Pyrite fossil of the trilobite Triarthrus eatoni. Eggs visible upper right. Scale bar 5 mm. Credit: Hegna et al. 2017

One of the enduring mysteries of the beloved but long-extinct animals called trilobites is this: how did they reproduce?

Despite long searching, no eggs or obvious reproductive equipment has ever observed on the scuttling, armored animals, whose fossils are famous for both their abundance and frequently spectacular detail. It is possible to see the calcite crystal eye lenses of trilobites. Why no trilobite junk? And how did they, you know, make little trilobites?

Over the years, a few fossils surfaced containing trilobite “associations” some scientists interpreted as animals caught in flagrante delicto. The only other possible sign of reproduction were suggestions of gendered shape differences in the animals’ exoskeleton that some said hinted they might have had brooding pouches.

Now a fossil hunter searching rocks near Watertown in upstate New York may have helped solve this mystery, according to a paper he and scientists at Western Illinois and Vanderbilt Universities published in the journal Geology last month. This bed is part of the Lorraine Group -- a geological layer that contains a suite of beds dating to the early Paleozoic (the Ordovician, to be exact) in which invertebrates were fossilized in fool’s gold. Thanks to a stroke of profound geochemical luck, their brassy bodies are remarkably well preserved and many specimens show soft tissues. The first bed of this type to be discovered was the famed Beecher’s Trilobite Bed (I wrote about one exquisitely well preserved ostracod from this bed and her little brood here).

After collecting and preparing trilobite fossils from a different bed in the Lorraine Group, discoverer Markus Martin observed clusters of three to nine tiny round and elliptical shaped objects under the trilobite’s head plate in two specimens. Each object – which he and co-authors Thomas Hegna and Simon Darroch are calling eggs – is just under two-tenths of a millimeter in size. The clusters of these objects can appear on the right or left side of the trilobite.

Close-up of suspected trilobite eggs, center. Scale bar 2 mm. Credit: Hegna et al. 2017

Three to nine is probably an undercount, the authors say, because some eggs may have been destroyed in preparing the fossil or lie still hidden within the rock.

Credit: Hegna et al. 2017

In CT scans, the eggs betray no internal detail, nor do they appear bound by sac or tether. The authors do believe the eggs were located outside the trilobite because they say it is rare for pyritized trilobites to have their guts preserved.

What else could the “eggs” be? The structures are too large to be microbes, the authors claim, and their location and distribution rules out an encrusting symbiont, trilobite poop, or abnormal pyrite growth. Their size is comparable to modern arthropod eggs, though on the small side.

Intriguingly, the eggs were found near the corner of the head plate in a location near where modern horseshoe crabs release their unfertilized eggs. (Did you know the ovarian network of the female horseshoe crab is located in her head? Neither did I.) Horseshoe crabs also happen to be one of trilobites’ closest living relatives, and are famously “living fossils”, animals who have visibly changed little since their evolution 450 million years ago. So the location of their egg launch tube may reflect where it was located historically, and perhaps also preserve the location in the first arthropods.

As a result, the authors believe it is likely trilobites released eggs and sperm through a pair of left and right genital pores located somewhere near the back of the head. If this is the case, and if this species – Triarthrus eatoni – is representative of other trilobites, they also think it’s likely trilobites spawned externally. That would make the previous mating “associations” the result of scientists going Freud on some poor, dead trilobites simply going about their business. If the authors of this new paper are right, trilobite mating was instead about getting together with your opposite-sex buds to sip tea, listen to smooth jazz, and exude gametes. Very chill.

However, there’s an important caveat here. Because pyritization favors the preservation of external features over internal features, any evidence of internal reproduction or egg brooding is unlikely to be preserved, they say. This would bias the fossil record against trilobites that employed this method.

Still, other recent studies of the oldest arthropods (with the notable exception of eurypterids – the extinct sea scorpions) have reached the same conclusion: the ancestral mating method for arthropods, the massive group of joint-legged animals that today encompasses all the insects, arachnids, and crustaceans, was external fertilization. Eggs and embryos seem to have been carried under the carapace or attached to limbs or filaments. It’s enough to make a female bedbug sigh.

Reference

Hegna, Thomas A., Markus J. Martin, and Simon AF Darroch. "Pyritized in situ trilobite eggs from the Ordovician of New York (Lorraine Group): Implications for trilobite reproductive biology." Geology (2017): G38773-1.