Hymenocarina is anorder of extinct marinearthropods known from theCambrian. They possess bivalvedcarapaces, typically with exposed posteriors. Members of the group are morphologically diverse and had a variety of ecologies, including asfilter feeders and as predators. Recent research has generally considered them to be stem orcrown group members ofMandibulata, due to the presence ofmandibles in well-preserved species.
Hymenocarines are characterized by the combination of the following characters: bivalved, convex carapace coveringcephalothoracic (combined head and thorax) region; cephalothorax bearing multisegmentedantennae (though as an exception antennae are absent inOdaraia[1]) and roundedmandibles and likelymaxillae, post maxillae limbs with spiny, subdivided basis andendopods (lower, leg-like braches of limbs) with well-developed terminal claws; absence of appendages between antennae and mandibles; median sclerite and lobate protrusions located betweencompound eyes; and posteriortagma (abdomen) with ring-like segments with the posterior of the body ending with pair of well-developedcaudal rami (often in the form of tail flukes).[2][3]
The group was very diverse in shape, with some forms likeWaptia somewhat resembling shrimp,[3] and others likeOdaraia having a largecarapace and trifurcate tail.[4] The appendages showing various degrees of specialization across the group, ranging from the feathery gills ofWaptia[3] to the robustpincers (chelae) ofTokummia.[2] They also had a wide range of sizes with some likeFibulacaris reaching a length of up to 2 cm (0.79 in) long,[5] while largestBalhuticaris 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 andsuspension feeding niches.[6]
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 asmandibulates (the group that contains insects, crustaceans, centipedes and millipedes, among others) after the discovery of theirmandible-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, includingWaptiidae[3] andProtocarididae.[2] The internal relationships of Hymenocarina are unstable, and it is unclear whether the group ismonophyletic orparaphyletic.[26][6]
Chuandianella? (lacks mandibles, may not be a hymenocarine)
Cambrian bivalved arthropods are now recognised to be apolyphyletic group, with other groups of bivalved arthropods such as theIsoxyida,Bradoriida andPhosphatocopina only distantly related to Hymenocarina.[21][27][28][29]Chuandianella a bivalved arthropod morphologically similar toWaptia and long thought to be closely related[3][25] was reinterpreted as a non-hymenocarine euarthropod based on a restudy published in 2022, which found that it definitely lackedmandibles, characteristic of true hymenocarines.[27]
Cladogram after O'Flynn et al, 2023. showing possible placement of Hymenocarina within Mandibulata:[30]
^Briggs, D. E. G. (1981). "The Arthropod Odaraia alata Walcott, Middle Cambrian, Burgess Shale, British Columbia".Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences.291 (1056):541–582.Bibcode:1981RSPTB.291..541B.doi:10.1098/rstb.1981.0007.JSTOR2395403.
^Ortega-Hernández, Javier (2014-12-21). "Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848".Biological Reviews.91 (1):255–273.doi:10.1111/brv.12168.PMID25528950.S2CID7751936.
