THE APPEARANCE OF VERTEBRATE ANIMALS.
Confessedly the highest style of animal is that which possesses a skull
and backbone, with brain and nerve system to match, and which embodies
the general plan of structure employed in man himself. Yet among the
fishes, which constitute the lowest manifestation of this type, are some
so rudimentary that the brain is scarcely developed, and the skeleton is
merely a cord of gristle. These are represented in the modern world only
by the Lancelot,[33] a creature which has sometimes been mistaken for a
worm, and by a slightly more advanced type, that of the Lampreys.[34] In
these animals the Vertebrates make the nearest approach to the lower
domains of the animal kingdom, collectively known as Invertebrates. We
should naturally expect that since the vertebrates succeed the inferior
animals in time, their lower types should appear first, and that these
should be aquatic rather than terrestrial. On the other hand, as the
oldest fishes that are certainly known are strongly protected with bony
armour, and had to contend against formidable Crustaceans and Cuttles,
we might suppose that the Lancelot and the Lampreys are rather degraded
types belonging to the modern period, than the true precursors of the
other fishes.
[Illustration: FIG. 102.aEuro"Siluro-Cambrian Conodonts. Magnified.aEuro"After
Pander.]
But if fishes like the Lancelot preceded all others, we may never find
in a fossil state any traces of their soft and perishable bodies; and
even the Lampreys have no hard parts except small horny teeth, which
might easily escape observation. But palA|ontologists have sharp eyes,
and it has not escaped them that certain microscopic tooth-like bodies
are somewhat widely distributed in the older rocks. In Russia, Pander
has found in the Upper Cambrian and Lower Silurian, and also in the
Devonian and Carboniferous, minute conical and comb-like teeth, to which
he has given the name of _Conodonts_ (Fig. 102), and which he supposes
to be the teeth of ancient Lampreys. Similar teeth have been found by
Moore and others in the Carboniferous of England, and by Newberry in
Carboniferous shales in Ohio. In point of form, these bodies certainly
resemble the teeth of the humble fishes to which they have been
referred. In the case of the Carboniferous specimens from OhioaEuro"the only
ones I have had an opportunity to examinethe material is calcium
phosphate, and the structures are more like those of teeth of Sharks
than of Lampreys, so that there can be no doubt that they are really
teeth of fishes, and probably of fishes of somewhat higher grade than
the Lampreys.[35] The Cambrian and Silurian specimens are said to be
composed of calcium carbonate, which would render it more probable that,
as has been suggested by Prof. Owen, they may have been teeth of some
species of Sea-snail destitute of shell. It is, however, possible that
they may have originally been horny, and that the animal matter has been
replaced by carbonate of lime. Rohon and Zittel have recently shown that
many of these are more allied to the teeth of worms than of any other
animals.[36]
[Illustration: FIG. 103.aEuro"Lower Carboniferous Conodont.
Magnified.aEuro"After Newberry.]
If these older Conodonts were really teeth of fishes, they carry the
introduction of these nearly as far back as that of the Mollusks and
Crustaceans. If they were not, then the earliest known representatives
of this class belong to a much later age, that of the Silurian. Here we
have undoubted remains of fishes belonging to two of the higher orders
of the class; and in the succeeding Devonian these became multiplied and
extended exceedingly.
Besides the inferior tribes already referred to, the modern seas and
rivers present four leading types of fishes:aEuro"first, the ordinary bony
fishes (Teleostians), such as the Cod, Salmon, and Herring; secondly,
the Ganoid fishes, protected with bony plates on the skin, as the
Bony-pike[37] and Sturgeon; thirdly, the Sharks and their allies, the
Dog-fishes and Rays; fourthly, the peculiar and at present rare group of
semi-reptilian fishes to which the name of _Dipnoi_ has been given, on
account of their capacity for breathing both in air and in water.
Of these four types the first is altogether modern, and includes the
great majority of our present fishes. It does not make its appearance
till the Cretaceous age, and then is at once represented by at least
three of the modern families, those of the Salmon, Herring, and Perch.
