Arthropoda		Xiphosura
Arthropoda		Xiphosura

Xiphosura

Chelicerata  
|--Merostomata
|  |--Xiphosura 
|  |  |--Bunodidae
|  |  `--+--Bellinurina
|  |     `--Limulina
|  |        |--Paleolimulidae
|  |        `--Limulidae
|  `--+--Chasmataspida
|     `--Eurypterida
`--Arachnida  

Taxonomy

Class Xiphosura Latreille, 1802

Limulus polyphemus (Linnaeus)

Contents

Introduction to the Xiphosura
Phylogeny
Systematics
Order Synziphosurida
family Weinberginidae
family Bunodidae
family Pseudoniscidae
family Kasibelinuridae

Order Xiphosurida
superfamily Bellinuroidea (including Euproopacea)
family Bellinuridae
family Euproopidae
family Elleriidae

superfamily unspecified
family Rolfeiidae

superfamily Limuloidea
family Paleolimulidae
family Moravuridae
family "Valloisellidae"
family Austrolimulidae
family Heterolimulidae
family Limulidae

Links

Remarks

There are only 3 genera and 5 species of Xiphosura left alive today, but they were much more numerous and diverse during the Palaeozoic era.  The surviving horseshoe crabs (Limulus) are 'living fossils', barely changed in some 250 million years (since early Triassic time).  Members of this class have a large shield that covers the cephalothorax; the carapace is hinged between the cephalothorax and abdomen.  The exoskeleton generally consists of three parts, the large, semicircular cephalothorax, or prosoma, the usually smaller, subtriangular and in earlier forms "trilobite"-like opisthosoma, and the long stout tail-spine or telson (which is actually the end part of the opisthosoma).

The prosoma contains both head and visceral organs. The compound eyes are small (and absent in some early forms), and there are six pairs of legs (in the living Limulus) but no antennae.  The second pair of appendages, the pedipalps, resemble walking legs. Respiration is via 5 pairs of book gills, the flaps of which beat in a metachronal rhythm to produce a vigorous current.  Recent xiphosurids (Horseshoe crabs) feed on worms and other small invertebrates.  They are often used as laboratory animals by physiologists.  It has been argued that because of their unique status as prehistoric "living fossils" they deserve special conservation status.

Evolution

 The Evolution of the Xiphosura during the Paleozoic era, showing representative genera. This cladogram is from Anderson & Selden, reproduced courtesy Lyall Anderson.

Paleomerus and Lemoneites are very early forms that were either Aglaspids or transitional between the Aglaspida and the Xiphosura. The Furongian (Caerfai epoch) marine family Eolimulidae is generally considered a true Xiphosuran, but again more research needs to be done if more is to be known about the early history of this interesting group.

Systematics

There are only two orders (or sub-orders, depending on your preference) of Xiphosura, the primitive and ancestral Synziphosurina/-ida, and the Lumulina/-ida/Xiphosurida/-ina. The latter group includes modern horseshoe crabs and their immediate ancestors.

The Aglaspida, for a long time considered a very primitive order of Xiphosura, are now thought to be a distinct group, and may actually be closer in fact be closer to the trilobites, or alternatively an ancestral lineage of Merostomata. The Chasmataspida, previously considered Xiphosura, appear to be closer to the Eurypterida, or possibly represent an independent subclass. With modern cladistic analysis even this Linnean arrangement is being discarded.

The following family level Linnean classification presented here is a sort of compromise between the Treatise of Invertebrate Paleontology, the Fossil Record I and II, and the recent work of Lyall Anderson (who presents a cladistic arrangement of the group). In view of the fact that this material is taken hither-thither from other sources it should not be considered an authoritative review of the group.

Order Synziphosurida (Basal Xiphosura)

Silurian to Early Devonian.

Cyamocephalus loganensis Currie 1927
Length about 5 cm - family Pseudoniscidae
Late Llandovery or possibly early Wenlock, Silurian period, Scotland (Euramerica)

The Synziohosurida are a small, fairly diverse, paraphyletic / ancestral group of primitive Xiphosura. Rather trilobite-like in appearance.  Large prosoma (headshield), simple eyes only. 9 or 10 opisthosomal (abdominal) segments, most or all of which are free (unfused).  The segments are not chelate.  Mostly brackish or freshwater environments, although some forms were marine (marginal marine?).  Usually found in association with eurypterids and ostracoderms.

Family Weinberginidae

Family Bunodidae

Small elongate forms, ornamented prosoma, abdomen with broad axis, 9 free segments, post-abdomen with 3 or 4 segments. 

