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.

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.

Data for this study contains two files:

Laibl-etal_length-measurements - contains all biomeric measurements for 99 specimens of Fritzolenellus and specifies numbers and housing of the measured material.

Laibl-etal_landmark-data - contains landmark data in a common format for 65 specimens of Fritzolenellus.