The history of the other three groups is precisely the opposite of this.
They abound exceedingly at an early period, and dwindle to a much
smaller number in the modern time. This is especially the case with the
Ganoids and the Dipnoi. It is also remarkable that these groups of
old-fashioned fishes[38] are in some respects the highest members of the
class, approaching the nearest to the reptiles; but this accords with a
well-known palA|ontological law, namely, that the higher members of low
groups give way on the introduction of more elevated types, while the
lower members may continue. Thus the decadence of these higher fish
begins with the incoming of the reptiles, just as the decadence of the
higher Mollusks and predaceous Crustaceans began with the incoming of
the fishes. Further, the modern Ganoids and Dipnoi are mostly
fresh-water animals, though the Sharks are largely pelagic. In the
PalA|ozoic there seem to have been abundance of marine species of all
these types; but though marine, they probably flourished most in bays
and estuaries and on shallow banks; and the existence of these implies
continental masses of land. This explains the curious coincidence that
the introduction of fishes and of an abundant land flora synchronise,
and that the ocean was still dominated by Invertebrates long after the
fishes had become supreme in bays, estuaries, and rivers.
[Illustration: FIG. 104.aEuro"_a_, Head-shield of an Upper Silurian fish
(_Cyathaspis_). _b_, Spine of a Silurian Shark (_Onchus tenui-striatus_,
Agass.). _c_, _d_, Scales of _Thecodus_, enlarged.]
The first fishes that we certainly know are the Ganoids and Sharks,
which appear near the close of the Upper Silurian, in the English Ludlow
for example (Fig. 104). The Ganoids found here all belong to an extinct
group, characterised by the covering of the head and anterior part of
the body with large bony plates. They are mostly small fishes, and
probably fed at the bottom, and used their long or rounded bony snouts
for grubbing in the mud for food. In this respect they present a
singular resemblance to the Trilobites, so that we seem to have here
animals of an entirely new type, the Vertebrate, and with bony instead
of shelly coverings, taking up the _rA'le_ and, to some extent, the
external form of a group about to pass away. Yet I presume that no
derivationist would be hardy enough to affirm that the Trilobites could
have been the ancestors of these fishes. Nor indeed is any ancestry even
hypothetically known for them, for the doubtful Lampreys of the Cambrian
Silurian are too remote and uncertain to be used in that way. The
head-shield copied in outline in Fig. 104, and the restoration after
Lankester in the frontispiece to this chapter, may serve to represent
these curious primitive Ganoids, which are continued in the Devonian
fishes represented in Figs. 105, 106.
[Illustration: FIG. 105.aEuro"_Cephalaspis Dawsoni_ (Lankester). Lower
Devonian of GaspA(C).]
Along with these, and not improbably their enemies, were certain Sharks
(Fig. 104), known to us only by the spines which were attached to their
fins as weapons of defence, and by detached bony tubercles which
protected their skin. These remains are chiefly interesting as
indications that two of the great leading divisions of the class of
fishes originated together.
In the Devonian age the Ganoids and Sharks, thus introduced in the
Silurian, may be said to culminate. The former, more especially, are
represented by a great variety of species, some of them nearly allied to
their Silurian predecessors (Fig. 106), others of forms and structure
not dissimilar to those of the few surviving representatives of the
order, or altogether peculiar to the Devonian (Fig. 107). So numerous
are these fishes, and of so many genera and speciesaEuro"and this not merely
in one region, but in widely separated parts of the worldaEuro"that the
Devonian has not inaptly been called the reign of Ganoids. As an
illustration at once of the very peculiar forms of some of these fishes
and of their wide distribution, I figure here along with the British
species a _Cephalaspis_ (Fig. 105) found in the Lower Devonian of GaspA(C),
in the same beds with some of the antique Devonian plants described in
the last chapter.
[Illustration: FIG. 106.aEuro"Devonian Placoganoid Fishes (_Pterichthys
cornutus_, _Cephalaspis Lyelli_), from Scotland.]
