CHAPTER II
THE VERTEBRAE
The spinal column or backbone of reptiles, as of all other air-breathing vertebrates, is made up of a variable number of separate segments called vertebrae. A vertebra (Fig. 73 b) is composed of a body, or centrum, and an arch, or neurapophysis, each ossifying separately and uniting at variable times, the neurocentral sutures more persistent than in most mammals, young or aquatic reptiles always (Fig. 87 b, c), adult land reptiles often showing them.[1]
Fig. 73. Anterior dorsal and cervical vertebrae: A, B, Sphenodon (Rhynchocephalia), anterior dorsal from the side and front; C, D, Iguana (Lacertilia), anterior dorsal from the side and front; E, F, Ophidia, anterior dorsal from behind and in front; G, Pteranodon (Pterosauria), cervical from the side, after Eaton.
Projections from the vertebrae, called processes or apophyses, serve for the attachment of muscles or ligaments, for articulation with adjacent vertebrae, or for the support of ribs, and are often characteristically different in different reptiles. Two pairs of processes springing from the arch, one in front and one behind, are known as zygapophyses. The pair in front, the prezygapophyses (az), always has the flat or concave articular surface directed upward, that is, toward the dorsal side, or upward and inward; while that of the posterior pair, the postzygapophyses (pz), is turned downward, that is, toward the ventral side, or downward and outward. The zygapophyses may be obsolete or even absent in the posterior part of the column of aquatic reptiles.
The vertebrae of all snakes, some lizards, and some mosasaurs, have additional articulations, or rather, extensions of the zygapophysial articulations about their inner ends, known as zygosphenes (Fig. 73 d, f) and zygantra (Fig. 73 e).
Fig. 74. Dorsal vertebra: Diadectes (Cotylosauria) from behind, showing diapophyses, postzygapophyses, and hyposphene.
The zygosphene is a wedge-shaped process at the anterior end of the arch, above and between the zygapophyses, which fits into a corresponding cavity, the zygantrum, at the posterior end of the next preceding vertebra. Zygosphenes and zygantra strengthen the articulations, though restricting vertical flexure. They occur, as is seen, only in reptiles with a long, flexible vertebral column,[2] and are absent in those mosasaurs in which the column is less elongate and flexuous. Zygosphenes are also known to occur in certain aquatic Stegocephalia with long, slender vertebral columns.
In certain other reptiles this arrangement is reversed, in that the wedge-shaped median process, called hyposphene (Fig. 74) is below and between the inner ends of the postzygapophyses, fitting into a cavity, the hypantrum, at the front end of the next succeeding vertebra. Hyposphenes and hypantra are especially characteristic of certain cotylosaurs, placodonts, and dinosaurs, where they were first recognized and described.
The later pterodactyls have another pair of articulating processes, called exapophyses (Fig. 73 g), at each end of the cervical vertebrae on the ventral side, their articulating surfaces facing in opposite directions to those of the zygapophyses above them. They strengthen the articulations, but limit torsion, and are substitutes for the peculiar saddle-shaped articulations of the cervical vertebrae of birds.
On the dorsal side of the arch, in the middle, is the spine or neurapophysis, of extremely variable size and length, sometimes rudimentary, sometimes very long. As a rule, the spines are longest and stoutest at the beginning of the dorsal series, for the attachment of muscles and ligaments controlling the neck and head. The spines are always short in legless or slender crawling reptiles (Fig. 73 d–f) and are never long or slender in aquatic reptiles, in front at least. The spines of most sauropod dinosaurs in front of the sacrum are broadly divided, V-shaped, doubtless for the lodgment of stout muscles and ligaments used in controlling the long neck.
A longer or shorter process on the sides of the arch for the support in part or wholly of the ribs is known as a diapophysis (Fig. 73 b, 75). A like process or facet on the side of the centrum anteriorly for articulation of the head of the rib is called a parapophysis (Fig. 73 f). Either is commonly called a transverse process, and the same term is often applied to a like process on the sides of the caudal vertebrae, of which probably the anterior ones, at least, in all cases are merely coössified ribs.
