A Manual of Elementary Geology Part 24

In some places they lie imbedded in loose earth, but they are usually included in a breccia.

The remains found most abundantly are those of the kangaroo, of which there are four species, besides which the genera _Hypsiprymnus_, _Phalangista_, _Phascolomys_, and _Dasyurus_, occur. There are also bones, formerly conjectured by some osteologists to belong to the hippopotamus, and by others to the dugong, but which are now referred by Mr. Owen to a marsupial genus, allied to the _Wombat_.

[Ill.u.s.tration: Fig. 126. _Macropus atlas_, Owen.

_a._ permanent false molar, in the alveolus.]

[Ill.u.s.tration: Fig. 127. Lowest jaw of largest living species of kangaroo.

(_Macropus major._)]

In the fossils above enumerated, several species are larger than the largest living ones of the same genera now known in Australia. The annexed figure of the right side of a lower jaw of a kangaroo (_Macropus atlas_, Owen) will at once be seen to exceed in magnitude the corresponding part of the largest living kangaroo, which is represented in fig. 127. In both these specimens part of the substance of the jaw has been broken open, so as to show the permanent false molar (_a._ fig. 126.) concealed in the socket. From the fact of this molar not having been cut, we learn that the individual was young, and had not shed its first teeth. In fig. 128. a front tooth of the same species of kangaroo is represented.

[Ill.u.s.tration: Fig. 128. Incisor of _Macropus_.]

Whether the breccias, above alluded to, of the Wellington Valley, appertain strictly to the Pliocene period cannot be affirmed with certainty, until we are more thoroughly acquainted with the recent quadrupeds of the same district, and until we learn what species of fossil land sh.e.l.ls, if any, are buried in the deposits of the same caves.

The reader will observe that all these extinct quadrupeds of Australia belong to the marsupial family, or, in other words, that they are referable to the same peculiar type of organization which now distinguishes the Australian mammalia from those of other parts of the globe. This fact is one of many pointing to a general law deducible from the fossil vertebrate and invertebrate animals of the eras immediately antecedent to the human, namely, that the present geographical distribution of organic _forms_ dates back to a period anterior to the creation of existing _species_; in other words, the limitation of particular genera or families of quadrupeds, mollusca, &c., to certain existing provinces of land and sea, began before the species now contemporary with man had been introduced into the earth.

Mr. Owen, in his excellent "History of British Fossil Mammals," has called attention to this law, remarking that the fossil quadrupeds of Europe and Asia differ from those of Australia or South America. We do not find, for example, in the Europaeo-Asiatic province fossil kangaroos or armadillos, but the elephant, rhinoceros, horse, bear, hyaena, beaver, hare, mole, and others, which still characterize the same continent.

In like manner in the Pampas of South America the skeletons of Megatherium, Megalonyx, Glyptodon, Mylodon, Toxodon, Macrauchenia, and other extinct forms, are a.n.a.logous to the living sloth, armadillo, cavy, capybara, and llama. The fossil quadrumana, also a.s.sociated with some of these forms in the Brazilian caves, belong to the Platyrrhine family of monkeys, now peculiar to South America. That the extinct fauna of Buenos Ayres and Brazil was very modern has been shown by its relation to deposits of marine sh.e.l.ls, agreeing with those now inhabiting the Atlantic; and when in Georgia in 1845, I ascertained that the Megatherium, Mylodon, _Harla.n.u.s america.n.u.s_ (Owen), _Equus curvidens_, and other quadrupeds allied to the Pampean type were posterior in date to beds containing marine sh.e.l.ls belonging to forty-five recent species of the neighbouring sea.

There are indeed some cosmopolite genera, such as the Mastodon (a genus of the elephant family), and the horse, which were simultaneously represented by different fossil species in Europe, North America, and South America; but these few exceptions can by no means invalidate the rule which has been thus expressed by Professor Owen, "that in the highest organized cla.s.s of animals the same forms were restricted to the same great provinces at the Pliocene periods as they are at the present day."

However modern, in a geological point of view, we may consider the Pleistocene epoch, it is evident that causes more general and powerful than the intervention of man have occasioned the disappearance of the ancient fauna from so many extensive regions. Not a few of the species had a wide range; the same Megatherium, for instance, extended from Patagonia and the river Plata in South America, between lat.i.tudes 31 and 39 south, to corresponding lat.i.tudes in North America, the same animal being also an inhabitant of the intermediate country of Brazil, where its fossil remains have been met with in caves. The extinct elephant, likewise, of Georgia (_Elephas primigenius_) has been traced in a fossil state northward from the river Alatamaha, in lat. 33 50' N.

