Xiphosura is an
orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of marine
cheliceratesThe subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites. They originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from in the...
which includes a large number of extinct lineages and only four recent species in the family
LimulidaeLimulidae is the only recent family of the order Xiphosurida and contains all the 4 living species of horseshoe crabs. Horseshoe crabs fossils have been first seen in the late Devonian. Horseshoe crabs are found in shallow water on soft sandy or muddy bottoms...
, which include the
horseshoe crabThe horseshoe crab or Atlantic horseshoe crab is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America...
s. The group has hardly changed in millions of years; the modern horseshoe crabs look almost identical to prehistoric genera such as the
JurassicThe Jurassic is a geologic period and system that extends from about Ma to Ma, that is, from the end of the Triassic to the beginning of the Cretaceous. 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...
Mesolimulus, and are considered to be
living fossilLiving fossil is an informal term for any living species of organism which appears to be the same as a species otherwise only known from fossils and which has no close living relatives. These species have all survived major extinction events, and generally retain low taxonomic diversities...
s.
Xiphosura are traditionally placed in the class
MerostomataMerostomata is a class of marine Chelicerata which includes horseshoe crabs and eurypterids. It includes only four living species but dozens of fossil species mainly from the Paleozoic. Some favor the abandonment of the term class Merostomata, in favour of splitting the class into two classes,...
, although this term was originally intended to encompass also the eurypterids, whence it denoted what is now known to be an unnatural (
paraphyleticIn phylogenetics, a group of organisms is said to be paraphyletic if the group contains its most recent common ancestor but does not contain all the descendants of that ancestor.- Relation to monophyletic groups :...
) group.
Xiphosura is an
orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of marine
cheliceratesThe subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites. They originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from in the...
which includes a large number of extinct lineages and only four recent species in the family
LimulidaeLimulidae is the only recent family of the order Xiphosurida and contains all the 4 living species of horseshoe crabs. Horseshoe crabs fossils have been first seen in the late Devonian. Horseshoe crabs are found in shallow water on soft sandy or muddy bottoms...
, which include the
horseshoe crabThe horseshoe crab or Atlantic horseshoe crab is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America...
s. The group has hardly changed in millions of years; the modern horseshoe crabs look almost identical to prehistoric genera such as the
JurassicThe Jurassic is a geologic period and system that extends from about Ma to Ma, that is, from the end of the Triassic to the beginning of the Cretaceous. 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...
Mesolimulus, and are considered to be
living fossilLiving fossil is an informal term for any living species of organism which appears to be the same as a species otherwise only known from fossils and which has no close living relatives. These species have all survived major extinction events, and generally retain low taxonomic diversities...
s.
Xiphosura are traditionally placed in the class
MerostomataMerostomata is a class of marine Chelicerata which includes horseshoe crabs and eurypterids. It includes only four living species but dozens of fossil species mainly from the Paleozoic. Some favor the abandonment of the term class Merostomata, in favour of splitting the class into two classes,...
, although this term was originally intended to encompass also the eurypterids, whence it denoted what is now known to be an unnatural (
paraphyleticIn phylogenetics, a group of organisms is said to be paraphyletic if the group contains its most recent common ancestor but does not contain all the descendants of that ancestor.- Relation to monophyletic groups :...
) group. Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this usage should be discouraged (e.g. Boudreaux, 1979).
Description
Modern Xiphosurans reach up to in adult length, but the
PaleozoicThe Paleozoic or Palaeozoic Era is the earliest of three geologic eras of the Phanerozoic eon...
species were often far smaller, some as little as long.
The body is covered with heavy mineralized cuticle, and is divided into an anterior prosoma and a posterior
opisthosomaThe opisthosoma is the posterior portion of the arachnids body behind the prosoma . The number of segments and appendages on the opisthosoma vary. Scorpions have 13, but the first is only seen during its embryological development. Other arachnids have twelve or less...
, or abdomen. The upper surface of the prosoma is covered by a semicircular
carapaceA carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids as well as vertebrates such as chelonians, order Testudines, turtles and tortoises.-Crustaceans:In crustaceans, the carapace is a part of the exoskeleton...
