Zoologists admit as a fact that Pinnipeds originate in true
land-animals. We are convinced that these land-animals were long-tailed
Viverrine animals. The tail must have been longer than one half of
the total length. This is no impossibility, as we have still living
forms, the tail of which is as long as half of the total length of the
animal, e. g. _Herpestes Widdringtonii_. The dentition must have been
the typical carnivorous one: i 3/3, c 1/1, m 7/7; or there were more
molars, perhaps 8/8, as a genus of wild dogs, _Otocyon_, has 8 molars
on each side of each jaw; its dentition is i 3/3, c 1/1, m 8/8. (The
_Cynoidea_, or dog-like animals are also considered as having their
origin in Viverrine animals.)
Some of the descendants of these long-tailed Viverrine animals had
gradually got such characters, that zoologists would term them
long-tailed Musteline animals. They may be called _long-tailed
ancestors of weasels and stoats_, for our common weasel (_Putorius
vulgaris_ L.) and our common stoat (_Putorius ermineus_ L.) are still
living descendants of them, though the tail has become very short, most
probably because they have accustomed themselves to live in holes. The
long tail has shown itself to be an inconvenient organ for this new
manner of living, and therefore has gradually become shorter.
Some of these _long-tailed ancestors of weasels and stoats_ took to
another manner of living, compelled thereto by certain circumstances.
They viz. took to eating fresh water fish. Gradually this grew to be a
habit; they learned to swim, which happened by vertical undulations,
they paddled with the feet, and used the tail as a rudder. This group
may be called _long-tailed ancestors of polecats and minks_, for
our common polecat (_Putorius putorius_ L.) and the Russian minks
(_Putorius lutreolus_ L.) are still living descendants of them, though
the tail has become short, because they have accustomed themselves to
live in holes. The long tail has shown itself to be an inconvenient
organ for such a manner of living, and therefore has gradually become
shorter, not so short, however, as in weasels and stoats. Zoologists
place the polecats and minks in the same genus as the weasels and
stoats. The minks live especially in the neighbourhood of rivers and
brooks, often go into the water and swim exceedingly well. Besides on
poultry and rats, they feed on frogs, crabs, cray-fish, and all kinds
of fish.
Some of these _long-tailed ancestors of polecats and minks_ got so
used to the water, that it finally became their proper element, and
they came ashore only to rest from swimming, to bask themselves in the
sun, or to find another brook or river. They began to feed on fish,
crayfish, and frogs, and only when driven by hunger they fed on rats
and poultry. It is evident that those individuals which by nature were
best adapted to their new element, must gradually have survived their
less privileged brethren, and so we may admit that a form gradually
arose, which swam very easily with vertical undulations, using the tail
as a rudder and as propelling organ. Also of great advantage must have
been more sharply pointed teeth, and more jagged molars, smaller ears,
a more woolly skin, and toes on the hind limbs, which were capable
of expansion and more or less provided with a web. This group may be
called _long-tailed ancestors of otters_, for all the known species
of otters (_Lutra_) are still living descendants of them, though the
tail has become shorter, shorter than one third of the total length of
the animal, because they too like to live in holes. The face greatly
resembles that of the polecat and mink, but the upper lips are thicker
and the whiskers are longer and stronger. The change was not only great
enough for zoologists to create for this group a new genus: _Lutra_
STORR, but even to establish for it a new subfamily _Lutrina_ GRAY.
These _long-tailed ancestors of otters_ were again survived by their
congeners which were still better adapted to the new medium, so that
from them another group gradually arose, which had broader webs on
the hind feet. This group may be called _long-tailed ancestors of
fin-tailed otters_, for the fin-tailed otter (_Lutra Sanbachii_
GRAY) is a still living descendant of them. The tail of this animal
is shorter than that of its ancestors, longer, however, than that
of the otters (_Lutra_), surpassing one third of the animal’s total
length. Moreover it is somewhat flattened and shows on its hindmost
half lateral fin-like dilatations. The change was great enough for
zoologists to place the animal into a new genus: _Pteronura_ GRAY.
Its ancestors, however, were not provided with these lateral fin-like
dilatations on the tail.