to the polar regions, and then again in the eastern hemisphere from Siberia to the south of Europe. If it be objected that, notwithstanding the adaptation of such quadrupeds to a variety of climates and geographical conditions, their great size exposed them to extermination by the first hunter tribes, we may observe that the investigations of Lund and Clausen in the ossiferous limestone caves of Brazil have demonstrated that these large mammalia were a.s.sociated with a great many smaller quadrupeds, some of them as diminutive as field mice, which have all died out together, while the land sh.e.l.ls formerly their contemporaries still continue to exist in the same countries. As we may feel a.s.sured that these minute quadrupeds could never have been extirpated by man, so we may conclude that all the species, small and great, have been annihilated one after the other, in the course of indefinite ages, by those changes of circ.u.mstances in the organic and inorganic world which are always in progress, and are capable in the course of time of greatly modifying the physical geography, climate, and all other conditions on which the continuance upon the earth of any living being must depend.[158-A]

The law of geographical relations.h.i.+p above alluded to, between the living vertebrata of every great zoological province and the fossils of the period immediately antecedent, even where the fossil species are extinct, is by no means confined to the mammalia. New Zealand, when first examined by Europeans, was found to contain no indigenous land quadrupeds, no kangaroos, or opossums, like Australia; but a wingless bird abounded there, the smallest living representative of the ostrich family, called the Xivi, by the natives (_Apteryx_). In the fossils of the Post-Pliocene and Pleistocene period in this same island, there is the like absence of kangaroos, opossums, wombats, and the rest; but in their place a prodigious number of well preserved specimens of gigantic birds of the struthious order, called by Owen Dinornis and Palapteryx, which are entombed in superficial deposits. These genera comprehended many species, some of which were 4, some 7, others 9, and others 11 feet in height! It seems doubtful whether any contemporary mammalia shared the land with this population of gigantic feathered bipeds.

To those who have never studied comparative anatomy it may seem scarcely credible, that a single bone taken from any part of the skeleton may enable a skilful osteologist to distinguish, in many cases, the genus, and sometimes the species, of quadruped to which it belonged. Although few geologists can aspire to such knowledge, which must be the result of long practice and study, they will nevertheless derive great advantage from learning what is comparatively an easy task, to distinguish the princ.i.p.al divisions of the mammalia by the forms and characters of their teeth. The annexed figures, all taken from original specimens, may be useful in a.s.sisting the student to recognize the teeth of many genera most frequently found fossil in Europe:--

[Ill.u.s.tration: Fig. 129. _Elephas primigenius_ (or Mammoth); molar of upper jaw, right side; one third of nat. size.

_a._ grinding surface.

_b._ side view.]

[Ill.u.s.tration: Fig. 130. _Mastodon angustidens_ (Norwich Crag, Postwick, also found in Red Crag, see p. 149.); second true molar, left side, upper jaw; grinding surface, nat. size. (See p. 149.)]

[Ill.u.s.tration: Fig. 131. Rhinoceros.

_Rhinoceros leptorhinus_; fossil from freshwater beds of Grays, Ess.e.x (see p. 147.); penultimate molar, lower jaw, left side; two-thirds of nat. size.]

[Ill.u.s.tration: Fig. 132. Hippopotamus.

Hippopotamus; from cave near Palermo (see p. 154.); molar tooth; two-thirds of nat. size.]

[Ill.u.s.tration: Fig. 133. Pig.

_Sus scrofa_, Lin. (common pig); from sh.e.l.l-marl, Forfars.h.i.+re; posterior molar, lower jaw, nat. size.]

[Ill.u.s.tration: Fig. 134. Horse.

_Equus caballus_, Lin. (common horse); from the sh.e.l.l marl, Forfars.h.i.+re; second molar, lower jaw.

_a._ grinding surface, two-thirds nat. size.

_b._ side view of same, half nat. size.]

[Ill.u.s.tration: Fig. 135. Tapir.

_Tapirus America.n.u.s_; recent; third molar, upper jaw; nat. size.]

[Ill.u.s.tration: Fig. 136. _a._ _b._ Deer.

Elk (_Cervus alces_, Lin.); recent; molar of upper jaw.

_a._ grinding surface.

_b._ side view; two-thirds of nat. size.]

[Ill.u.s.tration: Fig. 137. _c._ _d._ Ox.

Ox, common, from sh.e.l.l marl, Forfars.h.i.+re; true molar upper jaw; two-thirds nat. size.

_c._ grinding surface.

_d._ side view.]

[Ill.u.s.tration: Fig. 138. Bear.

_a._ canine tooth or tusk of bear (_Ursus spelaeus_); from cave near Liege.

_b._ molar of left side, upper jaw; one third of nat. size.]

[Ill.u.s.tration: Fig. 139. Tiger.

_c._ canine tooth of tiger (_Felis tigris_); recent.

_d._ outside view of posterior molar, lower jaw; one-third of nat. size.]

[Ill.u.s.tration: Fig. 140. _Hyaena spelaea_; second molar, left side, lower jaw; nat. size. Cave of Kirkdale. (See p. 154.)]

[Ill.u.s.tration: Fig. 141. Teeth of a new species of _Arvicola_ (field-mouse); from the Norwich Crag. (See p. 149.)

_a._ grinding surface.

_b._ side view of same.

_c._ nat. size of a and b.]

FOOTNOTES:

[147-A] Morris, Geol. Soc. Proceed., 1849.

[147-B] Woodward's Geology of Norfolk.