, while the underside bears five pairs of walking legs and a pair of pincer-like chelicerae. The mouth is located below the forward tip of the carapace, and lies behind a lip-like structure called the
labrumFor other uses, see: Labrum Labrum as a surname is either of Olde English or of medieval French origin...
.
Xiphosurans have up to four eyes, located in the carapace. There are a pair of compound eyes on the side of the prosoma, and one or two median ocelli towards the front. The compound eyes are simpler in structure than those of other arthropods, with the individual ommatidia not being arranged in a compact pattern. They can probably detect movement, but are unlikely to be able to form a true image. In front of the ocelli is an additional organ that probably functions as a chemoreceptor.
The first four pairs of legs end in pincers, and have a series of spines, called the gnathobase, on the inner surface. The spines are used to
masticateMastication or chewing is the process by which food is crushed and ground by teeth. It is the first step of digestion and it increases the surface area of foods to allow more efficient break down by enzymes. During the mastication process, the food is positioned between the teeth for grinding by...
the food, tearing it up before passing it to the mouth. The fifth and final pair of legs, however, have no pincers or spines, instead having structures for cleaning the gills and pushing mud out of the way while burrowing. Behind the walking legs are a sixth set of appendages, the
chilariaChilaria is a pair of reduced appendages located just posterior and medial to the last walking legs in Xiphosura...
, which are greatly reduced in size and covered in hairs and spines. These are thought to be vestiges of the limbs of an absorbed first opisthosomal segment.
The opisthosoma is divided into a forward mesosoma, with flattened appendages, and a metasoma at the rear, which has no appendages. In modern forms, the whole of the opisthosoma is fused into a single unsegmented structure. The underside of the opisthosoma carries the genital openings and five pairs of flap-like gills.
The opisthosoma terminates in a long caudal spine, commonly referred to as a
telsonThe telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...
(though this same term is also used for a different structure in
crustaceanCrustaceans are a very large group of arthropods, comprising almost 52,000 described species , and are usually treated as a subphylum . They include various familiar animals, such as crabs, lobsters, crayfish, shrimp, krill and barnacles...
s). The spine is highly mobile, and is used to push the animal upright if it is accidentally turned over.
Internal anatomy
The mouth opens into a sclerotised oesophagus which leads to a
cropA crop is a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion that is found in many animals, including gastropods, earthworms, leeches, insects, and birds.- Bees :...
and
gizzardThe gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; rocks are also...
. After grinding up its food in the gizzard, the animal regurgitates any inedible portions, and passes the remainder to the true
stomachIn most mammals, the stomach is a hollow, muscular organ of the gastrointestinal tract , between the esophagus and the small intestine. It is involved in the second phase of digestion, following mastication . The word stomach is derived from the Latin stomachus, which derives from the Greek word...
. The stomach secretes digestive enzymes, and is attached to an
intestineIn anatomy, the intestine is the segment of the alimentary canal extending from the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
and two large caeca that extend through much of the body, and absorb the nutrients from the food. The intestine terminates in a sclerotised
rectumThe rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
, which opens just in front of the base of the caudal spine.
Xiphosurans have a well developed circulatory system, with numerous
arteriesArteries are blood vessels that carry blood away from the heart. All arteries, with the exception of the pulmonary and umbilical arteries, carry oxygenated blood.The circulatory system is extremely important for sustaining life...
that send blood from the long tubular
heartThe heart is a muscular organ found in all vertebrates that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
to the body tissues, and then to two longitudinal sinuses next to the gills. After being oxygenated, the blood flows into the body cavity, and back to the heart. The blood contains haemocyanin, a blue copper-based pigment performing the same function as haemoglobin in vertebrates, and also has blood cells that aid in clotting.
The excretory system consists of two pairs of
coxal glandThe coxal gland is a gland in some arthropods for collecting and excreting urine. The excretion can take place at the base of the legs or, in some higher crustaceans, of the antennae or maxillae .The green gland is divided into three parts namely the end sac, the...
s connected to a
bladderBladder may refer to:* A bladder is a pouch or other flexible enclosure with waterproof or gasproof wallsIn Anatomy:*Urinary bladder, which collects urine for excretion*Gall bladder, which stores bile for digestion...
that opens near the base of the last pair of walking legs. The brain is relatively large, and, as in many arthropods, surrounds the oesophagus. In both sexes, the single
gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
lies next to the intestine and opens on the underside of the opisthosoma.
