THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 20:
THE PLIOCENE PERIOD.
The highest division of the Tertiary deposits is termed the
Pliocene formation, in accordance with the classification
proposed by Sir Charles Lyell. The Pliocene formations contain
from 40 to 95 per cent of existing species of Mollusca,
the remainders belonging to extinct species. They are divided
by Sir Charles Lyell into two divisions, the Older Pliocene and
Newer Pliocene.
The Pliocene deposits of Britain occur in Suffolk, and are known
by the name of "Crags," this being a local term used for certain
shelly sands, which are employed in agriculture. Two of these
Crags are referable to the Older Pliocene, viz.,
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the White and Red Crags,—and one belongs to the Newer
Pliocene, viz., the Norwich Crag.
The White or Coralline Crag of Suffolk is the oldest of
the Pliocene deposits of Britain, and is an exceedingly local
formation, occurring in but a single small area, and having a
maximum thickness of not more than 50 feet. It consists of soft
sands, with occasional intercalations of flaggy limestone. Though
of small extent and thickness, the Coralline Crag is of importance
from the number of fossils which it contains. The name "Coralline"
is a misnomer; since there are few true Corals, and the so-called
"Corals" of the formation are really Polyzoa, often of
very singular forms. The shells of the Coralline Crag are mostly
such as inhabit the seas of temperate regions; but there occur
some forms usually looked upon as indicating a warm climate.
The Upper or Red Crag of Suffolk—like the
Coralline Crag—has a limited geographical extent and a
small thickness, rarely exceeding 40 feet. It consists of
quartzose sands, usually deep red or brown in colour, and
charged with numerous fossils.
Altogether more than 200 species of shells are known from the
Red Crag, of which 60 per cent are referable to existing species.
The shells indicate, upon the whole, a temperate or even cold
climate, decidedly less warm than that indicated by the organic
remains of the Coralline Crag. It appears, therefore, that a
gradual refrigeration was going on during the Pliocene period,
commencing in the Coralline Crag, becoming intensified in the Red
Crag, being still more severe in the Norwich Crag, and finally
culminating in the Arctic cold of the Glacial period.
Besides the Mollusca, the Red Crag contains the ear-bones of
Whales, the teeth of Sharks and Rays, and remains of the Mastodon,
Rhinoceros, and Tapir.
The Newer Pliocene deposits are represented in Britain
by the Norwich Crag, a local formation occurring near
Norwich. It consists of incoherent sands, loams, and gravels,
resting in detached patches, from 2 to 20 feet in thickness,
upon an eroded surface of Chalk. The Norwich Crag contains a
mixture of marine, land, and fresh-water shells, with remains of
fishes and bones of mammals; so that it must have been deposited
as a local sea-deposit near the mouth of an ancient river. It
contains altogether more than 100 marine shells, of which 89
per cent belong to existing species. Of the Mammals, the two
most important are an Elephant (Elephas meridionalis),
and the characteristic Pliocene Mastodon
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(M. Arvernensis), which is hitherto the only Mastodon
found in Britain.
According to the most recent views of high authorities, certain
deposits—such as the so-called "Bridlington Crag" of
Yorkshire, and the "Chillesford beds" of Suffolk—are to
be also included in the Newer Pliocene, upon the ground that
they contain a small proportion of extinct shells. Our knowledge,
however, of the existing Molluscan fauna, is still so far
incomplete, that it may reasonably be doubted if these supposed
extinct forms have actually made their final disappearance,
whilst the strata in question have a strong natural connection
with the "Glacial deposits," as shown by the number of Arctic
Mollusca which they contain. Here, therefore, these beds will
be included in the Post-Pliocene series, in spite of the fact
that some of their species of shells are not known to exist at
the present day.
The following are the more important Pliocene deposits which have
been hitherto recognised out of Britain:—
1. In the neighbourhood of Antwerp occur certain "crags," which
are the equivalent of the White and Red Crag in part. The lowest
of these contains less than 50 per cent, and the highest 60 per
cent, of existing species of shells, the remainder being extinct.
2. Bordering the chain of the Apennines, in Italy, on both sides
is a series of low hills made up of Tertiary strata, which are
known as the Sub-Apennine beds. Part of these is of Miocene age,
part is Older Pliocene, and a portion is Newer Pliocene. The
Older Pliocene portion of the Sub-Apennines consists of blue or
brown marls, which sometimes attain a thickness of 2000 feet.
