Context: Horseshoe crabs are marine arthropods of the family Limulidae and order Xiphosura or Xiphosurida, that live primarily in and around shallow ocean waters on soft sandy or muddy bottoms. They occasionally come onto shore to mate. They are commonly used as bait and in fertilizer. In recent years, a decline in the population has occurred as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America. Tetrodotoxin may be present in the roe of species inhabiting the waters of Thailand. Because of their origin 450 million years ago (Mya), horseshoe crabs are considered living fossils., James Dwight Dana FRS FRSE (February 12, 1813  April 14, 1895) was an American geologist, mineralogist, volcanologist, and zoologist. He made pioneering studies of mountain-building, volcanic activity, and the origin and structure of continents and oceans around the world., Arachnids are a class (Arachnida) of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. All arachnids have eight legs, although the front pair of legs in some species has converted to a sensory function, while in other species, different appendages can grow large enough to take on the appearance of extra pairs of legs. The term is derived from the Greek word ("aráchn"), meaning "spider". Spiders are the largest order in the class., Mesolimulus is an extinct genus of arthropod. The best known examples are found in Solnhofen limestone near Solnhofen, Bavaria, Germany. Originally assigned to the living genus "Limulus", they are related to and look virtually identical to the modern horseshoe crab. Other species assigned to "Mesolimulus" have been recorded from Spain, Siberia and (questionably) from Lebanon.
Kouphichnium.
The unusual trackways left by these animals at Solnhofen were originally thought to have been made by birds or pterosaurs, since they included a cross-shaped marking comparable with the impression of a typical bird foot. Eventually they were correctly identified as arthropod footprints and named "Kouphichnium walchi", as proved by some remarkable 'death traces' showing a "Mesolimulus" circling round on itself before dying. The last leg of more advanced horseshoe crabs is modified into a so-called 'pusher' which consists of four plates at the tip which push against the soft sediment rather like a snow-shoe. This was what left the unusual bird-like footprints. 
Horseshoe crabs in general date to the Ordovician Period, more than 440 million years ago, and late Paleozoic "Euproops" fossils indicate that they have changed little over the last 300 million years. Fossils preserved in Solenhofen limestone are unusual because soft body parts and skeletons are clearly represented., The Jurassic (from Jura Mountains) is a geologic period and system that spans 56.3 million years from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period Mya. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles. The start of the period is marked by the major TriassicJurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian/Toarcian event in the Early Jurassic, and the Tithonian event at the end; however, neither event ranks among the "Big Five" mass extinctions., The Xiphosura are an order of marine chelicerates that includes a large number of extinct lineages and only four extant species in the family Limulidae, which includes the horseshoe crabs. The group has hardly changed in millions of years; the modern horseshoe crabs look almost identical to prehistoric genera such as the Jurassic "Mesolimulus", and are considered to be living fossils. The most notable difference between ancient and modern forms is that the abdominal segments in present species are fused into a single unit in adults., A volcanologist or vulcanologist is a geologist who studies the processes involved in the formation and eruptive activity of volcanoes and their current and historic eruptions. Volcanologists frequently visit volcanoes, especially active ones, to observe volcanic eruptions, collect eruptive products including tephra (such as ash or pumice), rock and lava samples. One major focus of enquiry is the prediction of eruptions; there is currently no accurate way to do this, but predicting eruptions could alleviate the impact on surrounding populations., An arthropod (from Greek "arthro-", joint + "podos", foot) is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages (paired appendages). Arthropods form the phylum Arthropoda, which includes the insects, arachnids, myriapods, and crustaceans. 
Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80% of all described living animal species, some of which, unlike most animals, are very successful in dry environments., Merostomata is a class of chelicerate arthropods that contains the extinct Eurypterida ( sea scorpions ) and the extant Xiphosura ( horseshoe crabs ) . The term was originally used by James Dwight Dana to refer to Xiphosura only , but was emended by Henry Woodward to cover both groups ., Subject: merostomata, Relation: parent_taxon, Options: (A) ash (B) bird (C) chelicerata (D) horseshoe crab (E) limulus (F) mya (G) xiphosura
Output: chelicerata