Some of these _long-tailed ancestors of the fin-tailed otter_ which
in their migration had reached the sea-shore, probably by following
the course of rivers, began to accustom themselves to eat sea-fish,
and ended by feeding on them exclusively. The sea-water became
their home, and their resting places and nests were found on the
strand, and among sea-weed; they seldom came ashore to sleep or to
sun themselves. Besides on sea-fish, they fed on crabs, lobsters,
mussels, and some sea-weed. They left off eating poultry, frogs, and
rats. The long tail was of great profit, as they used it as a rudder
and as propelling organ in swimming with vertical undulations. Of
course those individuals which were the best adapted to this new
manner of living, survived the less privileged by nature, and so a
group gradually arose which had a sharper dentition, and smaller
ears; the skin was also changed to the most valuable fur, the toes of
the hind-legs had become more webbed, and with such legs the animals
could swim more easily; those of the fore-limbs had sharper nails,
and with such nails the animals could more easily crawl upon the
rocks; the eyes were larger, and with such eyes the owners could see
better in great and dark depths, and in the sea-water near the shore,
which is commonly troubled; the whiskers were longer and stronger,
consequently the upper-lips, in which these whiskers were planted and
which contained numerous and thick sensorial nerves, were very thick,
and with such whiskers the animals could exceedingly well touch and
feel when searching for their food between stones and sea-weed, and
in the sandy bottom. The face resembled but little more that of the
otters and fin-tailed otters, and the large eyes and shorter ears gave
it a slight resemblance to seals. This group may be called _long-tailed
ancestors of sea-otters_, for our sea-otters (_Lutra lutris_ L.) are
still living descendants of them. But as these animals have accustomed
themselves to live more among sea-weed, ice, and rocks than their
direct ancestors, the long tail must have been inconvenient, so that
individuals with a shorter tail must have survived the others, and
finally a species arose with a tolerably short tail: our sea-otter. To
make up for this loss of tail, the hind-feet had become more webbed,
and were gradually stretched more backwards, and, modified in this
way, they were valuable swimming organs. The change was great enough
for zoologists to create a new genus for this animal, which is called
_Enhydra_ CUV.
Some of the _long-tailed ancestors of sea-otters_ took to a still more
aquatic, or better pelagic life, migrated more towards the north,
accustomed themselves to the icy regions, to swim greater distances
and to remain longer under water. The consequences of this change in
the manner of living were that all little adapted to this new life
became extinct, and that all which were better privileged survived
them, so that at last a group of animals arose of which we may safely
admit that they had the following characters: The head and fore-feet
resembled still more those of seals, the hind-legs were still more
able swimming organs, and less fit for terrestrial locomotion, they
were smaller than the fore-legs, because they were not always used in
swimming, as the best manner of swimming must have been by means of
vertical undulations; the long tail surpassed half of the total length,
and served as a rudder and as propelling organ; the ears were still
smaller, the dentition was still a normal carnivorous one (i 3/3, c
1/1, m 7/7), especially the molars were sharp and pointed, and on the
skull were found peculiar modifications which are so distinctly visible
on that of Pinnipeds. The animals must have resembled our common seals,
having, however, small external ears, and a tail, surpassing one half
of the total length. It is difficult to believe that these animals,
which I will call _Propinnipedia_, moved on land; probably they came
from time to time aland or on the ice to rest with the fore-part of
the body on it, leaving, however, most certainly the long tail in the
water. These _Propinnipedia_ gave origin to two groups of animals,
which are marked below with A and B.
A.--This group, by their having lived almost constantly far from land,
and having come only very seldom near the shore to rest, supporting
themselves on the chest or breast, clinging with the nails of the
fore-legs to the beach, rock, or ice, changed in such a way, that
zoologists can hardly reckon them any longer among Pinnipeds, but
generally consider them as a link between Pinnipeds and Whales.
