CHAPTER V
THE LIMBS
Two pairs of limbs are almost always present in reptiles, composed, as in mammals, of four analogous segments: the arm and thigh bones, conveniently called propodials; the forearm and leg bones, or epipodials; the wrist and ankle bones, or mesopodials; the manus and pes, composed of metacarpals and metatarsals, or metapodials; and a variable number of finger and toe bones, known as phalanges.
The limbs are best understood and described as though directed outward from the long axis of the body (Fig. 128), the palms of the hands and soles of the feet turned downward or to the ventral side, the epipodials parallel, the thumb or pollex, and the big toe or hallux, on the anterior or preaxial side, the little finger and little toe on the posterior or postaxial side. The terms outer and inner are often applied to the anterior extremity, as though directed backward in the axis of the body, the thumb on the outer side. The hind extremities are sometimes described as though parallel with the long axis of the body, with the big toe on the inner side. As the hallux is analogous with the pollex, this nomenclature places them on the opposite sides and should not be used for any vertebrates.
The fore and hind limbs of terrestrial reptiles are of approximately equal length, the hind pair the longer. In aquatic reptiles [e.g., ichthyosaurs, mosasaurs] the front pair are often the larger, and usually the longer; in volant reptiles [pterosaurs] they are much longer than the hind pair. In bipedal reptiles [e.g., later Theropoda] or those usually assuming this posture in locomotion, they are smaller or very much smaller. In climbing and cursorial reptiles the limbs are more or less, sometimes very much, elongated and slender (Fig. 155). The digits of fleet, crawling reptiles are long; those of the more upright-walking kinds (Figs. 145, 141 i), in which the digits of the two sides are brought more nearly parallel to each other, are short. The articular surfaces of the limb joints of aquatic reptiles (Figs. 149, 158) are poorly developed, unextensive, and more or less cartilaginous.
Swimming reptiles with propelling tails [e.g., ichthyosaurs, mosasaurs] have short propodials, sometimes very short; on the other hand, the propodials of limb-propelling water reptiles [e. g., plesiosaurs, proganosaurs] are elongated. The epipodials of ordinary terrestrial reptiles are always somewhat shorter than the propodials. Greater shortening of these bones is indicative of swimming habits, possibly also of burrowing; and in strictly aquatic reptiles they are always very short; indeed the degree of water adaptation may be gauged by the proportional lengths of the epipodials. On the other hand, in springing, leaping, or volant reptiles, they may be considerably longer than the propodials (Fig. 155).
The limbs of some Lacertilia and most Ophidia are wholly absent; some snakes have vestiges of the hind pair, and some lizards only vestiges of either pair or the front pair only. All other known reptiles have four functional limbs.
Primitively (e. g., Figs, 1, 128) reptiles were pentadactylate, with the phalangeal formulae 2, 3, 4, 5, 3 for the front, 2, 3, 4, 5, 4 for the hind pair, the fourth digit the longest and strongest; and most reptiles still retain these characters. The first digit to be lost is the fifth, and only in a few dinosaurs is the first digit wholly lost. In the more upright-walking kinds, those in which the feet of the two sides are brought more nearly parallel in walking, the greater strength of the foot passes more to the preaxial side, and both the fourth and fifth digits may be obsolete or lost, and very rarely the third also. This weakening of the postaxial digits is especially noticeable in the dinosaurs (Figs. 141, 156) and turtles (Fig. 154), in which the posture in locomotion is more like that of mammals. The same character is also observed in the Crocodilia (Figs. 140 a, 157), unlike other crawling reptiles, and tends to confirm Huene's contestation that the ancestors of these reptiles were originally more upright in locomotion than are their descendants.
As a rule the hind limbs of terrestrial reptiles, as of terrestrial mammals, are more specialized than the anterior ones; that is, there are fewer bones, and the ones remaining are more developed than those of the front feet. Among aquatic and volant reptiles, on the other hand, where locomotion is chiefly effected by the fore limbs, these are more specialized. In certain lizards (Phelsuma) the first digit has become vestigial, the others are well developed.
Fig. 128. Captorhinus (Cotylosauria): Skeleton, from below. One half natural size.
Fig. 129. Theromorph limbs: Naosaurus, humerus, dorsal side, femur, ventral side. One half natural size.
Propodials
The humerus (Figs. 129–131), or first bone of the anterior extremity, articulates in the glenoid fossa of the scapular girdle, usually by a more or less complete, free, ball-and-socket joint, permitting rotation. In most of the Cotylosauria (Figs. 128, 130, 132) and stouter-limbed Theromorpha (Figs. 129, 131, 134) the articular surface is more or less spiral-like, extending around the head from the ventral postaxial to the dorsal preaxial side, permitting movement in an antero-posterior direction with a concomitant partial rotation as the hand, directed forward obliquely, is brought backward in walking. The bone was not depressible below a horizontal plane without dislocation. The articular surface of the pterodactyl humerus (Fig. 141) is saddle-shaped, permitting motion in two planes only—antero-posterior and dorso-ventral.
Fig. 130. Seymouria (Cotylosauria). Humerus, femur, tibia. A, right humerus, a from side, b from front; B, left tibia, ventral side; C, radius; D, right femur, from behind; E; left femur, from Cacops bone-bed, natural size; F, undetermined.
At the upper or proximal end of the bone, near its articular part, are two more or less prominent processes for the attachment of muscles. That on the preaxial ventral side (Fig. 131), usually situated above the middle third, but often descending nearly to the middle or even below the middle in stout-limbed reptiles, is the radial or lateral tuberosity or process. On the opposite side, nearer the head, and often not well marked, is the ulnar or medial tuberosity (Figs. 129, 130). Between the two, on the ventral side, is the bicipital fossa (Fig. 131). Immediately below the lateral tuberosity the shaft is usually round or oval in cross-section. Among all reptiles the lateral process is most developed in the pterodactyls (Fig. 141). It is also largely developed in the Cotylosauria (Figs. 128, 130, 133), Theromorpha (Figs. 129, 131, 134), and Anomodontia, sometimes descending below the middle of the bone.
Fig. 131. Ophiacodon mirus Marsh (Theromorpha). A, left humerus, ventral side, one half natural size. B, left humerus, distal end, one half natural size. C, left ulna, radius and carpus, ventral side, one half natural size. D, left carpus, dorsal side, three fourths natural size.
The expanded extremities of the humerus are in divergent planes, the angle sometimes slight, at other times approximating or even exceeding a right angle, the bicipital fossa in such cases looking more dorsad than ventrad. The width of the more expanded distal extremity may be less than an eighth of the length of the bone, or may nearly equal it in stout-limbed reptiles like the Cotylosauria (Figs. 130, 133). The distal expansion is always great in the Cotylosauria and Anomodontia, as also in some Theromorpha (Figs. 129, 131). Doubtless in these animals, or some of them at least, the peculiar humerus is to be correlated with the screw-like motion in the glenoid fossa, the horizontal position of the humerus in locomotion, and the more turtle-like mode of progression. The digits in such animals are never long, and the ungual phalanges are short and stout.