3. In the valley of the Arno, above Florence, are both Older
and Newer Pliocene strata. The former consist of blue clays and
lignites, with an abundance of plants. The latter consist of sands
and conglomerates, with remains of large Carnivorous Mammals,
Mastodon, Elephant, Rhinoceros, Hippopotamus, &c.
4. In Sicily, Newer Pliocene strata are probably more largely
developed than anywhere else in the world, rising sometimes to a
height of 3000 feet above the sea. The series consists of clays,
marls, sands, and conglomerates, capped by a compact limestone,
which attains a thickness of from 700 to 800 feet. The fossils of
these beds belong almost entirely to living species, one of the
commonest being the Great Scallop of the Mediterranean (Pecten
Jacobœus).
5. Occupying an extensive area round the Caspian, Aral,
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and Azof Seas, are Pliocene deposits known
as the "Aralo-Caspian" beds. The fossils in these beds are partly
freshwater, partly marine, and partly intermediate in character,
and they are in great part identical with species now inhabiting
the Caspian. The entire formation appears to indicate the former
existence of a great sheet of brackish water, forming an inland
sea, like the Caspian, but as large as, or larger than, the
Mediterranean.
6. In the United States, strata of Pliocene age are found in
North and South Carolina. They consist of sands and clays, with
numerous fossils, chiefly Molluscs and Echinoderms.
From 40 to 60 per cent of the fossils belong to existing species.
On the Loup Fork of the river Platte, in the Upper Missouri region,
are strata which are also believed to be referable to the Pliocene
period, and probably to its upper division. They are from 300 to
400 feet thick, and contain land-shells, with the bones of numerous
Mammals, such as Camels, Rhinoceroses, Mastodons, Elephants, the
Horse, Stag, &c.
As regards the life of the Pliocene period, there are
only two classes of organisms to which our attention need be
directed—namely, the Shell-fish and the Mammals. So far as
the former are concerned, we have to note in the first place that
the introduction of new species of animals upon the globe went
on rapidly during this period. In the Older Pliocene deposits,
the number of shells of existing species is only from 40 to 60
per cent; but in the Newer Pliocene the proportion of living
forms rises to as much as from 80 to 95 per cent. Whilst the
Molluscs thus become rapidly modernised, the Mammals still all
belong to extinct species, though modern generic types gradually
supersede the more antiquated forms of the Miocene. In the second
place, there is good evidence to show that the Pliocene period
was one in which the climate of the northern hemisphere underwent
a gradual refrigeration. In the Miocene period, there is evidence
to show that Europe possessed a climate very similar to that
now enjoyed by the Southern United States, and certainly very
much warmer than it is at present. The presence of Palm-trees
upon the land, and of numerous large Cowries, Cones, and other
shells of warm regions in the sea, sufficiently proves this. In
the Older Pliocene deposits, on the other hand, northern forms
predominate amongst the Shells, though some of the types of hotter
regions still survive. In the Newer Pliocene, again, the Molluscs
are such as almost exclusively inhabit the seas of temperate
or even cold regions; whilst if we regard deposits like the
"Bridlington Crag" and
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"Chillesford beds"
as truly referable to this period, we meet at the close of this
period with shells such as nowadays are distinctively
characteristic of high latitudes. It might be thought that the
occurrence of Quadrupeds such as the Elephant, Rhinoceros, and
Hippopotamus, would militate against this generalisation, and
would rather support the view that the climate of Europe and
the United States must have been a hot one during the later
portion of the Pliocene period. We have, however, reason to
believe that many of these extinct Mammals were more abundantly
furnished with hair, and more adapted to withstand a cool
temperature, than any of their living congeners. We have also
to recollect that many of these large herbivorous quadrupeds
may have been, and indeed probably were, more or less migratory
in their habits; and that whilst the winters of the later portion
of the Pliocene period were cold, the summers might have been
very hot. This would allow of a northward migration of such
terrestrial animals during the summer-time, when there would be
an ample supply of food and a suitably high temperature, and a
southward recession towards the approach of winter.
The chief palæontological interests of the Pliocene deposits,
as of the succeeding Post-Pliocene, centre round the Mammals of the
period; and amongst the many forms of these we may restrict our
attention to the orders of the Hoofed Quadrupeds (Ungulates),
the Proboscideans, the Carnivora, and the
Quadrumana. Almost all the other Mammalian orders are
more or less fully represented in Pliocene times, but none of them
attains any special interest till we enter upon the Post-Pliocene.