Professor D’ARCY W. THOMPSON (_Studies from the Museum of Zoology in
University College_, Dundee, Vol. I, N^o. 9, 1890) rejects any affinity
of this group to Whales. I should like to go still farther and pretend
that it has just as much claim to the title of Pinnipeds, as seals,
sea-elephants, sea-bears, sea-lions, and sea-serpents. The skull was
somewhat lengthened in front; the brain-case diminished in size; the
deciduous dentition probably cut the gum; the permanent dentition was
the typical heterodont carnivorous one (i 3/3, c 1/1, m 7/7); the
nostrils were placed on the tip of the nose, as in seals, but directed
upwards; the fore-limbs were perfectly those of seals, and provided
with nails; but the rest of the body must have _resembled_ that of a
slender and elongated dolphin, or whale, with an enormous _pointed_
tail. The most successful manner of swimming for these animals was by
means of vertical undulations, which, as the forepart of the body (head
and trunk) was somewhat bulky, and therefore somewhat inflexible, were
strongest in the tail-part of the animal; consequently the hind-legs,
used less and less, disappeared, if not quite, at least for the greater
part. The animals were still hairy, though the hairs were most probably
thinly scattered; the whiskers remained on the lips. The head was
relatively large, not with regard to the animal’s total length, but to
the trunk, and therefore the neck was very short. Externally the neck
must not have been plainly visible. The animals could not move the head
as easily as seals and sea-lions do, and therefore it was of great
advantage that the nostrils were directed upwards. The vertebrae have
the type of those of the Pinnipeds.--Such animals are now extinct, but
their fossil remains are found and called _Basilosaurus_ by HARLAN in
1824 (afterwards OWEN gave them the name of _Zeuglodon_, 1839).
B.--This second group is called _Pinnipedia_ by ILLIGER in 1811, and
ALLEN gives of it the following characters:
“Limbs pinniform, or modified into swimming organs, and enclosed to or
beyond the elbows and knees within the common integument. Digits of the
manus decreasing in length and size from the first to the fifth; of
those of the pes, the first and fifth largest and longest, the three
middle ones shorter and subequal. Pelvis with the iliac portion very
short, and the anterior border much everted; ischia barely meeting by
a short symphysis (never anchylosed) and in the female usually widely
separated. Skull generally greatly compressed interorbitally; facial
portion usually short, rather broad, and the brain-case abruptly
expanded. Lachrymal bone imperforate and joined to the maxillary,
enclosed wholly within the orbit. Palatines usually separated by a
vacuity, often of considerable size, from the frontals. Tympanic bones
separated also by a vacuity from the exoccipitals. Dentition simple,
generally unspecialized, the molars all similar in structure. Deciduous
dentition rudimentary, never truly functional, and generally not
persistent beyond the foetal stage of the animal. Permanent incisors
usually 6/4 or 4/4, sometimes 4/2 (_Cystophora_ and _Macrorhinus_) or
even 2/2 (_Odobaenus_); canines 2/2; molars 5/5, 6/5, or 5/3.”
And we may add: Tail either very long: about as long as one half of the
animal’s total length, or very short: almost disappearing between the
hind-legs.
Already very early the Pinnipeds divided themselves into two different
branches, marked below with I and II.
I.--The members of this branch changed their manner of living. They
very often crawled on land, ice, and rocks; the long tail was a very
unconvenient organ in their new manner of living, consequently all
the individuals with a somewhat shorter tail than their congeners’
were better adapted to the new manner of living and survived them, so
that at last a group of animals arose of which the tail has become
very short, almost disappearing between the hind-legs, and to make up
for this loss the hind-legs grew much larger than the fore-legs, were
turned hindwards, gradually grew incapable of being turned forwards,
and of no use in terrestrial locomotion. This branch is called
_Inauriculata_ by PÉRON in 1816 (afterwards called _Phocidae_ by GRAY
in 1825, and _Reptigrada_ by ELLIOT COUES, invited thereto by ALLEN in
1880). The characters are described by ALLEN as follows:
“Hind-legs not capable of being turned forward, and not serviceable for
terrestrial locomotion. Neck short. Skull with the mastoid processes
swollen, but not salient, and without distinct alisphenoid canals.
Anterior limbs smaller than the posterior, the first digit little,
if any, longer than the next succeeding ones, all armed with strong
claws, which are terminal. Hind feet capable of moderate expansion,
short; digits (usually) all armed with strong claws, and without
terminal cartilaginous flaps. Femur with no trace of the trochanter
minor. Without external ears. Postorbital processes wanting, or very
small. Incisors variable (6/4, 4/4, or 4/2). Deciduous dentition not
persistent beyond foetal life.”
The group includes all true seals and sea-elephants.
