A Reduced Labrum in a Cambrian Great-Appendage Euarthropod

Area of Science:

• Paleontology
• Evolutionary Biology
• Anatomy

Background:

• The euarthropod head's evolution is complex, involving multiple appendage-bearing segments.
• The labrum, an anteromedian flap, is a key structure for understanding anterior segmental organization.
• Megacheirans, an extinct group with great appendages, were thought to lack a labrum, complicating head evolution hypotheses.

Purpose of the Study:

• To investigate the presence and position of the labrum in the megacheiran Leanchoilia illecebrosa.
• To clarify the homology of megacheiran appendages and their role in early euarthropod head evolution.
• To strengthen the phylogenetic placement of megacheirans within euarthropods.

Main Methods:

• Micro-computed tomography (micro-CT) was used to analyze three-dimensionally preserved juvenile specimens of Leanchoilia illecebrosa.
• Anatomical landmarks, including the labrum and mouth opening, were identified and analyzed in relation to other cephalic appendages.

Main Results:

• A three-dimensionally preserved labrum was identified associated with the mouth opening in juvenile Leanchoilia illecebrosa.
• The labrum's position indicates that the great appendages of L. illecebrosa are deutocerebral, homologous to the first appendage pair in extant euarthropods.
• The findings support the interpretation of megacheirans as stem-group chelicerates.

Conclusions:

• The discovery of a labrum in megacheirans resolves a significant gap in understanding early euarthropod head morphology.
• This research refines hypotheses on the evolution of the arthropod head and the phylogenetic relationships of extinct groups.
• The study highlights the importance of exceptionally preserved Cambrian fossils for reconstructing evolutionary history.