Samuel Wendell Williston.

The osteology of the reptiles online

. (page 6 of 18)
Online LibrarySamuel Wendell WillistonThe osteology of the reptiles → online text (page 6 of 18)
Font size
QR-code for this ebook


remained functional in all Amniota as the basal piece or "body" of
the atlas.

Intercentra are characteristic of deeply amphicoelous or noto-
chordal dorsal vertebrae, that is, in the more primitive vertebrae, and
never occur in procoelian, amphicoelian, or opisthocoelian reptiles.
They occur in many precocious lizards throughout the neck, often
in their normal places between the centra but frequently shifted for-
ward on the preceding centrum, either loosely attached or coossified
with an exogenous outgrowth, forming with it a functional hypa-
pophysis. Where they occur between the centra they may be elon-
gated into false hypapophyses. A similar condition is known in some
Chelonia on the first two to four vertebrae, where they are usually
paired. Double intercentra have also been observed in the anterior
vertebrae of Procolophon, a cotylosaur, and in the young of certain
plesiosaurs. In the Ichthyosauria, though the centra are deeply
biconcave, only two to four intercentra have been observed. They
have also been found in the anterior vertebrae of some plesiosaurs.

It is now universally believed that the undivided or holospondylous
vertebrae of reptiles were evolved from divided or temnospondylous
vertebrae of the Stegocephalia. It was Cope who first recognized
the identity of the parts and his views are now generally accepted,
though not by all.

Temnospondylous vertebrae are of two kinds, called by Cope
embolomerous (Fig. 76 a-c) and rhachitomous (Fig. 76 d). The former
are known in only a few amphibians, from the Mississippian, Penn-



THE VERTEBRAE 97

sylvanian, and Lower Permian, but best in Cricotus (Fig. 76 a-c)
from Illinois and Texas. Rhachitomous vertebrae are much more
widely known in numerous forms from the Pennsylvanian and
Permian of various parts of the world.

An embolomerous vertebra is composed of two subequal, noto-
chordal disks, the anterior one the intercentrum, or hypocentrum,
bearing the exogenous chevron in the tail; the posterior one the
pleurocentrum; and the arch or neurocentrum, resting upon both the
intercentrum and pleurocentrum, but chiefly the latter. The artic-
ular surface for the head or capitulum of the ribs is chiefly on the
intercentrum; the surface for the articulation of the tubercle of the
rib, on either the arch or diapophysis.

A rhachitomous vertebra (Fig. 76 d) differs in that the intercen-
trum or hypocentrum is more or less wedge-shaped, with its base on
the ventral line, its apex not reaching the dorsal side; while the
pleurocentra behind are paired, with the basal side above and their
apices reaching the ventral line only narrowly or not at all. The
neurocentrum, as in the embolomerous forms, is borne by all three
bones, but chiefly by the pleurocentra. The head of the ribs articu-
lates with the intercentrum, the tubercle with the diapophysis of the
neurocentrum.

The earliest known amphibian vertebrae are embolomerous ; rha-
chitomous and holospondylous vertebrae appearing later, so far
as our present knowledge goes. And this is one of the reasons why it
would seem that the embolomerous type is the more primitive, giv-
ing origin directly to the reptilian holospondylous type, as was first
suggested by Cope ; that the rhachitomous type was derived from it
by the loss of the upper part of the intercentrum and the lower part
of the pleurocentrum and the division of the latter into two lateral
parts. This reversion of the pleurocentrum to a more primitive onto-
genetic condition is the chief objection to this theory, nevertheless it
is the more probable. We have seen that the more primitive phylo-
genetic condition of the intercentra persists longest in the neck and
tail. In the caudal vertebrae of Eryops (Fig. 76 d), and probably
other rhachitomous amphibians, there is an intermediate condition
between the embolomerous and rhachitomous types, in which the
single pleurocentrum is typically embolomerous, that is, disk-like
and perforated for the notochord; while the intercentrum bearing