Amongst the Odd-toed Ungulates, in addition to the remains of
true Tapirs (Tapirus Arvernensis), we meet with the bones
of several species of Rhinoceros, of which the Rhinoceros
Etruscus and R. Megarhinus (fig. 249) are the most
important. The former of these (fig. 249, A) derives its specific
name from its abundance in the Pliocene deposits of the Val d'Arno,
near Florence, and though principally Pliocene in its distribution,
it survived into the earlier portion of the Post-Pliocene period.
Rhinoceros Etruscus agreed with the existing African forms
in having two horns placed one behind the other, the front one
being the longest; but it was comparatively slight and slender in
its build, whilst the nostrils were separated by an incomplete bony
partition. In the Rhinoceros megarhinus (fig. 249, B), on
the other hand, no such partition exists between the nostrils, and
the nasal bones are greatly developed in size. It was a two-horned
form, and is found associated with Elephas meridionalis and
E. Antiquus in the Pliocene deposits of the
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Val d'Arno, near Florence. Like the preceding, it survived, in
diminished numbers, into the earlier portion of the Post-Pliocene
period.
Fig. 249.—A. Under surface of the skull of Rhinoceros
Etruscus, one-seventh of the natural size—Pliocene,
Italy.; B, Crowns of the three true molars of the upper jaw,
left side, of Rhinoceros megarhinus (R. Leptorhinus,
Falconer), one-half of the natural size—Pliocene, France.
(After Falconer.)
The Horses (Equidœ) are represented, both in Europe
and America, by the three-toed Hipparions, which survive from the
Miocene, but are now verging upon extinction. For the first time,
also, we meet with genuine Horses (Equus), in which each
foot is provided with a single complete toe only, encased in a
single broad hoof. One of the American species of this period
(the Equus excelsus) quite equalled the modern Horse in
stature; and it is interesting to note the occurrence of indigenous
horses in America at such a comparatively late geological epoch,
seeing that this continent certainly possessed none of these
animals when first discovered by the Spaniards.
Amongst the Even-toed Ungulates, we may note the occurrence of
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Swine (Suida), of forms allied to
the Camels (Camelidœ), and of various kinds of Deer
(Cervidœ); but the most interesting Pliocene Mammal
belonging to this section is the great Hippopotamus major
of Britain and Europe. This well-known species is very closely
allied to the living Hippopotamus amphibius of Africa,
from which it is separated only by its larger dimensions, and by
certain points connected with the conformation of the skeleton.
It is found very abundantly in the Pliocene deposits of Italy
and France, associated with the remains of the Elephant, Mastodon,
and Rhinoceros, and it survived into the earlier portion of the
Post-Pliocene period. During this last-mentioned period, it
extended its range northwards, and is found associated with the
Reindeer, the Bison, and other northern animals. From this fact
it has been inferred, with great probability, that the
Hippotamus major was furnished with a long coat of hair
and fur, thus differing from its nearly hairless modern
representative, and resembling its associates, the Mammoth and
the Woolly Rhinoceros.
Passing on to the Pliocene Proboscideans, we find that the great
Deinotheria of the Miocene have now wholly disappeared,
and the sole representatives of the order are Mastodons and
Elephants. The most important member of the former group is the
Mastodon Arvernensis (fig. 250), which ranged widely over
Southern Europe and England, being generally associated with
remains of the Elephas meridionalis, E. antiquus, Rhinoceros
megarhinus, and Hippopotamus major.
Fig. 250.—Third milk-molar of the left side of the upper
jaw of Mastodon Arvernensis, showing the grinding surface.
Pliocene.
The lower jaw
seems to have been destitute of incisor teeth; but the upper
incisors are developed into great tusks, which sometimes reach a
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length of nine feet, and which have the simple curvature of the
tusks of the existing Elephants. Amongst the Pliocene Elephants
the two most important are the Elephas meridionalis and the
Elephas antiquus. Of these, the Elephas meridionalis
(fig. 251) is found abundantly in the Pliocene deposits of Southern
Europe and England, and also survived into the earlier portion
of the Post-Pliocene period.
Fig. 251.—Molar tooth of Elephas meridionalis,
one-third of the natural size. Pliocene and Post-Pliocene.
Its molar teeth are of the type of
those of the existing African Elephant, the spaces enclosed by the
transverse enamel-plates being more or less lozenge-shaped, whilst
the curvature of the tusks is simple. The Elephas antiquus
(fig. 252) is very generally associated with the preceding, and
it survived to an even later stage of the Post-Pliocene period.
