Sperm carriers in Silurian sea scorpions

Abstract

Invasion of the land by arachnids required adaptations of numerous organs, such as gills evolving into lungs, as well as mechanisms facilitating sperm transfer in a terrestrial environment. Many modern arachnids use spermatophores for this purpose, i.e. sperm transmitters detached from the body. Exceptionally preserved Silurian (423 Ma) fossils of Eurypterus tetragonophthalmus Fischer, 1839 (Chelicerata: Eurypterida) preserve so-called 'horn organs' which we here demonstrate as being equivalent to the spermatophore-producing parts of the genital tract in certain modern arachnids. This clarifies a long-running debate about sexing eurypterids based on the shape of the median abdominal (or genital) appendage. To our knowledge this is also the oldest direct evidence for spermatophore-mediated sperm transfer in the fossil record and suggests that eurypterids had evolved mating techniques using spermatophores as early as the Silurian, a valuable prerequisite for life on land. Spermatophores are absent in sea spiders (Pycnogonida) and horseshoe crabs (Xiphosura); thus the shared presence of sclerotized sperm-transfer devices in eurypterids and arachnids is a novel character, newly elucidated here, which offers explicit support for (Eurypterida + Arachnida). For this clade the name Sclerophorata n. nov. is proposed. Arachnida can be further defined by fusion of the originally paired genital opening.

Demecology in the Cambrian: synchronized molting in arthropods from the Burgess Shale

“…It does not appear to be a ground pattern feature of Xiphosura; so, based on phylogenetic reasoning, there is no indication for synchronized mating in eurypterids [53]. Newer evidence indicates that spermatophores were involved in eurypterid mating, so their mating was more similar to that in scorpions and other arachnids than to that in xiphosurans [56]; considering this aspect, mating and spawning was most probably decoupled in eurypterids. Although we cannot completely exclude a coupling of synchronized molting and mating in eurypterids or in trilobites, these data do not provide positive evidence for such a coupling.…”

Segmentation and tagmosis in Chelicerata

“…3, Fig. 24e25) which was interpreted by Kamenz et al (2011) as a sclerotized precursor to a spermatophore. The mode of sperm transfer in chasmataspidids is unknown although as noted above at least some taxa also have a eurypterid-like genital appendage (see 3.8.1), albeit without obvious horn organs.…”

“…The name Sclerophorata was introduced by Kamenz et al (2011) for Eurypterida and Arachnida and defined as euchelicerates which transfer sperm via a spermatophore, as opposed to just releasing their sperm into the water as per horseshoe crabs. Note that several arachnids e e.g.…”

The fossil record of ecdysis, and trends in the moulting behaviour of trilobites

“…The novel analytical approach employed by Tetlie et al (2008) allowed for comparison with the moulting modes in aquatic chelicerates and scorpions, in line with a chelicerate affinity for the eurypterids. Further study is needed to identify if ecdysial mode provides insight into the phylogenetic uncertainty for the position of eurypterids within Chelicerata, namely whether they are most closely related to arachnids (Weygoldt and Paulus, 1979;Shultz, 1990Shultz, , 2007Kamenz et al, 2011;Lamsdell, 2013), and the scorpions in particular (Kjellesvig-Waering, 1986;Dunlop and Webster, 1999) or the more traditional view of being sister taxa with the xiphosurans (Woodward, 1867(Woodward, , 1872Strømer, 1944Strømer, , 1955 in Merostomata, which has also received recent support (Garwood and Dunlop, 2014). Eurypterus shares in common with modern Limulus the ventral anterior location of the prosomal suture and the tendency of the ecdysial suture to close after egress (Loveland, 2002;Shuster and Sekiguchi, 2004;Tetlie et al, 2008), but also shows similarity to scorpion ecdysis in that the more anterior opisthomal segments remain attached to the cephalic carapace during formation of the anterior ecdysial opening (Tetlie et al, 2008).…”