Microanatomy of Early Devonian book lungs

Area of Science:

• Paleontology
• Arachnology
• Evolutionary Biology

Background:

• Book lungs are respiratory organs found in arachnids.
• The evolutionary origin of arachnid book lungs is debated, particularly regarding convergence versus common ancestry.
• Trigonotarbids are an extinct order of arachnids from the Paleozoic era.

Purpose of the Study:

• To investigate the microanatomy of book lungs in a fossil arachnid from the Early Devonian Rhynie cherts.
• To determine the terrestrial adaptations of trigonotarbids.
• To provide evidence for the evolutionary origin of arachnid book lungs.

Main Methods:

• Non-destructive imaging techniques were employed on exceptionally preserved fossil arachnids.
• Three-dimensional modeling was used to reconstruct fine structures from serial imaging data.
• Comparative studies of extant arachnid respiratory organs were conducted.

Main Results:

• Fossilized trabeculae, cuticular pillars within book lungs, were identified for the first time.
• The microanatomy of these ancient book lungs is indistinguishable from modern arachnids.
• Explicit characters, including trabeculae and lamellar edge spines, were identified in both fossil and extant arachnid respiratory organs.

Conclusions:

• Trigonotarbids were unequivocally terrestrial, possessing fully developed book lungs.
• The findings support the hypothesis that book lungs evolved once from a common terrestrial ancestor.
• The study resolves the controversy regarding convergence versus common ancestry in arachnid book lung evolution.