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Trilobite Sex

March 13, 2017

Just as people have a difficult time understanding those who speak a foreign language, the general public has a hard time understanding science, since scientists have their own language. An example of this, as devised by science fiction author and biochemist, Isaac Asimov (1920-1992), is how people interpret the word, unionized. Non-scientists will invariably think of a trade union, while scientists will think of a neutral atom (not ionized).

When I left academia and joined a corporate research laboratory, I found that many of my new colleagues had adopted some of the language of the corporate business administrators with whom we shared our campus. This was the time before the Six Sigma management fad and its concurrent expressions like, "Work smarter, not harder," but the first memorable proverb of the modern age I heard was "No amount of careful planning can beat pure luck."

Indeed, many scientific discoveries were accidental, a phenomenon commonly known as seredipity. Wikipedia has a short page listing a few historical examples. One of my favorites as a materials scientist is the supposed discovery of stainless steel by English metallurgist, Harry Brearley (1871-1948).

Harry Brearley (1871-1948)

Photo of English metallurgist, Harry Brearley (1871-1948), from a monument at the former Brown Firth Research Laboratories, Sheffield, England.

Brearley discovered stainless steel, supposedly by accident.

(Wikimedia Commons image by David Morris, modified for artistic effect.)

Brearley noticed a piece of non-rusted metal in a pile of alloy samples he had prepared for erosion resistance testing. This chromium-containing sample marked the invention of stainless steel. Biologists have a similar example of Alexander Fleming's (1881-1955) discovery of penicillin.

Paleontologists are more at the mercy of serendipity than any other group of scientists. These fossil hunters can only study what they find in the ground, as good an example of a "needle in a haystack" motif as any. Fortunately, they're guided in where to look by observations of geological strata conducted over the centuries. In this way they can pinpoint the layers of earth corresponding to a particular epoch.

Burgess Shale Formation at Mount Stephen, British Columbia

The trilobite beds at Mount Stephen, British Columbia, a part of the Burgess Shale Formation.

The Burgess shale contains many Middle Cambrian period fossils of organisms that existed half a billion years ago.

(Wikimedia Commons image.)

The so-called Cambrian explosion, starting about 541 million years ago, was a time of rapid diversification of organisms from simple cell colonies to large animals, including trilobites. The Burgess Shale Formation in British Columbia is important to paleontology, since the shale preserved fossils of soft body parts, such as legs, gills, and antennae, of these animals.

Trilobites are marine arthropods in the subphylum Trilobitomorpha. They are invertebrates having an exoskeleton, a segmented body, and jointed appendages. The name, "trilobite," derives from the Greek words τρι- (tri-, "three") and λοβος (lobos, "lobe"), and they resemble the horseshoe crab, an arthropod of the subphylum Merostomata. Trilobite fossils are found worldwide, and several thousand trilobite species are known. A trilobite even made an appearance in the first Godzilla movie (Godzilla, 1954, Ishiro Honda, Director).[1-2]

Trilobite illustration from Système silurien du centre de la Boh&ecircum;me by Joachim Barrande (1799-1883).

Trilobite illustration (Plate 3, vol. I, suppl. II) from the 1852 Système silurien du centre de la Bohême by Joachim Barrande (1799-1883).

Barrande was a French paleontologist who published this landmark study of trilobites of the Cambrian, Ordovician and Silurian periods of Bohemia.

(Portion of a Wikimedia Commons image.)

One interesting feature of trilobites that's been discovered from their fossils is the nature of the lenses of their eyes. As I wrote in a previous article (The Trilobite Eye, May 3, 2013), trilobites had compound eyes that resembled insect eyes. The lenses were an array of closely-packed hexagonal facets formed from calcite.[3] Although calcite is transparent, it's optically birefringent; that is, non-polarized light entering a birefringent crystal is split into two beams, one of which (the "ordinary" beam) follows the usual refraction laws, with the other ("extraordinary" beam) going off at a different angle.

For 600 nm light, which is roughly in the middle of the visible spectrum, calcite has an ordinary refractive index no of 1.486, and an extraordinary index ne of 1.660, so the beam paths will diverge when the incident beam is not along the optical axis of the crystal. This meant that trilobites had good forward vision, but poor peripheral vision, and this deficit was probably mitigated by the compound nature of the lens. Since a trilobites's existence involved just eating things in front of it and avoiding large predators that would be loom large in any peripheral vision, the calcite lenses were adequate, as evidenced by the fact that trilobites existed for 270 million years.

While female horseshoe crabs are abundant egg layers, from an ovarian network within their head, there has been no fossil evidence of trilobite eggs or their genitalia. This is not surprising, since the preservation of soft body structures in the fossil record is rare. Now, a team of scientists from Western Illinois University (Macomb, Illinois), and Vanderbilt University (Nashville, Tennessee) has published images of fossilized eggs of the trilobite species, Triarthrus eatoni, from the Lorraine Group in Upstate New York.[4-5]

This remarkable preservation was achieved since the exoskeletons were replaced with pyrite.[4-5] The 0.2 millimeter eggs were found to be spherical and ellipsoidal in shape, and they were clustered ventrally at the head. The small size of the eggs suggests that trilobites may have hatched without a protaspis shield, the precursor to the articulated shell.[4]

Fossil evidence of trilobite eggs (Thomas A. Hegna et al., © 2017, the Geological Society of America)

Egg images of pyritized trilobite specimens (Triarthrus eatoni) from the Ordovician Whetstone Gulf Formation of the Lorraine Group in Upstate New York. (Portion of an image by Thomas A. Hegna et al., © 2017, the Geological Society of America.)

The location of the eggs is like that of horseshoe crabs, which release their unfertilized eggs from the ovarian network within their head.[4-5] Trilobites likely released either eggs or sperm through a genital pore likely located near the posterior boundary of the head, and they spawned through external fertilization.[4-5]


  1. Godzilla (1954, Ishiro Honda, Director), on the Internet Movie Database.
  2. Trilobite Discovery (1954), YouTube video by Spike Genalo, May 26, 2011.
  3. Vernon L.Williams, "Evolution and the Calcite Eye Lens," arXiv, April 3, 2013.
  4. Thomas A. Hegna, Markus J. Martin, and Simon A.F. Darroch, "Pyritized in situ trilobite eggs from the Ordovician of New York (Lorraine Group): Implications for trilobite reproductive biology," Geology, vol. 45, no. 3 (March, 2017), pp. 199-202, doi: 10.1130/G38773.1.
  5. Trilobite Eggs in New York, Geological Society of America Press Release No. 17-05, February 23, 2017.

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