Trilobite species Olenoides serratus, has been well studied since 1909 and is one of the most fully understood trilobites in the fossil record, due to the exceptional preservation of soft tissues (labile tissue) at the Burgess Shale. Now after over 100 years of studying the species, and half a billion years since its extinction, scientists have cracked the case on how trilobites had sex.
Finding explicit evidence of sexual reproduction in the fossil record is rare. Paleontologist must find either organisms that have been preserved in the act of mating (ie. they died while mating and were subsequently fossilized); or the preservation of morphology (technical word for body shape and the arrangement of parts of an organism) that inform scientists about how a species mated. In the case of Olenoides Serratus, it is the latter that has been found; specifically, soft tissue specialized appendages were preserved that were used during copulation.
Published on May 6th, 2022 in the journal Geological Society America, Claspers in the mid-Cambrian Olenoides serratus indicate horseshoe crab-like mating in trilobites https://doi.org/10.1130/G49872.1; Sarah Losso and Javier Ortega-Hernandez argue that modified appendages in male Olenoides serratus were uses as claspers to grasp on to the female’s posterior pygidial spines (spikes on her back) to improve the success of external fertilization of the eggs.
Olenoides serratus had a bio-mineralized exoskeleton made of calcite. Calcite preserves readily, and consequently, the trilobite fossil record is mostly fragments of exoskeletons; conversely, soft tissues are exceptionally rare in the fossil record. The Burgess Shale is one of the few locations in the world where hard and soft tissues have been fossilized, enabling scientists the unusual opportunity to view an organism in its entirety with remarkable clarity, providing insight on how these amazing creatures lived.
Trilobites are arthropods. Arthropods by definition have an exoskeleton, a segmented body, and paired jointed appendages. Trilobites’ jointed appendages are roughly all the same size in their thorax (body) and pygidium (back end). Losso and Ortega Hernandez have found a specimen with diminutive appendages in the trunk region (central or main part of the body where internal organs are) that they interpret to be specialized for mating. Their interpretation relies on convergent evolution, the concept that natural selection will select for similar features in organisms that did not exist in a shared ancestor. An example of convergent evolution is flight in insects and bats; the common ancestor of the two did not possess flight, but natural selection selected for this feature in both organisms. Extant (not extinct) arthropod species have specialized limbs used in mating that are similar to the clasper in the trilobite Olenoides serratus. Due to the overwhelming evidence of convergent evolution in the fossil record, Losso’s hypothesis is plausible that Olenoides serratus’ differentiated appendages were used for mating.
On our hikes to the Burgess Shale we’re frequently asked questions regarding ecosystem relationships. Questions like: “What are the predator-prey relationships?”, “what did this species eat?”, “how did these organisms mate?” While some of these ecological relationships are understood, finding evidence to complete the full ecological picture is challenging. Most of our limited understanding of mid-Cambrian ecological relationships pertains to interspecies competition, predation, and symbiotic relationships. Evidence for mating and brooding strategies is scant.
To the best of my knowledge there are only two recent published papers regarding mating and brooding strategies in the Burgess Shale Cambrian. Losso’s paper discussed in this current blog post, and Caron and Vanier’s 2015 paper regarding brood care in Waptia fieldensis, published in the journal Current Biology https://doi.org/10.1016/j.cub.2015.11.006, and discussed on our blog here.