II.--This branch is called _Gressigrada_ by ELLIOT COUES in 1880, who
was thereto invited by ALLEN, though this skilled zoologist was then
unaware of the existence of the sea-serpent, or at least must have
doubted its belonging to this branch. I have not a single reason to
give another name to it; I purposely keep the name of _Gressigrada_, to
avoid the increase of synonyms. The early forms of the _Gressigrada_
must have had hind-legs which were smaller than the fore-legs, and a
tail, which was as long as the head, neck and trunk together. They
had also small external ears, and a somewhat lengthened neck. Further
characters are: “Hind-legs capable of being turned forward and used
in terrestrial locomotion. Neck lengthened (especially in section b).
Skull with the mastoid processes large and salient (especially in the
males), and with distinct alisphenoid canals. Anterior feet either
nearly as large as the posterior, or much larger, their digits rapidly
decreasing in length from the first to the fifth, with distinct claws,
and with a broad cartilaginous border extending beyond the digits.”
(They are called flappers.) “Hind-feet susceptible of great expansion,
the three middle digits only with claws, and all the digits terminating
in long, narrow, cartilaginous flaps, united basally.” (The hind-feet
may also safely be called flappers.) “Femur with the trochanter minor
well developed”.--Already at a very early date the branch of the
_Gressigrada_ divided itself into two sections, which are marked below
with a and b.--
a.--The members of this section changed their manner of living. They
very often crawled on ice, land, and rocks; the long tail was a very
inconvenient organ in their new manner of living, consequently all the
individuals with a somewhat shorter tail than their congeners’ were
better equipped, and survived the others, so that at last a group of
animals arose of which the tail has become very short, scarcely, if
at all, visible, being enclosed within the tegument of the body, and
to make up for this loss, the hind-flappers grew much larger than the
fore-flappers. The further characters for this group are: “Without
external ears. Form thick and heavy. Anterior portion of the skull
greatly swollen, giving support to the enormously developed canines,
which form long, protruding tusks. Incisors of deciduous (foetal)
dentition 6/6; of permanent dentition 2/6. No postorbital processes,
and the surface of the mastoid processes continuous with the auditory
bullae.”--This section is called _Trichecidae_ by GRAY in 1821
(afterwards it was named _Trichechidae_ by GRAY in 1825, _Broca_ by
LATREILLE in 1825, _Campodontia_ by BROOKES in 1828, _Trichecina_ by
GRAY in 1837, _Trichechoidea_ by GIEBEL in 1847, _Trichechina_ by GRAY
in 1850, _Rosmaridae_ by GILL in 1866, _Rosmaroidea_ by GILL in 1872,
and _Odobaenidae_ by ALLEN in 1880!!!). The section contains only the
walruses.
b.--The early forms of this section must have had hind-flappers which
were smaller than the fore-flappers, and a tail which was as long as
the head, neck and trunk together. The animals were very slender and
elongated in form, the neck being _somewhat more elongated_; external
ears, though small, were still present. Further characters are:
“Anterior portion of the skull not unusually swollen, and the canines
not highly specialized.” They came very seldom aland, and when doing
so, they must have only supported themselves on their breast and on
their fore-flappers, leaving the long tail always in the water. They
swam with vertical undulations, using also sometimes the flappers.--For
this section I choose the name of _Tenuia_, or Animals which are
slender.--Very early the section of the _Tenuia_ divided itself into
two smaller divisions which are marked below with 1 and 2.--
1.--The members of this division changed their manner of living.
They very often crawled on ice, land and rocks; the tail was a very
inconvenient organ in their new manner of living, consequently all
the individuals with a somewhat shorter tail than their congeners’,
were better equipped and survived the others, so that at last a group
of animals arose of which the tail has become very short, almost
disappearing between the hind-legs which on the contrary to make up for
this loss of tail, gradually became larger, so as to become even larger
than the anterior feet. The further characters of this group are: “With
small external ears. Incisors of deciduous dentition 6/4, only the
outer on either side cutting the gum; of permanent dentition 6/4, the
two central pairs of the upper with a transverse groove. Postorbital
processes strongly developed. Surface of the mastoid processes not
continuous with the auditory bullae.”--This division was called
_Auriculata_ by PÉRON in 1816, (afterwards also called _Otariina_ by
GRAY in 1825, _Otariadae_ by BROOKES in 1828, _Arctocephalina_ by GRAY
in 1837, and _Otariidae_ by GILL in 1866) containing the sea-bears and
sea-lions.
