Messorocaris

As of 2020, Messorocaris known only by two specimens. The holotype UU 17042.01 preserved the cephalon (head) and anterior half of the trunk.^[1] The second specimen KUMIP312406 preserved only the trunk region, and was previously assigned to the similar-looking Dicranocaris.^[3] It is a large arthropod, with a trunk measured up to 12 cm long.^[2]

The body comprise of a cephalic shield and a long, segmented trunk. All of the dorsal exoskeleton (tergite) have well-developed axial regions (vaulted middle sections) and tergopleurae (lateral extensions). The cephalic shield possess an anteriorly constricted axial region and wide pleural region, giving it a lens-shaped outline. The trunk posses 12 body segments, divided by a series of overlapping tergites. The tergopleurae are somewhat reduced on the first segment, widely spaced and sickle-shaped at the second to fourth segments, increasingly recurved and narrower posteriorly from the fifth, and eventually become 3 pairs of posteriorly projecting spines at the last 3 segments. The telson (tail) elongated and narrow at the base, widen and bifurcate posteriorly.^[2]

Appendages are only known by the partially preserved cephalic endopods. The anteriorly clustered position, terminal claws and posteriorly increased size resembling those of a sanctacaridid.^[1]

Based on the aforementioned similarity, Messorocaris is tentatively classified as a member of Habeliida (Sanctacarididae in the original description^[1]), a group that increasingly evident to be a member of stem-group chelicerates.^[4][5][6] Within putative habeliids, Dicranocaris might be a close relative of Messorocaris, differ by its semicircular cephalic shield and non sickle-shaped tergopleurae. Outside of habeliids, another similar-looking arthropod is Falcatamacaris (identified as an artiopod), particularly its sickle-shaped tergopleurae, but distinct by multiple features such as flatten cephalon, abrupt change of width between tergite 8–9 and calcitic exoskeleton.^[2]