CHAPTER IV
THE PECTORAL AND PELVIC GIRDLES
The Pectoral or Shoulder Girdle
(Figs. 95–113)
Fig. 95, Diadectes (Cotylosauria). Pectoral girdle, right side.
Those bones which form the framework for the support of the anterior extremities in vertebrate animals are collectively called the pectoral or shoulder girdle. In our own skeleton, as in that of most mammals, there are but two on each side, the scapula, or shoulderblade, and the clavicle, or collar-bone. A third bone, or possibly two, on each side, are represented in most mammals by mere vestiges, which early unite with the scapula to form the coracoid process. In the lowest living mammals, of which Ornithorhynchus and Echidna are the only examples, there are in addition to the clavicles three well-developed bones on each side, the scapula and two bones articulating with it at its lower end, the anterior of which, originally named epicoracoid by Cuvier, is generally known as the procoracoid; the posterior one helping to form the articulation for the arm bone, known as the true coracoid.
Fig. 96. Pectoral girdles: A, Cacops (Temnospondyli), from above. One half natural size. B, Seymouria (Cotylosauria), from below. One half natural size. C, Diadectes (?), from below. One half natural size. D, Varanops (Theromorpha), from above. One half natural size.
The homologies of these, or rather of the epicoracoid, are yet doubtful, and will be discussed later. There is also a median, unpaired bone in these mammals, the interclavicle, unknown in other mammals.
Primitively (Figs. 95, 96), that is, in the oldest known reptiles, the pectoral girdle is composed of eleven separate and distinct bones, at least in early life: the median interclavicle and a clavicle and cleithrum on each side, all five of dermal origin, together composing the secondary or clavicular girdle; and three bones on each side, the scapula and two coracoids,[1] all of endoskeletal origin, composing the primary or scapular girdle.
The cleithrum (Fig. 95), a relic from the fishes, disappeared in Triassic times, after long existence as a mere vestige. The posterior of the two coracoids also disappeared in late Triassic times, in reptiles at least, though a vestige may possibly be present in our own shoulder girdle. The scapula, clavicles, and anterior one of the two coracoids, the so-called procoracoid, are still present in most reptiles; in snakes only are they wholly absent, though much reduced and non-functional in some lizards.
Clavicular Girdle
The clavicular girdle is variable among the temnospondyl amphibians, dependent, as in reptiles and higher vertebrates, upon the habits of the animals. In the aquatic types of all Stegocephalia the clavicles and interclavicles are rugose [on the ventral side], heavy and broad, forming more or less of a pectoral buckler—a peculiar adaptation to their water habits, perhaps in a measure analogous to the plastron of the turtles or the extraordinary development of the coracoids in the plesiosaurs. In such forms also, the cleithrum is reduced. The girdle in the adult land forms, of which Eryops (Fig. 108) and Cacops (Fig. 96 a) may be taken as types, is almost indistinguishable from that of their contemporary cotylosaurs, except that the cleithrum is larger and the interclavicle less elongate. They are smooth throughout in Cacops, the more terrestrial form.
Cleithrum. The cleithrum so generally characteristic of the Stegocephalia (Figs. 96 a, 108) was doubtfully ever functional in reptiles, whatever may have been its function in the amphibians; and it was never large. It is known only in certain members of the Cotylosauria, Theromorpha, Dinocephalia, and Anomodontia, best developed perhaps of all in Diadectes and its allies of the Cotylosauria (Fig. 95), where its somewhat spatulate upper extremity partly overlies the front, upper border of the scapula, articulating below with the stem of the clavicle. It is vestigial in some forms and seems to be quite wanting in others. Among the Theromorpha it has been observed in Edaphosaurus (Fig. 98) as a rod-like bone at the upper front border of the scapula. In the Anomodontia and Dinocephalia (Fig. 107 d) it is a feeble splint, clearly a vestige. There have been several theories as to what has become of it, but none is demonstrable. Its vestigial condition in various cotylosaurs indicates its entire disappearance.
Fig. 97. Clavicles and interclavicle of Ophiacodon
(Theromorpha).
Clavicles. Clavicles are usually present in reptiles. They are absent in the Crocodilia, serpents, Mosasauria, and some Sauria; more or less vestigial in some lizards; and either absent or vestigial in the Pterosauria and Dinosauria.