A process, paired or single, on the under side of the vertebrae, is properly called a hypapophysis (Figs. 73 e, 75 a). Hypapophyses are characteristic of snakes, often as far back as the tail; in some instances they are developed to serve as a sort of masticatory apparatus for the crushing of eggs in the stomach.[3] They also often occur on the cervical vertebrae of lizards, crocodiles, and turtles. Paired hypapophyses (lymphapophyses) are characteristic of the caudal vertebrae of snakes, where they replace the absent chevrons.[4]
When the ends of the centra are concave, as they are in all early reptiles, nearly all fishes, and most amphibians, the vertebrae are known as amphicoelous (Fig. 74). If the cavities are deeply concave, communicating with each other through the centrum, the vertebrae are called notochordal; that is, the notochord was continuous in life. And this was the primitive condition found in the Cotylosauria (Fig. 74) and Triassic Ichthyosauria and continuous to the present time in the living gecko lizards. More usually, since middle Permian times the cavities are shallow, bowl- or saucer-like, or almost flat (platycoelous) or even quite flat (amphiplatyan).
Until after the middle Jurassic times the vertebrae of all known reptiles were amphicoelous. A ball-and-socket joint appears at that time, so far as we yet know, with the concavity in front, the ball or convexity behind. This kind of vertebra, called procoelous, gradually became the prevailing one, all reptiles since early Eocene times, except the geckos among lizards, the turtles, and Sphenodon, possessing them. Procoelous vertebrae appeared among the Crocodilia in early Cretaceous times (Hylaeochampsa), amphicoelian types, however, persisting until early Eocene (Dyrosaurus). The vertebrae of all known snakes (Fig. 73 e, f), dating from Lower Cretaceous, are procoelous, as are also the presacral vertebrae of the Pterosauria, dating possibly from early Jurassic times. The caudal vertebrae of some turtles are procoelous. Procoelous vertebrae, however, are not restricted to reptiles, some modern frogs having them. They doubtless arose in terrrestrial crawling reptiles with a flexuous column, and it was doubtless from such ancestors that the aigialosaurs and mosasaurs, aquatic reptiles, inherited them. Possibly the pterodactyls acquired the ball-and-socket articulations after the attainment of flight.
The presacral vertebrae of the sauropod, as also the cervical vertebrae of many theropod and orthopod dinosaurs, have the convexity of the centrum at the front end, just the reverse of procoelous. Such vertebrae have been called opisthocoelous, and are doubtfully known in other reptiles, save the cervicals and caudals of certain turtles. They do occur, however, in the cervical region of certain Triassic Stegocephalia, and in some modern fishes and many modern salamanders.
Most remarkable are the cervical vertebrae of the Chelonia. The earliest that we know had amphicoelous vertebrae throughout the column, but most others have an extraordinary combination of all types, amphicoelous, procoelous, opisthocoelous, plano-concave, plano-convex, and even biconvex, otherwise known in only the first caudal vertebra of the procoelian crocodiles. Platypeltis (=Amyda) spinifera, a living river-turtle, has opisthocoelous cervical vertebrae, and certain pleurodiral turtles have saddle-shaped articulations. In no other order of reptiles are the variations so great as in this.
Fig. 75. Dorsal vertebra of Diplodocus (Saurischia). After Hatcher. One tenth natural size.
The pleurocoelous (Fig. 75) presacral vertebrae of the sauropod dinosaurs are peculiar in having large cavities in the centra, separated by a median partition, with an oval or round orifice at each side. Not only is the centrum thus lightened in these dinosaurs, but the arch is curiously strengthened by plates and buttresses. Certain other South African reptiles (Tamboeria) also have pleurocoelous vertebrae. It is supposed that this hollowness and lightness of the cervical and dorsal vertebrae, correlated with the otherwise solid or cancellous skeleton, served to keep the body erect in water, their natural habitat in wading or swimming.