98



THE OSTEOLOGY OF THE REPTH^ES



its exogenous chevron is typically rhachitomous, in that it is wedge-
shaped. And this very probably represents the real intermediate
condition between the embolomerous and holospondylous vertebrae.
Evidence that reptilian vertebrae arose in this way is also seen in the
dorsal vertebrae of a young Seymouria, the most amphibian-like,




Fig. 76. Vertebrae: A, B, C, Cricotus (Temnospondyli), dorsal, basal
caudal, and median caudal, from the side and front. D, Eryops
(Temnospondyli), caudal, from the side. E, Seymouria (Cotylo-
sauria), median dorsal, from the side. F, Dimetrodon (Pelycosauria),
dorsal intercentrum from behind and below. G, Trimerorhachis
(Temnospondyli), intercentrum from side and below.



otherwise, of all known reptiles (Fig. 76 e). The intercentrum is here
remarkably large for a reptile, nearly half as long as the notochordal
centrum or pleurocentrum. And it is also almost the condition found
in the first vertebra of primitive reptiles, the atlas (Fig. 79), as will
be shown in the discussion of that bone. Additional evidence is fur-
nished by the fact that while truly embolomerous vertebrae occur in
fishes, in the modern Amia, for instance, real rhachitomous vertebrae



THE VERTEBRAE 99

are known only among amphibians. Certain ancient fishes {Eury-
cormus), it is true, with dorsal embolomerous vertebrae, have in the
tail pseudo-rhachitomous vertebrae, composed of two half-disks, the
one with its base below, the intercentrum, the other with it above,
the undivided pleurocentrum.

The evolution, then, of the holospondylous reptilian vertebra from
the temnospondylous amphibian vertebra seems clear : by the simple
increase in size of the notochordal centrum and the progressive de-
crease of the intercentrum to a wedge-shaped, subvertebral bone,
and its final loss everywhere in the column save in the atlas and
chevrons of the tail; and thus the term hypocentrum becomes purely
a" synonym of the earlier term intercentrum. The retrogression of
the disk-like pleurocentrum into the paired pleurocentra of the
Rhachitomi, is paralleled by the separation of the primitively single
intercentrum into pairs, observed in Procolophon, many turtles, and
some plesiosaurs.

Cervical Vertebrae

(Figs. 77-81)

The number of vertebrae in the neck or cervical region of reptiles
is not always easily determinable. In those reptiles having a sternum,
the first rib attached to it definitely determines the beginning of the
thorax. The distinction is almost as definite in those in which there
is a change in the articulation of the rib from the centrum to the
arch, as in the Sauropterygia and Archaeosauria. But the early
reptiles had no sternum, and free ribs were continuous from the atlas
to the sacrum without change in their mode of articulation. In such,
the changes in their shapes, with other modifications, may indicate
approximately the beginning of the dorsal series. Better evidence,
however, is found in the position of the pectoral girdle as found in
the rocks.

The number is very variable, more so than that of the dorsal verte-
brae. The Cotylosauria, like the Temnospondyli, have but one or
two vertebrae which may properly be called cervical, since the pec-
toral girdle is almost invariably found lying immediately back of the
skull, the front end of the interclavicle, indeed, between the angle of
the jaws. Primitive reptiles, then, like their immediate ancestors,
the Stegocephalia, had practically no neck, and but little motion of
the head in a lateral direction.



lOO



THE OSTEOLOGY OF THE REPTD^ES



The Theromorpha have a longer neck, with at least six and prob-
ably seven vertebrae (Fig. 77), as shown by the lengths of the ribs,
by the diapophyses, and mored efinitely by the position of the scap-
ula and clavicles as observed in various specimens. These numbers,
six or seven, are those given for the Therapsida, as this order is im-
perfectly known, and seven is the number that has remained so per-
sistently in their descendants, the mammals. Modern chameleons
have but five; true lizards, the Chelonia and Rhynchocephalia, eight;
the monitor lizards, Crocodilia, Theropoda, Iguanodontia, and




Fig. 77. Notochordal cervical vertebrae, with intercentra, of Ophiacodon, a
primitive theromorph: pa, proatlas; an, arch of atlas; o, odontoid; ax, axis.



