The Student's Elements of Geology Part 38

There are two fossil specimens of lower jaws of this genus evidently referable to two distinct species extremely unequal in size and otherwise distinguishable.

The Plagiaulax Becklesii (Figure 306) was about as big as the English squirrel or the flying phalanger of Australia (Petaurus Australis, Waterhouse). The smaller fossil, having only half the linear dimensions of the other, was probably only one-twelfth of its bulk. It is of peculiar geological interest, because, as shown by Dr. Falconer, its two back molars bear a decided resemblance to those of the Tria.s.sic Microlestes (Figure 389 Chapter 19), the most ancient of known mammalia, of which an account will be given in Chapter 21.

Up to 1857 all the mammalian remains discovered in secondary rocks had consisted solely of single branches of the lower jaw, but in that year Mr. Beckles obtained the upper portion of a skull, and on the same slab the lower jaw of another quadruped with eight molars, a large canine, and a broad and thick incisor. It has been named Triconodon from its bicuspid teeth, and is supposed to have been a small insectivorous marsupial, about the size of a hedgehog.

Other jaws have since been found indicating a larger species of the same genus.

Professor Owen has proposed the name of Galestes for the largest of the mammalia discovered in 1858 in Purbeck, equalling the polecat (Mustela putorius) in size.

It is supposed to have been predaceous and marsupial.

Between forty and fifty pieces or sides of lower jaws with teeth have been found in oolitic strata in Purbeck; only five upper maxillaries, together with one portion of a separate cranium, occur at Stonesfield, and it is remarkable that with these there were no examples in Purbeck of an entire skeleton, nor of any considerable number of bones in juxtaposition. In several portions of the matrix there were detached bones, often much decomposed, and fragments of others apparently mammalian; but if all of them were restored, they would scarcely suffice to complete the five skeletons to which the five upper maxillaries above alluded to belonged. As the average number of pieces in each mammalian skeleton is about 250, there must be many thousands of missing bones; and when we endeavour to account for their absence, we are almost tempted to indulge in speculations like those once suggested to me by Dr. Buckland, when he tried to solve the enigma in reference to Stonesfield; "The corpses," he said, "of drowned animals, when they float in a river, distended by gases during putrefaction, have often their lower jaw hanging loose, and sometimes it has dropped off. The rest of the body may then be drifted elsewhere, and sometimes may be swallowed entire by a predaceous reptile or fish, such as an ichthyosaur or a shark."

As all the above-mentioned Purbeck marsupials, belonging to eight or nine genera and to about fourteen species, insectivorous, predaceous, and herbivorous, have been obtained from an area less than 500 square yards in extent, and from a single stratum no more than a few inches thick, we may safely conclude that the whole lived together in the same region, and in all likelihood they const.i.tuted a mere fraction of the mammalia which inhabited the lands drained by one river and its tributaries. They afford the first positive proof as yet obtained of the co-existence of a varied fauna of the highest cla.s.s of vertebrata with that ample development of reptile life which marks all the periods from the Trias to the Lower Cretaceous inclusive, and with a gymnospermous flora, or that state of the vegetable kingdom when cycads and conifers predominated over all kinds of plants, except the ferns, so far, at least, as our present imperfect knowledge of fossil botany ent.i.tles us to speak.

TABLE 19.2. NUMBER AND DISTRIBUTION OF ALL THE KNOWN SPECIES OF FOSSIL MAMMALIA FROM STRATA OLDER THAN THE PARIS GYPSUM, OR THAN THE BEMBRIDGE SERIES OF THE ISLE OF WIGHT.

TERTIARY:

Headon Series and beds between the Paris Gypsum and the Gres de Beauchamp: 14: 10 English, 4 French.

Barton Clay and Sables de Beauchamp: 0.

Bagshot Beds, Calcaire Grossier, and Upper Soissonnais of Cuisse-Lamotte: 20: 16 French, 1 English, 3 United States (I allude to several Zeuglodons found in Alabama, and referred by some zoologists to three species.)

London Clay, including the Kyson Sand: 7 English.

Plastic Clay and Lignite: 9: 7 French, 2 English.

Sables de Bracheux: 1 French.

Thanet Sands and Lower Landenian of Belgium: 0.

SECONDARY:

Maestricht Chalk: 0.

White Chalk: 0.

Chalk Marl: 0.

Chloritic Series (Upper Greensand): 0.

Gault: 0.

Neocomian (Lower Greensand): 0.

Wealden: 0.

Upper Purbeck Oolite : 0.

Middle Purbeck Oolite : 14 Swanage.

Lower Purbeck Oolite: 0.

Portland Oolite: 0.

Kimmeridge Clay: 0.

Coral Rag: 0.

Oxford Clay: 0.

Great Oolite: 4 Stonesfield.

Inferior Oolite: 0.

Lias: 0.

Upper Trias: 4 Wurtemberg, Somersets.h.i.+re. N. Carolina.

Middle Trias: 0.

Lower Trias: 0.

PRIMARY.

Permian: 0.

Carboniferous : 0.

Devonian: 0.

Silurian: 0.

Cambrian: 0.

Laruentian: 0.

Table 19.2 will enable the reader to see at a glance how conspicuous a part, numerically considered, the mammalian species of the Middle Purbeck now play when compared with those of other formations more ancient than the Paris gypsum, and, at the same time, it will help him to appreciate the enormous hiatus in the history of fossil mammalia which at present occurs between the Eocene and Purbeck periods, and between the latter and the Stonesfield Oolite, and between this again and the Trias.

The Sables de Bracheux, enumerated in the Tertiary division of the table, supposed by Mr. Prestwich to be somewhat newer than the Thanet Sands, and by M.

Hebert to be of about that age, have yielded at La Fere the Arctocyon (Palaeocyon) primaevus, the oldest known tertiary mammal.

It is worthy of notice, that in the Hastings Sands there are certain layers of clay and sandstone in which numerous footprints of quadrupeds have been found by Mr. Beckles, and traced by him in the same set of rocks through Suss.e.x and the Isle of Wight. They appear to belong to three or four species of reptiles, and no one of them to any warm-blooded quadruped. They ought, therefore, to serve as a warning to us, when we fail in like manner to detect mammalian footprints in older rocks (such as the New Red Sandstone), to refrain from inferring that quadrupeds, other than reptilian, did not exist or pre-exist.

But the most instructive lesson read to us by the Purbeck strata consists in this: They are all, with the exception of a few intercalated brackish and marine layers, of fresh-water origin; they are 160 feet in thickness, have been well searched by skillful collectors, and by the late Edward Forbes in particular, who studied them for months consecutively. They have been numbered, and the contents of each stratum recorded separately, by the officers of the Geological Survey of Great Britain. They have been divided into three distinct groups by Forbes, each characterised by the same genera of pulmoniferous mollusca and cyprides, these genera being represented in each group by different species; they have yielded insects of many orders, and the fruits of several plants; and lastly, they contain "dirt-beds," or old terrestrial surfaces and vegetable soils at different levels, in some of which erect trunks and stumps of cycads and conifers, with their roots still attached to them, are preserved. Yet when the geologist inquires if any land-animals of a higher grade than reptiles lived during any one of these three periods, the rocks are all silent, save one thin layer a few inches in thickness; and this single page of the earth's history has suddenly revealed to us in a few weeks the memorials of so many species of fossil mammalia, that they already outnumber those of many a subdivision of the tertiary series, and far surpa.s.s those of all the other secondary rocks put together!

LOWER PURBECK.

(FIGURE 307. Cyprides from the Lower Purbeck.