0000000000154188
AUTHOR
Neil H. Landman
Ammonite extinction and nautilid survival at the end of the Cretaceous
One of the puzzles about the end-Cretaceous extinctions is why some organisms disappeared and others survived. A notable example is the differential extinction of ammonites and survival of nautilids, the two groups of co-occurring, externally shelled cephalopods at the end of the Cretaceous. To investigate the role of geographic distribution in explaining this outcome, we compiled a database of all the occurrences of ammonites and the nautilid genus Eutrephoceras in the last 0.5 m.y. of the Maastrichtian. We also included recently published data on ammonite genera that appear to have briefly survived into the Paleocene. Using two metrics to evaluate the geographic range of each genus (first…
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…
Ammonites on the Brink of Extinction: Diversity, Abundance, and Ecology of the Order Ammonoidea at the Cretaceous/Paleogene (K/Pg) Boundary
We examined the stratigraphic distribution of ammonites at a total of 29 sites around the world in the last 0.5 myr of the Maastrichtian. We demarcated this interval using biostratigraphy, magnetostratigraphy, cyclostratigraphy, and data on fossil occurrences in relation to the K/Pg boundary in sections without any facies change between the highest ammonites and the K/Pg boundary. The ammonites at this time represent all four Mesozoic suborders comprising six superfamilies, 31 (sub)genera, and 57 species. The distribution of ammonites is dependent on the environmental setting. Recent data suggest that ammonites persisted to the boundary and some species may have survived for several tens of…
The complex nature of progenetic species — examples from Mesozoic ammonites
Several examples of progenetic species among Mesozoic ammonites are investigated. The ammonites are Middle Jurassic kosmoceratids, Upper Jurassic oppeliids, and Upper Cretaceous scaphites. As assessed through outgroup comparison, the progenetic species in each of these examples is characterized by small size at maturity and the presence of morphological features typical of those of the juveniles of more primitive species. However, in none of these examples is the progenetic species a duplicate form of the juvenile stage of more primitive species. There are at least three kinds of features of progenetic species that differ from those of the juveniles of more primitive species: (1) symplesiom…
Mature Modifications and Dimorphism in Ammonoid Cephalopods
The shell of an ammonoid is a kind of autobiography of the animal that once occupied it. Different parts of the shell tell different parts of the life history. The growth lines and the tiny intervals in between, along with the shape of the shell itself, record what was happening at the anterior end of the body. The septa and their sutures relate the tale of the other extremity.