DOI: 10.5061/dryad.98sf7m0tk
title: Data from: Which arthropods have feet and why? Addressing an argument for aquatic fossil scorpions

description: Scorpions are the first arguably terrestrial animals identifiable in the
 fossil record, with important implications for arachnid evolution and
 terrestrial ecosystem development.  However, ongoing debate
 persists on whether some fossil scorpions from the Silurian and Devonian
 were, in fact, aquatic.  This study assesses the claim that a
 digitigrade (‘footless’) posture implies an aquatic habitat for early
 scorpions by evaluating the distribution of feet among ambulatory
 arthropods and how their presence correlates with factors including
 aquatic versus terrestrial habitat as well as body size, number of legs,
 cuticle mineralization, and time since terrestrialization.  The
 results demonstrate that these variables in isolation are poor predictors
 of leg posture, but become highly statistically significant in certain
 combinations.  However, with as many lineages diverging from the
 usual pattern of plantigrady as adhering to it, predictive power is
 weak.  Therefore, all factors influencing plantigrady must be
 accounted for when discussing its implications in fossil arthropods, and
 digitigrady alone does not provide compelling support for an aquatic
 habitat in fossil scorpions.  We further argue that other lines
 of evidence for an aquatic habitat in Palaeozoic scorpions are equivocal
 and that the weight of phylogenetic and anatomical evidence supports a
 terrestrial origin for the total group Scorpiones.
description: # Data from: Which arthropods have feet and why? Addressing an argument
 for aquatic fossil scorpions **Data_1_-_Body_Size_Measurements.csv** Table
 containing measured body lengths and body widths (excluding appendages) of
 all taxa included in this study. Mean log body length and mean log body
 volume per leg were calculated and averaged over all taxa included in a
 given phylogenetic tree tip (tips correspond to the tree in files S4 and
 S5) for use in the phylogenetic general linear regression analysis. Column
 descriptions: Class - Taxonomic class of specimen. Order - Taxonomic order
 of specimen. Family - Taxonomic family of specimen; "?" if
 unidentified to family level. Genus - Taxonomic genus of specimen;
 "?" if unidentified to genus level. Species - Taxonomic species
 of specimen; "?" if unidentifed to species level. Specimen
 number - An internal index used to order datapoints during figure
 generation. Adult - Indicates whether specimen was mature; "Y"
 for yes (mature), "N" for no (immature), "?" for
 undetermined. Sex - Sex of specimen, if known; "M" for male,
 "F" for female, "?" for undetermined. Feet - Indicates
 the leg posture of the specimen (digitigrade or plantigrade) if extant,
 otherwise that the specimen was a fossil. Habitat - Indicates the habitat
 of the specimen (aquatic or terrestrial) if extant, otherwise that the
 specimen was a fossil. Cuticle - Indicates whether the specimen cuticle
 was mineralized or non-mineralized, if extant, otherwise that the specimen
 was a fossil. Body length [mm] - The measured body length in millimeters
 of the specimen (single segment for Arthropleura). Body width [mm] - The
 measured body width in millimeters of the specimen (single segment for
 Arthropleura). Leg count - The number of walking legs of the specimen
 (single segment for Arthropleura, considering both diplopodous and
 non-diplopodous conditions). Estimated body volume - Rough estimate of
 body volume in cubic millimeters, obtained by multiplying body length by
 the square of body volume. Estimated body volume per leg - Ratio of
 estimated body volume to leg count in cubic millimeters per leg. Log body
 length - Base ten logarithm of body length, not applicable for
 Arthropleura due to single-segment measurements. Log body volume - Base
 ten logarithm of body volume, not applicable for Arthropleura due to
 single-segment measurements. Log leg count - Base ten logarithm of leg
 count calculated for mathematical consistency, not applicable for
 Arthropleura due to single-segment measurements. Log body volume per leg -
 Base ten logarithm of body volume per leg. Mean log body length - Log body
 length averaged over all specimens included in the given tip of the tree
 used for phylogenetic logistic regression, listed only for the first
 member of the tree tip (other members listed with "-"). Fossils
 were not included in phylogenetic logistic regression models and are
 therefore listed as "N/A." Mean log body volume - Log body
 volume averaged over all specimens included in the given tip of the tree
 used for phylogenetic logistic regression, listed only for the first
 member of the tree tip (other members listed with "-"). Fossils
 were not included in phylogenetic logistic regression models and are
 therefore listed as "N/A." Mean log leg count - Log leg count
 averaged over all specimens included in the given tip of the tree used for
 phylogenetic logistic regression, listed only for the first member of the
 tree tip (other members listed with "-"). Fossils were not
 included in phylogenetic logistic regression models and are therefore
 listed as "N/A." Mean log volume per leg - Log volume per leg
 averaged over all specimens included in the given tip of the tree used for
 phylogenetic logistic regression, listed only for the first member of the
 tree tip (other members listed with "-"). Fossils were not
 included in phylogenetic logistic regression models and are therefore
 listed as "N/A." Tree tip - Tip of the tree used for
 phylogenetic logistic regression in which the specimen is included.
