PRESENCE OF JUVENILE AMMONITES AT LATE CRETACEOUS METHANE SEEPS (WESTERN INTERIOR SEAWAY)

Cold methane seeps were abundant in the Late Cretaceous Western Interior Seaway. Expressed as large limestone masses surrounded by smaller limestone concretions, they provided a habitat for a diverse array of fauna. Recent research has demonstrated that ammonites lived at these sites, however, it is still uncertain if they hatched at the seeps or only arrived from elsewhere later in ontogeny. Through a combination of isotopic analysis and an inventory of juvenile ammonites from matrix, we have tried to answer this question. In this study we examined cold methane seep deposits and age-equivalent non-seep deposits in the Upper Cretaceous (Campanian and Maastrichtian) Pierre Shale of South Dakota and Montana. Focusing on material containing newly hatched specimens and juveniles, collections were broken down and examined under a microscope to identify juvenile Baculites and Hoploscaphites. Ammonites are abundant in the seep deposits, but less so in the surrounding sediment. Additionally, samples of jaws and hook-like structures are also present in the material, indicating rapid, in-situ burial.

Individuals were identified as juveniles only if both the phragmocone and body chamber were present. Many were incomplete. The maximum diameter (LMAX) was recorded. The smallest specimens include hatchlings with their ammonitellas. Other specimens range in length from less than 1 mm to ~2.5 cm (Baculites) and up to ~1 cm (Hoploscaphites). In the collected material, small juveniles are abundant but larger juveniles are less common. The relatively small number of larger juveniles may indicate that many of these ammonites died immediately after hatching at the seep or soon after arrival. Those that did not die apparently survived into maturity. The abundance of small juveniles suggests that the seeps could have acted as nurseries. Alternatively, the small juveniles were more prone to environmental perturbations common to seeps, such as the sudden release of H₂S.

Further studies will analyze the isotopic composition of well-preserved shell material of juveniles at different ontogenetic stages to determine if it exhibits the low values of δ¹³C characteristic of seep habitats.