Ceratopsia, nine; the Pterosauria and Phytosauria, eight or nine;
the Pseudosuchia, eight to ten; the Trachodontia and Sauropoda, as
many as fifteen. It must be remembered, however, that in some
cases these numbers are only approximately correct, dependent
upon the interpretation of what constitutes a cervical vertebra by
different observers.

On the other hand, among strictly amphibious or aquatic reptiles
there has been an increase or decrease in the number, the latter in
the tail-propelling aquatic types. The ancient proganosaurs have
ten or eleven; the dolichosaur lizards, thirteen; the nothosaurs, six-
teen to twenty-one or twenty- two ; the plesiosaurs, from thirteen to
as many as seventy-six; probably also the increase in number among
the trachodont and sauropod dinosaurs may be attributed to water



THE VERTEBRAE



lOI




Fig. 78. Ophiacodon. Proatlas, axis,
and ribs.



habits. The marine crocodile, with a fin-hke tail, lost two, like the
mosasaurs and aigialosaurs, having seven; Pleurosanrus probably
had but five ; and the ichthyosaurs, the
most specialized of all aquatic reptiles,
had practically no neck.

The first two or three of the cervical
vertebrae are markedly differentiated
in all reptiles, as in the higher animals.
The first of these, the proatlas, is in-
constant and vestigial, and has not
been included in the numbers above
given. The second, the first of our
usual nomenclature, is the atlas. The
third, more or less closely united with
the atlas, is the axis, or epistropheus.
The following cervical vertebrae, when
present, are differentiated more or less
from the dorsal series by their less
erect or shorter spines, transverse pro-
cesses, or the slenderness and mode of
rib articulation. The cervicals of the
later pterodactyls have additional ar-
ticulations on their ventral sides, as
has been described above (p. 91).

Proatlas. The proatlas (Figs. 79 c,
80 D, l) is a small, more or less vesti-
gial neural arch between the arch of
the atlas and the occiput, usually
paired. It is believed to be the arch
of a vertebra formerly intercalated be-
tween the atlas and the skull; by
some, homologous with the so-called
atlas of the Amphibia; by Baur, as the
representative of a vertebra fused with

the occiput in the reptiles; by others, as merely the separated spine
of the atlas; by others, as the arch of a vertebra whose centrum is
represented by the anterior end of the odontoid. Another theory,
which has less to commend it, is that of Jaekel, namely, that the




Fig. 79. Theromorph vertebrae: A,
Dimetrodon, atlas and axis; B, the
same atlas, from the front; C, the
same proatlas, from the side; D,
Sphenacodon, neurocentrum of atlas,
inner side, i, intercentrum; o, pleu-
rocentrum (odontoid); n, neurocen-
trum (arch).



I02 THE OSTEOLOGY OF THE REPTn.ES

centrum of the proatlas is the so-called intercentrum of the atlas,
necessitating the view that the axial intercentrum is merely an ac-
cessory or provisional bone developed below the odontoid to fill out
what would otherwise be an unoccupied space!

Positive evidence of the proatlas has been discovered in several
genera of the Cotylosauria, but no complete specimen has yet been
discovered ; it is doubtless present throughout the order. It is present
in many if not all forms of the Theromorpha and Therapsida. In
Ophiacodon (Fig. 78) and Dimetrodon (Fig. 79) of the former group, it
is a small bone on each side, articulating in front by a facet on the
exoccipital, behind with an anterior zygapophysis on the arch of the
atlas, both surfaces looking more or less downward. These articular
surfaces appear to be present in all known genera. In the Crocodilia,
occurring as far back as Jurassic times, it is a single bone in the adult,
roof-shaped, arising from paired cartilages. In Iguanodon (Fig. 80 l),
of the predentate dinosaurs, as also in several genera of the Sauro-
poda, and the Triassic Plateosaurus of the Theropoda, it is paired, as
in the modern Sphenodon (Fig. 80 D),also articulating with the atlas.
A roof -shaped, unpaired proatlas has been described in Rhampho-
rhynchus, a Jurassic pterosaur. It has also been reported in the cha-
meleon lizards and the mammals Erinaceus and Macacus. As an
abnormal element it was also found by Baur in a trionychoid turtle
{Platypeltis spinifer, Fig. 32), partially fused with the occiput, and
articulating with the arch of the atlas in the primitive way, from
which he concluded that the real body of this vertebra had become
permanently fused with the basioccipital. Probably it will be even-
tually discovered in many other extinct reptiles.