In crawling reptiles (Figs. 96 b–99) they are usually curved bones, with a dilated mesial extremity, articulating on the ventral side of the end of the interclavicle; and a more or less slender stem which articulates with the front border of the scapula, or its acromion when present, and also with the lower end of the cleithrum when that bone is present. In modern lizards the clavicles articulate usually with the front border of the cartilaginous suprascapula (Fig. 99). The inner end in some lizards is broad and perforated (Fig. 99 c).
Fig. 98. Edaphosaurus novomexicanus (Theromorpha). Pectoral girdle, two fifths natural size: c, cleithrum; cl, clavicle; sc, scapula.
The clavicles of the Chelonia are known as the epiplastra of the plastron (Fig. 100). In the Nothosauria (Fig. 101) they are normal but very stout, firmly united with the scapula and with each other. The clavicles of the Plesiosauria (Fig. 102) are remarkable in some respects. Usually they are a pair of thin, triangular bones, lying upon the inner or visceral surface of the proscapular process of the scapula (corresponding to an acromion), of the interclavicle and sometimes also of an anterior process from the coracoid; they may be absent. In the Ichthyosauria (Fig. 103), they are slender, sometimes coössified with each other; nor are they expanded mesially in either the Phytosauria or Choristodera (Fig. 104), and all water reptiles. Doubtful vestiges of the clavicles have been reported in the pterodactyls.
Fig. 99. Pectoral girdles (Lacertilia): A, B, Iguana; C, Zonosaurus (after Siebenrock). Natural size.
Fig. 100. Primitive chelonian pectoral girdle: Stegochelys.
After Jaekel.
Fig. 101. Pectoral girdle of Nothosaurus (Nothosauria), from photograph by E. Fraas: icl, interclavicle; cl, clavicle; sc, scapula; cor, coracoid.
Fig. 102. Pectoral girdle of Trinacromerum (Plesiosauria), from above: ic, interclavicle; cl, clavicle; sc, scapula; c, coracoid.
Fig. 103. Pectoral girdle of Ichthyosaur, Baptanodon (Ophthalmosaurus).
After Gilmore.
Fig. 104. Pectoral girdle of Champsosaurus (Choristodera).
After Brown.
Interclavicle. The interclavicle in the earliest-known reptiles (Fig. 96 b, c, d)
Fig. 105. Pantylus (Cotylosauria): interclavicle (icl) and coracoid (cor). Natural size.
is an elongate bone with a dilated but not T-shaped anterior extremity. The stem underlies the approximated mesial borders of the coracoids, usually extending beyond them. In a specimen referred to Pantylus (Fig. 105), a primitive cotylosaur, the interclavicle is forked in front and somewhat fan-shaped behind, shaped very much like that of the monotremes. In the later cotylosaurs the front end is more dilated, as usual with all later reptiles. In the known forms of the Therapsida (Fig. 107 c) the shape is usually like that of the Theromorpha and Cotylosauria. It is very short and fan-shaped in Lystrosaurus of the Anomodontia (Fig. 94 d), where Broom attributes its reduction to water habits.
In the Chelonia it is the entoplastron (Fig. 100.) In the Crocodilia (Fig. 121 d) and Mosasauria it is slender and free at the anterior end. The stem is short in the Ichthyosauria (Fig. 103), vestigial in the Nothosauria (Fig. 101). When present in the plesiosaurs it is an oval or triangular bone, in the earlier forms imperforate, in the later ones with a median interclavicular notch or foramen (Fig. 102). The interclavicle is absent in the Pterosauria, Dinosauria, chameleon lizards, and some plesiosaurs.
Scapular Girdle
The scapular girdle, or scapulo-coracoid of the aquatic temnospondyl amphibians of early Permian times, like that of the aquatic reptiles, is broad and short, but that of the terrestrial types is practically indistinguishable from the girdle of the contemporary reptiles. Each side, in both the amphibians and early reptiles, is composed of three bones more or less closely fused: a dorsal one, the scapula, and two ventral ones; the anterior one commonly called the procoracoid; and a posterior one, often called metacoracoid. The posterior bone was lost in all reptiles by the close of Triassic times.[2]
The three bones of the land Stegocephalia (Figs. 96 a, 108) are so firmly coössified that their sutural distinctions have rarely been observed. Among the Cotylosauria (Fig. 96 b, c) the union was less firm, or became invisible later in life; their sutural divisions have occasionally been observed. Among the Theromorpha, the posterior coracoid, the metacoracoid, is often found separated (Fig. 106), or united by a loose suture; in some forms (Fig. 96 d) it remained cartilaginous throughout life, and in all forms it probably did not ossify till growth was far advanced. Among most of the Therapsida the three bones (Fig. 107 a, b, d) fuse in maturity, but not in some, if not all, the Dinocephalia (Fig. 107 d). In the Proganosauria the division between the two bones, if present, has never been observed. In the Eunotosauria of the Upper Permian, the two bones are distinct. In no other reptile has the metacoracoid been certainly observed, though it has been affirmed in the Rhynchocephalia (Hyperodapedon), an error.
