# The Osteology of the Reptiles/Chapter 3

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CHAPTER III
THE RIBS AND STERNUM
The ribs of reptiles, like those of the amphibians, primitively articulate with all vertebrae, at least as far back as the middle of the tail. The first to become fixed or closely united with the vertebrae, after the sacral, were the caudal, next the lumbar, and last of all the cervical. The dorsal ribs are free, except in the Chelonia, some Pterosauria, and some armoured dinosaurs.
Fig. 86. Vertebrae and ribs: A, Clidastes (Mosasauria), posterior cervical vertebra, from behind; B, Cymbospondylus (Ichthyosauria), anterior dorsal vertebra, from the side, after Merriam; C, Ichthyosaurus (Ichthyosauria), middle dorsal vertebra, from the side (after Broili); D, Dimetrodon (Theromorpha), anterior dorsal rib; E, Diadectes (Cotylosauria), anterior dorsal rib.
The ribs of the Temnospondyli (Fig. 86) articulate with intercentrum and arch, usually without differentiation of the articular surfaces. And this was the original mode among reptiles. With the diminution in size of the intercentrum, the head, or capitulum, joins the adjacent ends of two centra across the intervertebral cartilage, the articular surface, however, continuous to the tubercle, which articulates with the end of the diapophysis. This continuous articulation from the intercentral space to the arch was the almost ​invariable rule among the Cotylosauria (Fig. 86 e) and occurs occasionally in the Theromorpha and even in the recent Sphenodon. Such ribs, though usually called single-headed, are not really so since both capitulum and tuberculum are present, though connected. A better name for them is holocephalous. Soon, however, the articular surfaces become restricted to the head and tubercle, that is, there is an emargination of the articular surface between them, the so-called neck, and the rib is truly double-headed, or dichocephalous (Fig. 86 d). Strictly speaking, single-headed ribs are those which have lost either the head or the tubercle.
Fig. 87. Dorsal vertebra of Thalattosaurus. (After Merriam.) This early mode of articulation of double-headed ribs, the head across the intervertebral cartilage, the tubercle to the diapophysis of the arch, has continued through those reptiles [see above] and through the mammals. And this is essentially the mode of rib articulation in the Diaptosauria.
In many reptiles, however, perhaps in part because of the closer articulation of the vertebrae, the head has migrated backward to articulate with a facet or process on the anterior end of the centrum, the parapophysis, and there it has remained in the cervical vertebrae of most reptiles, and in the dorsal vertebrae of the Squamata and their allies. In the dorsal region there have been many modifications. In those reptiles which are here classed under the Parapsida, that is, the Ichthyosauria, Proganosauria, Pleurosaurus, and Squamata (Figs. 80 n, 73 c–f), the tubercular part of the articulation has been largely or wholly lost, and the single-headed ribs remained attached more or less wholly to the centrum. In the later Ichthyosauria and later Plesiosauria, it is true, the ribs are often dichocephalous (Fig. 86 c), both articulations uniting with the centrum. There is, however, in such forms no real tubercle. The ribs of Araeoscelis, a Lower Permian reptile, are single-headed and central in the cervical region, imperfectly double-headed in the dorsal region. So also, the ribs are described as single-headed in Pleurosaurus, Protorosaurus, the Proganosauria, and Thalattosauria (Fig. 87), probably all with a single, typical, upper temporal opening. The dorsal ribs (Figs. 80 o, 89) of the ​Sauropterygia articulate with the diapophysis (d) exclusively by a single head, the cervical ribs (Fig. 89 r) exclusively with the centrum, usually also by a single head.
The dorsal ribs of the Archosauria, that is, the Pseudosuchia, Parasuchia (Fig. 88), Crocodilia (Fig. 90), Dinosauria (Fig. 80 p, and Pterosauria, are double-headed, the anterior ones at least, but both articulations are with the arch or diapophysis. And this mode of articulation would seem to exclude their immediate ancestral relationship to the birds, Fig. 88. Dorsal vertebra of phytosaur: az, anterior zygapophysis; pz, posterior zygapophysis; d, c, articulations of rib. in which the head of the ribs articulates with the centrum throughout.
Atlantal ribs, present in all early reptiles, have been lost in modern ones, except the Crocodilia, where they are attached exclusively to the intercentrum, in the ancient Metriorhynchus to both arch and intercentrum. In the Dinosauria, some of them at least, the first intercentrum bears a small rib (Fig. 81).
Axial ribs are more often present, but are lost in not a few reptiles, articularly the Pterosauria (Fig. 80 e) and Chelonia (Fig. 80 m). In early crocodiles the axial rib articulated with diapophysis and parapophysis; in later crocodiles the diapophysial articulation is lost, though a vestige often remains, and the single headed rib has migrated forward on the odontoid.