The molar teeth are of the type of the existing Indian Elephant,
with comparatively thin enamel-ridges, placed closer together
than in the African type; whilst the tusks were nearly straight.
Fig. 252.—Molar tooth of Elephas antiquus, one-third
of the natural size. Pliocene and Post-Pliocene.
Amongst the Pliocene Carnivores, we meet with true Bears
(Ursus Arvernensis), Hyænas (such as Hyœna
Hipparionum), and genuine Lions (such as the Felis
angustus of North America); but the most remarkable of the
beasts of prey of
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this period is the great "Sabre-toothed Tiger" (Machairodus),
species of which existed in the earlier Miocene,
and survived to the later Post-Pliocene. In this remarkable
form we are presented with perhaps the most highly carnivorous
type of all known beasts of prey. Not only are the jaws shorter
in proportion even than those of the great Cats of the present
day, but the canine teeth (fig. 253) are of enormous size, greatly
flattened so as to assume the form of a poignard, and having
their margins finely serrated. A part from the characters of the
skull, the remainder of the skeleton, so far as known, exhibits
proofs that the Sabre-toothed Tiger was extraordinarily muscular
and powerful, and in the highest degree adapted for a life of
rapine. Species of Machairodus must have been as large
as the existing Lion; and the genus is not only European, but
is represented both in South America and in India, so that the
geographical range of these predaceous beasts must have been
very extensive.
Fig. 253.—A, Skull of Machairodus cultridens,
without the lower jaw, reduced in size; B, Canine tooth of the
same, one-half the natural size. Pliocene, France.
Lastly, we may note that the Pliocene deposits of Europe have
yielded the remains of Monkeys (Quadrumana), allied to
the existing Semnopitheci and Macaques.
The following list comprises a small selection of some of the
more important and readily accessible works and memoirs relating
to the Tertiary rocks and their fossils. With few exceptions,
foreign works relating to the Tertiary strata of the continent
of Europe or their organic remains have been omitted:—
| (1) |
'Elements of Geology.' Lyell. |
| (2) |
'Students' Elements of Geology.' Lyell. |
| (3) |
'Manual of Palæontology.' Owen. |
| (4) |
'British Fossil Mammals and Birds.' Owen. |
| (5) |
'Traité de Paléontologie.' Pictet. |
| (6)
| 'Cours Elémentaire de Paléontologie.'
D'Orbigny. |
| (7) |
"Probable Age of the London Clay," &c.—'Quart.
Journ. Geol. Soc.,' vol. iii. Prestwich. |
| (8) |
'Structure and Probable Age of the Bagshot
Sands'—Ibid., vol. iii. Prestwich. |
| (9) |
'Tertiary Formations of the Isle of Wight'—Ibid.,
vol. ii. Prestwich. |
| (10) |
'Structure of the Strata between the London Clay and the
Chalk,' &c.—Ibid., vols. vi., viii., and x.
Prestwich. |
| (11) |
'Correlation of the Eocene Tertiaries of England, France,
and Belgium'—Ibid., vol. xxvii. Prestwich. |
| (12) |
'On the Fluvio-marine Formations of the Isle of
Wight'—Ibid., vol. ix. Edward Forbes. |
| (13) |
'Newer Tertiary Deposits of the Sussex Coast'—Ibid.,
vol. xiii. Godwin-Austen. |
| (14) |
'Kainozoic Formations of Belgium'—Ibid., vol. xxii.
Godwin-Austen. |
| (15) |
'Tertiary Strata of Belgium and French
Flanders'—Ibid., vol. viii. Lyell. |
| (16) |
'On Tertiary Leaf-beds in the Isle of Mull'—Ibid.,
vol. vii. The Duke of Argyll. |
| (17) |
'Newer Tertiaries of Suffolk and their Fauna'—Ibid.,
vol. xxvi. Ray Lankester. |
| (18) |
'Lower London Tertiaries of Kent'—Ibid., vol. xxii.
Whitaker. |
| (19) |
"Guide to the Geology of London"—'Mem. Geol. Survey.'
Whitaker. |
| (20) |
'Memoirs of the Geological Survey of Great Britain.' |
| (21) |
'Introductory Outline of the Geology of the Crag District'
(Supplement to Crag Mollusca, Palæontographical
Society). S. V. Wood, jun., and F. w. Harmer. |
| (22) |
"Tertiary Fluvio-marine Deposits of the Isle of Wight."