Fig. 106. Dimetrodon (Theromorpha): scapula (sc), coracoid (cor), and metacoracoid (mcor).
It is probable that the three bones early acquired a firm union, both ontogenetically and geologically, and that there was a progressive separation and delayed ossification of the posterior bone in the line leading toward the modern reptiles at least. It is known that in Ophiacodon from the Permocarboniferous, ossification of the metacoracoid did not occur till late, and that in Varanops (Fig. 96 d) it never ossified. This doubtless explains its absence in all known specimens of Paleohatteria, formerly placed among the Rhynchocephalia. Paleontological evidence that it is the posterior bone which has functionally disappeared in all modern reptiles, and not a fusion of the two, now seems complete. The coracoid of lizards, crocodiles, and Sphenodon is homologous with the anterior of the two bones, the so-called procoracoid. It was Howes and Lydekker who first reached this conclusion, and who proposed the name metacoracoid for the posterior bone. Whether this conclusion is the right one so far as the monotreme mammals are concerned is still a debatable question. The two coracoids in these mammals seem, and generally are considered to be, homologous with those of the early reptiles. Broom has suggested that in the evolution of the mammals the posterior bone, that is, the metacoracoid, was retained, though lost in the reptiles.
Fig. 107. Pectoral girdles (Therapsida): A, Galeops (Dromasauria). Natural size. B, Galechirus (Dromasauria). Natural size. C, Galepus (Dromasauria). About three fourths natural size. D, Moschops (Dinocephalia). One fifth natural size.
Gregory, however, has offered another solution of the problem that would homologize the anterior or "procoracoid" of the reptiles with the posterior bone of the Monotremata. He thinks that three elements are involved in the problem of their evolution:
"(a) The epicoracoid of Sphenodon, lizards and monotremes, a sheet of bone lying immediately above the clavicles, and never reaching the glenoid surface.
"(b) The true coracoid, or so-called procoracoid, lying behind the clavicles, originally pierced by the coracoid foramen, primitively forming at least the front part of the glenoid, often articulating with the sternum.
"(c) The metacoracoid of Permian reptiles, originally forming the back part of the glenoid region, lost in later reptiles (Williston), and in mammals except when preserved as a vestigial element."
It is true that such an element as the epicoracoid has not been found ossified in the early reptiles, but neither have numerous other bones in the mesenchyme of mammals, and its ossification in mammals would be nothing remarkable. A comparison of the epicoracoid of lizards (Fig. 99 b) with that of monotremes will show their identity in relations. And doubtless a similar epicoracoid filled in the interval between the coracoids above the clavicles and interclavicles in the early reptiles (Fig. 96 d). Should it eventually result that Broom's theory is the correct one, that both coracoids have remained in the Monotremata, the posterior one of which presumably represents the chief ossification of the coracoid process of higher mammals, then modern reptiles have no true coracoid, and the bone so called must be known as the procoracoid. The author believes that Gregory's theory is more probable. But, until the real homologies are fully determined, and to save confusion for the present, the terms procoracoid for the anterior bone, metacoracoid for the posterior are adopted in this work.
In all known reptiles possessing a metacoracoid, the suture separating it from the procoracoid enters the glenoid fossa (Fig. 106), except in certain therapsids (Fig. 107), where it joins the scapular suture a little in front of the articular surface. It passes directly inward to terminate in the free border. The scapula-procoracoid suture, in all the Cotylosauria and Theromorpha (Fig. 106) at least, divides nearly equally the glenoid surface in front of the metacoracoid, and is thence directed forward and upward to terminate in the front border.
The supracoracoid foramen, always present in the procoracoid (Figs. 95, 96, 99, 100, 106, 107), though not in the epicoracoid of the monotremes, and usually present in the coracoid of later reptiles (Figs. 112, 113), is absent in the Chelonia (except the Triassic Stegochelys), the Pterosauria, Ichthyosauria, Plesiosauria, Rhynchosauria (Howesia), many Phytosauria, and the Thalattosauria—chiefly water reptiles, it is seen. It may, in some, be represented by a notch between the scapula and coracoid (Figs. 103, 111), doubtless its original position.