At the distal extremity of the humerus (Figs. 131, 133), on the preaxial and more or less ventral side, there is a more or less convex surface, the radial condyle, or capitellum, for the articulation of the radius. Contiguous with it on the postaxial side, but more distal and dorsal, is the ulnar condyle or trochlea, for articulation of the ulna. In aquatic reptiles (e. g., Fig. 158 c, d) both of these are simple facets at the extremity of the humerus. The projection or process on the radial or preaxial side, above the radial condyle (Figs. 129 a, 131), in the short-limbed cotylosaurs and theromorphs as also the temnospondyl amphibians, sometimes turned more dorsad, is known as the radial epicondyle, ectocondyle, ectepicondyle, or pre-epicondyle. In the very stout-limbed cotylosaurs (Figs. 130, 133 sc.p.) and theromorphs (Fig. 131), as also the stout-legged temnospondyls (Fig. 136), there is a stout process on the radial side above the epicondyle. It is especially correlated with short digits and doubtless a more turtle-like mode of progression. It may be known as the supracondylar process. The distal expansion of the humerus on the ulnar or postaxial side is commonly known as the entocondyle or entepicondyle, misleading terms (Figs. 130, 131, 133 ent.).
Piercing the condylar expansions more or less obliquely (Fig. 129 a) are very characteristic foramina in most reptiles. That on the ulnar side, the entepicondylar foramen (entep. f.), for the passage of the median nerve, occurs in all Cotylosauria, Proganosauria, Theromorpha, and most therapsids, and in not a few mammals. A similar foramen on the radial side, the ectepicondylar foramen (Fig. 129 a, ectep. f.), for the passage of the radial nerve, is characteristic of most Lacertilia, Chelonia, Choristodera, and Phytosauria. In some of these it is replaced by a groove, and the latter is present in the Mosasauria and young Plesiosauria. Both the ectepicondylar and entepicondylar foramina occur in some Theromorpha and Anomodontia, the Nothosauria, Rhynchocephalia, Araeoscelis, Pleurosaurus, etc. The Pterosauria, Dinosauria, Crocodilia, Ichthyosauria, and Plesiosauria have no epicondylar foramina.
The humeri of many known temnospondylous amphibians differ but little from those of the Cotylosauria, save in the absence of the entepicondylar foramen. This foramen is reported for Cochleosaurus, a rhachitomous temnospondyl, and is known in Diplocaulus of the Lepospondyli, but is known in no other amphibian. An ectepicondylar foramen is quite unknown in the class.
Femur. The thigh-bone, or femur (e. g., Figs. 129 b, 135), like the humerus, is variable in shape. Its articulation in the acetabulum is by a more or less convex head. The femur of most reptiles is turned outward from the long axis of the body in locomotion, with the articulation at the extremity; or if the bone is directed more or less upward, as well as outward, the convexity is more on the dorsal side, as in the Chelonia. The two femora of a lizard, for instance, cannot be brought parallel with each other in the same direction without dislocation from the socket. There is, consequently, in such reptiles, no real neck, so characteristic of birds and mammals. The dinosaurs (Fig. 132) and pterodactyls (Fig. 155) only, because of the more or less vertical or antero-posterior position of the femora, have the head set off from the shaft of the bone by a more or less well-marked neck, most noticeable in the bipedal types of dinosaurs, but also apparent in the quadrupedal. The absence, then, of a neck to the femur is indicative of crawling or aquatic habits. Many of the Therapsida (Fig. 132), though without a differentiated neck, have the proximal preaxial border of the femur more or less curved, with the articulation more on the preaxial side, giving evidence of a more upright, mammal-like mode of progression. Pariasaurus of the Cotylosauria has also been restored in a more upright posture, but its femur is quite like that of the earlier cotylosaurs,[1] and like them it probably never was brought below a horizontal position in walking, though, as in Diadectes, the mode of locomotion was probably more like that of the turtles, accounting, perhaps, for the reduction of the phalangeal formula in that genus. So also, the propodials of Lystrosaurus and doubtless of other Anomodontia were directed horizontally in locomotion.
On the preaxial ventral side, usually on the upper third of the bone, but sometimes, as in the short-limbed Cotylosauria, 
Fig. 132. Therapsid femora: A, Moschops; B, Aelurosaurus; C, Cynognathus. After Gregory and Camp. Scales various. Upper row, proximal end; middle row, dorsal; lower row, ventral.
descending below the middle, there is, especially in crawling forms, a rugosity or eminence, the lesser trochanter,[2] from which usually a more or less pronounced ridge or roughening descends toward, or nearly to, the postaxial condyle (Figs. 129 b, 132). It corresponds to the linea aspera of mammals and may be called the adductor ridge or crest.
Fig. 132 bis. Dinosaur femur: Camptosaurus, right femur. After Gilmore. One sixth natural size.
On the opposite side, and nearer the head, obsolete or even absent in ordinary crawling reptiles but well developed in the Chelonia (Fig. 154) [and in certain Therapsida, Fig. 132], is the great trochanter. Between the two there is a depression or fossa [intertrochanteric], at the upper extremity in turtles (Fig. 154), but broadly ventral in most other forms.
The femora of the dinosaurs (Fig. 132 bis), especially the bipedal Predentata, but also indicated in the Sauropoda, have near the middle on the ventral preaxial side a rugosity or prominence, the fourth trochanter, sometimes, as in Camptosaurus, long and pendent.
The condyles, at the distal extremity of the femur, are separated by a groove in front and another behind (Fig. 129 b). The preaxial condyle, usually the smaller, gives articulation to the tibia; the postaxial condyle, to the fibula, and in part to the tibia behind. The shaft of the femur is sometimes markedly curved (Figs. 155, 157), sigmoidally in the more slender kinds. It is always longer and more slender than the humerus, its distal width seldom if ever equal to more than half the length of the bone.
The femur of the temnospondylous amphibians (Fig. 151 a) is sometimes indistinguishable from that of the cotylosaurs, but usually the adductor ridge is more strongly developed, and the articular ends are less well ossified.
Epipodials
Radius and Ulna. The two bones of the forearm or antibrachium are always complete in reptiles and movable upon each other, freely in most terrestrial reptiles, flexibly in the aquatic types, that is, without rotation of the radius; they may be more or less fixed in the chelonians (Fig. 145 a), though not crossed. The forearm in this order has a peculiar twist on the humerus by which the dorsal surface of the forearm, wrist, and hand is turned forward at right angles to the humerus without pronation or rotation of the radius (Fig. 145 a).
|
|
| Fig. 133. Cotylosaur limb: Limnoscelis, left foreleg, ventral side. One fourth natural size. | Fig. 134. Ophiacodon: Anterior extremity, as mounted. One half natural size. |
The radius (Figs. 133, 134), on the thumb, radial or preaxial side, articulates with the preaxial condyle of the humerus by a more or less concave and rotating joint, as in the pentedactylate mammals; distally, normally with the radiale of the carpus. The ulna (Figs. 133, 134), on the postaxial side, articulates with the trochlear condyle of the humerus, as in mammals, by a hinge, but somewhat spiral joint; distally, normally with the intermedium and ulnare of the carpus, usually also at its distal postaxial angle with the pisiform. In terrestrial reptiles the ulna is produced more or less into an olecranon, or elbow.
In the aquatic reptiles the two bones, like the posterior epipodials, are shortened, sometimes losing all resemblances to the terrestrial forms. They retain some of their land characters in the early plesiosaurs and ichthyosaurs, but in the more specialized of both groups (Figs. 158 c, d, 159), they are wider than long, articulating with each other throughout their adjacent sides. In some of the later plesiosaurs a third and even a fourth bone, whose homologies are ill understood, may articulate with the distal end of the humerus on the postaxial side. A third bone is also known in some ichthyosaurs—an accessory epipodial (Fig. 158 c).
The radius and ulna of the temnospondylous amphibians (Fig. 136) present no characters by which they can be distinguished from those of the Cotylosauria; the olecranon is but feebly or not at all produced.