Atlas (Figs. 78, 79, 80). There is no vertebra in the known amphib-
ians which can be homologized with the atlas of reptiles. By some
the so-called atlas of the amphibians is thought to be represented by
the proatlas; or it may have entirely disappeared. In the earliest
reptiles (Fig. 79), the atlas is temnospondylous in structure, that is,
composed of a paired arch resting in part upon a large, wedge-
shaped intercentrum, in part upon a single large, embolomerous,
notochordal pleurocentrum, all of them loosely connected with the
axis, the arch of the atlas or neurocentrum articulating in the usual
way by zygapophyses.

In its highest development, in the mammals, the arch and inter-




Fig 8o Atlas, axis, and ribs: A, Trinacromerum (Plesiosauria); B, Platecarpus (Mosasauria);
C, 'Baptanodon (Ichthyosauria), after Gilmore; C, Cymbospondylus (Ichthyosauna), after
Merriam; \i,Sphenodon (Rhynchocephalia); Y.,Nyctosaurus (Pterosauna); Y,Champsosaurus
(Choristodera), after Brown; G, Gavialis (Crocodilia); H, Enaliosuchus (Crocodilia), atter
Jaekel; I, J, Diplodocus (Dinosauria), after Marsh; K, Camptosaurus (Dinosauna), atter Gil-
more; Uhuanodon (Dinosauria), after Dollo; M, Chrysemys (Chelonia); l<i. Iguana (Lacer-
tilia); O, Trinacromerum (Plesiosauria"); P, Apatosaurus (Dinosauna), after Riggs.



I04



THE OSTEOLOGY OF THE REPTILES



centrum are fused into a ring, which revolves about its pleurocen-
trum, the odontoid, a small, tooth-shaped, or spout-shaped bone
firmly fused with the axis in front and usually described as a part of
it. Long ago, however, the odontoid was recognized by Cuvier as
really the body of the axis. In no reptile did the atlas attain the spe-
cialization of the mammals, even approximately, but it most nearly
approached it in the Theriodonts. In very few do the two bones of
the arch fuse with the intercentrum into a complete arch ring, or
does the pleurocentrum unite with the axis as a real odontoid. In

few is there any degree of ro-
tation about it, not more than
between the axis and the fol-
lowing vertebra. This lack of
torsion, in most reptiles at
least, was compensated for
by the ball-and-socket joint
between the single condyle
>cand the atlas, lost in mam-
mals.

In the primitive Ophiaco-
don (Fig. 78) and Dimetrodon
(Fig. 79) the condylar cup is
formed by the intercentrum
and arch, completed in the
middle by the front end of
the odontoid, that is, the
pleurocentrum or true centrum, which has no independent motion
whatever, and is not united with the axis. The arch bears a rib upon
its diapophysis, and the large odontoid is perforated for the noto-
chord, as in the embryonic cartilage of mammals. The pleurocen-
trum or centrum, large and notochordal primitively, reaching the
ventral side of the vertebra, grew progressively smaller till it finally
disappeared wholly from side view in the Pterosauria (Fig. 80 e) ,
most Dinosauria, and the Squamata (Figs. 80 b, l). In the Rhyn-
chocephalia (Fig. 80 d), Choristodera (Fig. 80 r), and Phytosauria
it is yet largely visible from the side, but the first and second inter-
centra have become contiguous below it. In the Crocodilia (Fig.
80 g) and Chelonia (Fig. 80 m) the pleurocentrum still retains its




Fig. 81. Atlas and axis of D//)/oi/o(:«J' (Saurischia).
After Holland. One fourth natural size.



