VII. Parka decipiens , its Origin, Affinities, and Distribution

Abstract

With # Plate (No. VIII.), and a Section.[Read 8th February, 1897.] SINCE the publication of papers by Sir William Dawson and Professor Penhallow on Parka decipiens, renewed attention has been drawn to the origin and affinities of this deceiving fossil, and though the authors of the present paper had in their own minds been perfectly satisfied with the general conclusions arrived at by these writers, they are aware that doubts are still cast upon the propriety of associating certain stems and leaves with this organism as their fruit. Indeed, one eminent botanist, after all the evidence which has been produced, has been unable to accept it, and still expresses doubts of the vegetable origin of Parka at all.It is principally, then, to meet such objections that the present writers have thought of bringing together, and re-stating, in the following paper, the whole argument in favour of the vegetable origin and rhizocarpian affinities of this remarkable fossil.Parka decipiens was first discovered by Dr. Fleming, who described it in 1831.* It received its generic name from Parkhill, near Newburgh, on the Fife shore of the Firth of Tay, where Dr. Fleming had found it some two years previously in a bed of sandstone which underlies the Ochils, and stretches from Balmerino on the east to Abernethy and Dron on the west. Its specific name was given on account of its deceiving nature, a character which, as we shall see, it has since well maintained.Hugh Miller, in his " Testimony of the Rocks," t quotes Fleming's description of Parka, as follows: -"These organisms occur in the form of circular flat patches, not equalling an inch in diameter, and composed of numerous smaller contiguous pieces. They are not unlike what might be expected to result from a compressed berry, such as the bramble or the rasp. As, however, they are found lying adjacent to the narrow leaves of gramineous [-looking] vegetables, and chiefly in clay slate, originally lacustrine silt, it is probable that they constituted at Carleton University http://trngl.lyellcollection.org/ Downloaded from 106 TRANSACTIONS-GEOLOGICAL SOCIETY OP GLASGOW. the conglobate panicles of extinct species of the genus Junicus or Sparzanium."Sir Charles Lyell, in 1855,* referred to the occurrence of Parka in the sandstones of Fifeshire and Forfarshire, mentioning its resemblance to the eggs of Natica, a gasteropodous mollusc, and also figuring a specimen of the latter to show the resemblance between the two forms. But he says that, as no gasteropodous shells have been detected in the same formation, the Parka has, probably, no connection with this class of organisms. He also cites Mantell's theory of their batrachian origin.Hugh Miller, in 1857,t showed that the egg-theory of the origin of Parka had been first propounded by the quarrymen of Carmylie in Forfarshire, who likened it to "puddock [frog] spawn," an idea which was largely adopted by many of our leading geologists, but he himself favoured the vegetable origin, and compared it to a strawberry or raspberry. We may note here...

On Parka decipiens

“…in width. The entire 22,2016 at Ohio University on June http://jgslegacy.lyellcollection.org/ Downloaded from part 4~ ozr P~R~ D~CWI~s. 655 thallus, apart from the margin, is composed of small ' discs,' oval and thickened, with)a,_ thinner tissue intervening between them (' intervening tissue While, however, the size of the thallus is extremely variable, the size of the component discs tends to be constant.…”

“…The various features of the anatomy of Parka, in so far as they are revealed in the tlaC'tened films of carbonized tissue, have now been described. But it is obvious that, if we wish to obtain any ~dequate conception of the relationship of the parts, and of the 2x2 22,2016 at Ohio University on June http://jgslegacy.lyellcollection.org/ Downloaded from general form and functior~s of the tissues, the facts brought together above must be interpreted and discussed. Now, a vegetable s~ructure, however flat it may be originally, cannot be pressed into a thin film without its form in some way becoming obscured.…”

“…Now, it seems quite'clear that _Parka developed by marginal growth. Apar~ fl'om the difllcult~r of supposing development to have taken place otherwise, the markedly-radial an~ngement of 22,2016 the cells of the 'epidermal' layer makes this conclusion inevitable. But an examination of certain specimens reveals a somewhat ~ eculiar fact: namely, that from the undifferentiated marginal rill arise directly, not only the outer intervening and investing layers, but the sporogenous tissues as well.…”

“…The facts might be explained, for instance, by conveniently altering the form of Parka to suit the case the sporangia might be regal~led as concavo-eonvex, or the intermediate tissue might be made to project on one side and to occupy hollows on the other. But a fairly satisfactory solution is possible on the assumption that Parka was flat ventrally, and that the spore-masses steod 22,2016 at Ohio University on June http://jgslegacy.lyellcollection.org/ Downloaded from LIEUT. A. W. ~.…”

“…On the other hand, profuse masses of simple euticularized spores, so far as I am aware, are quite unknown among modern aquatic thallophyta. Nevertheless, on more mature reflection, it seems to be most reasonable to regard these cells as what they appear to be, in view of the fact that certainly no ot~er type of vegetable cell is 22,2016 at Ohio University on June http://jgslegacy.lyellcollection.org/ Downloaded from ~.UT..k.W.R. DON AND DR. G. HICKLLNG [VOI.…”