Tibia and Fibula. The tibia on the preaxial or big-toe side of the hind-leg is always the larger in terrestrial reptiles (Fig. 135), unlike the radius, which is more often the smaller. It articulates with both condyles of the femur, though chiefly with the preaxial, especially in bipedal forms. Its proximal extremity is expanded into a more or less prominent cnemial crest on the dorsal side for the immediate attachment of the extensor muscles, since there is no patella, and rarely sesamoid bones of any kind, in reptiles. The distal extremity (Figs. 135, 151, 153) articulates exclusively with the astragalus, or the astragalar part of the fused bone. This joint in the early reptiles was extensive and loose, permitting a wide range of lateral movement in the foot; in later reptiles it is closer and firmer.
The fibula, on the postaxial or little-toe side, is more slender than the tibia in land reptiles. It articulates proximally exclusively with the postaxial condyle (Figs. 135, 151). It has more of a sliding or arthrodial joint in land reptiles rotating the foot in extension. It articulates distally—primitively with the calcaneum and astragalus. Its lower end in those reptiles, especially cursorial reptiles, in which the astragalus is closely united or fused with the tibia, is obsolete or lost; in the later pterodactyls (Fig. 155 c) the whole bone has disappeared, as a separate ossification at least.
Fig. 135. Theromorph limbs: A, Varanops; B, Casea. One half natural size.
Fig. 136. Temnospondyl limb: Eryops, left foreleg. A, Cope's original specimen, in American Museum of Natural History; dorsum, or upper side. B, under side. C, reconstruction. After Gregory, Miner, and Noble. One third natural size.
The posterior epipodials in aquatic reptiles (Figs. 159, 158) are almost indistinguishable from the anterior ones, except that they are somewhat, or much, smaller. As in the front leg there may be accessory epipodials in both the plesiosaurs and ichthyosaurs. This shortening of the epipodials, so characteristic of aquatic animals, is seen to a moderate extent in the earliest known reptile, Eosauravus (Fig. 151 b) from the middle Pennsylvanian, as also in the Proganosauria (Fig. 153 a) and Choristodera. It is much more pronounced in the Mosasauria (Figs. 146–148, 158 a, b), Aigialosauria, Thalattosauria, and Thalattosuchia (Fig. 150). The elongation of the tibia and fibula, so characteristic of the cursorial or leaping forms, reached the maximum in the Pterosauria (Fig. 155 c).
The tibia and fibula of some temnospondylous amphibians are quite indistinguishable from those of many Cotylosauria.
Mesopodials
Numerous modifications have occurred in the structure of the carpus and tarsus of reptiles in adaptation to diverse habits of life. The carpus (Fig. 134) or wrist of the earliest known reptiles is composed of eleven freely articulated bones, none small: four in the first row, called respectively, from the preaxial to the postaxial side, the radiale, intermedium, ulnare, and pisiform, corresponding quite to the scaphoid, lunar, pyramidal, and pisiform of the human wrist; two in the second row, the radial or first, and the ulnar or second, centrale; and five in the third row, the carpalia, the first four corresponding to the trapezium, trapezoid, magnum, and unciform of the human wrist. Watson has recognized a small third centrale in the curious genus Broomia (Fig. 137 e) from South Africa, unknown as an ossified element in other reptiles, though perhaps represented by a cartilage in the young of the modern Sphenodon.
Carpus
The carpus is known in but two temnospondylous amphibians, Eryops (Fig. 136) and Trematops. In both, the preserved bones are the same in number as in the early reptiles and some modern ones. The radius of Eryops, however, articulates with three bones, the supposed radiale, intermedium, and ulnare, while the pisiform is large, and an articular surface on the postaxial distal margin of the ulna seems to indicate the original presence of another bone that would correspond better in position with the real pisiform. Unfortunately, the single known specimen of the carpus of Trematops has but two bones preserved in the proximal row. Two centralia, and two only, were present in Trematops, while the specimen of Eryops, here figured by the kindness of Dr. Gregory, has a small space which may represent a small third centrale. There are five well-developed carpalia in both forms, proving conclusively the presence of five digits in the hand.[3]
In some of the early cotylosaurs and anomodonts, supposed water habits have delayed the ossification of some of the mesopodial bones—supposedly, but it is a curious fact that all have similar feet, short, broad toes and ungual phalanges, very broad humeri, and short epipodials. It is not impossible, it seems to the author, that similar walking or digging habits, more after the mode of the tortoises, had, even this early, brought about a modified structure in the carpus and tarsus of Limnoscelis (Fig. 133), Diadectes, and Lystrosaurus.
It is the general belief that the loss of mesopodial bones has been due to their fusion with adjacent ones; it is doubtless true for the most part, but not always. That there has been an actual loss of the first centrale and fifth distale, the first to be lost in both carpus and tarsus, seems certain, as shown in specimens of various early reptiles, where unoccupied spaces for cartilaginous elements have been preserved. Moreover, their actual loss in living reptiles has long since been affirmed. Primitively both centralia were large (Figs. 133, 134, 137 e), as was also the fourth carpale, as correlated with the longest and strongest digit. And this carpale is the most persistent bone in the carpus, as it also is the first to be ossified in the human wrist. Every other bone of the carpus may be absent in different reptiles, but not the fourth carpale, unless it be in certain quadrupedal dinosaurs like the Sauropoda (Fig. 141 f) and Stegosauria (Fig. 141 i, j).
There was a perforating foramen between the second centrale, intermedium, and ulnare that was very persistent, retarding or preventing the fusion of these three bones (Fig. 134).
The first centrale and fifth carpale are always absent in mammals, at least since early Eocene times, but the second centrale is often 
Fig. 137. Therapsid limbs: A, B, Galechirus, front and hind feet. After Broom. Natural size. C, Galesphyrus, left tarsus. After Broom. Natural size. D, Broomia, left tarsus. After Watson. Natural size. E, Broomia, left carpus. After Watson. About twice natural size.
present. In Procolophon only, of the Cotylosauria, is this centrale absent, as is also affirmed of the radiale. Throughout the Theromorpha, as known in numerous forms, the carpus is primitively complete, save that the first centrale and fifth carpale remained cartilaginous in one known genus, Varanops.
Fig. 138. Limbs: A, Theriodesmus (Therocephalia), front leg, dorsal side (rearranged from Seeley). One half natural size. B, Scymnognathus (Therocephalia), front foot. After Broom. One third natural size. C, Protorosaurus (Protorosauria), front leg. After von Meyer. One half natural size. D, hind leg of same. E, F, Araeoscelis (Protorosauria), part of tarsus, F probably immature. Nine eighths natural size. G, Sceloporus (Lacertilia). Enlarged.
Among the reptiles collectively known as the Therapsida, the carpus is ill known. In Galechirus of the Dromasauria (Fig. 137 a) as figured by Broom, the primitive structure is retained, as it also is in Dicynodon and its allies of the Anomodontia. The carpus of Scymnognathus of the Theriodontia, as figured by the same writer (Fig. 138 b) has a small intermedium, and the fifth carpale is represented as fused with the fourth, an error. A small element found near the first carpale was referred to a possible prepollex, or a radial sesamoid. Among reptiles there is no evidence of a lost prepollex.[4] Theriodesmus (Fig. 138 a) of the same group, as restored by the author from Seeley's figures, also lacks the fifth carpale, though the intermedium is not small. The complete carpus is unknown in other members of the Therapsida.
Fig. 139. Rhynchocephalian limbs: A, B, Sphenodon. After Howes and Swinnerton. About seven eighths natural size. C, Sauranodon. After Lortet. Nine eighths natural size. D, Pleurosaurus. After Lortet. Nine eighths natural size.
