Trilobite tagmosis and body patterning from morphological and developmental perspectives

Area of Science:

• Paleontology
• Evolutionary Biology
• Developmental Biology

Background:

• Trilobites possessed a cephalic region with differentiated segments and a dynamic trunk region with similar appendages.
• Ventral appendages were biramous, with exceptions for anterior and posterior pairs.
• Some trilobite clades showed trunk segmentation into two batches, sometimes aligning with the thorax and pygidium.

Purpose of the Study:

• To investigate evolutionary trends in trilobite body patterning regulation.
• To explore the implications of trilobite Hox gene distribution for arthropod evolution.
• To present an alternative model to existing hypotheses on trilobite body plan organization.

Main Methods:

• Comparative morphological analysis of trilobite exoskeletons and appendages.
• Phylogenetic analysis to understand evolutionary relationships.
• Inference of gene expression patterns based on extant arthropod models.

Main Results:

• Trilobites displayed distinct cephalic and trunk tagmosis, with independent evolution of trunk segmentation in several clades.
• The arrangement of Hox genes in trilobites may have overlapped similarly to extant arachnates.
• Trilobites likely possessed ten Hox genes, challenging models based on eight body divisions.

Conclusions:

• Repeated independent evolution of trunk segmentation in trilobites highlights convergent body patterning.
• Trilobite Hox gene organization provides a novel perspective on early arthropod body plan development.
• The study proposes an alternative framework for understanding trilobite segmentation and evolution.