2.--The members of this division did not accustom themselves to live in
the midst of ice and rocks, consequently they retained the long tail,
and small hind-legs. As the animals retained also their slenderness and
extraordinary litheness, a long neck with a relatively small head must
have been of great use to them, and consequently those individuals
which had a longer neck than the others survived their less privileged
congeners, so that at last a group arose with a very long neck and a
comparatively small head. It seems that the external ears disappeared.
They never came aland or on ice-floes. They even abandoned the cold
regions and currents of the ocean, better liking the warmer parts.
Their ordinary mode of swimming is with vertical undulations. Seldom do
they swim with the body in a straight line, by means of their flappers.
This little division for which I propose the name of _Longicaudata_,
or Long-tailed Animals, consists only of one genus: _Megophias_ RAF.,
including only one species _Megophias megophias_ (RAF.) OUD., the
sea-serpent.
I purposely have not mentioned the genera _Squalodon_ and _Stenodon_,
and the group of _Plagiuri_ (ART., 1735; _Physeteres_, KLEIN,
1741; _Cetacea_, BRISS, 1756; _Cete_, LINN, 1758), as the recent
cetologists still differ in opinions as to their relation to
_Basilosaurus_ and the _Pinnipedia_.
I think the following phylogenetic table will in a more practical
manner show the rank which in my opinion sea-serpents occupy in the
System of Nature.
To many of my readers the above sketch of the rank of the sea-serpent
in the System of Nature will no doubt seem to be too bold. They will
say that the affinity of the sea-serpent to sea-lions and sea-bears
(to the _Auriculata_) is expressed here too decisively, that,
scientifically spoken, the sea-serpent is not yet known, that at best
its existence is only beyond a doubt, and that when a specimen fell
into the hands of men, it might be shown that the close affinity to
the _Auriculata_ was only apparent, and that in reality the relation
is more remote. I confess that there is much to say in favour of this
reasoning, but _at all events the sea-serpent is a true Pinniped_.
It has four flappers, a hairy skin, and strong whiskers. Its head
_resembles_ that of a sea-lion, its long neck _resembles_ that of
a sea-lion, its trunk and its foreflappers _resemble_ those of a
sea-lion. But these _resemblances_ may be explained as resulting from
convergency. When viewed in this way it seems to be more careful to
consider the origin of the sea-serpent in the following manner.
Putorius vulgaris.
Putorius ermineus.
|
| Putorius putorius.
| Putorius lutreolus.
| |
| | Lutra.
| | |
| | | Pteronura Sanbachii.
| | | |
| | | | Enhydra lutris.
| | | | |
| | | | | Inauriculata.
| | | | | |
| | | | | | Trichecidae.
| | | | | | |
| | | | | | | Auriculata.
| | | | | | | |
| | | | | | | | Longicaudata.
| | | | | | | | |
| | | | | | | | |
--+------+-----+-----+----+-------------+------+-----+-------+-------
| | | | |Basilosaurus.| | | |
| | | | | | | | | |
| | | | | | | | Long-tailed ancestors
| | | | | | | | of Tenuia.
| | | | | | | | |
| | | | | | | Long-tailed ancestors
| | | | | | | of Gressigrada.
| | | | | | | |
| | | | | | Long-tailed early
| | | | | | forms of Pinnipedia.
| | | | | | |
| | | | | Propinnipedia, long-tailed
| | | | | ancestors of
| | | | | Pinnipedia and Basilosaurus.
| | | | | |
| | | | Long-tailed ancestors
| | | | of sea-otters.
| | | | |
| | | Long-tailed ancestors
| | | of fin-tailed
| | | otters.
| | | |
| | Long-tailed ancestors
| | of otters.
| | |
| Long-tailed ancestors
| of polecats and
| minks.
| |
Long-tailed ancestors
of weasels and
stoats.
|
Long-tailed
Viverrine
ancestors.
The ancestors of _Pinnipedia_ and _Basilosaurus_, which I have called
_Propinnipedia_, had most probably hind-legs which were smaller than
the fore-legs, and most certainly a tail which was nearly as long as
the head, neck and trunk together. They had small external ears. Their
most successful manner of swimming must have been by means of vertical
undulations. It is difficult to believe that the _Propinnipedia_ moved
on land; probably they came only from time to time aland, or on the
ice, to rest, leaving, however, most probably the long tail in the
water. These _Propinnipedia_ divided themselves into two branches.