Fig. 108. Pectoral girdle of Eryops (Temnospondyli). Two thirds natural size.
The scapular girdle of the terrestrial temnospondylous (Fig. 108) amphibians has three foramina piercing it: the supracoracoid foramen, already mentioned, entering a little in front of and below the glenoid fossa and opening on the inner side at the lower end of the subscapular fossa; the glenoid foramen, entering the glenoid fossa and opening on the inner side in front of the subscapular fossa; and the supraglenoid foramen entering the supraglenoid fossa near the hind border and opening at the upper end of the subscapular fossa. The glenoid foramen has not been observed in reptiles. The supraglenoid foramen is present in the Cotylosauria (Fig. 95), Theromorpha (Fig. 96 d), probably the Therapsida, in most modern Lacertilia (Fig. 99), and in Sphenodon. It will probably be found in many other forms when searched for. Its external orifice, however, varies much, even in the Theromorpha. In Ophiacodon only, so far as has been observed, does it enter the supraglenoid fossa back of the border; more usually, as in many modern reptiles, it is on the outer face of the scapula in front of the border, at a variable distance above the glenoid surface. A small artery traverses it, according to Douthitt.
In the early cotylosaurs and theromorphs (Fig. 106) the glenoid articulation is more or less spiral or "screw-shaped." In most other reptiles it is a simple, oval cavity. In the pterosaurs (Fig. 109) it is saddle-shaped, concave in the dorsoventral, convex in the conjugate, diameter, permitting motion of the arm in two planes only, dorso-ventral and antero-posterior.
The double coracoids are never elongated transversely. Turned inward at nearly a right angle from the plane of the scapula, they were approximated along their mesial borders (Fig. 96 d), as shown by many specimens in which they have been found in place. Doubtless epicoracoid cartilages occupied the interval in front.
In the single coracoid of later reptiles the glenoid articulation has been completed from behind. In the modern lizards there are emarginations of the mesial border (Fig. 99), the deeper one opposite the supracoracoid foramen; this emargination is very variable in the mosasaurs. It has also been observed in the procoracoid of the theromorphs. The coracoid of the Pterosauria (Fig. 109 a), Chelonia (Fig. 109 b), and Crocodilia (Fig. 112) is elongate. When the sternum is present the coracoid articulates with its anterior lateral border.
The coracoids, presumably the precoracoids only, are extraordinarily developed in the Plesiosauria (Fig. 102), where they sheathe the whole under side of the pectoral region, meeting in a firm median symphysis; in most plesiosaurs throughout their lengths, but in the Elasmosauridae broadly separated posteriorly by a deep emargination, apparently a specialization (Fig. 110).
Fig. 109. Pectoral girdles: A, Nyctosaurus (Pterosauria). One half natural size. B, C, Testudo (Chelonia). One half natural size. D, Stegochelys (Chelonia). After Jaekel. One fourth natural size.
In most reptiles the single coracoid is fused with the scapula in adult life, but it is free in the crocodiles, and more or less suturally loose in the early pterosaurs, dinosaurs (Fig. 113), phytosaurs (Fig. 111), and rhynchocephalians.
Fig. 110. Pectoral girdle (in part) and front paddles of Elasmosaurus (Plesiosauria). After Riggs. sc, scapula; h, humerus; cor, coracoid; r, radius; u, ulna.
The scapula of the plesiosaurs (Figs. 102, 110) is peculiar in the development of a strong proscapular process projecting downward, forward, and inward, and often meeting its mate in a median symphysis, a character unique among vertebrates. The blade is short and small, of little service for muscular attachment, unlike the scapulae of tail-propelling aquatic reptiles.
Probably the great development of the ventral elements of the pectoral and pelvic girdles in the plesiosaurs implies greatest development of the ventral muscles, used in the antero-posterior and downward movement of the paddles. A clavicular process of the coracoids of the later plesiosaurs (Fig. 102) extends forward to articulate with the proscapular process or with the clavicles. The mode of development of the proscapular process, as shown by Andrews, proves that it is an exogenous process of the scapula, corresponding to the acromion and not to the procoracoid, as it was once thought to do. The scapulae of tail-propelling aquatic reptiles are always short and broad, fan-shaped (Figs. 85, 112). The scapula of the Chelonia is also peculiar (Fig. 109 b, c, d). Enclosed within the thoracic cavity it has two rather slender branches, one extending toward the roof; the other, the proscapular process, springing from near the articular fossa, is directed downward and inward to be attached by ligaments to the interclavicle or entoplastron. Formerly this process was also supposed to be a separate ossification, the procoracoid, fused with the scapula, and on the strength of it a relationship was found with the plesiosaurs. It is now known to be an exogenous process of the scapula. The coracoid is more or less flattened and dilated at its extremity. It is directed inward and backward, and is connected with its mate by ligaments. In Stegochelys, a Triassic turtle, the proscapular process is small (Fig. 100).
