page_title: Eurypterid
section_title: Respiration
hierarchical_section_title: Eurypterid / Biology / Respiration
caption_attribution_description: English: This image shows a 0.4 inch (10 mm) large male Common woodlouse (Oniscus asellus). Deutsch: Dieses Bild zeigt eine 10 mm große männliche Mauerassel (Oniscus asellus). Danke an Andreas Böttger für das Beschaffen des Tieres und danke an Achim Raschka für die Bestimmung. Français&#160;: l'Aselle des murs, mâle, 10mm Nederlands: Kelderpissebed (Oniscus asellus), mannetje, 10mm Camera data Camera Nikon D70 Lens Tamron SP AF 90mm/2.8 Di Macro 1:1 Focal length 90 mm Aperture f/32 Exposure time 8 s Sensivity ISO 200
context_page_description: Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is likely to have appeared first either during the Early Ordovician or Late Cambrian period. With approximately 250 species, the Eurypterida is the most diverse Paleozoic chelicerate order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the Silurian, from which the majority of eurypterid species have been described. The Silurian genus Eurypterus accounts for more than 90% of all known eurypterid specimens. Though the group continued to diversify during the subsequent Devonian period, the eurypterids were heavily affected by the Late Devonian extinction event. They declined in numbers and diversity until becoming extinct during the Permian–Triassic extinction event 251.9 million years ago.
  Although popularly called "sea scorpions", only the earliest eurypterids were marine; many later forms lived in brackish or fresh water, and they were not true scorpions.
context_section_description: In eurypterids, the respiratory organs were located on the ventral body wall (the underside of the opisthosoma). Blatfüsse, evolved from opisthosomal appendages, covered the underside and created a gill chamber where the kiemenplatten (gill tracts) were located. Depending on the species, the eurypterid gill tract was either triangular or oval in shape and was possibly raised into a cushion-like state. The surface of this gill tract bore several spinules (small spines), which resulted in an enlarged surface area. It was composed of spongy tissue due to many invaginations in the structure.
  Though the kiemenplatte is referred to as a "gill tract", it may not necessarily have functioned as actual gills. In other animals, gills are used for oxygen uptake from water and are outgrowths of the body wall. Despite eurypterids clearly being primarily aquatic animals that almost certainly evolved underwater (some eurypterids, such as the pterygotids, would even have been physically unable to walk on land), it is unlikely the gill tract contained functional gills when comparing the organ to gills in other invertebrates and even fish. Previous interpretations often identified the eurypterid "gills" as homologous with those of other groups (hence the terminology), with gas exchange occurring within the spongy tract and a pattern of branchio-cardiac and dendritic veins (as in related groups) carrying oxygenated blood into the body. The primary analogy used in previous studies has been horseshoe crabs, though their gill structure and that of eurypterids are remarkably different. In horseshoe crabs, the gills are more complex and composed of many lamellae (plates) which give a larger surface area used for gas exchange. In addition, the gill tract of eurypterids is proportionally much too small to support them if it is analogous to the gills of other groups. To be functional gills, they would have to have been highly efficient and would have required a highly efficient circulatory system. It is considered unlikely, however, that these factors would be enough to explain the large discrepancy between gill tract size and body size.
  It has been suggested instead that the "gill tract" was an organ for breathing air, perhaps actually being a lung, plastron or a pseudotrachea. Plastrons are organs that some arthropods evolved secondarily to breathe air underwater. This is considered an unlikely explanation since eurypterids had evolved in water from the start and they  would not have organs evolved from air-breathing organs present. In addition, plastrons are generally exposed on outer parts of the body while the eurypterid gill tract is located behind the blatfüssen. Instead, among arthropod respiratory organs, the eurypterid gill tracts most closely resemble the pseudotrachae found in modern isopods. These organs, called pseudotrachae, because of some resemblance to the trachae (windpipes) of air-breathing organisms, are lung-like and present within the pleopods (back legs) of isopods. The structure of the pseudotrachae has been compared to the spongy structure of the eurypterid gill tracts. It is possible the two organs functioned in the same way.
  Some researchers have suggested that eurypterids may have been adapted to an amphibious lifestyle, using the full gill tract structure as gills and the invaginations within it as pseudotrachea. This mode of life may not have been physiologically possible, however, since water pressure would have forced water into the invaginations leading to asphyxiation. Furthermore, most eurypterids would have been aquatic their entire lives. No matter how much time was spent on land, organs for respiration in underwater environments must have been present. True gills, expected to have been located within the branchial chamber within the blatfüssen, remain unknown in eurypterids.
caption: The supposed "gill tracts" of eurypterids have been compared to the air-breathing pseudotrachae present in the posterior legs of modern isopods, such as Oniscus (pictured).
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