Except in the snakes and legless lizards, where but two regions are recognized, the caudal and precaudal, the spinal column of reptiles is divisible into cervical, dorsal, sacral, and caudal regions, and sometimes lumbar also, as in mammals. The cervical region is that in front of the shoulder-girdle, the dorsal that between the shoulder and hip girdles, the sacral that which supports the hip girdle, and the caudal that of the tail.
We may hardly venture to guess as to the primitive number of vertebrae in reptiles. We are quite sure that there has been an increase in number in some, a decrease in others. The land temnospondylous amphibians that we know have but one real cervical vertebra, the so-called atlas, twenty-two to twenty-five dorsals, one or two sacrals, and a short or moderately long tail. Trimerorhachis, an aquatic Lower Permian temnospondyl, has thirty-one precaudal vertebrae and no differentiated sacrals. The earliest reptile that we know, Eosauravus, subaquatic in habit, had at least twenty-four or twenty-five precaudals, two sacrals, and a long tail. In no embolomerous amphibian is the number of vertebrae known.
The numbers of presacral and sacral vertebrae in reptiles may be tabulated as follows:
| | Presacral | | Sacral | |
| Cotylosauria | 23–26 | 1–3 | ||
| Chelonia | 18 | 2–3 | ||
| Theromorpha | 23–27 | 2–3 | ||
| Therapsida | 25–28 | 2–7 | ||
| Nothosauria | 40–42 | 2 | ||
| Plesiosauria | 40–105 | 3–4 | ||
| Proganosauria | 29–34 | 2 | ||
| Ichthyosauria | 40–65 | 0 | ||
| Sauranodon (Saphaeosaurus) | 22–23 | 2 | ||
| Kionocrania (Lacertilia) | 22–74 | 0–2 | ||
| Rhiptoglossa | 16 | 2 | ||
| Dolichosauria | 29 | 2 | ||
| Mosasauria | 29–42 | 0 | ||
| Rhynchocephalia | 25 | 2 | ||
| Rhynchosauria | 23–24 | 2 | ||
| Choristodera | 23–26 | 2 | ||
| Pseudosuchia | 23–26 | 2 | ||
| Phytosauria | 26 | 2 | ||
| Eusuchia | 23–24 | 2 | ||
| Thalattosuchia | 25 | 2 | ||
| Theropoda | 23 | 2–5 | ||
| Sauropoda | 26 | 4–5 | ||
| Stegosauria | 27 | 3–4 | ||
| Trachodontia | 30–34 | 8–9 | ||
| Iguanodontia | 24–28 | 4–5 | ||
| Ceratopsia | 24 | 7 |
The earliest reptiles had functional ribs and a sacrum, and we may omit the very variable tail in our comparisons. The majority of terrestrial reptiles, it is seen, have between twenty-three and twenty-six presacral vertebrae. In all probability the earliest reptiles were lowland and crawling in habit, and it is legitimately presumable that they had not less than twenty-three nor more than twenty-six vertebrae in front of the sacrum, a single sacral, and not more than sixty caudals, the largest number found in any early reptile, or altogether between eighty and ninety vertebrae in the whole column, as against thirty-five in modern turtles and four hundred and fifty in some modern snakes. The smallest number of presacral vertebrae known in any reptile—sixteen—is recorded for Brooksia, a recent chameleon lizard.
Intercentra. The earliest reptiles probably all have a small or vestigial, more or less wedge-shaped bone intercalated between the adjacent ventral margins of the centra throughout the column, to which Professor Cope in 1878 gave the name intercentrum (Fig. 76 e). Intercentra had previously long been known as "intervertebral" or "subvertebral wedge-shaped bones," but their significance was ill understood. With the more complete ossification of the vertebral centra they began to disappear in the dorsal region, in early or middle Permian times, but have remained to modern times in the gecko lizards and in Sphenodon. They have persisted in nearly all reptiles in the tail as the chevrons, and more or less in the neck, having been entirely lost as simple intercentra only in the crocodiles and a few other reptiles. The intercentrum of the first vertebra has remained functional in all Amniota as the basal piece or "body" of the atlas.
