Hymenocarina

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[1] and Protocarididae.[2] The internal relationships of Hymenocarina are unstable, and it is unclear whether the group is monophyletic or paraphyletic.[22][23]

Cambrian bivalved arthropods are now recognised to be a polyphyletic group, with other groups of bivalved arthropods such as the Isoxyida, Bradoriida and Phosphatocopina only distantly related to Hymenocarina.[17][24][25][26] Chuandianella a bivalved arthropod morphologically similar to Waptia and long thought to be closely related[1][21] was reinterpreted as a non-hymenocarine euarthropod based on a restudy published in 2022, which found that it definitely lacked mandibles, characteristic of true hymenocarines.[24]

• 
  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 Waptia somewhat resembling shrimp,[1] and others like Odaraia having a large carapace and trifurcate tail.[27] The appendages showing various degrees of specialization across the group, ranging from the feathery gills of Waptia[1] to the robust claws 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,[28] while largest Balhuticaris reached 24.5 cm (9.6 in) long.[23] Hymenocarines are thought to have been ecologically diverse, with various forms occupying scavenging, predatory, deposit feeding and suspension feeding niches.[23]