[Illustration: FIG. 107.aEuro"Devonian Lepidoganoid Fishes (_Diplacanthus_
and _Osteolepis_). After Page and Nicholson.]
[Illustration: FIG. 108.aEuro"Modern Dipnoi.
_a_, _Ceratodus Fosteri_. Australia. _b_, _Lepidosiren annectus_.
Africa.]
A new and interesting light has recently been cast upon some of the most
anomalous of the ancient fishes by the study of the now rare and
peculiar species of the group of Dipnoi. Two of these, belonging to the
genus _Lepidosiren_, are the aEurooeMud-fishesaEuroŁ of the rivers of tropical
Africa and America (Fig. 108, _b._) These creatures have an elongated
and elegant form, and the body is covered with overlapping horny scales
like those of ordinary fishes; but the pectoral and ventral fins are
rod-like, and are supported by simple cartilaginous rays, while the
tailfin forms a fringe around the posterior part of the body. Unlike
ordinary fishes, they have lungs as well as gills, and their mouths are
armed with sharp, bony, beak-like teeth (Fig. 115), with which they can
inflict terrible bites on the small fishes and frogs which furnish them
with food. Their most remarkable habit is that of burying themselves in
the mud of dried-up ponds, thus forming a sort of water-chamber or
aEurooecocoon,aEuroŁ in which they remain in a torpid state until the return of the
rainy season sets them free.
Another example of these Dipnoi is the Barramunda, or _Ceratodus_ of the
Australian rivers (Fig. 108_a_). This fish resembles the _Lepidosiren_ in
many essential points of structure; but its fins have lateral rays, and
are consequently of some breadth, though of peculiar form, and its mouth
is armed with flat, pavement-like teeth, wherewith it browses on aquatic
grasses.
[Illustration: FIG. 109.aEuro"Anterior part of the palate of _Dipterus_.
Showing the dental plates at _a_, Devonian.aEuro"After Traquair.]
These modern fishes have enabled us to understand several mysterious
forms met with in the older rocks. In the first place, they show the
meaning of certain flat-toothed fishes, like _Dipterus_ of the Devonian
(Fig. 109), _Conchodus_ of the Carboniferous (Fig. 110), and _Ceratodus_
of the Carboniferous and Trias (Figs. 111, 112), previously of very
doubtful character. These must all have been of similar structure and
habits with the Barramunda, which is thus the sole survivor, perhaps
itself verging on extinction, of a group of herbivorous fishes
introduced, it may be, contemporaneously with the first stream affording
the requisite vegetable food, and which have continued almost without
improvement or deterioration to the present time. These fishes are,
however, very closely connected with the Ganoids, and there are some of
these, with fringed fins and overlapping scales, which, while regarded
as true Ganoids, resemble the Dipnoi very closely.
[Illustration: FIG. 110.aEuro"Dental plate of _Conchodus plicatus_ (Dn.).
Coal-formation of Nova Scotia. _Acadian Geology_.]
[Illustration: FIG. 111.aEuro"Dental plate of _Ceratodus Barrandii_.
Coal-formation of Bohemia. After Fritsch.]
[Illustration: FIG. 112.aEuro"Dental plate of _Ceratodus serratus_. From the
Trias.]
[Illustration: FIG. 113.aEuro"Jaws of _Dinichthys Hertzeri_ (Newberry).
Laterally compressed; one-sixth natural size.]
Again, certain huge fishes, whose remains are found in the Devonian of
Ohio,[39] had jaws on the same plan with those of _Lepidosiren_, but of
enormous size and strength (Figs. 113, 114, 115), so that in this and
some other points of structure they may be regarded as colossal
Mud-fishes, and they must have had the same destructive powers, but on a
far grander scale. They were besides clothed with heavy armour of bony
scales, having some resemblance to that of those mailed fishes of
smaller size already referred to, and indicating that, huge though they
were, and formidable in destructive power, they also had enemies to be
dreaded. These plates serve to ally them with the Ganoids, as their jaws
do with _Lepidosiren_.