Sphenodon (Fig. 139 b) of the Rhynchocephalia is the only modern reptile which has retained the primitive structure and arrangement of the carpal bones. Extinct members of the order and its allies of the Diaptosauria are not sufficiently well known to determine whether this primitive structure is general, though doubtless it has been for the most part. In Rhynchosaurus, in a specimen figured by Newton, traces of the missing bones have been shown in dotted lines, indicating a primitive carpus save for the pisiform which was doubtless present.
The carpus of the Crocodilia has been strangely modified (Fig. 140 a). It is composed of four bones only in all forms so far as known: the radiale, ulnare, pisiform, and fourth carpale, as they are usually called. The radiale is very large and elongate, dilated at its ends and articulating with the radius and preaxial border of the ulna. It is supposed to be the fused radiale and intermedium but possibly is the intermedium only. The ulnare, of similar shape, but smaller, is approximated to the middle part of the distal border of the ulna, articulating also with the pisiform and radiale; distally with the fourth carpale only. The pisiform, of considerable size, articulates with the postaxial border of the ulna and ulnare. The first three carpalia, and perhaps also the centrale, are represented by cartilage, which fills out the interval between the end of the radiale and the metacarpals. This structure is a very ancient one as shown in the carpus of Alligatorellus (Fig. 140 b) and Crocodeleimus from the Jurassic, where, indeed, the two carpals are yet more elongate.
Fig. 140. Limbs: A, Alligator (Crocodilia). One half natural size. B, Alligatorellus (Crocodilia). Twice natural size. C, D, Amblyrhynchus (Lacertilia). Natural size.
Fig. 141. Dinosaur pedes: A, Plateosaurus (Saurischia). After von Huene. One eighth natural size. B, Gryponyx (Saurischia). After Broom. One fifth natural size. C, Allosaurus (Saurischia). After Gilmore. About one sixth natural size. D, Allosaurus (Saurischia). After Gilmore. One fourth natural size. E, Gorgosaurus (Saurischia). After Lambe. One twelfth natural size. F, Morosaurus (Saurischia). After Riggs. One twelfth natural size. G, Morosaurus (Saurischia). After Marsh. One eighth natural size. [Should be Brontosaurus.] H, Trachodon (Ornithischia). After Brown. One ninth natural size. I, J, K, Stegosaurus (Ornithischia). After Gilmore. One eighth, one eighth, and one fourth natural size. L, Thescelosaurus (Ornithischia). After Gilmore. One fourth natural size. M, Leptoceratops (Ornithischia). After Brown. One half natural size.
Fig. 142. Pterosaur limbs: A, Pterodactylus. American Museum of Natural History. Nearly three times natural size. B, Rhamphorhynchus. After Plieninger. One half natural size.
The carpus in the Dinosauria (Fig. 141) has suffered greater reduction than in any other order of terrestrial reptiles, doubtless because of the upright posture. In no form has a centrale been reported, and the fifth carpale is doubtfully present in any (Camptosaurus), as would be expected from the constantly reduced fifth finger. In the quadrupedal forms there are but two proximal bones, both large and massive. In Stegosaurus (Fig. 141 i, j) the postaxial one of the two has been found in young specimens in three parts, the intermedium, ulnare, and pisiform; perhaps that was also the case in the Sauropoda (Fig. 141 f). A small bone may possibly represent a vestigial intermedium in Leptoceratops (Fig. 141 m) of the Ceratopsia. In the Theropoda, and iguanodont orthopods, that is, bipedal forms, the radiale, intermedium, and ulnare seem distinct in all, though not large. The second row of carpals has disappeared in the Sauropoda (Fig. 141 f) and Stegosauria (Fig. 141 i). Two have been found in all other known forms, except the Trachodontidae; in most cases the third and fourth carpale, though identified as the first and second in Ornitholestes and its immediate allies of the Theropoda. The carpus in the Trachodontidae (Fig. 141 h) is more reduced than in any other reptiles, unless it be some aquatic mosasaurs, but two small bones remaining, probably the radiale and fourth carpale.
Fig. 143. Squamata, Rhiptoglossa. Limbs, etc., of Chameleon, much enlarged. A, right hand, dorsal; B, right hind foot, dorsal, with tibia, fibula, and tarsus; C, right scapulocoracoid; D, left innominate.
The most remarkable modifications of the carpus are those of the volant Pterosauria (Fig. 142). The earliest stages we do not know, though certain progressive modifications are observable from the earlier to the later. In Pteranodon and its allies of the Upper Cretaceous, the carpus is reduced to three bones: a proximal one, articulating with both radius and ulna, and perhaps to be homologized with all the bones of the proximal row except the pisiform; a distal one, composed either of the greatly enlarged fourth carpale, or a fusion of two or three, probably the former; a third carpale, on the radial side, articulating chiefly with the [distal] carpale, may be either the first carpale, the centrale, or possibly neither. In the earlier Rhamphorhynchus (Fig. 142 b), there are two distal carpals, the first articulating with the first three metacarpals, the second with the fourth or wing metacarpal. This is also the structure in Pterodactylus (Fig. 142 a), except that in some forms there are five bones, two in the proximal, two in the distal row, and the usual lateral one supporting the pteroid. This great consolidation of the carpus in pterodactyls resulted in a maximum of firmness with but little mobility, which was not needed in the volant hand.
The carpus of the Pseudosuchia and Phytosauria is practically unknown.
Fig. 144. Chelonia, Pleurodira: Thalassochelys, right front and hind legs.
Marked modification in the structure of the carpus is also characteristic of the Lacertilia (Fig. 140 c). There are but three bones in the proximal row, which may also be interpreted as the radiale, ulnare, and pisiform. No intermedium is visible in the various forms examined. It is reported to be present only in the family Lacertidae. A centrale is usually present, though sometimes small. The first centrale is also identified in some lizards. The fourth centrale [carpale], as usual, is large; the second, third, and fifth are usually large. The first is absent, unless it be the element sometimes called the first centrale.
In the curious hands of the highly specialized perching Rhiptoglossa (Fig. 143 a) the carpus is reduced to four functional bones, the radiale, ulnare, and posteriorly placed pisiform in the first row, and a large, hemispherical, fused third and fourth carpale in the distal row, around which the metacarpals revolve. Between the first metacarpal and radiale there are in the more specialized types two minute bones, which may represent the first and second carpalia, or the second and the centrale, probably the latter.
In the marine Chelonia (Fig. 144) the carpus is broad and flat, and is least reduced, though much modified. The radiale and intermedium are more or less elongate, the ulnare is small, the centrale large. The pisiform is greatly enlarged and has lost its primitive location between the ulna and ulnare, becoming attached to the ulnare and fifth metacarpal or the latter alone. This was the structure of the marine turtles as far back as the Cretaceous in Protostega, except that the proximal bones were less elongate.
At the opposite extreme, among the terrestrial tortoises (Fig. 145 a) the radiale has disappeared until nothing is left of it but a nodule of cartilage united with the first centrale, which has usurped its place. At least, this is the explanation given by Baur, who found in Emydura the two centralia in their normal positions, though enlarged. The two centralia are often present, often fused into the large single bone. The fused centralia in such early forms as Idiochelys, from the Jurassic, reached almost to the radius, and the radiale was doubtless cartilaginous. The fifth carpale may be absent, fused with the fourth, or separate and distinct. Indeed, in some old animals the third, fourth, and fifth carpalia and the pisiform may all be coössified.