Subfamily Limuloidinae

Subfamily Bunodinae

Family Pseudoniscidae

Family Kasibelinuridae

Order Xiphosurida

Liomesaspis laevis Raymond 1944
Length about 4 cm
A common coal swamp form - Family Euproopidae
Late Bashkirian ? and early Moscovian to early Gzhelian (Euramerica)

The Order Xiphosurida/Limulida includes most Xiphosura, and all post-Devonian forms. These animals were quite common during the Carboniferous, and some forms seem to have been amphibious, although other types (e.g. Paleolimulus) were fully marine. Beginning from small ancestral types they increased in size through the Mesozoic and Cenozoic, and modern horseshoe crabs are giants compared to Paleozoic forms (the horse also has shown a similira tendency to increase in size but reduce in diversity throughout the Tertiary and Quaternary periods), There is the tendency towards fusion of the opisthosomal tergites (free abdominal segments) to form a thoracetron or fused plate. There are several superfamilies and a greater number of families, but only one lineage made it into the post-Paleozoic world. Cladistic analysis so far indicates that the Xiphosurida are a monophyletic taxon.

Suborder Bellinurina

Anderson and Selden consider the Bellinuroidea a suborder (Bellinurina) which contains only two families, the Bellinuridae and the Euproopidae. His Euproopidae would seem to be equivalent to the Treatise's Euproopacea, as it includes the genera Euproops and Liomesaspsis (usually each put in a seperate family of the Euproopacea). These were small animals, with short bodies, only partially fused abdominal segments, and wide rounded horseshoecrab-like headshields, giving them a rather trilobite-like resemblence, apart from the long tail-spine or telson. They frequented the coal swamps and were in all likelihood amphibious, perhaps living part of their life-cycle on land. 

Elleriidae?

time range: Frasnian
habitat: Marginal Marine ? Brackish and Fresh water. Amphibious?
representative taxa:
description: Abdominal axis and pleural area segmented; the rear of the abdominal shield deeply indented. In the Treatise placed in the superfamily Euproopacea. In view of the early date this group may perhaps turn out to be a separate early off-shoot.

Family Euproopidae

Family Bellinuridae

Suborder Limulina

The Limulina represent the most advanced lineage of the group, descended from early Carboniferous transitional forms like Rolfeia.  The cephalothorax is wide and arched, with the cardiophthalmic region poorly defined. .  The abdominal segments are usually fully fused, although the marginal spines are movable. Includes the 5 living species of "horseshoe crabs" (Limulus).  These creatures live in a shallow marine environment (although some Paleozoic forms may have been brackish water inhabitants).  Jurassic Xiphosurids are extremely similar to those found today.

Anderson and Selden distinguish between the superfamily Paleolimuloidea - defined by the pyramidal cheek node on the carapace - and the superfamily Limuloidea (which Includes all Mesozoic and Cenozoic Xiphosura). 

Family Rolfeiidae

These are paraphyletic basal limulines.

Family Paleolimulidae

Family Moravuridae

time range: Serpukhovian
habitat: Marginal Marine
representative taxa: Xaniopyramis linseyi Siveter and Selden, 1987

Probably a branch of the Paleolimulidae

Family Undetermined (New family? - "Valloisellidae")

This, and the following two families are staem limulids, of uncertain monophyly.

Family Austrolimulidae

Family Heterolimulidae

Family Limulidae

Subfamily Mesolimulinae Størmer, 1952

Subfamily Limulinae Zittel, 1885

Tribe Tachypleini Pocock, 1902

Tribe Limulini Zittel, 1885

Links and References

Links and References

Xiphosura - Horseshoe crabs presents the best general introduction to the group on the Web

Lyall I. Anderson's Home Page - excellent material on Paleozoic Xiphosura, and heaps of links

Anderson, LI (1997), The xiphosuran Liomesaspis from the Montceau-les-Mines Konservat Lagerstatte, Massif Central, France.  N. Jahrb. Geol. Palaontol., Abh. 204: 415-436.  abstract

Anderson, LI (1998), A new specimen of the Silurian synziphosurine Cyamocephalus.  Proc. Geol. Assoc.  110: 211-216. abstract

Anderson, LI & PA Selden (1997), Opisthosomal fusion and phylogeny of Palaeozoic Xiphosura.  Lethaia 30: 19-31. abstract

Euproops danae, from the Mazon Creek deposits of Illinois (Moscovian age)  Euproops danae was a small form, length about 2 cm, that is relatively common in the Mazon Creek deposit. It seems to have been freshwater or even semiaquatic, living part of its life on land