Trilobite’s eggs (or Thomas and Markus’s ‘Eureka!’ moment)

In the previous three weeks, trilobites (those beloved paleozoic arthropods) have taken the paleontological front stage, yet once more. The discovery of what is believed to be the first record of trilobite eggs and with it, the suggestion of a possible mechanism of reproduction, was then brought to the general public and sparked considerable enthusiasm and commotion. Well, sadly, the parody by Glendon Mellow (see image below) was just that: a parody, and no gigantic and shiny blue eggs, neatly tucked on a nest closely guarded by the eye of the dedicated parent were found. Nonetheless, what was indeed found is undeniably amazing.

Trilobite Eggs by Glendon Mellow

Markus J. Martin, an amateur fossil collector, found two pyritized specimens of Triarthrus eatoni (Hall, 1838), a trilobite belonging to the Ptychopariida Order (Olenidae Family) upon cracking open a few rocks he had collected from the Ordovician Whetstone Gulf Formation (Lorraine Group), upstate New York (USA).

Thomas Hegna, a paleontologist at Western Illinois University, Markus’s friend, was then called to analyze the finding. Both would ultimately team up with Simon Darroch of Vanderbilt University in the paper that reports the discovery to the scientific community. In a article, Hegna is quoted, describing the moment when he saw the specimens for the first time:

“After Markus showed me the pictures of what he found we had a ‘eureka’ moment”, “my first thought was ‘What else could they be?’ People have found trilobites before but never found the actual animal and eggs together”.

The specimens were collected and prepared using an air abrasion system. A micro-CT scanner was then used to perform a digital dissection. The identification of the “spherical to elliptical” entities “nearly 200 µm in size, and (…) clustered in the genal area of the cephalon” as probable eggs, was made after the excessive size ruled out a microbial origin and their regular distribution and localization made an inorganic explanation highly improbable.

YOU MAY ALSO LIKE  Job | Science Summer Camp Developers & Instructors in Paleontology

The probable eggs were thought to be of an adequate size on account of its compatibility with egg sizes previously described from modern arthropods. They were, however, ruled too small to be compatible with the current description of the early protaspis phase of the olenids’ life cycle, justifying the proposal of an unknown previous stage.

Several suggestions are made in terms of the probable reproductive biology of trilobites. The authors consider that eggs were likely to he held externally, and that “trilobites may have had innocuous paired genital pores hidden at the base of one of the anterior appendages, much like modern horseshoe crabs, rather than external genitalia”, pores that were probably “located near the back of the cephalon”. Finally, they consider that “external fertilization was likely for trilobites, and was probably the primitive mode for fertilization in Arthropoda”.

Sources:  Western Illinois UniversityGeological Society of America,, Earth Touch News Network, RedOrbit

Image credit: click for source

Title: “Pyritized in situ trilobite eggs from the Ordovician of New York (Lorraine Group): Implications for trilobite reproductive biology”

Authors: Thomas A. Hegna, Markus J. Martin, & Simon A.F. Darroch

Publisher: Geology

Read it online: Full-text available here.


“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.”

Lurdes Fonseca

Assistant Professor and Researcher at University of Lisbon
Sociologist (PhD), Paleontologist (Researcher in Micropaleontology), Majors in Sociology and Biology, Minor in Geology. Main interests in Paleontology: Microfossils, Molecular fossils, Paleobiology and Paleoecology. (read more about me)