# Early conch morphology of a gigantic Cretaceous ammonoid, Pachydesmoceras denisonianum (Desmoceratidae)

## TL;DR

This study examines the early shell structure of a giant Cretaceous ammonoid to better understand its growth and lifespan.

## Contribution

The study provides new insights into the ontogeny of a giant ammonoid through analysis of its early conch morphology.

## Key findings

- Constrictions in the early conch of Pachydesmoceras denisonianum likely indicate periods of slowed or halted growth.
- The frequent occurrence of constrictions suggests a longer lifespan than previously assumed for this ammonoid species.
- Ontogenetic patterns of septal spacing may not reflect growth halts in this species.

## Abstract

Gigantic ammonoids, with conch diameters exceeding 1 m, remain one of the most enigmatic groups of extinct organisms. Their paleoecology has been the subject of ongoing debate, with some uncertainties arising from preservation biases, especially of an early conch. This study focuses on an exceptionally preserved early conch of the giant Cretaceous ammonoid Pachydesmoceras denisonianum from southern India. Conch morphology and the ontogenetic trajectories of constrictions and septal spacings were examined. The results indicate that constrictions were frequently present in the early conch; based on the shell layers observed in the cross-section, these constrictions likely resulted from periods of halted or slowed growth. The common occurrence of constrictions during early ontogeny suggests that Pachydesmoceras lifespan may have been longer than previously assumed. Additionally, the ontogenetic patterns of septal spacing might not reflect these growth halts or slowdowns in the examined species.