Sperm carriers in Silurian sea scorpions.

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.

Heavy metal--a contrasting substance for micro-tomographical visualization of scorpion book lungs.

Remarkable results obtained while searching for suitable contrasting agents for tomographical imaging are presented. Book lungs, the respiratory organs of scorpions (Scorpiones, Arachnida), served as challenging objects in terms of fragility and preparatory requirements, as well in terms of size differences between the entire organ and its functional subunits (the lamellae). The problem of tissue contrast was investigated using various metals, but most successfully addressed using osmium. Gaseous osmium tetroxide exposure with subsequent ethanol treatment yielded the best results. A constant and thorough staining of the book lungs was achieved, easing book-lung visualization using 3D-reconstructions from microCT scans.

Microanatomy of Early Devonian book lungs.

The book lungs of an exceptionally preserved fossil arachnid (Trigonotarbida) from the Early Devonian (approx. 410 Myr ago) Rhynie cherts of Scotland were studied using a non-destructive imaging technique. Our three-dimensional modelling of fine structures, based on assembling successive images made at different focal planes through the translucent chert matrix, revealed for the first time fossil trabeculae: tiny cuticular pillars separating adjacent lung lamellae and creating a permanent air space. Trabeculae thus show unequivocally that trigonotarbids were fully terrestrial and that the microanatomy of the earliest known lungs is indistinguishable from that in modern Arachnida. A recurrent controversy in arachnid evolution is whether the similarity between the book lungs of Pantetrapulmonata (i.e. spiders, trigonotarbids, etc.) and those of scorpions is a result of convergence. Drawing on comparative studies of extant taxa, we have identified explicit characters (trabeculae, spines on the lamellar edge) shared by living and fossil arachnid respiratory organs, which support the hypothesis that book lungs were derived from a single, common, presumably terrestrial, ancestor.

Reinterpreting the morphology of the Jurassic scorpion Liassoscorpionides.

The fossil scorpion (Arachnida: Scorpiones) Liassoscorpionides schmidti Bode, 1951, the youngest scorpion to be assigned to a putative aquatic clade, from the early Jurassic (Toarcian) of Hondelage near Braunschweig, Germany, is redescribed. It is the only unequivocal Jurassic scorpion, but there is in fact no evidence to support an aquatic mode of life. Some features hint that it might be the last of the mesoscorpions-a largely Palaeozoic lineage with a few Mesozoic "relict" taxa. Our reinvestigation revealed that the holotype was over-interpreted by Kjellesvig-Waering. Key characters in his redescription (e.g. eye position, chelicerae, gill slits, etc.) are dubious at best and we follow previous authors in treating L. schmidti as an incertae sedis species. To investigate the hypothesis that mesoscorpions were habitually terrestrial, macerated cuticle fragments originally assigned to Mesophonus infans E Wills, 1947 from the Late Triassic Lower Keuper Sandstone Series of Bromsgrove, UK were re-examined. These were drawn as having book lungs in the original description, but the original figures are misleading and the expected lung lamellae could not be resolved.

The book lungs of Scorpiones and Tetrapulmonata (Chelicerata, Arachnida): evidence for homology and a single terrestrialisation event of a common arachnid ancestor.

The question of whether Arachnida (Chelicerata) conquered terrestrial habitats only once or several times is controversial. The key group in this respect is the Scorpiones. Several authors claim that they became terrestrial independently of other arachnid lineages. This argumentation uses two lines of evidence. One is that book lungs of scorpions and other arachnids are considered non-homologous because they occur on different segments. The other line is based on fossil evidence which suggests that early scorpions were aquatic, together with a putative sister group relationship between scorpions and the aquatic Eurypterida. To address this problem we undertook a comparative scanning electron microscopical and histological study of the book lungs of scorpions, amblypygids, uropygids, and mesothelid spiders. In addition, we included the book gills of a xiphosuran. We found several detailed similarities in the book lungs shared by all arachnid taxa studied. Based on these findings we conclude that arachnid book lungs are homologous. Furthermore, we suggest that the apomorphic book lungs of arachnids indicate a single terrestrialisation event in the stem lineage leading to Arachnida.