THE VERTEBRAE 105

primitively large size, reaching the ventral side, doubtless because
of the loss, fusion, or great decrease in the size of the axial inter-
centrum. In the marine crocodiles (Fig. 80 h) the pleurocentrum
is more reduced. Among the Chelonia the atlas may fuse into an
independent vertebra, articulating with the axis. At other times
the odontoid is more or less united with the axis, with no motion be-
tween it and the ring of the atlas. The axial intercentrum may be
paired or single, fused with the odontoid or apparently absent.
When paired they are more or less elongated, forming pseudo-hypa-
pophyses, serving for the attachment of neck muscles.

In the Plesiosauria (Fig. 80 a) the odontoid is to a greater or less
extent visible from the side, but is much reduced. In both the plesi-
osaurs and pterodactyls the atlas and axis are fused, indistinguish-
ably so in the adult; both are slender-necked animals with small or
vestigial cervical ribs. In the short-necked Ichthyosauria the atlas
and axis show a progressive fusion from the earlier forms (Fig. 80 c),
in which a complete disk represents the atlas, to those in which the
bodies of atlas and axis are imperfectly or indistinguishably fused
(Fig. 80 c).

Axis (Figs. 78-81). The axis differs from the following vertebrae
in its broader and stouter spine, its usually more elongated centrum,
and in its relations with the atlas. Its prezygapophyses are small
and turned outward at the base of the spine. In the Cotylosauria
and Theromorpha the front end of its centrum is deeply concave, the
persistent notochord continuous through the notochordal odontoid.
In procoelian, opisthocoelian, and platycoelian vertebrae the front
end is flattened for sutural or ligamentous union with the odontoid.
Its centrum is usually longer and usually bears a rib, though in the
modern cocodiles (Fig. 80 g) and the dinosaurs (Fig. 81) its rib has
migrated forward.

The axial intercentrum is nearly always present, primitively
larger than the following intercentra, and is intercalated between the
bodies of the atlas and axis in the usual way. Among the crocodiles
(Fig. 80 G, h), anomodonts, and some lizards it has disappeared or is
represented by the merest vestige. It is small in the dinosaurs and
chelonians.



io6



THE OSTEOLOGY OF THE REPTILES



Dorsal Vertebrae

(Fig. 82)

The smallest number of dorsal vertebrae known in reptiles is
that of the Chelonia, invariably ten. In the chameleon lizards there
are as few as eleven; in the pterodactyls about twelve. In the lat-




FiG. 82. Ophiacodon mirus Marsh (Theromorpha). Seventh to
twentieth vertebrae, from the side. One half natural size.



ter order three or more of the anterior ones may be more or less
immovably united for the support of the pectoral arch, forming the
notarium. In the Chelonia they are fused throughout in the cara-
pace. The largest number of dorsal vertebrae in reptiles having a
sacrum, forty-one or forty-two, is found in Pleurosaurus, a slender,
aquatic Jurassic reptile. About thirty is the usual number in the
plesiosaurs. In terrestrial reptiles the number never exceeds twenty-
two or twenty-three and is usually about eighteen. In reptiles lack-



THE VERTEBRAE



107



ing a sacrum the number between
the girdles may be much greater,
thirty-five in the mosasaurs, and
as many as seventy-four in some
terrestrial, legless lizards.

As has been said, there is not
often the same distinction between
thoracic and lumbar vertebrae
that there is in mammals. There
are, however, even in the Coty-
losauria, examples (Fig. 164) of
true lumbar vertebrae, that is,
vertebrae in front of the sacrum
not bearing ribs of any kind.

