The specific epithet 'extincta' refers to the belief that the first specimens collected were subfossil. In his description, Odhner noted that the same species was also found in New Guinea:
The occurrence of one and the same species in a subfossil state at these two separate localities, the one situated in New Guinea and the other in North Queensland, is a fact of considerable interest, as it offers the best thinkable proof of the theory of a land bridge between the two countries in a geological period immediately preceding the present time. A careful scientific investigation of the caves at Chillagoe will certainly reveal many other facts of importance as to our knowledge of the zoo-geography of Australia.
I'm not sure whether Odhner was correct in the assessment that the specimens from New Guinea and Australia belonged to the same species but the cave micro-snails do show a strong biogeographic association between northern Australia and South East Asia. Pleuropoma is widespread. Two other cave micro-snails — Gyliotrachela australis and Georissa minuta are related to similar species in tropical Asia — whereas the fourth species, Stenopylis coarctata is thought to range over a wide area of the tropics.
The larger snails, most of which are camaenids, are a little more conservative. As far as we know, their closest relatives are all in Australia.
More snails later.
Odhner, N.H. (1917). Results of Dr E. Mjöberg's Swedish scientific expeditions to Australia. 1910-1913. K. Sven. Vetensk.-Akad. Handl. 52(16): 1-115 pls 1-3