The changes of the wrist and hand in adaptation to aquatic life are more profound than those of terrestrial reptiles. The earliest observed effect of water habits is delayed ossification, not only of the mesopodial bones, but of the bones of the skeleton in general, a large amount of cartilage remaining in the joints. Partial chondrification of the wrist and ankle occurred as early as the cotylosaurian Limnoscelis (Fig. 133) and Diadectes, probably marsh animals. In Clidastes (Fig. 146) of the Mosasauria, essentially a surface-swimming lizard, the four proximal bones of the wrist are ossified, but the centralia, first and fifth carpalia were not. In Platecarpus (Fig. 147) a more advanced aquatic type, the ulnare, pisiform, and second carpale have also disappeared, leaving only the radiale, intermedium, third and fourth carpalia. In Tylosaurus (Fig. 148) the most highly specialized of all mosasaurs, there are but one or two bony nodules left, one of which is certainly the fourth carpale. All the others disappeared as bones but remained as cartilage, since space is left for them in many specimens as they have been found in the rocks.
Fig. 145. Chelonia, Pleurodira: Testudo, A, front leg, dorsal side; B, the same, radial side; C, hind foot (tarsus, etc.) dorsal side.
 |
Fig. 148 |
 |
Fig. 147 |
 |
Fig. 146 |
Fig. 146. Clidastes (Mosasaur), left front paddle: c, coracoid; h, humerus; r, radius; sc, scapula; u, ulna.
Fig. 147. Platecarpus (Mosasaur), right front paddle.
Fig. 148. Tylosaurus (Mosasaur), left front paddle.
In the swimming feet of Lariosaurus (Fig. 149) of the Nothosauria the carpus is also reduced, the radiale apparently the most conspicuous for its absence.
Fig. 149. Nothosaurian limbs: Lariosaurus. About four times natural size.
Very interesting are the modifications of the wrist and hand in the marine Crocodilia (Fig. 150). But two carpals remain, corresponding to the elongated ossified bones of the terrestrial forms; the first of them, the supposed radiale, is very broad and flat.
Fig. 150. Geosaurus (Thalattosuchia). Elongate left hind leg, and paddle-like left front leg. After Fraas.
The carpus and hand of the strictly aquatic or marine reptiles are so like the ankle and foot that they may be discussed together (p. 193).
Tarsus
The earliest known tarsus is that of Eosauravus (Fig. 151 b), presumably a cotylosaur reptile, though the skull is not known, from the middle Pennsylvanian. It has but two bones in the proximal row, corresponding quite to the astragalus and calcaneum of mammals and the typical reptiles. Beyond these, six, and only six, bones are visible, five of which are undoubted tarsalia; one may be a centrale. The whole number, eight, was the most known in any reptile until recently. Nine bones are present in the tarsus of Ophiacodon (Fig. 152), from the uppermost Pennsylvanian or basal Permian of New Mexico: two in the proximal row, the astragalus and calcaneum, two centralia in the middle row on the tibial side; and five tarsaha in the distal row, one corresponding to each metatarsal. Since this discovery two centralia have also been found by Watson in the genus Broomia (Fig. 137 d), from the Permian of South Africa; and probably also two in the cotylosaurian genus Labidosaurus. The second centrale is usually retained in later reptiles, but the fifth tarsale is absent in all reptiles since Triassic times, and a free centrale is absent in all living reptiles, though present in most mammals.
 |
Fig. 152 |
 |
Fig. 151 |
Fig. 151. Limbs: A, Trematops (Temnospondyli). One half natural size. B, Eosauravus (Cotylosauria). About twice natural size.
Fig. 152. Ophiacodon (Theromorpha): right hind leg, from mounted skeleton. A little less than one third natural size, a, astragalus; c, calcaneum.
The mammalian foot, in this respect, is even more primitive than that of the lizards, turtles, and crocodiles, the navicular corresponding to the second centrale, the cuneiforms and cuboid to the four tarsalia. The fourth distale, primitively, as in the carpus and as a general rule in all reptiles, is the largest of the series, corresponding to the greater length and strength of the fourth toe.
The tarsus is known in but two temnospondylous amphibians, both from later rocks than Eosauravus. Trematops (Fig. 151 a), and Archegosaurus. In the former, and according to Baur in the latter also, there are three bones in the proximal row, the tibiale, intermedium, and fibulare; four centralia in the middle row; and five tarsalia in the distal—twelve in all.
Three of these have been lost in all known reptiles, the intermedium, or tibiale, and the third and fourth centralia. Nine bones, then, we may assume was the primitive number of tarsal bones in the reptiles. A separate intermedium has been accredited to certain reptiles, Howesia of the Rhynchocephalia, Oudenodon (Dicynodon) of the Anomodontia, and the ichthyosaurs and plesiosaurs. But, unless such forms have enjoyed an uninterrupted and independent descent of which we have no knowledge from the Amphibia, it is altogether improbable that both intermedium and tibiale have ever been present as separate bones in reptiles since early Pennsylvanian times. Otherwise we must assume that there has been a reversion from the specialized to the generalized condition of the Amphibia in these animals, a seeming impossibility in evolution. Moreover, there are but two bones in the proximal row of the tarsus of the Nothosauria (Fig. 149), and these reptiles are generally supposed to have a real genetic relationship with the plesiosaurs.
There have been various theories as to what has become of the additional bones of the amphibian tarsus.[5] Since Gegenbaur, it is generally believed that the intermedium is fused with the tibiale to form the astragalus. This is denied by Baur, who says there is no evidence of such union. Others have thought that the intermedium alone forms the astragalus, the tibiale represented by the tibial sesamoid, which occurs in certain mammals but is unknown as such in lower animals. In this uncertainty it is better to use the two mammalian names astragalus and calcaneum and abandon the names tibiale, fibulare, and intermedium for the reptilian tarsus. Of the
Fig. 153. Limbs: A, Mesosaurus (Proganosauria). Modified from McGregor. Natural size, B, Sauranodon (Protorosauria). Modified from Lortet. Three fourths natural size.
centralia the most probable theory is that the fourth of the amphibian tarsus has united with the astragalus, the third with the fourth tarsale. The second is known to fuse with the astragalus in the modern Chelonia (Fig. 154 c); perhaps at other times it is lost. And it is very probable that the first centrale of the amphibian and reptilian tarsus ceased very soon to be ossified, and is not represented, even in a fused condition, in any later reptilian tarsus. It has been shown by Baur and others that the fifth tarsale is not fused with the fourth, but has disappeared.
Among the Cotylosauria there are usually eight tarsal bones.[6] In Pariasaurus the centrale and fifth tarsale are not known with certainty. In the Theromorpha eight are present in all known forms except Ophiacodon (Fig. 152), which has nine. The centrale has not been recognized in the Proganosauria (Fig. 153 a), but there are five tarsalia; until their discovery four were the most known in any reptile. Indeed, Baur based the order Proganosauria chiefly upon this character. All other known reptiles, except certain Therapsida (like the mammals), have not more than seven tarsal bones, the fifth tarsale being invariably absent.
In Pariasaurus, Sclerosaurus, and Telerpeton of the later Cotylosauria, Sphenodon (Fig. 139 a) of the Rhynchocephalia, and most Lacertilia (Fig. 140 d) and Chelonia (Figs. 145 c, 154 d, g), the astragalus and calcaneum are fused into a single bone, and the calcaneum is either fused or lost in the Pterosauria (Fig. 155 d) and some Dinosauria (Fig. 156 i). A free centrale is absent in all modern reptiles, though sometimes suturally fused with the astragalus in the Chelonia (Fig. 154 c).
In the Chelonia the small calcaneum is sometimes free (Fig. 154 c). The centrale is never free. Four tarsalia are usually present, the third sometimes suturally united with the fourth. The fourth tarsale is always large.