Sacral Vertebrae

(Fig. 83)

The sacrum of land vertebrates
is composed of from one to four
or five vertebrae, either fused to-
gether or separate, bearing short,
stout ribs for the support of the
pelvis. Rarely among the am-
phibians are there more than one ;
certain temnospondyls and mod-
ern urodeles^ are known to have
two. It is quite certain, however,
that reptiles began their career
with but a single rib-bearing sacral
vertebra, inasmuch as Seymouria
of the Cotylosauria is known to
have no more (Fig. i). A second
vertebra (Fig. 84), however, was
soon added from the basal caudal
series by the enlargement of the
ribs to come in contact with the
ilium on each side. And this num-

^ [Also some frogs. — Ed.]




sri



Fig. 83. Sacrum and caudal vertebrae of
Macrochelys (Chelonia), seen from below.



THE VERTEBRAE I 09

ber, two, has remained persistent in most reptiles and even most
mammals to the present time. A third vertebra, from the cau-
dal series, was early united in many Theromorpha and the latest
Cotylosauria. Still another, and possibly two, were joined in the
Dinocephalia and Anomodontia. The Plesiosauria, purely aquatic
animals with propelling legs, have three or four sacrals. From one to
three additional vertebrae have been fused with the sacrum in front
in the Pterosauria (Fig. 118 d), and some Dinosauria, but they are
not true sacral vertebrae.

Not only may the sacral vertebrae be closely fused, but their
arches and spines may become almost indistinguishably united.
Usually, however, the zygapophyses remain visible and are some-
times functional. In Iguana, even the zygosphene and zygantrum
are present between the two sacrals. The sacrum is lost, not only
in the snakes and legless lizards, but also in the mosasaurs and late
ichthyosaurs, where hind legs have lost locomotive functions.

Caudal Vertebrae

(Figs. 76, 83-85)

The tail of the earliest known reptile, from the Coal Measures of
Ohio (Fig. 84), was long and slender. The Cotylosauria had, for the
most part, only a moderately long tail, with not more than sixty
vertebrae. The length of the tail, however, depends so much upon
habits that it may be extremely variable even in members of the
same order. Stumpy- tailed lizards (Trachysaurus) , for instance,
have practically no tail, while other skinks have a very long and
slender one. Invariably it is long in tail-propelling, swimming rep-
tiles; such reptiles move sinuously through the water, preventing
much use of the legs as propelling organs. Those with propelling
legs, on the other hand, have a broader and flatter body and short
tail, of use only as a steering organ. However, sauropod dinosaurs,
though supposed to be exclusively water animals, have a very long
and slender tail, more or less whiplash-like at the end. As a rule,
swift-moving, crawling reptiles have a long and slender tail, while
short-tailed reptiles are invariably slow in their movements upon
land.

The spines of the caudal vertebrae in land reptiles are seldom long;
certain chameleon lizards and the basilisc lizard are exceptions ; the



no



THE OSTEOLOGY OF THE REPTILES



vertebrae distally are more slender and the zygapophyses weak. One
of the first indications of swimming habits, at least in those rep-
tiles with long tails, is the widening and elongation of the caudal
spines throughout, [less] at first [anteriorly] and then more distally
until a terminal fin is developed with the end of the column in
the lower lobe (Fig. 85).

The basal caudal vertebrae, from one to six in number, those with-
out chevrons but with ribs, are called pygals. They have the ordinary
intercentra in those reptiles in which they [intercentra] are persistent
throughout; sometimes with rudimentary chevron-like processes >




Fig. 85. Tail, scapula {sc), and coracoid {cor) of Geosaurus (Thalattosuchia). After Fraas.



The cloaca in the living animal occupies the space below them. The
number is more or less reduced in modern reptiles; the Crocodilia
have but one, most lizards, two.

There is an unossified vertical septum through each caudal cen-
trum in many lizards, the Proganosauria Saphaeosaurus and Spheno-
don, along which it readily breaks, causing the easy loss of the distal
part. This septum was once supposed to represent the division
between the primitive component parts of the centrum. It is now
thought to be an acquired character, not occurring in the early
embryo.

Chevrons, or haemapophyses (Fig. 84) for the protection of the
vessels on the under side of the tail, really outgrowths from the inter-


1 2 3 4 6 8 9 10 11 12 13 14 15 16 17 18

Online LibrarySamuel Wendell WillistonThe osteology of the reptiles → online text (page 6 of 18)