In Eunotosaurus, a Permian genus of South Africa, that has been referred to the Chelonia in a wide sense, the pectoral girdle is of the primitive type, having a moderately long scapula, slender clavicles, and interclavicle, and the two coracoids approximating their mates in the median line.
A distinctly differentiated acromion process occurs in reptiles only among the Pariasauridae and especially the therapsids, mammal-like forms from South Africa. A distinct angular process on the front margin of the scapula in the Cotylosauria (Fig. 96 b, c) and Theromorpha (Figs. 96 d, 106), to which the clavicle is attached, however, corresponds to the acromion.
In general, the shorter and stouter are the legs, the shorter and broader are the scapulae. In upright-walking reptiles the scapula is
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| Fig. 111 | Fig. 112 |
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| Fig. 113 | |
Fig. 111. Scapula and coracoid of Rutiodon carolinensis, an American phytosaur. After McGregor.
Fig. 112. Scapula (sc) and coracoid (cor) of gavial (Crocodilia).
Fig. 113. Pectoral girdles (Dinosauria): A, Gorgosaurus (Saurischia). After Lambe. One sixteenth natural size. B, Allosaurus (Saurischia). After Gilmore. About one twelfth natural size. C, Triceratops (Ornithischia). After Marsh. One sixteenth natural size. D, Morosaurus (Saurischia). After Marsh. One twenty-eighth natural size.
more elongated, in bipedal forms slender. The scapula of the Cotylosauria (Figs. 95, 96, b, c) is relatively short and broad; that of the Theromorpha (Figs. 98, 106) more elongated, but never narrow; that of the therapsid reptiles (Fig. 107) relatively narrow, slender in the Dromasauria. The scapula of the Sauropoda (Fig. 113 d) is relatively long, that of the Predentata (Fig. 113 c) is much more slender, but it is most slender and bird-like of all in the Theropoda (Fig. 113 a, b). The scapula of the Pterosauria (Fig. 109) is always elongated, very slender and bird-like in some of the earlier forms, but stouter and firmly fused with the coracoid in the latest. In the most specialized of all pterodactyls (Pteranodon, Ornithocheirus) its enlarged distal extremity articulates with the fused spines of the dorsal vertebrae, the only known examples among vertebrates of the articular union of the pectoral girdle with the spinal column.
In the early reptiles the scapula was more nearly erect, or with a slight inclination backward. In the Crocodilia, Pterosauria, and bipedal reptiles, as also birds especially, it is very obliquely placed, the upper end turned backward over the ribs.
The Pelvic or Hip Girdle
(Figs, 114–127)
The pelvic girdle or pelvis, in reptiles, as in other air-breathing vertebrates, is composed of three bones on each side, more or less firmly coössified in the adult, and collectively known as the innominate; the girdle is completed by the sacrum on the dorsal side with which the pelvis is never closely united in reptiles, not even in the Pterosauria. The upper or dorsal bone of the three, that to which the sacral ribs or transverse process of the lumbar vertebra are attached, is the ilium; the one on the lower or ventral side in front is the pubis; that on the ventral side behind is the ischium. On the outer side, where the three bones meet, there is a cup-like depression, sometimes a hole, the acetabulum, for the articulation of the thigh bone. In only two groups of reptiles, the Crocodilia and Plesiosauria, is the pubis excluded from union with the ilium. In the snakes and snake-like lizards there are at most only vestiges of the pelvic bones.
The pelvis of the terrestrial temnospondylous amphibians (Fig. 114 a) is almost indistinguishable from that of the contemporary cotylosaur reptiles in early Permian times. The ilium of the rhachitomous forms is not dilated above, as in the reptiles, but even this distinction fails in the more nearly allied embolomerous Cricotus, in which the ilium is prolonged backward, quite as in the reptiles. The pubes and ischia meet in a close symphysis without openings of any kind, except the pubic foramen, a small hole through the pubis below the margin of the acetabulum, in front of the ischiatic suture, for the passage of the obturator nerve. This "plate-like" structure of the pelvis is characteristic of the Cotylosauria (Figs. 114 b, 115), and more or less of the Theromorpha (Figs. 114 c, 117), Therapsida (Fig. 119), Proganosauria, the Choristodera, and early Rhynchocephalia.
