From Wikipedia, the free encyclopedia
Jump to: navigation, search
Temporal range: Late Ordovician–Recent
Eurypterus Smithsonian.jpg
Reconstruction of Eurypterus (Eurypterida)
Limulus polyphemus.jpg
Five views of Limulus polyphemus (Xiphosura)
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Merostomata
Woodward, 1866

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.


The name "Merostomata" derives from the Greek roots μηρός (meros, "thigh") and στόμα (stoma, "mouth"), in reference to the animals' possession of appendages which are mouthparts at their proximal end, but swimming legs at their distal end.[1]


The scientific consensus at the beginning of the 20th century was that these two marine groups were closely related, and only more distantly related to the terrestrial Arachnida.[2] Some more recent analyses suggest the grouping Merostomata is not monophyletic, with Xiphosura being basal to a clade comprising Eurypterida and Arachnida.[3] Other recent analyses support the monophyly of this group.[4] The Xiphosura are estimated to have diverged from the Arachnida 480 million years ago.[5]

The shared features of the two groups traditionally grouped in the Merostomata are now thought to be retentions of primitive conditions (symplesiomorphies), thus the name Merostomata has been recommended to be abandoned.[3]


  1. ^ "Literary Notices". The Intellectual Observer: review of natural history, microscopic research, and recreative science. 11: 233–234. 1867.  |chapter= ignored (help)
  2. ^ J. A. Dunlop & P. A. Selden (1998). "The early history and phylogeny of the chelicerates". In Richard A. Fortey & Richard H. Thomas. Arthropod relationships. Volume 55 of Systematics Association Series. Springer Verlag. pp. 221–236. ISBN 978-0-412-75420-3.  Also available as PDF
  3. ^ a b Colin Tudge (2002). "Spiders, scorpions, mites, eurypterids, horseshoe crabs, and sea spiders. Subphylum Chelicerata and Subphylum Pycnogonida". The variety of life: a survey and a celebration of all the creatures that have ever lived. Oxford University Press. pp. 309–326. ISBN 978-0-19-860426-6. 
  4. ^ Garwood, Russell J.; Dunlop, Jason A. (2014). "Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders". PeerJ. 2: e641. PMC 4232842Freely accessible. PMID 25405073. doi:10.7717/peerj.641. Retrieved June 15, 2015. 
  5. ^ Davide Pisani; Laura L. Poling; Maureen Lyons-Weiler; S. Blair Hedges (2004). "The colonization of land by animals: molecular phylogeny and divergence times among arthropods". BMC Biology. 2: 1. PMC 333434Freely accessible. PMID 14731304. doi:10.1186/1741-7007-2-1. 

External links[edit]