Mandibulate convergence in an armoured Cambrian stem chelicerate.

Area of Science:

• Paleontology
• Evolutionary Biology
• Arthropod Systematics

Background:

• The evolutionary origins and early body plan of Chelicerata, a major arthropod group, remain poorly understood.
• It is debated whether key chelicerate features evolved early or arose from a more variable ancestral form.

Purpose of the Study:

• To reinvestigate the Cambrian arthropod Habelia optata to clarify its phylogenetic position and ancestral chelicerate features.
• To understand the early evolution of arthropod body plans and the diversification of major lineages like Chelicerata and Mandibulata.

Main Methods:

• Phylogenetic analysis using Bayesian methods.
• Detailed morphological examination of Habelia optata from the Burgess Shale.

Main Results:

• Habelia optata and Sanctacaris uncata are identified as stem chelicerates.
• Habelia exhibits a seven-segmented prosoma, a precursor to the chelicera, and specialized gnathobases, supporting a specific chelicerate ground pattern.
• Convergent features with mandibulates in Habelia's head anatomy suggest early variability and adaptation.

Conclusions:

• Habeliidans demonstrate early chelicerate features but also convergent traits with mandibulates, indicating plasticity in early euarthropod evolution.
• Specialization of the head for food processing was crucial for the divergence of Chelicerata and Mandibulata.
• The expansion of durophagous (hard-shelled prey) niches likely facilitated this diversification.