In the kionocrane Lacertilia (Fig. 143 b) there is a similar condition, the small calcaneum either indistinguishably fused with the astragalus, or suturally attached in the adult. There is no centrale or fifth tarsale, and the first and second tarsalia are either vestigial or lost. The tarsus of the chameleons (Rhiptoglossa), like the wrist, is very curiously modified (Fig. 143 b). But two bones remain in the highly specialized species, the astragalo-calcaneum and the large, hemispherical fourth tarsale, articulating together enarthrodially, around which all the short metatarsals closely articulate in two groups of two and three.
Fig. 154. Limbs and feet of Chelonia. Natural size. A, Chrysemys, hind leg from postaxial side. B, Chrysemys, front foot, dorsal side. C, Chelydra, hind foot, dorsal side. D, Cistudo, hind foot, dorsal side. E, Podocnemys, left hind leg, postaxial side. F, Podocnemys, right front forearm and foot, dorsal side. G, Trionyx, left hind foot, dorsal side.
The hind foot is poorly known in the Therapsida. In Galechirus (Fig. 137 b) of the Dromasauria the fifth tarsale is lost, but a small one has been recognized in the related genus Galesphyrus (Fig. 137 c). The Anomodontia have the astragalus and calcaneum, four tarsalia, and a small, frequently unossified centrale; the fifth tarsale is absent. The tarsus is unknown in other groups.
The tarsus of the modern Sphenodon (Fig. 139 a), unlike the carpus, is highly specialized. In addition to the fused calcaneum and astragalus, the centrale and fifth tarsale have disappeared and the first three tarsalia are fused in the adult.
The tarsus of the Pterosauria (Fig. 155 d), like the carpus, is highly specialized. In the early forms the astragalus is suturally united with the tibia, the calcaneum fused with the astragalus. In the later forms the astragalus is indistinguishably united with the end of the tibia, the calcaneum fused or lost as in birds, forming a large, pulley-like articulation. In the early pterodactyls there were at least three other tarsals; in the later ones, like Pteranodon or Nyctosaurus, there are but two free tarsalia, probably the fourth and the fused second and third, or fused first, second, and third. Centralia are unknown in all.
The tarsus of the dinosaurs (Fig. 156), like the carpus, has been much modified in adaptation to upright-walking habits. There is a tendency in all for the two proximal bones, the astragalus and calcaneum, to articulate closely with the leg bones. The astragalus of the Theropoda (Fig. 156 b, c, e) fits more or less closely in a depression or groove on the under and anterior side of the tibia; in the later forms (e. g., Ornithomimus, Fig. 156 e) developing a high ascending process in front, as in the young of birds—a parallel character which has no genetic value. In the Sauropoda (Fig. 156 i) there is a less close union, perhaps due to the larger amount of cartilage in the joints of these animals. The centrale and first and fifth tarsalia are always absent. The second and third tarsalia are often fused, apparently; the fourth is always single when present. The tarsalia, like the carpalia, are absent in the Trachodontidae (Fig. 156 g); even the fourth is said to be wanting—possibly a vestige yet remains. If really absent it is the only known example among reptiles of the absence of all the bones of the distal row.
Fig. 155. Limbs: A, Araeoscelis (Protorosauria). Three fourths natural size. B, Hallopus (Dinosauria). After Marsh. One half natural size. C, Pteranodon (Pterosauria). About one third natural size. D, Pteranodon. About five sixths natural size.
Fig. 156. Dinosaur pedes: A, Plateosaurus (Saurischia). After Huene. One twelfth natural size. B, Anchisaurus (Saurischia). After Marsh. One eighth natural size. C, Allosaurus (Saurischia). After Osborn. One seventeenth natural size. D, Struthiomimus (Saurischia). After Osborn. A little more than one sixth natural size. E, Ornithomimus (Saurischia). After Marsh. One sixth natural size. F, Thescelosaurus (Ornithischia). After Gilmore. One fifth natural size. G, Trachodon (Ornithischia). After Brown. About one nineteenth natural size. H, Monoclonius (Ornithischia). After Brown. One sixteenth natural size. I, Morosaurus (Saurischia). After Hatcher. About one eighth natural size. [Brontosaurus.]
The calcaneum of the Crocodilia (Fig. 157 a, b) is produced into a heel-like process; the first and fifth tarsalia and the centrale are absent, the second tarsale is small. Hallopus (Fig. 155 b), usually referred to the Dinosauria, also has a heel-like calcaneum, as is the case in Scleromochlus, and other genera of the Pseudosuchia, Araeoscelis (Fig. 155 a), Broomia (Fig. 137 d), and other leaping or springing reptiles.
Fig. 157. Crocodilian limbs: A, B, left hind limb of Alligator, dorsal and postaxial; C, left femur dorsal; D, Alligatorellus. After Lortet. About three fourths natural size.
In the web-footed Mosasauria the tarsus, like the carpus (Figs. 146–148), progressively became more cartilaginous. In Platecarpus (Fig. 158 a) and Clidastes (Fig. 158 b) the astragalus, calcaneum, and fourth tarsale alone remain, with the divaricated fifth metatarsal, as in land lizards. In Tylosaurus, the most specialized of mosasaurs, but one, or at most two, small bones remain. Other tarsal bones remained unossified, though represented by cartilage in the adult.
Fig. 158. Limbs of aquatic reptiles: A, Platecarpus (Mosasauria), right hind leg. About one sixth natural size. B, Clidastes (Mosasauria), right hind leg and tarsus. One third natural size. C, Ophthalmosaurus (Ichthyosauria), left front paddle. One eighth natural size. D, Ichthyosaurus platydactylus (Ichthyosauria), left front paddle. One sixth natural size.
Not more than six bones of the plesiosaurs can be called tarsals, and their homologies are doubtful (Fig. 159 b, c). They have the same shapes and relations as the carpal bones and cannot be distinguished from them except by their smaller size. The three in the first row are usually called the tibiale, intermedium, and fibulare; a fourth, on the postaxial side, has sometimes been called the pisiform in both front and hind limbs, but as there never was in any terrestrial reptiles a pisiform in the tarsus, that name is of course incorrect. There are valid reasons for doubting the reappearance of the intermedium after its loss in the terrestrial ancestors of the plesiosaurs. It may be the enlarged centrale. The bones in the distal row may be the first, fused second and third, and the fourth tarsalia. The homologies of the mesopodial bones of the Ichthyosauria (Fig. 158 c, d), where a like similarity between the front and hind limbs exists, are even more doubtful. There is the same objection to the recognition of an intermedium tarsi in this order as in the plesiosaurs, whatever may be the corresponding bone in the carpus.
Fig. 159. Paddles of Plesiosaurs: A, right hind paddle of Thaumatosaurus, after Fraas. B, right hind paddle of Trinacromerum. C, right front paddle of same individual. f, femur; fb, fibula; t, tibia; h, humerus; r, radius; u, ulna.
Metapodials and Phalanges
The most primitive hand or manus known is that of the Cotylosauria, from the Permocarboniferous (Figs. 128, 133). The five metacarpals increase in length to the fourth; the fifth is shorter, but is not markedly divaricated. There are two phalanges in the thumb or pollex, three in the second digit, four in the third, five in the fourth, and but three in the fifth. The first and fifth metacarpals are more freely movable on the wrist than are the other three.
Of the Temnospondylous amphibians no complete hand is known. That there were five functional digits is certain,[7] since there are five functional carpalia in both Eryops (Fig. 136) and Trematops. It is often assumed that all amphibians of the past, as of the present, had but four fingers, as is known to be the case in some of the ancient Stegocephalia. The phalangeal formula was either 2, 3, 4, 4, 3 or 2, 3, 3, 4, 3, in the rhachitomous temnospondyls. It must be remembered, however, that we know nothing whatever of the hands or feet of the earliest amphibians, and it is purely an assumption that the reptilian hands and feet were evolved from forms like the later ones of Permocarboniferous times. In all probability the embolomerous ancestors of the reptiles had the phalangeal formulae of both front and hind feet like those of the known earliest reptiles. We can hardly conceive of an increase either of the number of digits or number of phalanges in the earliest reptiles.
