STATOLITH GEOCHEMISTRY OF MODERN AND ANCIENT CEPHALOPODS: A VIEW FROM MODERN ARCHITEUTHIS DUX AND SPECIMENS FROM THE CALLOVIAN (JURASSIC) CHRISTIAN MALFORD LAGERSTÄTTE

Cephalopod statoliths are small (~150 µm in maximum length) and incrementally precipitated within statocysts, a pair of balance and acceleration-sensing organs in the head. Statoliths are analogous to fish otoliths and contain similar records of environmental and internal conditions like temperature and metabolic rate in geochemical proxies across the entire lifespan of the individual. Although present in all cephalopods, these carbonate archives of life history remain relatively understudied, especially in deep time, compared to other parts of cephalopods. Their study may be limited by difficulties in serial sampling due to their small size and taxonomic uncertainties.

Here we present a diagenetic assessment of statoliths from the Callovian (Jurassic) Christian Malford Lagerstätte from Wiltshire (UK) and compare these data to modern giant squid (Architeuthis dux) statoliths. We find that, although the surfaces of the fossil statoliths appear to be well preserved, the core of many has been replaced by pyrite. We suggest that this pyrite replacement is due to intra-statolith organic matter degradation during early diagenesis. Analysis of remaining carbonate material for δ¹⁸O by SIMS had low ion yields and, therefore, may provide unreliable measurements.

Modern A. dux statoliths from 5 individuals that were collected near Aotearoa (New Zealand) show intact carbonate with growth banding throughout. We successfully measured δ¹⁸O and δ¹³C from transects spanning the entire life history of these squid. In each statolith, we observed a plateau in δ¹⁸O values near -3‰ (VPDB) which suggests that early life stages were spent in a warm, shallow habitat. A gradual shift towards higher values of up to +2‰ is observed toward the rim which correlates to movement to deeper waters. We observe uniform δ¹³C of -7‰ in the core of each statolith, increasing to +2‰ by the rim, which implies that metabolic rate decreases through ontogeny.

Our diagenetic assessment and modern results highlight promising paths for future research on statolith geochemistry. We suggest that these underutilized archives are important for understanding change in coleoid ecology across major environmental perturbations and can be calibrated across timescales.