The Cheloniellida Broili, 1932 are an unranked taxon (usually referred to as a class or order) of extinct Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 arthropods, currently (Dunlop & Selden, 1997; Dunlop, 2002) thought to be a member of the Arachnomorpha

Arachnomorpha

Arachnomorpha Lameere 1890 is a subdivision or clade of Arthropoda, comprising the monophyletic group formed by the trilobites, other great appendage arthropods and trilobite-like families , and a diverse sister clade including the chelicerates...

 and related to the Chelicerata

Chelicerata

The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites...

 and the Trilobita, while not included in any of those.
There are six included genera, dating from Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

, Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 and Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

.

Included genera

• Cheloniellon Broili, 1932 - Lower Devonian, Germany
• Duslia Jahn, 1893 - Upper Ordovician, Czech Republic
• Neostrabops Caster & Macke, 1952 - Upper Ordovician, USA
• Paraduslia Dunlop, 2002 - Lower Devonian, Russia.
• Pseudarthron Selden & White, 1984 - Upper Silurian
• Triopus Barrande, 1872 - Upper Ordovician

Relationships - varied proposals

"Originally Cheloniellon was believed to be similar to trilobites (Broili 1932), though subsequent authors have suggested suggested that it is related to trilobites and Cambrian “merostomoids” (Størmer 1944), that it is a late representative of a trilobitomorph
group giving rise to Chelicerata (Stürmer & Bergström 1978), that it is related to “loricate”
crustaceans (Delle Cave & Simonetta 1991) or that it is the sister group to Chelicerata (Wills et al. 1995; Dunlop & Selden 1997)." [Account by Dunlop, 2002].

• Class Cheloniellida, sister group to class Crustacea (Boudreaux, 1979).
• Order Cheloniellida in subclass Petalopleura
  Petalopleura
  Petalopleura is a genus of moth in the family Arctiidae.-References:*...
  
   in class Artiopoda, this being sister to class Marellomorpha (Hou & Bergström, 1997).
• One of the five major taxa of the Arachnomorpha
  Arachnomorpha
  Arachnomorpha Lameere 1890 is a subdivision or clade of Arthropoda, comprising the monophyletic group formed by the trilobites, other great appendage arthropods and trilobite-like families , and a diverse sister clade including the chelicerates...
  
  , sister group to the Chelicerata. The other three are Trilobita, Aglaspidida and Strabopida (Dunlop & Selden 1997, repeated by Budd, 1997).