Fig. 114. Pelvic girdles: A, Cacops (Temnospondyli), from below. One half natural size. B, Seymouria (Cotylosaur), from below. A little more than one half natural size. C, D, Varanops (Theromorpha), below and from the side.
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| Fig. 115. Limnoscelis paludis (Cotylosauria). Pelvis, from below. Two fifths natural size. Cross-section through pubes at a, cross-section through ischia at b. | Fig. 116. Limnoscelis paludis. Diagram of pelvis, from the side. il, ilium; pb, pubis; is, ischium. |
Fig. 117. Ophiacodon (Theromorpha). Pelvis. One half natural size. A, from the side; B, from above, pu, pubis; il, ilium; is, ischium.
Fig. 118. Pelves and sacrum: A, Varanus (Lacertilia), from the right. B, Erythrosuchus (Parasuchia), from the right. After Broom. One tenth natural size. C, Rutiodon (Phytosauria), from below. After McGregor. One eighth natural size. D, Nyctosaurus (Pterosauria), sacrum and right innominate bone from within; D′, anterior parasternal ribs of same; D″, prepubis of the same from below.
Fig. 119. Pelves (Therapsida): A, Galechirus (Dromasauria). After Broom. Nearly natural size. B, Diademodon (Cynodontia). After Broom. About one half natural size. C, Galepus (Dromasauria). After Broom. Nearly natural size.
A small opening soon appeared where the four bones meet below in the Theromorpha (Figs. 114 c, 117), and increased in size, till, in most reptiles, since Triassic times at least, this pubo-ischiatic opening extended on each side nearly to the acetabulum, leaving only a narrow connection between the pubis and ischium (Fig. 118). Later, the symphysial ends of the pubis and ischium became connected in many by ligaments, or cartilage (Fig. 120), and later in some by bone, producing a false obturator or thyroid vacuity on each side. A foramen or vacuity homologous with that in mammals, the so-called
Fig. 120. Pelvis and sacrum. A, Iguana (Lacertilia), pelvis from below; B, sacrum from below. About natural size. C, Dicynodon (Anomodontia), pelvis, from above and from the side. After Broili. Nearly one half natural size.
obturator foramen, that is between the pubis and ischium with which the real obturator or pubic foramen is merged, occurs in the Theriodontia (Fig. 119), Anomodontia, and later pterodactyls (Fig. 118 d). The formation of a thyroid vacuity in the theriodonts may be due to the gradual increase in size of the pubic or true obturator foramen and its recession backward, as in the Dromasauria, till it finally lies between the two bones, the pelvis still retaining its primitive plate-like character with only a small median pubo-ischiatic vacuity. But this will not explain the thyroid vacuity in Pteranodon and Nyctosaurus of the Pterosauria (Fig. 118), since it is inconceivable that these reptiles had an unbroken descent from forms without a median vacuity.
Fig. 121. Pelvic girdle and sternum: Alligator (Crocodilia). A, pelvic girdle, from the right; B, the same, from above, showing sacrum; C, the same, from below, with parasternals; D, sternum and interclavicle. One half natural size.
In no reptiles is the pelvis more aberrant than in the Crocodilia (Fig. 121). So characteristic is its structure that it at once distinguishes the order from all others. The ilium is a strong bone firmly united with the two pairs of stout sacral ribs, of which the posterior is the larger. Below, the ilium articulates with the ischium only, to form the acetabulum. In front of the acetabulum it is produced forward to join ligamentously with an anterior process of the ischium, enclosing between them a foramen of considerable size for the passage of the obturator nerve. The ischium is a rather long bone, with a thin, spatulate extremity which joins its mate in a median symphysis. Its anterior process, which may be in part the real pubis, articulates in front with the so-called pubis. This bone is slender, with a thin and dilated anterior extremity which touches, or is closely approximated to, its mate only at its inner anterior corner, and is continuous anteriorly, with a thin but strong plate of fascia joined to the parasternal ribs. With much reason it has long been urged that the anterior projection of the ischium represents the real pubis.[3] In early life it is largely cartilaginous, but becomes fully ossified in the adult. The so-called pubis is probably homologous with the prepubis of the pterodactyls. It has no pubic foramen.