The dorsal ribs of the Eunotosauria and all Chelonia have expanded to meet or fuse with each other, forming more or less of a carapace (Fig. 91). Peculiarly expanded and overlapping ribs in the posterior dorsal series occur in some of the Theriodontia. In Cynognathus the thirteenth to the seventeenth ribs shorten rapidly and project widely with a remarkable expansion near the proximal end, which overlaps the succeeding rib in a concavity on its anterior border. In the lumbar series (Fig. 92) they lose the free portion of the shaft, ending in wide, ​interlocking ends. Such ribs gave great strength to the lumbar region, and are perhaps analogous to the greatly expanded transverse processes of the crocodiles.
Fig. 89. Plesiosaur vertebrae: Polycotylus. Cervical vertebrae from the side and behind, and dorsal vertebrae from in front: az, anterior zygapophysis; pz, posterior zygapophysis; r, r, r, cervical ribs; d, articulation of dorsal rib.
Fig. 90. Vertebrae of gavial from the side (cervical), and from in front (dorsal): az, anterior zygapophysis; pz, posterior zygapophysis; d, diapophysis; r, cervical rib; c, articulation for head; t, for tubercle of dorsal rib.
The first four to six ribs of the Cotylosauria, and rarely also of the Theromorpha, are progressively flattened and dilated, sometimes, as in Diadectes and Limnoscelis (Fig. 95), remarkably so, for the direct support of the short and broad scapula. Not only are these ribs so remarkably dilated in Diadectes, but, continuing the expansion backward, there are three flat dermal plates overlying the following ribs.
The cervical ribs of the Crocodilia (Fig. 90) and Dinosauria are ​short and more or less "hatchet-shaped," either fused or more or less closely united to arch and centrum. The free cervical ribs of lizards and mosasaurs begin upon the axis. Only vestiges of ribs remain in the pterodactyls and turtles; they are nearly always fused. Three or four of the single-headed ribs of the Sauropterygia articulating with both centrum and arch are known as pectoral ribs.
Fig. 91. Inner side of carapace of Stegochelys (Chelonia). After Jaekel. About one sixth natural size.
In certain early cotylosaurs (Figs. 128, 164), four or five vertebrae in front of the sacrum bear no ribs of any kind; in others, Seymouria (Fig. 1) for instance, free ribs continue to the sacrum. Many other reptiles have a variable number of the presacral ribs coössified with the centrum, or centrum and arch, so-called transverse processes.
Fig. 92. Vertebrae: Cynognathus (Cynodontia), posterior dorsal vertebrae, from above. After Seeley. One half natural size.
Fig. 93. Nothodon lentus Marsh (Cotylosauria). Sacral vertebra, from in front. Natural size.
​arch. Real sacral ribs have been in all cases added from behind, since the caudal ribs have retained more or less in all animals their original attachments, while the lumbar or posterior dorsal ribs have often undergone changes. It is improbable that there has ever been any "migration" of the sacral vertebrae; that is, the first true sacral vertebrae of all animals are identical with the single sacral vertebra of Seymouria. Additional sacral ribs have been due to the gradual elongation of the basal caudal ribs and their articulation with the ilium, as shown in the tail of the alligator snapper turtle. The second and third pairs were added very early in the history of reptiles.
Remarkably, in the Lacertilia evidences of sacral ribs have not been found, the ilia being supported by transverse processes, outgrowths of the centra (Moodie).
Not only are the two sacral ribs of the Crocodilia (Fig. 121) primitive in their attachments, but the centra also have retained their primitively amphicoelous structure.
On the other hand, additional vertebrae have joined the sacrum in front, as many as three in some reptiles, but in such cases the ribs have not reverted to their primitive attachments if modified, though they may extend to the ilium. In the Ceratopsia three lumbar vertebrae have been fused with the sacrum, and their diapophyses with the ilium. Indeed in some instances (Monoclonius for instance) a vestigial free rib may remain on the first, so-called sacral vertebra. In the later pterodactyls there are several such sacro-lumbar vertebrae, and also in the Anomodontia (Fig. 119 c), groups that have been accredited with from seven to ten sacral vertebrae. In all these the ilium is greatly prolonged in front of the acetabulum. The projections from the vertebrae have often been called indiscriminately transverse processes, but that term is true only of the sacro-lumbars.
Whether or not the dinosaurs acquired the third or the fourth sacral vertebrae after their divergence from their immediate ancestral stock is perhaps a question. But two are accredited to Hallopus, a primitive type. There can be no question, however, but that the dinosaurs, both the Saurischia and the Ornithischia, descended from reptiles with but two sacral vertebrae, since the allied Crocodilia ​and Pseudosuchia have but that number, and since doubtless the Diapsida began with but two.
