The Trilobitomorpha is a subphylum of the phylum Arthropoda that includes the trilobites. Originally a variety of peculiar forms, mostly from the lower Cambrian, were included as the Class Trilobitoidea. However, the many species—known only from fossils—do not appear to be closely related to the trilobites or, in many cases, to each other either, and are now generally placed in separate subphyla when classified at all. Subphyla and Classes Subphylum Trilobitomorpha Trilobita - Trilobites (extinct) Subphylum Chelicerata Arachnida - Spiders, Scorpions, etc. ... Orders Agnostida Redlichiida Corynexochida Lichida Phacopida Proetida Asaphida Harpetida Ptychopariida Proposed order Nektaspida Trilobites are extinct arthropods in the class Trilobita. ... The Cambrian is a major division of the geologic timescale that begins about 542 million years before the present (BP) at the end of the Proterozoic eon and ended about 490 million years BP with the beginning of the Ordovician period. ...
As such, "Arachnomorpha" or "Arachnata" constitutes a broad taxon that includes the Trilobites, the Chelicerata and a number of primitive Cambrian to Devonian forms that are usually included in the Trilobitomorpha or the Xiphosura.
Included around the base of the "Crown Group" Chelicerata are a number of primitive forms called "Merostomoidea" - which seem to be transitional between Trilobites and Chelicerates proper.
"Trilobitomorpha with eurypterid or xiphosurid type of body, trilobation more or less distinct; tergites of trunk free or ankylosed into continuous shield, telson mostly styliform; 1st and last pair of trilobite appendages may be modified and others partly reduced"
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