Data from: Evidence for seasonal variation in δ13C and δ18O profiles of Baculites and implications for growth rate

The life histories of ammonites and the life strategies they employed are
difficult to assess without robust modern analogues but placing
constraints on ammonite growth rates provides a fundamental first step to
understanding this abundant, but poorly understood, fossil group. Here we
interpret periodic variations in carbon and oxygen stable isotope profiles
from Campanian and Maastrichtian ammonites (Baculites) as seasonally
driven and use these records to determine their rate of shell
precipitation. Several of these samples are housed in museums and were
originally prepared using sealants for display and preservation but
testing of these sealants indicated no alteration of the isotopic values
of treated carbonate. Diagenetic alteration, as determined by shell
microstructure, affected the preservation of isotopic signals, resulting
in the loss of seasonal variation in less well‐preserved specimens, and
the δ13C signal is more robust than δ18O. The periodicity of isotopic
profiles from Baculites shells presented here suggest that these organisms
grew at rapid rates (c. 340 mm per year), which may imply an r‐type life
strategy in which the animals reach maturity quickly, spawn large
quantities of progeny, and die at a young age. Because of the potential
mobility of Baculites, reconstructing palaeoenvironmental conditions from
these isotopic records is challenging and should be conducted cautiously.
Unfortunately, well‐preserved Baculites shells much longer than 350 mm are
rarely recovered, which complicates the statistical treatment of potential
periodicity in isotopic profiles.