Hymenocarina

Hymenocarines are characterized by the combination of the following characters: bivalved, convex carapace covering cephalothoracic (combined head and thorax) region; cephalothorax bearing multisegmented antennae (though as an exception antennae are absent in Odaraia^[1]) and rounded mandibles and likely maxillae, post maxillae limbs with spiny, subdivided basis and endopods (lower, leg-like braches of limbs) with well-developed terminal claws; absence of appendages between antennae and mandibles; median sclerite and lobate protrusions located between compound eyes; and posterior tagma (abdomen) with ring-like segments with the posterior of the body ending with pair of well-developed caudal rami (often in the form of tail flukes).^[2][3]

The group was very diverse in shape, with some forms like Waptia somewhat resembling shrimp,^[3] and others like Odaraia having a large carapace and trifurcate tail.^[4] The appendages showing various degrees of specialization across the group, ranging from the feathery gills of Waptia^[3] to the robust pincers (chelae) of Tokummia.^[2] They also had a wide range of sizes with some like Fibulacaris reaching a length of up to 2 cm (0.79 in) long,^[5] while largest Balhuticaris reached 24.5 cm (9.6 in) long.^[6] Hymenocarines are thought to have been ecologically diverse, with various forms occupying scavenging, predatory, deposit feeding and suspension feeding niches.^[6]

• 
  Life restoration of Waptia
• 
  Life restoration of Canadaspis laevigata
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  Tokummia is the earliest known animal bearing pincers
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  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
• 
  Life restoration of Odaraia, which had a trifurcate tail and probably habitually swam upside down
• 
  Life restoration of Tuzoia, a large hymenocarine

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.^{[7][8][9][10][11][12][13][14]} They later became widely accepted as mandibulates (the group that contains insects, crustaceans, centipedes and millipedes, among others) after the discovery of their mandible-bearing mouthparts in late 2010s.^{[2][3][15][16]} Since then, most phylogenetic analysis suggest hymenocarines represent part of the mandibulate stem-group,^{[2][17][3][18][19][20][21][22]} with some results suggest a rather crownward position such as stem-pancrustaceans,^{[3][23][24][22]} stem-myriapods,^[3] stem-hexapods^[20] or somewhere in-between the former taxa.^[25]

Several subgroups within the order are recognised, including Waptiidae^[3] and Protocarididae.^[2] The internal relationships of Hymenocarina are unstable, and it is unclear whether the group is monophyletic or paraphyletic.^[26][6]