The Evolution and Development of Cephalopod Chambers and Their Shape

Area of Science:

• Paleontology
• Evolutionary Biology
• Functional Morphology

Background:

• Ammonoidea, extinct cephalopods, are key to studying deep-time evolution.
• Planispiral shells and complex septa in ammonoids were hypothesized to increase chamber surface area for enhanced metabolic functions.
• These functions include chamber emptying, mineralization, and growth rates.

Purpose of the Study:

• To investigate ontogenetic changes in surface area to volume ratios in ammonoid phragmocone chambers.
• To compare these ratios with extant cephalopods, specifically *Spirula spirula* and nautilids.
• To test the hypothesis that ammonoids possess a persistently high relative chamber surface area.

Main Methods:

• Utilized nano-computed tomography (nano-CT) and synchrotron radiation-based micro-computed tomography (SR-μCT).
• Analyzed ontogenetic changes in surface area to volume ratios in multiple ammonoid species and extant cephalopods.
• Quantified siphuncular surface area to chamber volume ratios.

Main Results:

• Contrary to hypothesis, ammonoids do not exhibit persistently high relative chamber surface area.
• Functional chamber surface area is higher in early ontogeny compared to *Spirula spirula*.
• Ammonoids, particularly *Amauroceras sp.*, show a higher siphuncular surface area to chamber volume ratio than *S. spirula* or nautilids.
• Increased septal complexity correlates with increased chamber surface curvature, potentially enhancing refilling rates.
• A unique relationship between ammonoid chamber shape and size was identified, absent in *S. spirula* and nautilids.

Conclusions:

• Ammonoid chamber surface area is not persistently high but is elevated in early ontogeny, supporting faster growth potential.
• Increased chamber curvature due to septal complexity may facilitate quicker chamber refilling.
• The findings suggest a unique chamber function in ammonoids and have implications for the evolution of coleoid internal shells, linking soft-body and shell growth decoupling.