Vidal writes:

"The atractaspidids include about 11 genera and 70 species. The monophyly of the family (including the ‘‘aparallactines’’, which were previously considered to be ‘‘colubrids’’), is supported both by morphological and molecular data.[25 – 27] Only one genus, the genus Atractaspis, displays a frontfanged venom system, but it is not homologous to the solenoglyph apparatus of the vipers.[27,28] The remaining atractaspidids are either aglyphs or opisthoglyphs. Unfortunately, the phylogenetic relationships between the genus Atractaspis and the ‘‘aparallactines’’ are unresolved."



Heise et al.'s data show that Atractaspis is an elapid. From Vidal's description, the "Atractaspididae" is a morphologically heterogeneous family with some front fanged, some aglyphous and some opistoglyphous snakes. Since the relationship between the aparallactines and Atractaspis is unresolved, there is a strong possibility that the "Atractaspididae" is a polyphyletic family. If Atractaspis is an elapid, then that would explain its front fanged system, which is also found in elapids. If the "Atractaspididae" is polyphyletic, that would explain the differences in dentition within this taxon.



Reference:

Philip J. Heise, Linda R. Maxson, Herndon G. Dowling, and S. Blair Hedges (1995). Higher-level snake phylogeny inferred from mitochondrial DNA sequences of 12S rRNA and 16S rRNA genes.

Mol. Biol. Evol. 12: 259–265.



Vidal, Nicolas 2002. Colubroid Systematics: Evidence for an Early Appearance of the Venom Apparatus Followed by Extensive Evolutionary Tinkering. J. Toxicol.—Toxin Reviews, 21(1&2), 21–41

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