Intercentra are characteristic of deeply amphicoelous or notochordal dorsal vertebrae, that is, in the more primitive vertebrae, and never occur in procoelian, amphicoelian, or opisthocoelian reptiles. They occur in many procoelous lizards throughout the neck, often in their normal places between the centra but frequently shifted forward on the preceding centrum, either loosely attached or coössified with an exogenous outgrowth, forming with it a functional hypapophysis. Where they occur between the centra they may be elongated 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, Pennsylvanian, 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, notochordal 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 articular 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 intercentrum 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 articulates with the intercentrum, the tubercle with the diapophysis of the neurocentrum.
The earliest known amphibian vertebrae are embolomerous; rhachitomous 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, giving 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 ontogenetic condition is the chief objection to this theory, nevertheless it is the more probable. We have seen that the more primitive phylogenetic 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 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 (Cotylosauria), 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 furnished by the fact that while truly embolomerous vertebrae occur in fishes, in the modern Amia, for instance, real rhachitomous vertebrae are known only among amphibians. Certain ancient fishes (Eurycormus), 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 decrease 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 vertebrae. The Cotylosauria, like the Temnospondyli, have but one or two vertebrae which may properly be called cervical, since the pectoral 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.
The Theromorpha have a longer neck, with at least six and probably seven vertebrae (Fig. 77), as shown by the lengths of the ribs, by the diapophyses, and more definitely by the position of the scapula and clavicles as observed in various specimens. These numbers, six or seven, are those given for the Therapsida, as this order is imperfectly known, and seven is the number that has remained so persistently in their descendants, the mammals. Modern chameleons have but five; true lizards, the Chelonia and Rhynchocephalia, eight; the monitor lizards, Crocodilia, Theropoda, Iguanodontia, and 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.
Fig. 77. Notochordal cervical vertebrae, with intercentra, of Ophiacodon, a primitive theromorph: pa, proatlas; an, arch of atlas; o, odontoid; ax, axis.
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, sixteen 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 habits.
Fig. 78. Ophiacodon. Proatlas, axis, and ribs.
The marine crocodile, with a fin-like tail, lost two, like the mosasaurs and aigialosaurs, having seven; Pleurosaurus 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 inconstant 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 processes, or the slenderness and mode of rib articulation. The cervicals of the later pterodactyls have additional articulations on their ventral sides, as has been described above (p. 91).
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, pleurocentrum (odontoid); n, neurocentrum (arch).
Proatlas. The proatlas (Figs. 79 c, 80 d, l) is a small, more or less vestigial 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 between 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 centrum of the proatlas is the so-called intercentrum of the atlas, necessitating the view that the axial intercentrum is merely an accessory 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 Sauropoda, 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 Rhamphorhynchus, a Jurassic pterosaur. It has also been reported in the chameleon 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 eventually discovered in many other extinct reptiles.
Atlas (Figs. 78, 79, 80). There is no vertebra in the known amphibians 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 
Fig. 80. Atlas, axis, and ribs: A, Trinacromerum (Plesiosauria); B, Platecarpus (Mosasauria); C, Baptanodon (Ichthyosauria), after Gilmore; C', Cymbospondylus (Ichthyosauria), after Merriam; D, Sphenodon (Rhynchocephalia); E. Nyctosaurus (Pterosauria); F, Champsosaurus (Choristodera), after Brown; G, Gavialis (Crocodilia); H, Enaliosuchus (Crocodilia), after Jaekel; I, J, Diplodocus (Dinosauria), after Marsh; K, Camptosaurus (Dinosauria), after Gilmore; L, Iguanodon (Dinosauria), after Dollo; M, Chrysemys (Chelonia); N, Iguana (Lacertilia); O, Trinacromerum (Plesiosauria); P, Apatosaurus (Dinosauria), after Riggs.
intercentrum are fused into a ring, which revolves about its pleurocentrum, 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.
