrally with no dorsal brood pouch or recognized sexual dimorphism. The location of the eggs is consistent with where modern female horseshoe crabs release their unfertilized eggs from the ovarian network within their head. Trilobites likely released their gametes (eggs and sperm) through a genital pore of as-yet unknown location (likely near the posterior boundary of the head). If the T. eatoni reproductive biology is representative of other trilobites, they spawned with external fertilization, possibly the ancestral mode of reproduction for early arthropods. Because pyritization preferentially preserves the external rather than internal features of fossils, it is suggested that there is likely a bias in the fossil record toward the preservation of arthropods that brood eggs externally: arthropods that brood their eggs internally are unlikely to preserve any evidence of their mode of reproduction.
What’s so fascinating is the vividness of the photos, including eggs, genitalia, and other anatomy. Some of the pictures are easily imagined as of an animal that was alive yesterday. By way of comparison, here’s a picture of horseshoe crab eggs:
According to Wikipedia, crab roe is eaten as a delicacy in France and China. I’m not certain about horseshoe crabs.
What is certain is that as with so many other developments in science, the more we know, the more vivid and detailed our knowledge, the more tenuous conventional evolutionary thinking appears. We observed the other day:
As a plausible explanation of life’s complexity, Darwinian thinking emerged when cellular biology was a great blur. As what we know about cells and their contents has dramatically sharpened in detail and focus, orthodox evolutionary thinking correspondingly fades in its persuasiveness.
And that is true not only of cellular biology but of biology in general, including paleontology. As a rule, vagueness works to Darwinism’s advantage, vividness to that of design theory. A key player in the Cambrian explosion just got a little more vivid.