Edward Forbes. Edited by Godwin-Austen; with Descriptions of
the Fossils by Morris, Salter, and Rupert Jones—'Memoirs
of the Geological Survey.' |
| (23) |
'Geological Excursions round the Isle of Wight.'
Mantell. |
| (24) |
'Catalogue of British Fossils.' Morris. |
| (25) |
'Catalogue of Fossils in the Museum of Practical Geology.'
Etheridge. |
| (26) |
'Monograph of the Crag Polyzoa' (Palæontographical
Society). Busk. |
| (27) |
'Monograph of the Tertiary Brachiopoda' (Ibid.)
Davidson. |
| (28) |
'Monograph of the Tertiary Malacostracous Crustacea'
(Ibid.) Bell. |
|
Page 333
(29) |
'Monograph of the Tertiary Corals' (Ibid.) Milne-Edwards
and Haime. |
| (30) |
'Supplement to the Tertiary Corals' (Ibid.) Martin
Duncan. |
| (31) |
'Monograph of the Eocene Mollusca' (Ibid.) Fred. E.
Edwards. |
| (32) |
'Monograph of the Eocene Mollusca' (Ibid.) Searles V.
Wood. |
| (33) |
'Monograph of the Crag Mollusca' (Ibid.) Searles V.
Wood. |
| (34) |
'Monograph of the Tertiary Entomostraca' (Ibid.) Rupert
Jones. |
| (35) |
'Monograph of the Foraminifera of the Crag' (Ibid.) Rupert
Jones, Parker, and H. B. Brady. |
| (36) |
'Monograph of the Radiaria of the London Clay' (Ibid.)
Edward Forbes. |
| (37) |
'Monograph of the Cetacea of the Red Crag' (Ibid.)
Owen. |
| (38) |
'Monograph of the Fossil Reptiles of the London Clay'
(Ibid.) Owen and Bell. |
| (39) |
"On the Skull of a Dentigerous Bird from the London Clay
of Sheppey"—'Quart. Journ. Geol. Soc.,' vol. xxix.
Owen. |
| (40) |
'Ossemens Fossiles.' Cuvier. |
| (41) |
'Fauna Antiqua Sivalensis.' Falconer and Sir Proby
Cautley. |
| (42) |
'Palæontological Memoirs.' Falconer. |
| (43) |
'Animaux Fossiles et Géologie de l'Attique.'
Gaudry. |
| (44) |
"Principal Characters of the Dinocerata"—'American
Journ. of Science and Arts,' vol. xi. Marsh. |
| (45) |
'Principal Characters of the Brontotheridæ' (Ibid.)
Marsh. |
| (46) |
'Principal Characters of the Tillodontia' (Ibid.)
Marsh. |
| (47) |
"Extinct Vertebrata of the Eocene of
Wyoming"—'Geological Survey of Montana,' &c.,
1872. Cope. |
| (48) |
"Ancient Fauna of Nebraska"—'Smithsonian
Contributions to Knowledge,' vol. vi. Leidy. |
| (49) |
'Manual of Geology.' Dana. |
| (50) |
"Palæontology and Evolution" (Presidential Address
to the Geological Society of London, 1870)—'Quart.
Journ. Geol. Soc.,' vol. xxvi. Huxley.' |
| (51) |
'Mineral Conchology.' Sowerby. |
| (52) |
'Description des Coquilles Fossiles,' &c.
Deshayes. |
| (53) |
'Description des Coquilles Tertiaires de Belgique.'
Nyst. |
| (54) |
'Fossilen Polypen des Wiener Tertiär-beckens.'
Reuss. |
| (55) |
'Palæontologische Studien über die älteren
Tertiär-schichten der Alpen.' Reuss. |
| (56) |
'Land und Süss-wasser Conchylien der Vorwelt.'
Sandberger. |
| (57) |
'Flora Tertiaria Helvetica.' Heer. |
| (58) |
'Flora Fossilis Arctica.' Heer. |
| (59) |
'Recherches sur le Climat et la Végétation
du Pays Tertiaire.' Heer. |
| (60) |
'Fossil Flora of Great Britain.' Lindley and Hutton. |
| (61) |
'Fossil Fruits and Seeds of the London Clay.'
Bowerbank. |
| (62) |
"Tertiary Leaf-beds of the Isle of Mull"—'Quart.
Journ. Geol. Soc.,' vol. vii. Edward Forbes. |
| (63) |
'The Geology of England and Wales.' Horace B.
Woodward.[25] |
|
