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The legs of insects and myriapods are uniramous. In crustaceans, the first antennae are uniramous, but the second antennae are biramous, as are the legs in most species. For a time, possession of uniramous limbs was believed to be a shared, derived character, so uniramous arthropods were grouped into a taxon called Uniramia. It is now believed ...
The common ancestor of all arthropods likely had one pair of uniramous (unbranched) antenna-like structures, followed by one or more pairs of biramous (having two major branches) leg-like structures, as seen in some modern crustaceans and fossil trilobites. [3]
The discovery of fossil lobopods, determined to be intermediate between onychophorans and arthropods led to the splintering of the Lobopoda and Onychophora into separate groups. This redefined the Uniramia as strictly "true" arthropods with exoskeletons and jointed appendages. Uniramians have strictly uniramous appendages. [2]
Crustacean appendages are typically biramous, meaning they are divided into two parts; this includes the second pair of antennae, but not the first, which is usually uniramous, the exception being in the Class Malacostraca where the antennules may be generally biramous or even triramous.
Appendages may become uniramous, as in insects and centipedes, where each appendage comprises a single series of segments, or it may be biramous, as in many crustaceans, where each appendage branches into two sections. Triramous (branching into three) appendages are also possible. [1]
The three uniramous appendages had six segments (podomeres), each ending with a terminal claw, with the endopod of the fourth limb having a similar morphology. The trunk was divided up into 10 segments (tergites), all of approximately equal length and width, each of which were associated with pairs of biramous appendages.
The first pair of head limbs were biramous antennae, with around 400 annulations on the longer ramus. The next four pairs were all relatively similar, with around 15 short podomeres, each with a stout spine. These appendages appear to have been uniramous, and what seems to be the fifth pair shows bunches of smaller, inward-facing spines.
The head carries three pairs of biramous limbs, alongside a many-segmented pair of uniramous antennae at least two-thirds as long as the entire body. No eyes, mouth or hypostome are known. The biramous limbs are all equivalent in structure, with large proximal segments on the endopods and more hook-like distal segments.