Schindler Jochem. A thorny problem

Die Sprache. — 1977. — Vol. 23. — P. 25-35.Neogrammarians reconstructed for Proto-Indo-European a separate fricative *þ on the basis of the correspondence of a dental in Greek and
Irish with a sibilant elsewhere, as in the word for ‘bear’: *h2rkþo-: Sanskrit rksa-, Greek arktos, Latin ursus, Middle Irish art. However, the elucidation of Anatolian and Tocharian has provided further cognates to words in this set, and the reconstruction now looks much less straightforward, since both languages show sequences with dental before velar in these words. Schindler's paper is an important treatment of this vexed issue.