DOI: 10.5061/dryad.2z34tmphb
title: Post-embryonic development of Fritzolenellus suggests the ancestral morphology of the early developmental stages in Trilobita

description: Trilobite development has been intensively explored during past decades,
 but information about ancestral character combinations in the early
 developmental stages of trilobites remains virtually unknown. Trilobites
 of the superfamily Olenelloidea are one of the earliest diverging groups.
 Study of their development coupled with the development of other early
 diverging trilobite groups can provide crucial information about the
 ancestral morphology of trilobite early stages. Herein we describe
 numerous well-preserved specimens of the olenelloid trilobite
 Fritzolenellus lapworthi. The earliest stages have circular cephala
 bearing intergenal spines and lacking genal spines. During subsequent
 development, morphological changes comprise the modification of the
 cephalic shape from circular to semi-circular, expansion of LA, gradual
 shortening of intergenal spines and origin and prolongation of genal
 spines. Trunk development of Fritzolenellus suggest that the development
 of macropleurae and macrospines are two independent processes and that
 origin of the opistotrunk is linked with the onset of phase 5 of cephalic
 development. The morphology of the early developmental stages of
 Fritzolenellus and some related taxa differs in many aspects from the
 morphology of equivalent stages of some other members of Olenelloidea.
 Consequently, two basic morphotypes are recognized during the early
 development of Olenelloidea – the Fritzolenellus and the Olenellus
 morphotypes. Comparison with Fallotaspidoidea and Redlichiina reveals that
 early developmental stages of these taxa share character combinations that
 are typical for Fritzolenellus morphotype. Such a comparison suggests that
 characters defining Fritzolenellus morphotype are ancestral for Trilobita.
 The Olenellus morphotype is likely a derived condition within Olenelloidea
 and might be related to predator deterrence.
description: Biometric measurement - Dimensions were taken directly from
 photographs using the optical image analyser TpsDig2. Four dimensions were
 measured, specifically: cephalic length, cephalic width at the occipital
 ring, length of the preglabellar field (including anterior border) and
 intergenal spine separation. The measured parameters were plotted on
 bivariate plots using PAST software. Geometric
 morphometric - A total of thirteen landmarks were selected on the right
 sides (or digitally mirrored from the left sides) of the cephala of 65
 specimens and digitized using the optical image analyser TpsDig2. The
 landmarks locations are: 1, anteriormost point of cephalon on sagittal
 axis; 2, intersection of the marginal cephalic border furrow with sagittal
 axis; 3, anteriormost point of glabella on sagittal axis; 4, intersection
 of S3 with sagittal axis; 5, posterior margin of LO on sagittal axis; 6,
 intersection of axial furrow with anterior margin of ocular lobe; 7,
 intersection of axial furrow with S3; 8, intersection of axial furrow with
 S1; 9, intersection of axial furrow with S0; 10, intersection of axial
 furrow with posterior cephalic margin; 11, posterior tip of ocular lobe;
 12, adaxial side of the intergenal spine base; 13, adaxial side of the
 genal spine base. Subsequently, centroid sizes were calculated from the
 landmark configurations using TpsRelw. To visualise allometry, we
 calculated partial Procrustes distance (PPS) and plotted it against
 centroid size for each specimen in Regres8 package of Integrated
 Morphometrics Programs. As a reference for PPS, we used landmark
 configurations of the three smallest specimens in our sample.
description: Data for this study contains two files:
 Laibl-etal_length-measurements - contains all biomeric
 measurements for 99 specimens of <em>Fritzolenellus</em> and
 specifies numbers and housing of the measured material.
 Laibl-etal_landmark-data - contains landmark data in a common
 format for 65 specimens of <em>Fritzolenellus</em>.