In crawling reptiles the structure of the digits, it is seen, has not changed much to the present time, the modern Sphenodon (Fig. 139 a, b) as well as most modern lizards (Fig. 140 c, d) having the same number of bones arranged in the same ways. This primitive phalangeal formula is that of the Cotylosauria, Therocephalia, Theromorpha, Phytosauria, Pseudosuchia, Rhynchocephalia, Nothosauria, or at least some members of the group, and the group called by the author the Acrosauria, that is, the early Araeoscelis (Fig. 155 a), Protorosaurus (Fig. 138 d), Pleurosaurus (Fig. 139 d), and Sauranodon (Fig. 139 c). In the Crocodilia (Figs. 140 a, 157 a, b), the postaxial fingers are in all cases shorter and weaker, with fewer phalanges.
In no other reptiles has there been as great modification of the fingers as in the Pterosauria (Fig. 142), so great indeed that there is dispute as to the homologies of the ones that remained. The maximum of changes was reached in the latest forms, especially Nyctosaurus and Pteranodon, where there are three very short and weak fingers on the preaxial side, with two, three, and four phalanges, the terminal ones in the shape of strong claws. On the postaxial side the fourth finger is very long and strong, with four phalanges for the support of the patagium. This wing finger has generally been supposed to be the fifth, the first finger or pollex represented by a slender bone turned backward from the wrist toward the humerus and known as the pteroid. It seems more probable that the wing finger is the fourth, as originally so called by Cuvier, the fifth being absent. In the development of the patagium the claw of the wing finger would in all probability disappear, as in the bats, leaving the normal number for the fourth digit. If it is really the fifth, not only has the claw been converted into a long membrane-supporting phalange, but an additional phalange has been added; while each of the preceding three digits has lost one phalange. We can conceive of no cause for such hypo- and hyper-phalangy in the hand in these volant reptiles. One of the phalanges of the third finger is short, as in the third digit of the foot.
The first three metacarpals of the early pterodactyls articulated normally with the carpus (Fig. 142); in the later ones they were mere splints lodged loosely in the flesh at the distal end of the fourth metacarpal, only the first of them retaining a very slender connection with the wrist. The fourth metacarpal, on the other hand, progressively increased in length till it much exceeded the length of the forearm. Its distal articulation is a very perfect pulley-like joint, permitting flexion of the first phalange through almost one hundred and eighty degrees.
A general reduction of the postaxial digits of both front and hind feet is characteristic of the Dinosauria (Figs. 141, 156). Only in the primitive Anchisaurus and Plateosaurus (Fig. 141 a) is a nearly complete hand recognized, and even in these, two phalanges of the fifth finger are gone. The fifth finger is absent in all Theropoda since the early Jurassic, the fourth usually, the third sometimes. In Gorgosaurus (Fig. 141 c), from the uppermost Cretaceous, the hand is functionally didactyl, the extreme of specialization among reptiles. In the Theropoda (Fig. 141 a–e) the thumb is the stoutest digit, its claw the largest. In the herbivorous dinosaurs (Fig. 141 f, h, i, l, m) the hand is less preaxial, the first and second fingers being the larger. In the Trachodontidae (Fig. 141 h), indeed, the first finger is absent. In all herbivorous forms the outer fingers are reduced, though the fifth is seldom entirely absent, the phalangeal formula never exceeding 2, 3, 4, 3, 2, the claws lacking in the two postaxial digits. In Trachodon a greater reduction has occurred, almost the maximum among reptiles, the formula, according to Lambe, being 0, 3, 3, 2, 2. The ungual phalanges of both front and hind feet are characteristic, curved and sharply pointed in the Theropoda (Fig. 141 a–d), more obtuse in the Sauropoda (Fig. 141 f, g), for the most part hoof-like in the Predentata (Fig. 141 h–m).
The foot or pes of reptiles is similar in structure to the hand, the reduction of the toes being usually anticipatory of the fingers in the terrestrial forms. There was one more phalange in the fifth toe than in the fifth finger primitively. In Pariasaurus, only, of the Cotylosauria, the phalangeal formula is slightly reduced, though primitive in Propappus, a related genus.
The loss of the fifth toe is rare among reptiles, aside from the Dinosauria. The crocodiles (Fig. 157 a, b, d) have only the fifth metatarsal left, and the fourth toe has but four phalanges. A very few lizards also have lost the fifth toe. It is often reduced among the Chelonia (Fig. 154 c, d); usually one, sometimes two, of the normal phalanges are lost. The greater strength of the foot in this order as in the dinosaurs is more to the preaxial side, unlike most other reptiles.
The foot of dinosaurs (Fig. 156), so far as the reduction of phalanges is concerned, is less specialized than the hand, the Theropoda (Fig. 156 a–e) retaining the original formula, except in the fifth toe. Plateosaurus (a) and Anchisaurus (b), from the Trias, have the formula 2, 3, 4, 5, 1; Allosaurus (c), from the lower Cretaceous, and Struthiomimus (d), from the uppermost Cretaceous, 2, 3, 4, 5, 0, the fifth metatarsal a vestige. The known Sauropoda (i) have 2, 3, 4, 3, 1 phalanges. Among the Predentata (f–h) the phalanges of the fifth toe are invariably absent in known forms, the formula, 2, 3, 4, 5, 0 being the usual one, and in Trachodon (g), 0, 3, 4, 5, 0. Among the quadrupedal Sauropoda (i) the axis of the foot is more to the preaxial side; in other dinosaurs it is the third toe that is the stoutest, though less so in the oldest theropods (a, b), this arguing perhaps a more sauropod-like mode of progression.
The earliest pterodactyls had two or three phalanges in the fifth toe; the later ones (Fig. 155 d) have only the hook-shaped metatarsal left. The greatly elongated feet were adapted more for perching or clinging than for locomotion. A striking peculiarity is seen in the greatly reduced second phalange of the third toe and the second and third of the fourth toes, singularly identical with the corresponding phalanges of the hand of the therocephalian Scymnognathus (Fig. 138). Similar reduced phalanges are seen in the hand of the theropod Struthiomimus and the tree sloths among mammals, in all cases doubtless to be ascribed to the grasping or clinging habits.
A peculiarity of the fifth metatarsal among the Diapsida (Figs. 139 a, 153 b), or many of them, and the Sauria (Fig. 140 d) and Chelonia (Figs. 144 b, 145 c, 154) is the more or less hook-like shape, proximally, a character which has been adduced in proof of their phylogenetic relationships. In all such cases the metatarsal articulates with the fourth tarsale, and the fifth tarsale is absent. In those reptiles which have a fifth tarsale, either ossified or cartilaginous, the metatarsal is straight, and perhaps also in those reptiles in which the foot had become more or less erect or digitigrade before its entire loss.
Hypophalangy. In the Chelonia (Figs. 144, 145, 154), Dromasauria (Fig. 137 a, b), Anomodontia, Cynodontia, as in the mammals, the primitive phalangeal formula suffered a reduction to 2, 3, 3, 3, 3 in both front and hind feet, with a further reduction to 2, 2, 2, 2, 2 (i) (Fig. 145 a) in many tortoises. The river turtles (Trionychoidea. Fig. 154) have normally four phalanges in the fourth and the fifth digits, which may rather be ascribed to a secondary hyperphalangy. More than three phalanges have also been observed in some Pleurodira. The chameleon lizards have the phalangeal formula 2, 3, 4, 4, 3 for both fore and hind feet (Fig. 143), and various examples of partial reduction of the postaxial digits occur among the Cotylosauria (Pariasaurus), Crocodilia, and especially the Dinosauria, as has been mentioned above.
