Tag Archives: Permian

The crustacean that walks – and only in Tasmania

My last post introduced a giraffe who took a long walk. Since giraffes typically walk as part of their locomotion, the surprise of that story cannot be as great as the fact of the shrimp (prawn if you like) which walks around the high creeks and streams which flow down into the Derwent River in Tasmania.

Anaspides tasmaniae from Parks&Wildlife Service

(Photo courtesy of Parks & Wildlife Service Tasmania – http://www.parks.tas.gov.au/indeX.aspX?base=11244)

Geoffrey Smith, in his book A Naturalist in Tasmania (Oxford, 1909), describes the Anaspides tasmaniae. “A number of streams rise in the plateau of Mount Wellington, some of them attaining a considerable size before joining the estuary of the Derwent, and all of them characterised by the beautiful clearness of their water, owing to their beds being formed entirely of hard greenstone boulders.

In the pools of the upper reaches of these streams near the top of the mountain, a very peculiar shrimp-like animal is found.  It is now recognised as one of those survivals of a bygone age of which the Australian continent has furnished so many and such interesting examples. The nearest allies of this animal appear to be some marine shrimps which come down to us as fairly common fossils in the sand deposited round the Permian and Carboniferous seas of Europe and North America: subsequent to this very remote period they do not seem to have existed in the seas, at any rate in the northern hemisphere, so that an enormous passage of the earth’s history has occurred between their peopling the northern seas and their survival on the mountain tops of Tasmania.

The Tasmanian Mountain Shrimp (Anaspides tasmaniae) is sometimes as much as two inches in length, of a brown colour, and walks about on the stones and among the weeds at the bottom of the pools, browsing on the mosses and liver-worts and any small creatures it can catch; it very rarely swims, but when frightened it darts forward by flicking its tail and takes cover under a stone.

In other parts of the world no trace of the animal’s survival has been discovered.

Goethe somewhere remarks that the most insignificant natural object is, as it were, a window through which we can look into infinity. And certainly when I first saw the Mountain Shrimp walking quietly about its crystal-clear habitation, as if nothing of any great consequence had happened since its ancestors walked in a sea peopled with great reptiles … time for me was annihilated and the imposing kingdom of man shrunk indeed to a little measure.”

The website http://www.anaspides.net/other/website_name_why.html believes “Anaspides tasmaniae has remained unchanged for 250 million years (Triassic Period): it is a living fossil. The first published record of Anaspides tasmaniae was made in 1893 from alpine pools on Mt. Wellington behind Hobart.”  More details about this discovery, and about the later involvement of the author of the book excerpt above, Geoffrey Smith, can be read at http://www.tasfieldnats.org.au/TasNaturalist/Articles/1967/TasNat_1967_No8_Feb_pp1-2_Hewer_AnaspidesTasmaniae.pdf

Geological history and the Bedlam Walls area

For readers interested in the rock foundations of the Bedlam Walls Point the information from http://www.mrt.tas.gov.au/mrtdoc/dominfo/download/TR12_116_118/TR12_116_118.pdf may be of help.

“The area examined is rectangular and measures about 700 yards from SW to NE and about 600 yards from NW to SE. It is on an elevated promontory projecting into the Derwent estuary, bounded on the N by Shag Bay, on the W by the Derwent River and on the S by Geilston Bay.

The rocks present are fine light grey sandstone and siltstone of the Malbina Formation of Permian age. The sandstone forms hard, compact massive beds from one foot to six feet in thickness which alternate with siltstone beds generally less than two feet in thickness. The siltstone weathers more readily and so is readily distinguished in outcrop, but it is not markedly softer than the sandstone.

The rocks dip at low angles up to 6° to the W, that is, towards the Derwent. They possess vertical joints usually from nine inches to four feet apart although locally they may be as little as one inch or as far as six feet apart. Scattered isolated pebbles up to eight inches in diameter are present in the Formation Cliffs and on the W side of the area they plunge into the Derwent River. Along the foreshore in the NW corner are three shallow sea caves about 30 feet along any diameter. Their floors are now about twenty feet above sea level. There has been some collapse. No evidence of groundwater was seen either on the promontory or along the shoreline. The soil cover is a shallow silty grey loam overlaying thin stony clay. This forms a podsolic profile up to  eighteen inches and usually less than one foot in thickness.

Comments: In view of the rock types present, the low angle of dip and the well-drained nature at the rocks, the general stability of the area is not in doubt. Locally, the presence of the caves has been mentioned but they are superficial features caused by marine erosion, and do not extend underground for any distance. Except on the steepest slopes, the rocks are adequate to support normal industrial structures. Excavation will be assisted by the jointed and bedded nature of the rocks. Where there are large joint spacings, explosives will be required but in the closely jointed areas heavy earth moving equipment will be sufficient. Blocks up to six feet in diameter could be quarried from the massive sandstone. Groundwater supplies are not likely, and in any case would be limited in quantity to, at the most, 1,000 gallons per hour from a bore hole, and in quality by the proximity of sea water.”

The photo below was taken off the rocks at Bedlam Walls Point.