Susan M. Klofak

Mode and Rate of Growth in Ammonoids

In this chapter we discuss the mode and rate of growth in ammonoids, focusing primarily on postembryonic growth. We first discuss the general mode of growth and then describe the ontogenetic sequence of growth stages. These stages are recognized on the basis of changes in morphology. For example, a graph of the increase in size of whorl width versus shell diameter in an individual reveals changes through ontogeny that pinpoint the end of one growth stage and the beginning of another. We next discuss the overall rate of growth through ontogeny and establish a generalized growth curve. In this discussion, we refer to other cephalopods whose rate of growth is known. Fluctuations in the rate of growth that are superimposed on this growth curve are indicated in ammonoids by the presence of such shell features as varices and constrictions.

ANATOMY OF A CONCRETION: LIFE, DEATH, AND BURIAL IN THE WESTERN INTERIOR SEAWAY

ABSTRACT Concretions are the most characteristic mode of fossil occurrence in the Upper Cretaceous Western Interior of the United States. An in-depth analysis of a single concretion from the upper Campanian Pierre Shale, South Dakota, drawing upon sedimentology, paleontology, shell preservation, degree of encrustation, and geochemistry allows us to determine a time frame for the accumulation and burial of the organisms and the process of cementation and diagenesis of the concretion. The concretion is very fossiliferous and dominated by mollusks. Large ammonites are commonly broken up with pieces missing from the adapical end of the body chamber. This breakage pattern is widely interpreted as evidence of lethal damage, implying introduction into the burial site via predation. In contrast, smaller ammonites are nearly complete and may have died due to smothering in resuspended sediment produced by bottom currents. The concretion is rich in cephalopod jaws, which mostly appear as isolated occurrences, usually deformed, with the calcite covering (aptychus) missing. The preservation of jaws suggests that the organic debris did not remain in the taphonomically active zone for more than a few years. The concretion, thus, represents a time-averaged deposit of organisms derived from the local community. In contrast, host sediments contain fewer fossils, most of which are crushed. Oxygen and carbon isotopic composition of samples in the concretion and the host sediments reveals a two-stage diagenetic history of the concretion. First, cementation probably occurred at shallow burial depths in early diagenesis in association with the decomposition of organic matter and the oxidation of methane. Second, alteration of the shelly material and the formation of calcite crystals filling the empty chambers of ammonites probably occurred during later diagenesis in contact with meteoric water.

Embryonic Development of Primitive Ammonoids and the Monophyly of the Ammonoidea

A small egg-shaped initial chamber is a common feature of the Ammonoidea. This plus the shell up to the primary constriction defines the embryonic shell, called the am- monitella. All ammonitellas possess the same general characteristics, but their expression is different. This has both phylogenetic and ontogenetic implications. Most available data are from Mesozoic ammonoids, due to preservational biases. As a result, most models of embryonic development are based on the characteristics of the better preserved Mesozoic ammonoids and hypothesize a non-accretionary mode of growth. These models include the Paleozoic ammonoids by default or exclude them outright, postulating that they formed differently, namely by accretionary growth.

Inquilinism of a Baculite by a Dynomenid Crab from the Upper Cretaceous of South Dakota

ABSTRACT We describe a small crab inside the phragmocone of a heteromorph cephalopod Baculites sp. smooth from the Gammon Ferruginous Member (lower Campanian) of the Pierre Shale in Butte County, South Dakota. The crab Ferricorda kimberlyae (Bishop, 1987) is well preserved with its carapace and pereiopods parallel to and between two septa of the phragmocone. Because of its superb preservation, the specimen is unlikely to have been washed into the phragmocone. The crab probably retreated into the phragmocone chamber to avoid predation or to molt and was subsequently buried by an influx of rapid sedimentation. This is the first instance of inquilinism by a crab in a heteromorph ammonite. Despite the rarity of such fossils, the occurrence of crabs inside ammonite shells was probably not uncommon on sea floors during the Mesozoic. Morphological details of the specimen reveal that Ferricorda is a dynomenid crab.