All the members of the _first_ branch got a tendency to bulkiness.
The head grew longer and larger, consequently the neck grew shorter;
the jaws grew longer, consequently the teeth began to stand widely
apart; in consequence of the little mobility of the head the nostrils,
placed at the top of the nose, became turned upwards, or probably
got their seat a little more towards the top of the head; and in
proportion as the animal got a thick layer of bacon, the hairs became
thinly scattered. Probably it is here better to say: in proportion
as the animal lost its hairs, it got a thick layer of bacon. The
warm-blooded mammals are possessed of the hair, because hair was to
them what feathers are to a bird. The air enclosed between the hairs
and the feathers is a worse conductor of temperature than the hairs
or feathers themselves. As soon as the manner of living has changed
so much that air could no longer come between the hairs, the hairs
themselves lost their reason of existence, hence a thick layer of bacon
gradually replaced them. Probably this is a better way to explain the
presence of bacon and the absence of hair, than to say that the hair
disappeared because the animals obtained a layer of bacon, and could
therefore dispense with them, or that the layer of bacon checked the
development of hairs.--In short we may admit that the animals, of which
we treat at present, were thinly scattered with hairs. The whiskers
in all probability were still present, and even well developed. This
branch has wholly become extinct. The fossil remains were called
_Basilosaurus_.
All the members of the _second_ branch did not show a tendency to
bulkiness, they retained the relatively small head and well developed
neck, the head consequently could very well move on the trunk. These
are the _Pinnipedia_.
Already very early they divided themselves into two sections.
All the members of the _first_ section accustomed themselves to crawl
more on land, ice, and rocks, and as the long tail must have been an
inconvenient organ in this new manner of living, all the individuals
which had a smaller number of caudal vertebrae survived their
congeners; consequently a form at last originated with a very short
tail our well known order of Pinnipeds for which I now propose the name
of _Brevicaudata_.
All the members however, of the second section accustomed themselves
more to the sea, and therefore all the members which were best adopted
for this manner of living successively survived their less privileged
congeners, and finally the sea-serpents remained; animals which are
so excellently adapted to an aquatic life and rapid movement, that
their tendency to become extinct can only be explained by the singular
phenomenon that colossal animals bring forth very few young ones, only
two, or only one, at a time, and only after very long intervals. For
these animals I already proposed above the name of _Longicaudata_. They
form with the _Brevicaudata_ the order of _Pinnipedia_.
If this view is better, (and who will tell us this with any certainty?)
the phylogenetic table should be altered as follows:
Auriculata. Trichecidae.
| |
Living. | |
Gressigrada. Inauriculata.
| |
| |
Longicaudata. Brevicaudata.
| |
------------------+---------+------------------------------------
| |
Long-tailed early forms Basilosaurus.
of Pinnipedia. |
Extinct. | |
| |
Propinnipedia, long-tailed
ancestors of Pinnipedia
and Basilosaurus.
In the first table I have tried to show two things.
Firstly:--With a horizontal dotted line I have separated the still
living animals or groups from those who have become extinct; the former
are placed above, the latter beneath the dotted line.
And Secondly:--With the different lengths of the vertical dotted lines
I have tried to show the different relative lengths of time-periods
wanted by the different species or groups to be formed, so to speak,
from that species or group which in this table is placed exactly
beneath it, and with which it is united by a dotted line.
It is clear that the evolution must have happened, geologically spoken,
with extreme rapidity there, where the animals were entirely changing
their manner of living, be it from a terrestrial one into an aquatic
one, or otherwise; and that the evolution happened less rapid, or
even, geologically spoken, very slowly, where the animals remained
terrestrial or aquatic beings, and only changed their manner of living
in so far, that they became troglodytes or semi-troglodytes, or became
from carnivorous only piscivorous or semi-carni-semi-piscivorous. I
believe that by this hypothesis the problem is solved why remains of
_Basilosaurus_ are already found in Eocene layers together with remains
of Viverrine ancestors of _Carnivora_, _Pinnipedia_ and _Basilosaurus_,
whilst those of true _Pinnipedia_, _Lutrina_ and _Mustelina_ appear
for the first time during the Miocene period, and whilst remains of
true _Viverra_’s (the genus) do not seem to have made their appearance
before the Pliocene period.