 Fossils were not included in phylogenetic logistic regression models and
 are therefore listed as "N/A." Notes - General comments on
 specimens and measurements. Reference (See Data 2)/Provenance - Source of
 specimen.  Data was collected from observations of live animals observed
 in the wild (vicinity of Traverse City, MI, or Stanford, CA) or captivity
 (GT Butterfly House & Bug Zoo in Williamsburg, MI) and from specimens
 illustrated in the literature. Long-form versions of references are
 provided separately in file Data 2.
 **Data_2_-_References_for_Body_Size_Measurements.docx** Complete long-form
 reference list for body size measurements taken from the literature.
 **Data_3_-_Tree_File_for_Phylogenetic_Regression.nex** NEXUS file for the
 phylogenetic tree used in the phylogenetic general linear regression
 models.
 **Data_4_-*Data_for_Phylogenetic_Regression*(Dependent_Habitat).csv**
 Table of habitats, leg postures, and other predictor variable states for
 all included taxa for use in phylogenetic general linear regression models
 with habitat as the dependent variable. Column descriptions: Taxon - The
 taxon represented by a tree tip in the phylogenetic regression analyses.
 Feet - The leg posture (digitigrade or plantigrade) exhibited by members
 of the taxon. Min Legs - The minimum number of walking legs exhibited by
 any member of the taxon. Max Legs - The maximum number of walking legs
 exhibited by any member of the taxon. Mean log leg count - Base ten
 logarithm of leg count averaged over all measured specimens included in
 the taxon. Corresponds to data values in file Data 1. Habitat - Indicates
 whether the habitat of members of the taxon is aquatic (0) or terrestrial
 (1). This is the dependent variable used in the phylogenetic logistic
 regression that this file corresponds to; hence, it must be formatted
 numerically. Cuticle - Indicates whether the cuticle of members of the
 taxon is mineralized or nonmineralized. Mean log Body Length - Base ten
 logarithm of measured body length in millimeters, averaged over all
 measured specimens included in the taxon. Corresponds to data values in
 file Data 1. Mean log Volume - Base ten logarithm of estimated body volume
 in cubic millimeters, averaged over all measured specimens included in the
 taxon. Corresponds to data values in file Data 1. Mean log Volume Per Leg
 - Base ten logarithm of the ratio of estimated body volume to leg count,
 averaged over all measured specimens included in the taxon. Corresponds to
 data values in file Data 1. Min Time Since Terrestrialization - The
 minimum time in millions of years since the taxon became terrestrialized;
 "0" for aquatic taxa.
 **Data_5_-*Data_for_*Phylogenetic_Regression(Dependent_Posture).csv**: The
 same as file Data 4, but reformatted for analyses that treat leg posture
 as the dependent variable. Formatting differences include a numerical
 value for the "Feet" variable (0 for digitigrade, 1 for
 plantigrade) and a categorical value for the "Habitat" variable
 (aquatic/terrestrial). **Data_6_-_R_Code_for_Phylogenetic_Regression.R**:
 R code that performs phylogenetic general linear regressions using the
 hylolm package (Ho et al. 2014). Requires files Data 3, Data 4, and Data
 5. Scorpions are removed from the data to avoid circular argumentation.
 Doing so does not meaningfully change the statistical results.
 **Data_7_-_Statistical_Results_Comparisons.xlsx**: Tables of the
 statistical results generated by the code in file Data 6. The table on the
 first page is the same as Table S2 of the Supporting Information for the
 article. Additional tables contain the same data rearranged and
 colour-labelled for easy comparison between models.