DOI: 10.5061/dryad.3xsj3txhc
title: Bivalve body size distribution through the Late Triassic mass extinction event

description: The synergic relationship between physiology, ecology and evolutionary
 process makes the body size distribution (BSD) an essential component of
 the community ecology. Body size is highly susceptible to environmental
 change, and extreme upheavals, such as during a mass extinction event,
 could exert drastic changes on a taxon’s BSD. It has been hypothesized
 that the Late Triassic mass extinction event (LTE) was triggered by
 intense global warming, linked to massive volcanic activity associated
 with the Central Atlantic Magmatic Province. We test the effects of the
 LTE on the BSD of fossil bivalve assemblages from three study sites
 spanning the Triassic/Jurassic boundary in the UK. Our results show that
 the effects of the LTE were rapid and synchronous across sites, and the
 BSDs of the bivalves record drastic changes associated with species
 turnover. No phylogenetic signal of size selectivity was recorded,
 although semi-infaunal species were apparently most susceptible to change.
 Each size class had the same likelihood of extinction during the LTE,
 which resulted in a platykurtic BSD with negative skew.  The
 immediate post-extinction assemblage exhibits a leptokurtic BSD although
 with negatively skewed, where surviving species and newly appearing
 small-sized colonizers exhibit body sizes near the modal size. Recovery
 was relatively rapid (~100kyr), and larger bivalves began to appear during
 the Pre-Planorbis Zone, despite recurrent dysoxic/anoxic conditions. This
 study demonstrates how a mass extinction acts across the size spectrum in
 bivalves and shows how BSDs emerge from evolutionary and ecological
 processes.
description: Morphometric
 measurements of bivalves were collected from the base of the Westbury
 Formation (Rhaetian) to the Angulata Zone (Hettangian) in St. Audrie’s
 Bay; from the base of the Langport Member to the Angulata Zone in Pinhay
 Bay; and from the Westbury Formation to the Liasicus Zone (Hettangian) in
 Larne (Supplementary Figure 1 and supplemental files). One sample of 1.5 ±
 0.2 kg was collected from each major lithology (limestone and mudstone) on
 average every 1.98 ± 0.5m. In addition, to increase the sample size of
 each stratigraphic horizon, individual measurements were made in the field
 from bedding plane exposures using a tape measure (standard error ± 0.5
 mm). Samples from the
 different lithologies were processed in different ways. Limestone rock
 samples of 1.5 ± 0.2 kg (with dimensions of ca. 20cm length × 20cm width
 ×15cm height) were cut perpendicular to the bedding plane surface,
 generating slabs approximately 8cm wide (see Supplementary Figure 2A).
 Using a chisel-tipped hammer, each slab (20×15×8 cm) was then splitted
 parallel to the bedding plane, which generated sampling plates of
 approximately 2×20×8cm with specimens exposed on the surfaces of each slab
 for potential measurement (see Supplementary Figure 2A). This technique
 minimized damage to the specimens in order to preserve entire individuals
 for measurement. In contrast,
 mudstone samples were broken into ~49 cm<sup>2</sup> chips,
 and water was used to soak the rock and separate the fossils without
 damaging them. All the bivalves were classified from published information
 (Hodges 2000, Lord and Davis 2010, Swift and Martill 1999). In all cases,
 measurements of the length (maximum distance on the anterior–posterior
 axis) and height (maximum distance on the dorsal-ventral axis) were
 recorded only from complete, well-preserved specimens (Supplementary
 Figure 2B-C). All morphometric variables were measured using electronic
 calipers (Standard error ± 0.01 mm). Data Analysis: The body size of
 each specimen was calculated as the geometric mean (i.e., the square root
 of the product of length and width) and log<sub>2</sub>
 transformed to reduce the heteroscedasticity and to facilitate graphic
 representation (Piazza et al. 2019).
description: Supplementary information contains: 
 Dataset.Body.Size.Opazo&Twitchett2021.csv_Paleobiology contains
 one spreadsheet with all body size measurements used in this study. This
 file is made up of 17 columns which indicates:   
 Formation = Corresponds to stratigraphic units (and biozones)
 considered in this study:  WF: Westbury Formation/ CM: Cotham Member/ LM:
 Langport Member/ PPZ=Pre-Planorbis Zone/ PZ=Planorbis Zone/ LZ: Liasicus
 Zone/ AZ: Angulata Zone  Locality = NI: Larne,
 Northern Ireland/PB: Pinhay Bay, Devon/AB: St. Audrie's Bay (all from
 United Kingdom) Sample = Sample Code
 Heights (m) = Stratigraphic height Tier
 = Infaunal/Surficial/Semi-infaunal
 Taxonomy=Order/Family/Genus/Species  
 Ecological categories: Colonizers/Extinct/Survivors Length (mm) =
 Maximum distance on the anterior–posterior axis 
 Height (mm) = Maximum distance on the dorsal-ventral axis
 BS (mm) = Body size expressed as geometric mean
 (i.e., the square root of the product of length and width)
 log2 (BS) = log2 of the geometric
 mean Residuals =  Grand bivalves mean (log2) minus
 the body size (log2) of each specimen Lithology =
 Mudstone/Limestone 
 Opazo&Twitchett2021.Supplementary.text.pdf_Paleobiology:
 This file contains information regarding paleoenvironment, extinction
 event and the Triassic/Jurassic Boundary, and geological setting of the
 Triassic/Jurassic Boundary in the United Kingdom. Additionally, this file
 contains the R code to generate a null model completely random.<br>
     1.  
  Opazo&Twitchett2021.Supplementary.figures.pdf_Paleobiology<br>     2.    Opazo&Twitchett2021.Supplementary.tables.pdf_Paleobiology