Many temnospondylous amphibians have on the hind border of the dorsal ribs an angular uncinate process, like that so characteristic of birds. Such ossifications have never been observed among the older reptiles. They occur in the Jurassic Homoeosaurus and the modern Sphenodon of the Rhynchocephalia; imperfectly ossified processes also occur in the Crocodilia. In many other reptiles they doubtless remained unossified, and in much probability will yet be found in other reptiles as fused processes or separate ossifications.
Ventral or Abdominal Ribs
Many, perhaps most, of the Stegocephalia, especially the Branchiosauria, had on the under side of the body an armature of bony rods, or plates, of various forms, called by Baur gastralia, by Gegenbaur the parasternum, and ordinarily known as ventral or abdominal ribs. They are arranged in a V-shaped pattern with the apex in front, and may sheath the whole under side of the body, extending on the limbs. In some cases, exterior to these a distinct armor of dermal plates has been observed. Among the temnospondylous amphibians they occur more rarely. In Cricotus, an ambolomerous type, they sheath closely the under side of the abdomen, each composed of an unpaired median piece, and numerous lateral ones. Among the Rhachitomi they have been observed in Archegosaurus in the shape of slender rods. They are unknown in the Stereospondyli.
Ossified parasternal ribs, in greater or lesser numbers and complexity, occur in some members of every chief group of reptiles, though far from constantly in each group. Among the Cotylosauria they are known in three families, the Sauravidae, Captorhinidae, and Procolophonidae; they are certainly absent in some, if not many, of the known genera of the order. They have been observed in only a few of the Theromorpha, and are certainly absent in some of the families. They are known in Galechirus of the Dromasauria; among the Proganosauria; Protorosaurus, Kadaliosaurus, Pleurosaurus, and Saphaeosaurus of the subclass here called the Parapsida; in Aigialosaurus and some recent lizards; in the Choristodera, Homoeosaurus, Hyperadapedon, and Sphenodon of the Rhynchocephalia; in the Pseudosuchia (Scleromochlus), Phytosauria, Pterosauria, Crocodilia, ​and Theropoda, of the Archosauria; in the Sauropterygia, Ichthyosauria, and Chelonia. They are thus, it is seen, characteristic of the Reptilia as a whole, though frequently absent in forms related to those which possess them. No explanation has yet been given of their inconstancy.
Fig. 94. Sternum and parasternum: A, Theropleura (Theromorpha). About one half natural size. B, Sphenodon (Rhynchocephalia). Three halves natural size. C, Champsosaurus (Choristodera). One half natural size. D, Lystrosaurus (Anomodontia). One half natural size. E, Nyctosaurus (Pterosauria). Nearly one half natural size.
The most primitive parasternals known among reptiles are those of the Cotylosauria and Theromorpha (Fig. 94 a), slender, bony rods composed of several pieces on each side, meeting in a median, unpaired, V-shaped piece. They sheath closely the whole under side of the abdomen from the coracoids to the pelvis, twelve to fifteen times as numerous as the overlying vertebrae, and more than two ​hundred in number. Anteriorly they are covered or underlaid by the distal end of the interclavicle. In the modern Sphenodon (Fig. 94 b) there are about twenty-four such rods, each composed of a median, unpaired piece and a lateral splint, every alternate one of the first eleven attached to the end of a dorsal rib. In the Phytosauria they are similar, nineteen or twenty in number. The Choristodera (Fig. 94 c), Plesiosauria, and Ichthyosauria, aquatic reptiles, have larger and stouter parasternals, consisting of a straight or slightly curved median piece, and three or four lateral splints on each side. The Crocodilia (Fig. 121 c) have seven or eight pairs, each composed of two slender rods on each side (not joined in the middle). In earlier members of the order there was a larger number, and some of them, at least, were composed of the usual V-shaped median piece and a lateral splint on each side. The last pair is enclosed in a dense sheath of fascia continuous with the ends of the so-called pubes.
Among the modern lizards abdominal ribs are often present, especially in the chameleons, each composed of one broadly V-shaped piece, either connected with the dorsal ribs or free, sometimes paired and usually cartilaginous. Only in a few forms have they been observed as slender ossifications. Clearly endoskeletal in origin, they have been supposed to be not true parasternals, and have been called distinctively abdominal ribs. That they are not continuations of the dorsal ribs seems evident from the fact that they are sometimes much more numerous than the overlying vertebrae. These lacertilian ribs are located, it is said, in the rectus abdominis muscles. The parasternals of Sphenodon are in the superficial part of the rectus and external oblique muscles, and are united by a dense sheath of fascia.