[Illustration: FIG. 114.aEuro"Lower Jaw of _Dinichthys Hertzeri_. One-sixth
natural size.]
[Illustration: FIG. 115.aEuro"Jaws of _Lepidosiren_. Natural size.aEuro"After
Newberry.]
We are thus enabled to see in the streams, lakes, and bays of the
PalA|ozoic, harmless fishes, of the type of _Ceratodus_, feeding on plants,
and huge precursors of the Mud-fishes darting from the depths, and
provided with a dental apparatus more formidable than that of any modern
fish, sufficient to pierce the strongest armour of the Ganoids, and to
destroy and devour the largest aquatic animals. These huge fishes, armed
with shears two or three feet in length, and capable of cutting asunder
scale, flesh, and bone, are the _beau idA(C)al_ of destructive monsters of
the deep, far surpassing our modern Sharks; and if, by means of
supplementary lungs, they could breathe in air as well as in water, they
would on that account be all the more vigorous and voracious.
Newberry has well remarked that while in the Devonian the Ganoids and
Dipnoi were the real tyrants of the sea, as well as of the streams, in
the Carboniferous they already diminish in size, though still abundant
as to numbers, and are more limited to estuaries and fresh waters. Thus
their departure from power had already begun, and went on until in
modern times the proportion of Ganoids to ordinary fishes is, according
to GA1/4nther, nine out of 9,000. The Carboniferous, indeed, very specially
abounds in small Ganoids, though there are many large and formidable
species. One of these smaller species, a very beautiful little fish, of
fresh-water ponds and streams in the older part of the Carboniferous
age, is represented of the natural size in Fig. 116, and is not a
restoration, being found preserved entire, though flattened, in a fine
bituminous shale, which has perfectly preserved even the most delicate
sculpturing of its bony scales.
[Illustration: FIG. 116.aEuro"A small Carboniferous Ganoid (_PalA|oniscus
(Rhadinichthys) Modulus_ Dn.). Lower Carboniferous, New Brunswick.
_a_, Outline. _b_, _c_, _d_, Sculpture of scales magnified.]
[Illustration: FIG. 117.aEuro"Teeth and Spines of Carboniferous Sharks. Nova
Scotia.
_a_, _Diplodus penetrans_. _b_, _Psammodus_. _c_, _Ctenoptychius
cristatus_. _d_, Spine, _Gyracanthus magnificus_. One-eighth natural
size.aEuro"_Acadian Geology._]
The Sharks in the Carboniferous increase in number and importance. Fig.
117 shows a few examples of their teeth and spines. In the
Carboniferous, however, there is a great preponderance of those species
with flat, crushing teeth fitted for grinding shells,[40] which in
diminishing numbers continue up to the present time, when they are
represented by the Port Jackson Shark and a few other species. The
increase toward the modern time of the true Sharks[41] with sharp
cutting teeth, is obviously related to the increase of the ordinary
fishes which furnish them with food. Another curious difference,
connected probably with the same circumstance, is the fact that in the
sharp toothed Sharks of the Carboniferous the two side fangs of each
tooth are the largest, or are exclusively developed (Fig. 117, _a_),
while in later periods the central point becomes dominant, or is
developed to the exclusion of the others (Figs. 118, 119).
[Illustration: FIG. 118.aEuro"Teeth of Cretaceous Sharks (_Otodus_ and
_Ptychodus_).aEuro"After Leidy.]
[Illustration: FIG. 119.aEuro"Tooth of a Tertiary Shark (_Carcharodon_).]
[Illustration: FIG. 120.aEuro"A Liassic Ganoid (_Dapedius_).
Restored.aEuro"After Nicholson.]