The ilium of the Pterosauria, like that of all bipedal reptiles is produced anteriorly by the sides of the vertebrae, very much so in some forms. The ischium and pubis are closely united into a more or less broad plate, either with a thyroid foramen, as in Nyctosaurus (Fig. 118 d) and Pteranodon, or with a small pubic foramen below the acetabulum, as in Rhamphorhynchus, proving the normal structure of the pelvis, though sutures have not been observed. The prepubes, often called the real pubes, are either paired, as in Pterodactylus, or united in a ventral band, as in Rhamphorhynchus, Pteranodon and Nyctosaurus (Fig. 118 d). They articulated with a tuberosity on the front margin of the pubes and in all probability were continued in front with a ligamentous sheath that enclosed the parasternal ribs. The pubes and ischia meet in a symphysis below, though this has been disputed for some.
As remarkable as the pelvis of the crocodiles is that of the Dinosauria, or rather of that division called the Predentata, or order Ornithischia. In the other divisions, the Theropoda (Fig. 122 a) and Sauropoda (Fig. 122 b), the pubes have the normal reptilian structure, though unusually stout and strong, meeting in the middle below in a firm symphysis, much elongated in the Theropoda. The symphysis of the ischia is less strong.
Fig. 122, Pelves (Dinosauria): A, Ceratosaurus (Saurischia). After Marsh. One sixteenth natural size. B, Apatosaurus (Saurischia). After Marsh. One thirty-second natural size. C, Triceratops (Ornithischia). After Marsh. One twenty-fourth natural size. D, Stegosaurus (Ornithischia). After Marsh. One twentieth natural size. E, Trachodon (Ornithischia). One tenth natural size.
The pubes of the Ornithischia (Fig. 122 c–e) have been the subject of much dispute and speculation. Each is composed of two projections or processes: the anterior one, the so-called prepubis, or prepubic process, typically flattened and more or less spatulate distally, is directed forward and downward [upward] and does not join its mate in a median symphysis. At times it may be small or even vestigial (Ankylosaurus), but is broad and stout in the quadrupedal Ceratopsia, where apparently it again functions as the normal pubis. The postpubis, or postpubic process, typically is long and slender, directed backward immediately below, the slender ischium and not meeting its mate in a symphysis; that is, the pelvis is more or less open below, as in birds. The postpubis is vestigial in the heavy quadrupedal Ceratopsia, which have certainly descended from bipedal forms. It is, however, unusually stout in the quadrupedal Stegosaurus, possibly as a reinforcement to the ischia in the support of the heavily armored body.
When this peculiarity of the dinosaurian pelvis was first discovered by Hulke and Marsh it was hailed as a direct proof of the dinosaurian ancestry of birds. It may be, however, merely another of the many parallel characters brought about by similar causes. According to one view, the prepubic process is the real pubis, homologous with the pubis of the Saurischia; the postpubic process an outgrowth from it. According to another view, the postpubic process is the real pubis, corresponding to the pubis of birds, the prepubic process homologous with the prepubis of pterodactyls or crocodiles. There has never been, however, any evidence to show that it is derived from a separate center of ossification.
An analogous but not homologous structure is observed in many running birds, the ostriches, Geococcyx, etc., where, in addition to the normal, slender, posteriorly directed pubis similar to the postpubic process of the dinosaurs, a more or less prominent pectineal process, arising, however, from the ilium, is directed forward, like that of the dinosaurs. The pubis of birds in its embryonic development turns backward from its normal position. Whence it would appear that the development of the two processes in the dinosaurs has arisen in response to similar causes, and cannot be ascribed to a common heredity, as was once thought. Why the bipedal predentate dinosaurs should have acquired such a remarkable structure of the pelvis, and not the even more bipedal theropods, is not yet entirely clear. It has been ascribed to differences in the posture of the tail in running, but would seem, to the author at least, rather to have been due to differences in procreational methods, the open pelvis of the predentates permitting larger eggs to be extruded, as in the birds. It may be added that the acetabulum of all dinosaurs is perforate, as in the birds.
The pelvis of amphibious or aquatic reptiles is also modified not a little. It lost all connection with the spinal column in the Mosasauria and later Ichthyosauria, but is firmly connected, as usual, in other water animals. The slender ilium of the mosasaurs, like that of the ichthyosaurs, lay loosely in the flesh with its upper end in apposition or ligamentously connected with the end of a transverse process or rib of a single vertebra. The narrow ischia and pubes meet in a symphysis, and there is a pubic foramen.
