Problems of functional morphology and classification in stylonuroid eurypterids (Chelicerata, Merostomata), with observations on the Scottish Silurian Stylonuroidea

Abstract

SynopsisEurypterids of the Superfamily Stylonuroidea Diener 1924 sensu Størmer (1974, 373) from the Pentland Hills, Midlothian, are redescribed and the evidence which these forms may give concerning the life environment of the Gutterford Burn Eurypterid Bed (Upper Llandovery), from which most of them have been obtained, is considered. Five species are recognised. Parastylonurus ornatus (Laurie) is redescribed with special reference to the organs of locomotion and reproduction. A new form from the Gutterford Burn is described as Parastylonurus hendersoni sp. nov. Stylonurus macrophthalmus Laurie is designated the type species of the new genus Hardieopterus and Stylonurus knoxae Lamont as the type species of the new genus Lamontopterus. The unique holotype of Lamontopterus knoxae is of morphological interest in showing evidence of the gut.Stylonuroids from other Scottish Silurian localities are also considered. Two new species, Brachyopterella ritchiei sp. nov. from Seggholm and Hardieopterus (?) lanarki sp. nov. from the Logan Water are described. Stylonurella spinipes (Page) is redescribed and this species is reported for the first time from Seggholm.The new information provided by this study has raised a number of problems of classification and the criteria upon which eurypterids are classified, and particularly those applied to the stylonuroid eurypterids, are re-assessed and a new classification of the Stylonuroidea proposed. This has required the description of new taxa; a new family, the Parastylonuridae, is proposed to accommodate Parastylonurus, Hardieopterus and probably Lamontopterus which share a unique combination of characters of the prosomal appendages and metastoma. A new genus, Kiaeropterus, is described to accommodate certain species previously assigned to Stylonurella but displaced from that genus as now emended.A reconstruction of Parastylonurus ornatus in the walking position is attempted and has resulted in the recognition of a new generation of problems related to the functional morphology of these animals. Criteria which may appropriately be used in deciding the manner in which they walked and the posture which they adopted when doing so, are discussed. It is suggested that, in response to certain anatomical and physiological constraints which otherwise would have rendered the animals unstable in the walking position, the post-abdomen and telson have been specialised as hydrodynamic structures for monitoring water movements and vectoring the animal for maximum walking efficiency. The significance of this hypothesis is discussed in relation to the functional morphology and evolution of other members of the Stylonuroidea.