Research Highlight

Extinct trilobites had gills like modern-day crabs, lobsters

doi:10.1038/nindia.2021.54 Published online 28 April 2021

The well preserved trilobite fossil Triarthrus eatoni.

© Jin-Bo Hou

An imaging study of exceptionally well-preserved trilobite fossils shows that specific filaments on the upper limb of these marine creatures performed a respiratory function1.   

The upper-limb filaments, resembling gill structures in modern-day crabs, lobsters and shrimps, acted as gills in trilobites, an international research team has found.

The upper-limb filaments, the researchers say, oxygenated body fluids in trilobites, allowing them to thrive in ancient seas before they disappeared around 250 million years ago.  

Each trilobite had a segmented body with three parts – head, thorax and tailpiece. It also had distinct branches of upper and lower limbs. However, it remained unclear whether the upper limb had a respiratory function.   

To find out, the scientists, including a researcher from the Kolkata-based Indian Statistical Institute, examined the upper limb branches in fossils of two trilobite species – Triarthrus eatoni and Olenoides serratus.

Based on three-dimensional imaging of the fossils, the researchers found that the filaments of the upper limb branch, with its inflated marginal bulbs and narrow central region, mimicked the gill of crabs, lobsters and shrimps.

The inflated marginal bulbs supported the filament and provided channels for body fluid circulation. The narrow central region, they report, was used in gas exchange. Such an alignment restricted the movement of the upper limb but was sufficient for gill aeration.  

The researchers say that the movements of the upper and lower limbs during locomotion would have persistently forced water between the filaments of the upper limb, indicating its respiratory function.


1. Hou, J. et al. The trilobite upper limb branch is a well-developed gill. Sci. Adv. 7, eabe7377 (2021)