Friday, 3 August 2007

Pleuropoma extincta

A helicinid snail from the Chillagoe region. This the biggest of the four cave-associated species. (Although these specimens are under rocks at the base of an outcrop rather than near a cave. Move along please, there's nothing to see here.)

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


boobook48 said...

Hi Snail
When you have a minute could you explain what 'subfossil' means.

Snail said...

Subfossils are those remains that haven't become wholly mineralised. There's often traces of organic material associated with them.


The presence of the same genus even if they are not the same species in Australia & New Guinea probably means that they were present on Gondwanaland before it broke apart. They are old snails!

Odhner was correct in that sense although he(?) didn't know about continental drift.

Snail said...

Yep! Sure is an old group of snails but I suspect the best evidence for that is in the wider distribution rather than the NG - Oz connection.

Problem is, New Guinea (IIRC) is a composite land mass, with bits from other plates plus new land from volcanic activity. Southern New Guinea is part of the Australian plate. (Geoff, if you're around, can you confirm?) The Torres Strait, which now separates NG from N Oz is very shallow, so was dry during the glacial cycles of the Pleistocene. So the NG - Oz connection doesn't always indicate an ancient lineage. (Although it might, esp in species confined to the N or W of the land.)

I must sit down and re-read the literature on the geological development of SE Asia, Australia and Wallacea. With a geologist, I think, because all that stuff about back arcs and whatnot make no sense to me!

(Oh my goodness, I just got enthusiastic about something. That can't be right!)

Shorty CreeKI said...

settle down kiddo ... :))
And best regards to those who are enthusing this snaily person again.