Thanks to their easily fossilizable exoskeleton, trilobites largely dominate the fossil record of early complex animal life. However, the appendages of trilobites and the anatomy of their undersides are generally not well preserved, making it difficult to infer their mating and reproductive behaviors.
Until now, modern arthropods have been widely used as analogues to infer trilobite mating behavior, but a new study published Friday in Geology describes the discovery of a specialized limb in a mature male trilobite species that sheds light on trilobite mating behaviors for the first time.
Detailed study of a fossil specimen of the trilobite species Serrated Olenoides revealed two sets of particularly reduced appendages in the middle of its body. Each of these appendages is interpreted as a clasp-like limb, which mature males would use to grasp females during mating to ensure the male is in the best position for external fertilization of eggs.
“Once in a while you will get fossil specimens that are actually dead and preserved when mating, and there are a few insects that are preserved when mating, but other than that it’s hard to infer the behaviors of ‘mating,’ said lead author Sarah Losso. of the study. “There are about 20,000 described species of trilobites, but fewer than 40 species have retained appendages. This is the first time that truly significant appendage specialization has been observed in trilobites. The discovery tells us more about the behavior of trilobites and shows that this type of complex mating behavior already existed in the middle Cambrian.”
the O. serrated The fossil specimen is around 500 million years old and dates to the Cambrian period. The fossil specimen is currently housed at the Royal Ontario Museum in Toronto, Canada.
Losso systematically studied and photographed over 60 Burgess Shale trilobite specimens that had retained appendages. The only specimen of O. serrated was the only one to display these only specialized members. According to Losso, the fossil specimen is broken and missing most of the exoskeleton that covers the head and half of the body, but the fact that it was broken helped them see the clasp-like limbs, which otherwise would not have been visible. and would have been completely hidden. Until now, this special clasp-like appendage was otherwise unknown to O. serrated or other trilobite species to date.
Horseshoe crabs are commonly used as modern analogues of trilobites given their similar appearance and lifestyle, and this special trilobite member appears functionally similar to the clasps that male horseshoe crabs have and use to cling to the spines of a female during external fertilization. While it was previously assumed that trilobites had mating behaviors similar to horseshoe crabs, this fossil find provides evidence for the similarities in their reproductive strategies based on their anatomical features.
“Trilobites and horseshoe crabs aren’t particularly closely related to each other, but they share a similar overall organization and live in similar marine environments. It’s kind of like how a bat can fly and a bumblebee can fly. They both use wings. , but the wings themselves are quite different in how they are made and how they work. This finding suggests to some extent that if you are a helmet-like sea animal that lives on the sediments, there are only a number of ways to effectively mate externally,” said study co-author Javier Ortega-Hernández.
The discovery of this clasp-like limb in trilobites reveals that the complex mating behaviors seen in modern arthropods originated during the Cambrian Explosion more than 500 million years ago. It is the oldest record of an appendage of this type used for reproduction and represents a degree of specialization of the limbs for a non-food function.
“Traditionally, trilobites are considered examples of primitive animals. This finding shows that they may actually display complex behaviors for reproduction, similar to what some of the animals we have today do,” said Ortega-Hernandez. “It really contributes to a better understanding of the Cambrian environment as actually being a thriving and ecologically complex system, rather than a lesser version of the biosphere we have today.”
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