Fig. 81. Atlas and axis of Diplodocus (Saurischia). After Holland. One fourth natural size.
In no reptile did the atlas attain the specialization 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 rotation about it, not more than between the axis and the following vertebra. This lack of torsion, in most reptiles at least, was compensated for by the ball-and-socket joint between the single condyle and the atlas, lost in mammals.
In the primitive Ophiacodon (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 notochord, as in the embryonic cartilage of mammals. The pleurocentrum 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 Rhynchocephalia (Fig. 80 d), Choristodera (Fig. 80 f), and Phytosauria it is yet largely visible from the side, but the first and second intercentra have become contiguous below it. In the Crocodilia (Fig. 80 g) and Chelonia (Fig. 80 m) the pleurocentrum still retains its primitively large size, reaching the ventral side, doubtless because of the loss, fusion, or great decrease in the size of the axial intercentrum. 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 between 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-hypapophyses, 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 plesiosaurs and pterodactyls the atlas and axis are fused, indistinguishably so in the adult; both are slender-necked animals with small or vestigial cervical ribs. In the short-necked Ichthyosauria the atlas 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.
Dorsal Vertebrae
(Fig. 82)
Fig. 82. Ophiacodon mirus Marsh (Theromorpha). Seventh to twentieth vertebrae, from the side.
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 latter order three or more immovably united for the support of the pectoral arch, forming the notarium. In the Chelonia they are fused throughout in the carapace. 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 plesiosaurs. In terrestrial reptiles the number never exceeds twenty-two or twenty-three and is usually about eighteen. In reptiles lacking a sacrum the number between the girdles may be much greater,
Fig. 83. Sacrum and caudal vertebrae of Macrochelys (Chelonia), seen from below.
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 Cotylosauria, 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)
Fig. 84. Sacrum and tail vertebrae of Varanosaurus (Theromorpha).
number, two, has remained persistent in most reptiles and even most mammals to the present time. A third vertebra, from the caudal 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 sometimes 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 reptiles; 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 vertebrae distally are more slender and the zygapophyses weak. One of the first indications of swimming habits, at least in those reptiles 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 without 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 centrum in many lizards, the Proganosauria Saphaeosaurus and Sphenodon, 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 intercentra (Fig. 76 d), occur below and between the caudal centra in most reptiles. Usually single and Y-shaped—whence the name chevron—they may be paired in the Plesiosauria and Ichthyosauria. The medial ones of the Sauropoda have two Y-shaped, broadly divergent branches united at their base. More or less vestigial in the turtles, they are absent in snakes, replaced by a pair of vertical hypapophysial-like processes (lymphapophyses).
Chevrons articulate as a rule intercentrally, but sometimes exclusively to the distal part of the preceding centrum with which they may be coössified, as in some mosasaurs and lizards, especially those in which the cervical intercentra have migrated forward to articulate or be coössified with the median hypapophysis. Chevrons primitively, as in the temnospondyl amphibians, have their branches united above in an intercentrum-like bone, a condition found in the proximal chevrons of Sphenodon. In later reptiles, for the most part, the two branches articulate separately. At the tip of the tail they are vestigial or absent.
- ↑ [For the modern embryological viewpoint of the composition of reptilian vertebrae see Schauinsland, in Hertwig's Handbuch der Entwickelungsgeschichte der Wirbeltieren, etc., 1906.—Ed.]
- ↑ [Exceptions to this rule occur in recent lizards.—Ed.]
- ↑ [The eggs are cut, not crushed, and in the oesophagus, not the stomach (Fitzsimons).—Ed.]
- ↑ [The distinctions between lymphapophyses and hypapophyses break down in the embryology of modern lizards.—Ed.]
- ↑ [Also some frogs.—Ed.]