Ecology
Recent Xiphosura are marine
benthicBenthos are the organisms which live on, in, or near the seabed, also known as the benthic zone. They live in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths....
living on soft substrate, coming ashore for reproduction. Based on the sediment associations, it has been proposed that Paleozoic species were not marine, but lived in fresh and brackish water.
Reproduction
Xiphosurans move to shallow water to mate. The male climbs onto the back of the female, gripping her with his first pair of walking legs. The female digs out a depression in the sand, and lays from 200 to 300 eggs, which the male covers with sperm. The pair then separate, and the female buries the eggs.
The eggs are about across, and hatch into a larva that superficially resembles a
trilobiteTrilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. Trilobites first appear in the fossil record during the Early Cambrian period and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the...
. Through a series of successive
moultEcdysis is the moulting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support of the body and is inelastic, it is shed during growth and a new, larger covering is formed...
s, the larva develops additional gills, increases the length of its caudal spine, and gradually assumes the adult form. Modern xiphosurans reach sexual maturity after about three years of growth.
Taxa removed from Xiphosura
There are two groups originally included in the Xiphosura, but which have been assigned to separate classes:
- Aglaspida
The Aglaspida, more correctly termed Aglaspidida, were a group of small, horseshoe crab-like arthropods that were once regarded as basal or ancestral horseshoe crabs. Now, though, they are regarded as a distinct group, possibly being close to the trilobites...
Walcott 1911 (Cambrian to Ordovician).
- Chasmataspida Caster & Brooks, 1956 (Lower Ordovician).
Classification of the Xiphosura
Order Xiphosura Latreille, 1802
- †Suborder Synziphosurida
- Weinberginidae Richter and Richter, 1929 (Lower Devonian
The Devonian is a geologic period and system of the Paleozoic era spanning from . It is named after Devon, England, where rocks from this period were first studied....
)
- Bunodidae Packard 1886
- Bunodinae Packard 1886 (Upper Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician period, about 443.7 ± 1.5 Ma , to the beginning of the Devonian period, about 416.0 ± 2.8 Ma . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
to Downtonian)
- Limuloidinae Størmer, 1952 (Upper Silurian)
- Pseudoniscidae Packard 1886 (Upper Silurian)
- Kasibelinuridae Pickett, 1993 (Middle Devonian to Late Devonian)
- Suborder Xiphosurida
- †Infraorder Bellinurina
- Elleriidae Raymond, 1944 (Upper Devonian to Upper Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
)
- Euproopidae Eller, 1938 (= Liomesaspidae Raymond 1944) (Upper Carboniferous to Lower Permian)
- Bellinuridae Zittel and Eastman, 1913 (Middle Devonian to Upper Carboniferous)
- Infraorder Limulina
- †Rolfeiidae Selden and Siveter, 1987 (Early Carboniferous to Early Permian)
- †Paleolimulidae Raymond, 1944 (Carboniferous to Permian)
- †Moravuridae Pribyl, 1967 (Mississippian)
- †Austrolimulidae + †Heterolimulidae (Middle Triassic
The Triassic is a geologic period and system that extends from about 251 to 199 Ma . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
)
- Limulidae Zittel, 1885
- †Mesolimulinae Størmer, 1952 (Lower Triassic to Cretaceous
The Cretaceous , Latin language for "chalky", usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago . In the geologic timescale, the Cretaceous follows on the Jurassic period and is followed by the Paleogene period of the...
)
- Limulinae Zittel, 1885
- Tachypleini Pocock, 1902 (Miocene
The Miocene is a geological epoch of the Neogene period and extends from about 23.03 to 5.33 million years before the present . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the...
to Recent)
- Limulini Zittel, 1885 (Recent)
See also
- Synziphosurine
Synziphosurines are a paraphyletic group of xiphosurid ancestors from the Silurian to Carboniferous eras. The earliest accepted synziphosurines are Venustulus waukeshaensis, found in Early Silurian deposits in Wisconsin, and Bembicosoma pomphicus, from the same period, known from deposits in...
- List of xiphosurans
External links
- Peripatus - an overview of arthropod relationships.
- Paleos - a site with a synoptic account of the Xiphosura, focused on fossils.
- Xiphosura - the article from the UCMP Web Taxa project.