The Ganoids and Dipnoi still, however, occupy a very important place
through the Mesozoic ages (Fig. 120), and it is only at the close of the
Cretaceous that they finally give place to the Teleosts, or common
fishes, which, though perhaps more fully specialised in purely ichthyic
features, have dropped the reptilian characteristics of their
predecessors (Fig. 121). It is interesting to observe that these
old-fashioned fishes had culminated before the advent of air-breathing
Vertebrates, which appear for the first time in the Carboniferous. It is
further to be observed that groups of fishes furnished with means of
aiding their gills by rudimentary lungs were especially suited to waters
more charged with carbonic acid, and less with free oxygen, than those
of more recent times. This remark especially applies to the mephitic and
sluggish streams and lagoons of the Carboniferous swamps, where, in the
midst of a rank vegetation and reeking masses of decaying organic
matter, the half air-breathing fishes and the amphibious reptilian
animals met with each other and found equally congenial abodes. Thus,
independently of the fact that some of these fishes were probably
vegetable feeders, it is not altogether an accident, but a wise
adaptation, that caused the culmination of the reptilian fishes and
batrachian reptiles to coincide with the enormous development of the
lower forms of land-plants in the Devonian and Carboniferous. Another
curious illustration of the diminishing necessity for air-breathing to
the fishes, is the change of the tail from the unequally-lobed or
heterocercal form, which prevailed in the PalA|ozoic, to the more modern
equally-lobed (homocercal) style in the Mesozoic. The former is better
suited to animals which have to rise rapidly to the surface for air, and
is still continued in some modern fishes, which for other reasons need
to ascend and descend, or to turn themselves in the water; but the
homocercal form is best suited to the ordinary fish, whether Ganoids or
Teleosts (Fig. 122). It is curious also to find the beginning of the
dominancy of the ordinary fish to coincide with that of the broad-leaved
exogenous trees in the later Cretaceous, and to precede immediately the
appearance of the mammals on the land; all these changes being related
to the purer air, the clearer waters, and the more varied continental
profiles of the later geological periods. Thus physical improvement and
the changes of animal and vegetable life are linked together by
correlations which imply not only design, but prescience, whether we
attribute these qualities to a spiritual Creator or to mere atoms and
forces.
[Illustration: FIG. 121.aEuro"Cretaceous Fishes of the modern or Teleostian
type.
_a_, _Beryx Lewesiensis_. English chalk. _b_, _Portheus molossus_
(Cope). A large fish from the American Cretaceous. One twenty-eighth
natural size.]
The history of fishes extends further through geological time than that
of any other Vertebrates, and is perhaps more completely known to us, in
consequence of the greater facilities for the preservation of their
remains in aqueous deposits. If we receive PanderaEuro(TM)s Conodonts as
indicating a low type of cartilaginous fishes, these must have
continued for vast ages without any elevation, and struggling for a bare
existence amidst formidable Cuttle-fishes and Crustaceans, before, under
more favourable conditions, they suddenly expanded into the high and
perfect types of Ganoids and Sharks. If we reject the early Conodonts,
then the two last-mentioned types spring together and suddenly into
existence, like the armed men from the dragonaEuro(TM)s teeth of Cadmus. They
rapidly attain to numbers and grandeur unexampled in later times, and
become the lords of the waters at the time when there was probably no
Vertebrate life on the land. As the reptiles establish themselves on the
land and in the waters, the Ganoids diminish, but the Sharks hold their
own. At length the reign of reptiles is over, but the Ganoids, instead
of resuming their pristine numbers, give place to the Teleosts, and
become reduced to insignificance; while the Sharks, profiting by the
decadence of the great marine reptiles, remain the tyrants of the seas.
This history is strangely unlike a continuous evolution; but we are
anticipating facts which will fall to be discussed in a subsequent
chapter.
[Illustration: FIG. 122.aEuro"Modern Ganoids (_Polypterus._ Africa.
_Lepidosteus._ America).]
[Illustration: A MICROSAURIAN OF THE CARBONIFEROUS PERIOD (_Hylonomus
Lyelli_). RESTORED FROM THE SKELETON AND DERMAL APPENDAGES FOUND IN AN
ERECT SIGILLARIA. HALF NATURAL SIZE.]