Exceptionally preserved early Cambrian bilaterian developmental stages from Mongolia

Area of Science:

• Paleontology
• Developmental Biology
• Evolutionary Biology

Background:

• Fossilized embryonic and developmental stages are crucial for understanding the evolution of animal body plans.
• Such fossils are rare, primarily found in the Ediacaran-Cambrian transition, a key period for animal diversification.

Observation:

• A new assemblage of eggs, embryos, and bilaterian post-embryonic developmental stages was discovered in the early Cambrian Salanygol Formation, Mongolia.
• These fossils exhibit cellular fidelity, showing bilaterally arranged ridges comparable to camenellan sclerites.

Findings:

• The identified fossils represent early post-embryonic developmental stages of camenellans, an early clade of stem-brachiopods previously known only from isolated sclerites.
• This finding confirms previous reconstructions of camenellan scleritomes and their growth patterns.

Implications:

• The study supports the interpretation of camenellans' growth from juvenile stages with specific cellular morphology.
• It reinforces the hypothesis that molluscs and brachiopods evolved from a common, ancestral vermiform, slug-like body plan.