Hymenocarina

Hymenocarina Temporal range: Cambrian PreꞒ Ꞓ O S D C P T J K Pg N
Diagram of Waptia
Fossil of Canadaspis perfecta
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	Mandibulata
Order:	†Hymenocarina Størmer, 1944
Genera
Balhuticaris Canadaspis Ercaicunia Fibulacaris Jugatacaris[1] Nereocaris[1] Odaraia Pakucaris Perspicaris Plenocaris[1] Synophalos Xiazhuangocaris Pectocaris Clypecaris Vermontcaris ?Forfexicaris[1] ?Occacaris[1] ?Yunnanocaris[1] ?Erjiecaris ?Pseudoarctolepis Tuzoiidae Tuzoia Duplapex Protocarididae Tokummia Branchiocaris Protocaris Loricicaris Waptiidae Waptia Pauloterminus Synophalos?
Synonyms
Canadaspidida Novozhilov in Orlov, 1960

Hymenocarina is an

Hymenocarines are characterized by the combination of following characters: bivalved, convex carapace covering cephalothoracic region; cephalothorax bearing multisegmented antennules and rounded mandibles, alongside post-maxillular limbs with spiny, subdivided basis and endopods with well-developed terminal claws; absence of appendages between antennules and mandibles; median sclerite and lobate protrusions located between compound eyes; posterior tagma (abdomen) with ring-like segments and terminated by a pair of well-developed caudal rami.^[2][1]

Based on the interpretation of simple head region that possess only a few segments and appendages, hymenocarine taxa were thought to be part of the upper stem-group euarthropods in early and mid 2010s.^{[3][4][5][6][7][8][9][10]} They later became widely accepted as mandibulates (jawed arthropods) after the discovery of their mandible-bearing mouthparts in late 2010s.^{[2][1][11][12]} Since then, most phylogenetic analysis suggest hymenocarines represent part of the mandibulate stem-group,^{[2][13][1][14][15][16][17][18]} with some results suggest a rather crownward position such as stem-pancrustaceans,^{[1][19][20][18]} stem-myriapods,^[1] stem-hexapods^[16] or somewhere in-between the former taxa.^[21]

Several subgroups within the order are recognised, including Waptiidae

• 
  Life restoration of Waptia
• 
  Life restoration of Canadaspis laevigata
• 
  Tokummia is the earliest known animal bearing pincers
• 
  Only 2 cm-long Fibulacaris is suggested to have swum upside down
• 
  Large-sized Balhuticaris shows extreme multisegmentation with over 100 segments
• 
  Unlike other hymenocarines, Pakucaris had a separate pygidium carapace covering its posterior
• 
  Unlike other taxa, eyes of possible hymenocarine Erjiecaris were probably placed over carapace
• 
  The carapace of possible hymenocarine Pseudoarctolepis had wing-like projections

The group was very diverse in shape, with some forms like