The later pterodactyls have five or six flattened parasternals, the anterior ones broadly V-shaped, the posterior ones paired. In the earlier pterodactyls the unpaired median piece has one or two lateral splints. They have also been observed in various genera of theropod dinosaurs. In the Chelonia they are represented by the posterior three pairs of plastral elements, as usually accepted, but it is possible that these are really dermal elements and [not] true parasternals. The extinct Saphaeosaurus (Sauranodon) had a full armature of ossified parasternals similar to those of Sphenodon.
Parasternal ribs have long been considered to be of dermal origin, ​skin bones which have sunk into the muscles. The abdominal ribs of the lizards are undoubtedly true endoskeletal bones, and Fürbringer has suggested that in these animals they are new growths, supplanting the dermal parasternals which have long since disappeared, and that they represent the ends of the dorsal ribs, or outgrowths from them.
That they and the sternum to which they are supposed to have given origin are really the ends of true ribs is improbable, since no other tetrapods are known in which the dorsal ribs meet on the under side of the body, or even approach each other. It would seem more reasonable that the abdominal ribs of all reptiles are of parenchymatous or cartilaginous origin, and that the anterior ones fused to form the sternum. The so-called sternum of the modern amphibians (there was no sternum of any kind in the Stegocephalia) is an ossification of the myocomata, not derived from the dorsal ribs, and is thought not to be homologous with the sternum of reptiles.
Sternum
The earliest recorded occurrence of a sternum or breastbone in reptiles is in the Anomodontia (Fig. 94 d) where, according to Broom, it is generally present and ossified. It is figured in Keirognathus as a small, subquadrilateral bone lying over the posterior extremities of the coracoids and distal end of the interclavicle. Only rarely does it occur as an ossification in other reptiles, the best examples of which are the Pterosauria (Fig. 94 e) where, as a broad, shallow concave bone, it covers the whole under side of the thoracic region with a stout manubrium-like process in front, but without a true keel. On either side of the base of the median anterior protuberance it gives articulation to the elongate coracoid. Its lateral margins have articular facets for four or five, sometimes ossified, sternal ribs. Posteriorly in the middle it is contiguous with the parasternal ribs.
In many reptiles the sternum is wholly absent, even as a cartilaginous element. There was no space, even for a rudimentary one, in the Ichthyosauria and Sauropterygia back of the united coracoids and in front of the parasternals. It has been thought that its absence in these orders is due to its loss; it is more probable that their ​ancestors never possessed it. There could have been no sternum, even a small cartilaginous one, among the Cotylosauria and Theromorpha, since in several instances in both these orders the parasternals have been found sheathing the whole abdomen from the coracoids to the pelvis. Probably this was the condition in all the early reptiles; probably also the condition in the early Rhynchocephalia, since Rhynchosaurus had parasternal ribs reaching nearly to the coracoids, leaving little or no space for a sternum.
In the modern Lacertilia (Fig. 99) and in Sphenodon there is a more or less calcified, thin, rhomboidal plate articulating on each side with the coracoid in front and ending in a single or paired continuation, the xiphisternum. It gives articulation on each side to from one to four or five, exceptionally more, sternal ribs, also cartilaginous. Rarely, no ribs articulate with the sternum, and the sternum itself may be represented by a pair of small cartilaginous plates or may be wholly absent. Sometimes it has single or paired perforations. Similar cartilaginous sterna have been found in the Dolichosauria and Mosasauria, and doubtless it was present in most of the extinct members of the order.
In the Chelonia there is no trace of a sternum. In the living Crocodilia the sternum is a small, oval, cartilaginous plate, continued into a pair of cartilaginous xiphisternal rods to which six or eight dorsal ribs are attached by the intervention of cartilaginous sternal ribs. Nothing is known of the sternum in extinct crocodiles or phytosaurs; probably it was present as a cartilage.
The sternum has been found in not a few dinosaurs. Among the Sauropoda it has been recognized in a pair of oval, ossified plates.
* ↑ [For the morphology and variation of the ribs, in connection with the segmentation of the vertebrate body, see Bütschli, 1921, Vorlesungen über Vergleich Anat.; Kingsley, Compar.-Anat. Cert.—Ed.]
* ↑ [This leaves out of account the costal plates which enter into the formation of the carapace. See Gadow, "Reptiles and Amphibia," Cambridge Nat. Hist.; Procter, 1923, Proc. Zoöl. Soc.—Ed.]
* ↑ [But few contemporary morphologists would endorse this view. It certainly does not apply to the Amphibia and is very doubtful for the Chelonia.—Ed.]
* ↑ [For further support of this view, see C. L. Camp, 1923, Bulletin, Amer. Mus. Nat. Hist., vol. XLVIII, pp. 389–393]