No abstract

The relationship between the Chicxulub impact event and the Cretaceous-Paleogene (K-Pg) boundary mass extinction has been repeatedly questioned. Specifi cally, Cretaceous planktonic foraminifera occurring in high-energy sediments in the Gulf of Mexico related to the impact have been used to argue that the impact preceded the mass extinction. Here we address this dispute by comparing grain size data from two Gulf of Mexico sections and a distal, pelagic sequence. Signifi cantly larger grain sizes, combined with evidence for size sorting in the proximal sections, suggest that fossils in boundary deposits are redeposited and cannot be used to assign ages. Thus the grain size data support other evidence that indicates that high-energy deposits from around the Gulf of Mexico correlate stratigraphically with the Chicxulub impact and the K-Pg mass extinction.

Vieja melanura is a Neotropical cichlid occurring in the Petén-lake district systems of Guatemala, as well as the Río Grijalva-Usumacinta basin, and other systems in Southern México, Belize, and Guatemala. A caudal stripe, extending forward from the caudal peduncle, is characteristic of this species. This stripe is sloped downward in nearly all individuals of V. melanura, but the degree of the slope is highly variable throughout its range. The slope and shape of the stripe has previously been used in diagnosing and differentiating between species of Vieja. The purpose of this study was to use objective methods to investigate morphological variation in the caudal stripe and body shape throughout the range of V. melanura. We studied geometric morphometric analyses of body shape and empirical measurements of the slope of the caudal stripe in 215 specimens of V. melanura. We also used the mitochondrial cytochrome b marker to study population level patterns within V. melanura. Results from our analyses showed significant geographic variation in body shape and patterns of coloration with little mitochondrial phylogeographic structure. These patterns likely correspond to differences in riverine habitats throughout the species’ distribution. In conclusion, these results can be used to inform other studies of color and shape variation as it applies to taxonomy and systematics.