DOI: 10.5061/dryad.dfn2z358h
title: Morphometric analysis of the Late Cretaceous Placenticeras of Alabama, USA: Sexual dimorphism, allometry, and implications for taxonomy

description: A traditional typological approach to taxonomy often does not adequately
 account for intraspecific variation and can result in taxonomic
 oversplitting. For many groups, including ammonoids of the Placenticeras
 genus, intraspecific variation documented in recent studies (e.g.,
 ontogenetic changes, sexual dimorphism, polymorphism) challenges the
 historic proliferation of species names. Here, we used a population
 approach to taxonomy and quantitatively evaluated morphometric variation
 in a sample of Late Cretaceous (Santonian -Campanian) Placenticeras from
 Alabama and adjacent counties. We used linear mixed models (LMMs) to
 characterize how morphological variables scale with conch size across the
 sample, exploiting mixed longitudinal data to evaluate individual
 variation in growth and inform interpretations of multivariate analyses.
 Extended LMMs incorporating geological formation evaluated morphological
 changes through time. Principal component analysis and clustering analysis
 were then used to evaluate the number of distinct clusters that emerged in
 multivariate morphospace independent of previous taxon name assignments.
 Discontinuous scaling relationships and distinct clusters in multivariate
 space suggest sexual dimorphism characterized by differences in adult
 size, and secondarily, shape. Previous Stantonoceras and Placenticeras
 assignments broadly overlap in our morphospace, failing to justify this
 historic distinction (as sexual dimorphs, or as genera or subgenera).
 Placenticeras conch morphology and ornament placement changed through
 time, suggesting a potential utility for coarse (stage-level)
 biostratigraphy. However, temporal changes were not associated with
 distinct clusters in morphospace and our data fail to support the plethora
 of reported species names. As few as one or two (successive) species may
 be present in our sample (representing 130 years of collection effort). In
 addition to highlighting the need for a significant taxonomic revision of
 the Placenticeras genus, this study demonstrates the utility of LMMs for
 distinguishing between different sources of morphological variation,
 improving interpretations of morphospace under a population approach to
 taxonomy, and maximizing the amount of ontogenetic information that can be
 obtained non-destructively.
description: Morphometric measurements were collected with digital calipers.
 See associated manuscript for details.
description: # Morphometric analysis of the Late Cretaceous *Placenticeras* of Alabama,
 USA: Sexual dimorphism, allometry, and implications for taxonomy
 Supplemental Information
 accompanying the article: "Morphometric analysis of the Late
 Cretaceous *Placenticeras* of Alabama, USA: sexual dimorphism, allometry,
 and implications for taxonomy" Authors: Rachel C. Mohr Thomas S.
 Tobin Emily M. Tompkins ## Description of the data and file structure
 ##### "MOHR\_et\_al Supplement Text.pdf" Includes: Supplement
 Text Supplementary Tables 1-8 Supplementary Figures 1-25 #####
 "MOHR\_et\_al Supplementary Data.xlsx" Sheet 1:
 "Metadata": overview of contents, metadata about all variables
 Sheet 2: "Specimen Information": basic information about
 Placenticeras specimens (accession numbers, previous taxonomic
 identifications, locality information, etc.) Sheet 3: "Morphometric
 Data": morphometric data collected from n=112 Placenticeras specimens
 Sheet 4: "main PCA (n=105)": data included in the main PCA and
 associated clustering results Sheet 5: "main PCA - 'linear'
 version": data included in the Supplementary "linear"
 version of our main PCA, which includes only linear measurement variables
 Sheet 6: "main PCA - 'shape' version": data included
 in the Supplementary "shape" version of our main PCA, which
 includes only Whorl Height and size-standardized or dimensionless
 variables Sheet 7: "Ornament PCA (n=45)": data included in the
 secondary "Ornament PCA" and associated clustering results Cells
 contain "NA" when the information is "not available"
 or "not applicable". For morphometric measurement variable
 columns, a cell containing "NA" indicates that incomplete or
 poor preservation of the specimen prevented the measurement of the
 variable. For the "ID_by" column on Sheet 2 (indicating the
 individual who assigned any prior taxonomic identification),
 "NA" is used when no prior taxonomic identification exists, and
 "unknown" is used when a taxonomic identification was present,
 but it is unknown who made the taxonomic identification. For the
 "locality_code" column on Sheets 2 and 3 (indicating standard
 paleontological site numbers; further information about these site numbers
 are available at repositing institutions), "NA" is used in all
 cases where the locality was unknown, or if the locality was not assigned
 a site number by the repositing institution. Empty cells in columns
 "VT_present", "LT_present", and "UT_present"
 (on Sheet 3: "Morphometric Data") indicate instances in which
 the presence/absence of VTs (ventral tubercles), LTs (lateral tubercles),
 or UTs (umbilical tubercles), respectively, could not be determined,
 typically due to poor preservation of the specimen. The associated R code
 scripts require these empty cells (rather than "NA" values) in
 order to load and format the data correctly. ##### "MOHR\_et\_al R
 code scripts" ZIP Folder This folder includes 14 R code scripts used
 to analyze the data for the manuscript and produce the figures. Also
 includes the "Plac_Morphometric_Data.csv" file, which is Sheet 3
 from "MOHR_et_al Supplementary Data.xlsx", saved as a csv file.
 This csv file is the data used in the associated R scripts. See
 "Code" section below for more detailed information about the R
 scripts. ## Code The 14 R scripts and their outputs are described below:
 **"load and format data.R"** This script must be run in its
 entirety before any other R scripts are run Note that all analyses were
 performed in R version 4.0.4 (2021-02-15) -- "Lost Library Book"
 **"density histogram plots.R"** Produces Manuscript Figure 4
 **"LMM scatterplots.R"** Produces Manuscript Figures 6-8;
 Supplementary Figures 1, 22-24 Produces results included in Supplementary
 Tables 1, 3-5 **"LMM random slope model.R"** Produces results
 included in Supplementary Table 2 **"LMM summary figure.R"**
 Produces base of Manuscript Figure 5 **"LMM temporal models.R"**
 Produces results included in Supplementary Tables 7 & 8 **"LMM
 temporal model figure.R"** Produces Manuscript Figure 9 **"power
 single-slope.R"** Produces data required for Supplementary Figures 9
 & 10 **"power threshold.R"** Produces data required for
 Supplementary Figures 9 & 10 **"power plots.R"** Produces
 Supplementary Figures 9 & 10 **"PCA datasets.R"** Produces
 datasets for PCAs **"PCA figures.R"** Produces parts of
 Manuscript Figures 10 & 11; Supplementary Figures 11, 13, 14, 17 &
 18 Produces Manuscript Figures 12 & 13; Supplementary Figures 3-8, 12,
 15, 16 & 19 **"cluster figures.R"** Produces parts of
 Manuscript Figures 10 & 11; Supplementary Figures 11, 13, 14, 17 &
 18 **"dot plots.R"** Produces Supplementary Figure 25