This species is not rare but is much less common that its ubiquitous cousin, Elrathia kingii. This species is found in the Marjum Formation, which lies over the Wheeler Formation where Elrathia kingii is found. So this species came later and probably evolved from Elrathia kingii.

The differences between the two species are very small. E. kingii has a slight concavity in the posterior part of the profile of the pygidium, E. marjumi lacks this concavity. A slightly greater degree of ornamentation in the upper part of the pygidium sometimes results in small projections on each side of the anterior part of the pygidium, although this isn't seen in all specimens.