Fig. 123. Pelvis of Platecarpus (Mosasauria), from below.
Fig. 124. Pelvis of Nothosaurus (Nothosauria). After Andrews.
In the earlier ichthyosaurs the broad ischia and pubes were separated by the broad pubo-ischiatic opening, and the pelvis was connected with a sacrum. In the later forms, however, the pelvis was reduced, the rod-like ilium lay loosely in the flesh, and the pubes and ischia were united without a pubo-ischiatic opening.
In the Nothosauria (Fig. 124) the pelvis, of the usual type, shows only a moderate aquatic adaptation in the broad pubes and ischia. The ilium is firmly connected with the sacrum, and there is a pubic foramen; the pubo-ischiatic notch is small. In the Plesiosauria (Figs. 125, 126), the slender ilium, connected ligamentously with a sacrum of three or four vertebrae, articulates at its distal extremity with the
Fig. 125. Pelvic girdle of Trinacromerum osborni, an Upper Cretaceous plesiosaur, from above: p, pubis; is, ischium; il, ilium.
Fig. 126. Pelvic girdle of Elasmosaurus (Plesiosauria): p, pubis; is, ischium; il, ilium.
Fig. 127. Pelvis: Testudo (Chelonia); A, from below; B, from the side. One half natural size.
ischium only, and, like that of the Chelonia, is directed upward and backward. The pubes and ischia, like the coracoids, are very broad and flat, secondarily plate-like, meeting in a more or less horizontal symphysis. There is no pubic foramen, and usually the large pubo-ischiatic vacuity is broadly connected across the median line—probably separated by a ligament in life. In some genera, however, Sthenarosaurus or Thaumatosaurus, for instance, the two bones are secondarily broadly united at their symphyses, producing a false thyroid foramen with which the obturator foramen is confluent, as in mammals. The ischia are triangular or "hatchet-shaped," elongated in the short-necked forms, short in the long-necked.
The pelvis of the Chelonia (Fig. 127), like the pectoral girdle, has been modified by its peculiar relations to the carapace and plastron. There is a large pubo-ischiatic vacuity, often divided in the middle by a cartilaginous septum, but broadly ossified in the land tortoises, as in the plesiosaurian Sthenarosaurus. As in the plesiosaurs, there is no separate pubic foramen or notch, rarely absent in reptiles.
The ilium, like that of the plesiosaurs, is elongate and is directed upward and backward to the firm sacrum. The pubis is larger than the ischium and has a stout tuberosity which rests upon the plastron, or, in the Pleurodira, is coössified with it.
Usually in crawling reptiles (Figs. 114–118 a) there is no, or only a small, preacetabular process to the ilium, but always a postacetabular one. In upright-walking animals the preacetabular process is always well developed, sometimes at the entire expense of the postacetabular process. It is unusually long in the Anomodontia (Figs. 120 c, 119), Ceratopsia (Fig. 122 c, e), and Pterosauria (Fig. 118 d), where it is supported by the united or contiguous diapophyses of the lumbar vertebrae, false sacral vertebrae. The ilium is more or less helmet-shaped in the Saurischia (Fig. 122 a, b) as also in some Cotylosauria, Therapsida (Cynognathus), and Theromorpha (Casea)—all such forms have short toes; possibly it is due to the greater expansion of the gluteal muscles.
The evolution of the reptilian pelvis has been, as we have seen, from the primitive closed and plate-like type, by the progressive development of a vacuity between the ischia and pubes, by the elongation of the anterior process of the ilium, and by its closer union with additional true sacral or lumbar vertebrae.
- ↑ [According to Watson, the coracoid originally was a single piece which never became subdivided in the amphibians, cotylosaurs, or ordinary reptiles, the subdivision occurring only in the Theromorpha, Therapsida, and mammals.—Ed.]
- ↑ [For a different view of the fate of the two coracoids see Watson, 1917, Journ. Anat, vol. LII; Romer, 1922, Anat. Record, vol. XXIV, pp. 39–47.—Ed.]
- ↑ [The pubis of the Crocodilia gives attachment to a series of muscles which as a whole are homologous with those that are attached to the true pubis in Sphenodon and lizards (Gregory and Camp, Bulletin, Amer. Mus. Nat. Hist., 1918; Romer, ibid., 1923, p. 606). If the true pubis of Crocodilia has become vestigial and the prepubis has become the functional pubis, how did the prepubis capture the system of muscle attachments of its predecessor?—Ed.]