Hyperphalangy and Hyperdactyly. An increase of the phalanges above the normal number (hyperphalangy) and of the digits (hyperdactyly) is known only in swimming animals. In some if not all Proganosauria (Fig. 153 a) the fifth toe has two extra phalanges, that is, 2, 3, 4, 5, 6, possibly but very improbably a primitive character, as the earliest foot known (Eosauravus, Fig. 151 b) from the middle Pennsylvanian has the same number and arrangement of the phalanges as in the Cotylosauria (Figs. 128, 133) and modern lizards (Fig. 140). In Trionyx, a river turtle, five phalanges have been observed in the fourth toe, and as many as six in the fifth, certainly an acquired character, and the only examples of hyperphalangy in the order Chelonia. In web-footed swimming animals there is sometimes a tendency toward the elongation of the fifth toe, as observed in Eosauravus (Fig. 151 b), Lariosaurus (Fig. 149), and especially Mesosaurus (Fig. 153 a), and Tylosaurus (Fig. 148). It may perhaps indicate the use of the hind legs more as sculling organs after the manner of seals, sea otters, and the Cretaceous bird Hesperornis, in all of which the fifth toe is very long and strong, though without additional phalanges.
In all strictly aquatic reptiles (Figs. 158, 159) the digits are elongated, and except in the Chelonia, there was an increase of the number of phalanges in both front and hind feet, sometimes far beyond the normal number. A like hyperphalangy is observed in the marine mammals, one or two additional cartilaginous phalanges in the sirenians, and from two to ten ossified ones in the Cetacea. Various theories have been proposed to account for their origin. That they cannot be due to the ossification and separation of the normal epiphyses in reptiles is quite evident, for these reptiles at least had no epiphyses. Like the additional epipodials of the plesiosaurs and ichthyosaurs, they are accessory, new ossifications in the mesenchyme and not reversions to a primitive fish-like fin.
In the Mosasauria there was a progressive increase in hyperphalangy as observed in the genera Clidastes (Fig. 146), Platecarpus (Fig. 147), and Tylosaurus (Fig. 148) from one or two to as many as six or eight additional phalanges, concomitant with the progressive chondrification of the mesopodials. In certain plesiosaurs as many as twenty-two phalanges are known in the third digit, and certain ichthyosaurs have even more. Hyperdactyly, due to the same causes, is known in the ichthyosaurs only among reptiles (Fig. 158 c, d). More usually the feet are pentedactylate, but certain early forms have but three digits, while other later ones may have as many as nine. It is a question whether three was the primitive number, and that all above that number are accessory; more probably the hypodactyly occurred after the ichthyosaur paddle was essentially evolved. Some of these accessory digits seem to have arisen at the sides of the paddles; others by a splitting of the digits, as shown in Figure 158 c. The paddles of both the plesiosaurs and ichthyosaurs were oar-like and flexible; the feet of the mosasaurs were webbed, more like the feet of ducks and frogs.
In crawling reptiles (Figs. 1, 128) the feet are directed more outwardly, and the motion of the foot upon the epipodials is largely lateral. The structure of the feet in the early forms, even as late as Sauranodon (Fig. 153 b), with a large astragalus and calcaneum, shows an extensive lateral movement of the foot upon the leg in locomotion. In the modern lizards (Fig. 140 d) and Sphenodon (Fig. 139 a) the angle between the leg and foot in locomotion is acute, probably much more so than in the early forms, and this may account for the coössification of the heel bones. In such reptiles the toes always are and must be long, with the main axis of the foot more postaxial. On the other hand, the direction of the foot in the turtles, and especially the tortoises (Fig. 145), is more forward than lateral; the digits of the feet on the two sides are brought more nearly parallel in locomotion. The same acute angle between the foot and leg and the elevation of the heel bones have also resulted in the firmer ossification of the tarsal bones and their fusion. In such locomotion long toes would be a hindrance, and they have been shortened, both by a reduction in the numbers and by a shortening of the segments.
Doubtless this same more mammal-like or turtle-like mode of progression was characteristic of the Dromasauria, Anomodontia, and Theriodontia, and likewise resulted in the reduction of the phalanges and shortening of the toes. One can imagine the difficulties of locomotion if our toes were six inches long! The Cotylosauria have short and broad feet, and many of the later ones, like Pariasaurus and Telerpeton have the astragalus and calcaneum fused. Possibly the mode of progression was more turtle-like than lizard-like, and the results began to be seen in the reduced phalangeal formula of Pariasaurus. Except among the Sauropoda and Stegosauria, in which the toes have become shortened and the phalanges on the postaxial side reduced, the dinosaurs have rather long digits, but they had become distinctively digitigrade, shortening the portion resting upon the ground, like the reduction of the digital formula in the plantigrade reptiles. In all such reptiles with the more mammal-like mode of locomotion, the foot is more mesaxial or preaxial, as in the mammals, where the fourth is very seldom the strongest toe.
The chief joint between the foot and legs in mammals is between the end of the tibia and the first row of the tarsals. In reptiles it is intratarsal, that is, between the first and second rows of the tarsus. In those reptiles which walked more or less upon the toes, digitigrade, there was a progressively closer articulation between the tibia and the astragalus, giving a firmer and closer ankle which otherwise would have been subject to injury with the heel elevated far above the ground. In the bipedal Theropoda (Fig. 156 a, c), the astragalus, while perhaps never fully fused with the tibia, acquired a long ascending process which fitted closely into a groove in front of the distal end of the tibia. In the still more elongated feet of the pterodactyls (Fig. 155 c, d) the astragalus became indistinguishably fused with the tibia, as in birds, and the joint, while actually, as formerly, intratarsal, was functionally between the leg and tarsus as in mammals.
Short toes and reduction of phalanges, then, mean a more mammal-like mode of locomotion and posture of the feet. In the turtles this has been produced by the exigency of the immovable shell, and by the greater or lesser twisting of the epipodials upon the propodials.
- ↑ [But Romer (Bulletin, Amer. Mus. Nat. Hist., 1922, vol. XLVI, plate XLVI) shows that the pariasaur femur (Propappus) differs in significant features from the femora of cotylosaurs, while Amalitzky, Broom, and Romer are agreed that the femur of pariasaurs was directed obliquely downward.—Ed.]
- ↑ [Recent evidence (Romer, 1924) indicates that this process is not homologous with the true "lesser trochanter" of mammals. A better name for it is "internal trochanter."—Ed.]
- ↑ [For a different interpretation of the manus of Eryops see Gregory, Miner, and Noble in Bulletin, Amer. Mus. Nat. Hist., 1923, vol. XLVIII.—Ed.]
- ↑ [But see Steiner, Acta Zoöl., 1922, pp. 307–360.—Ed.]
- ↑ [For an excellent review of this subject see Broom, 1921, in Proc. Zoöl. Soc., London, pp. 143–155.—Ed.]
- ↑ [Watson (Proc. Zoöl. Soc., 1919) reports the presence of three bones in the proximal row of the tarsus of the very primitive Seymouria, and adopts the view that the true tibiale has disappeared in later reptiles, the astragalus representing the intermedium only.—Ed.]
- ↑ [For a different interpretation, however, see Gregory, Miner, and Noble in Bulletin, Amer. Mus. Nat. Hist., 1923, vol. XLVIII.—Ed.]