Skip to main content
Dryad

Post-embryonic development of Fritzolenellus suggests the ancestral morphology of the early developmental stages in Trilobita

Data files

Jul 07, 2020 version files 36.74 KB

Abstract

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.