According to Roe (1975, p.365) 'X. paululus' is very similar to Amallothrix robustipes Grice & Hulsemann, 1965; the author has examined the type of this latter species in the British Museum (Natural History). The shape of the cephalothorax is the same as that of the present females but unfortunately the slides of the appendages is not in the Museum's collection. Mx1 is very similar to that of the present species, and the swimming legs are apparently identical except for the absence of small spines on the endopod of P2. P5 differs in having the terminal spine longer than the internal spine. Without a description of the remaining mouthparts, especially Mx2, it is impssible to ascertain whether or not A. robustipes and 'X. paululus' are synonymus.
At the moment it is not possible to place this species into any of the four families which have sensory setae on the endopod of Mx2. Bradford (1973) was unable to assign either A. robustipes or X. paululus to either the Phaennidae or the Scolecitrichidae. This species differs from both these families (as difined by Bradford) in the following respects: (1) the endopod of Mx2 apparently lacks any brush-like setae; (2) there are no large spines on the posterior surfaces of the swimming legs; (3) the male P5 are biramous and have symmetrical basipodites. The presence of enlarged setae on lobes 4 and 5 of the Mx2 further distinguishes it from the Scolecitrichidae. It is distinct from the Diaixidae and Tharybidae in having reduced mouthparts in the male.
Markhaseva & Ferrari (2005, p.124) transfer Xanthocalanus paululus in the new genus Brodskius in Scolecitrichidae family. A doubt subsists concerning the male in Roe (1975).
Female: endopodite of Mx2 with 6 setae of which at least two are plumose but none, as far as can be seen, are brush-like.
Male: The endopod of Mx2 has 6 sensory setae, all of which are, apparently, worm-like.