Redescription ofDrepanopterus pentlandicusLaurie, 1892, the earliest known mycteropoid (Chelicerata: Eurypterida) from the early Silurian (Llandovery) of the Pentland Hills, Scotland

Abstract

ABSTRACTDrepanopterus pentlandicusLaurie, 1892 is redescribed from the original type material along with previously unfigured specimens. A cleft metastoma is confirmed as a characteristic of the genus, along with the armature of the second and third prosomal appendages being modified into flattened blades, while the species is shown to possess a somewhat enlarged second tergite and lateral prosomal margins that overlap the first opisthosomal tergite. Different ontogenetic stages ofD. pentlandicusare described, and reveal that these latter characters develop only later in ontogeny, suggesting that described specimens ofDrepanopterus abonensisSimpson, 1951 may represent juveniles. Cladistic analysis of Stylonurina shows the genusDrepanopterusto be monophyletic consisting ofD. pentlandicus,D. abonensisandD. odontospathussp. nov.: it forms a basal clade of mycteropoids. Hibbertopteroidea Kjellesvig-Waering, 1959 is shown to be a junior subjective synonym of Mycteropoidea Cope, 1886, with the synonymy of many of the hibbertopterid genera hypothesised andHibbertopterusKjellesvig-Waering, 1959 suggested to represent juvenile specimens ofCyrtoctenusStørmer & Waterston, 1968.Hibbertopterus permianusPonomarenko, 1985 is transferred toCampylocephalusEichwald, 1860. The role of heterochrony in the morphological development of the mycteropoid lineage is discussed, with both hibbertopterids and mycteropids suggested to be hypertrophic and pre-displacement peramorphs respectively.