DOI: 10.5061/dryad.jsxksn0br
title: Geographic and temporal morphological stasis in the latest Cretaceous ammonoid Discoscaphites iris from the U.S. Gulf and Atlantic Coastal Plains

description: We examine temporal and spatial variation in morphology of the ammonoid
 cephalopod Discoscaphites iris using a large dataset from multiple
 localities in the Late Cretaceous (Maastrichtian) of the United States
 Gulf and Atlantic Coastal Plains, spanning a distance of 2000 km along the
 paleoshoreline. Our results suggest that the fossil record of D. iris is
 consistent with no within species net accumulation of phyletic
 evolutionary change across morphological traits or the lifetime of this
 species. Correlations between some traits and paleoenvironmental
 conditions as well as changes in the coefficient of variation may support
 limited population-scale ecophenotypic plasticity, however where
 stratigraphic data are available, no directional changes in morphology
 occur prior to the Cretaceous/Paleogene (K/Pg) boundary. This is
 consistent with models of ‘dynamic’ evolutionary stasis. Combined with
 knowledge of life history traits and paleoecology of scaphitid ammonoids,
 specifically a short planktonic phase after hatching followed by
 transition to a nektobenthic adult stage, these data suggest that
 scaphitids had significant potential for rapid morphological change in
 conjunction with limited dispersal capacity. It is therefore likely that
 evolutionary mode in the Scaphitidae (and potentially across the broader
 ammonoid clade) follows a model of cladogenesis wherein a dynamic
 morphological stasis is periodically interrupted by more substantial
 evolutionary change at speciation events. Finally, the lack of temporal
 changes in our data suggest that global environmental changes (such as
 those possibly related to the emplacement of the Deccan Traps Large
 Igneous Province) had a limited effect on the morphology of North American
 ammonoid faunas during the latest Cretaceous prior to the K/Pg mass
 extinction event.
description: We assembled a large morphometric
 dataset consisting of 328 individual fossil specimens of the scaphitid
 ammonoid cephalopod <em>Discoscaphites iris</em> collected
 from nine localities in Texas, Missouri, Mississippi, and New Jersey,
 representing a ~2000 km transect from SW to NE and encompassing the full
 geographic range of this species. Morphometric parameters were measured on
 well-preserved adult specimens of two dimorphs (Macroconchs - presumably
 the female, and microconch, presumably the male). We took up to seven
 morphometric measurements, and calculated ratios that captured the size,
 shape, and degree of compression of each of these ammonoid shells from
 each different locality. We evaluated the coefficient of variation (the
 standard deviation divided by the mean) for size and shape ratios as well
 as compression ratios at each locality. We used non-parametric statistical
 tests [Mann-Whitney U] to evaluate the significance of changes in mean
 morphological trait values between localities. To correct for multiple
 comparisons we applied a Bonferroni correction and also controlled for the
 false discovery rate. We also explored relationships between morphological
 traits and several environmental variables using linear modelling. All
 analyses were conducted in the R programming environment.
description: Missing morphometric values are highlighted with NA in the
 dataset.