Trunk segmentation of Cambrian eodiscoid trilobites

Area of Science:

• Paleontology
• Developmental Biology
• Evolutionary Biology

Background:

• Trilobites are extinct marine arthropods crucial for understanding early animal evolution.
• Postembryonic development in trilobites typically involves sequential segmentation (anamorphosis).
• The conventional view posits that trunk segments form gradually after hatching.

Purpose of the Study:

• To investigate the developmental patterns of early trilobites.
• To examine the ontogeny of the eodiscoid trilobite Badiscus spinosus from Cambrian Series 2.
• To determine if early trilobites exhibited unique segmentation strategies.

Main Methods:

• Analysis of three-dimensionally phosphatized exoskeletons of Badiscus spinosus.
• Examination of protaspides (juvenile stages) to identify embryonic trunk segments.
• Comparative analysis of tagmosis and developmental patterns.

Main Results:

• Discovery of unusual "embryonic trunk segments" in protaspides of Badiscus spinosus.
• Evidence suggests specification of all body segments occurred during embryogenesis.
• This pattern of tagmosis is inconsistent with conventional anamorphosis.

Conclusions:

• Badiscus spinosus exhibits a previously unknown style of tagmosis.
• This finding implies epimorphic development (pre-formation of segments) in early trilobites.
• Trilobite trunk segmentation during ontogeny is more complex than previously understood.