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Merostomata (Merostomates)

      Merostomata is the name given to a grouping of the extinct Eurypterida (sea scorpions) and the 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. The scientific consensus at the beginning of the twentieth century was that these two marine groups were closely related, and only more distantly related to the terrestrial Arachnida.More recent analyses suggest that the grouping Merostomata is not monophyletic, with Xiphosura being basal to a clade comprising Eurypterida and Arachnida.[3] It is estimated that the Xiphosura diverged from the Arachnida 480 million years ago.The shared features of the two groups traditionally grouped in the Merostomata are now thought to be retentions of primitive conditions (symplesiomorphies), and thus it has been recommended that the name Merostomata be abandoned.

Source: http://en.wikipedia.org/wiki/Merostomata
aglaspis
Img. 70: A comparison of the aglaspidid Aglaspis spinifer with merostomate Kodymirus vagans
merostomata
Img. 70:
Acutiramus bohemicus p1380
Acutiramus bohemicus p1381
Acutiramus bohemicus JH154
Acutiramus bohemicus p5601
Acutiramus bohemicus p1368
Acutiramus bohemicus p5597
Acutiramus bohemicus JH157
Acutiramus bohemicus p1384
Acutiramus bohemicus p5592
Acutiramus bohemicus p1385
Acutiramus bohemicus JH155
Acutiramus bohemicus p1382
Acutiramus bohemicus p1383
Acutiramus bohemicus p1378
Acutiramus bohemicus p1379
Acutiramus bohemicus OZ161
Acutiramus bohemicus OZ165
Acutiramus bohemicus OZ166
Acutiramus bohemicus OZ167
Acutiramus bohemicus OZ168

Virtual museum of the Czech Geological Survey, www.geology.cz, (C) Czech Geological Survey, 2011, v.0.99 [13.12.2011]