Rhynchippidae
This family name is used for the three genera Rhynchippus, Morphippus, and Eurygeniops, which made up a part of Ameghino’s family, Notohippidae. These forms I find much simpler than Coresodon, Interhippus, Stilhippus, and Nesohippus, which, by their molars, should be associated with Nesodontidae, unless it should prove that they did not have the incisors enlarged to caniniform teeth, in which case another disposition will have to be made of them. Ameghino places the Rhynchippidae among his Hippoidea, leading to horses, but we found a nearly complete skeleton of Rhynchippus equinus which in all particulars is typically toxodont. In the Deseado we found fourteen specimens belonging to this family, and strangely enough they were all Rhynchippus, and all of the species R. equinus.
This family is distinguished by the brachydont, or nearly brachydont, molar teeth, being relatively simple, and the secondary cristae not being developed. The large basin in the upper premolars and molars is, therefore, not subdivided, but is deep, and rather narrow, usually appearing as an oblique pit in the centre of the crown. There is no enlargement of the incisors to make caniniform teeth. Both the upper incisors and the canine have in the crown a longitudinal groove, which on wear becomes a pit, and being shallow may disappear entirely. The lower teeth are those typical of all toxodonts. The feet are tridactyl, and compact.
The following three genera may be distinguished:
Rhynchippus Ameghino
Rhynchippus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 462.
The teeth of both jaws are rooted, but tend to be hypsodont. The elongated incisors of the upper series are characterized by the presence of a longitudinal furrow in the top of each tooth, which, with wear, becomes a pit, and, as it is shallow, disappears in old individuals. This is the only suggestion of a horse character in the genus, but the pit in a horse’s incisor is a late development, and here it is also probably a specialization due to eating grass. Incisor 1 is the largest and they decrease in size toward either side. The canine is small, and is also marked by having a furrow in the crown, but in this case it is transverse to the long axis of the jaw.
The premolars are peculiar in having on the anterior internal corner a highly developed cingulum, which so builds out the tooth that it is usually wide and is rectangular in outline. As this cingulum rises, it incloses a bay on the ant. int. corner of the tooth, which, with wear, becomes first a bay, then a pit, and lastly may disappear entirely in old age. On each premolar the anterior and posterior lobes are developed, inclosing between them an elongated basin, which with wear becomes a long narrow pit. On the molars, the cingulum on the ant. int. corner is wanting entirely. The external anterior corner of the tooth, however, is prolonged, so that the crown has a rhomboidal outline. The crown is made up in the typical manner of the wall, the anterior, and the posterior lobes, which inclose between them an elongated basin, which, as in the premolars, becomes, on wear, a pit extending obliquely across the tooth.
The lower incisors have no furrows in the crowns, but in this genus there is a small cingulum on the inner side just above the base of the enamel. The lower canine is incisiform, and also has the basal cingulum. Each of the premolars has, on the external side, a median vertical groove, beginning at the base of the enamel, and widening toward the top. This is progressive if less marked from pm. 1 to pm. 4. The premolars and molars consist essentially of two crescents, the shorter anterior, and the posterior which is about twice as long as the anterior. The details are as described on page 96, and seen in [figure 55].
The skull is of moderate height, nearly flat on top with wide zygomatic arches. The sagittal crest is moderately high, and slightly convex in the antero-posterior direction. The occipital region is overhanging and topped by short lambdoidal crests, which, extending to either side, unite with the zygomatic arches. The nasals are large, roughly rectangular, and slightly constricted just in front of the middle. The frontals are short, and project over the orbits in strong processes. The maxilla is large, bounding the front of the orbit, and extending backward in a strong zygomatic process which makes nearly half of the arch. The jugal, while stout, is short, and reaches from the long zygomatic process of the maxilla to the short one of the squamosum. The lachrymal bone is tiny, with a low tubercle, just below which is situated the lachrymal duct, just on the margin of the orbit. Just behind the zygomatic arch, the squamosum is inflated and contains a large hollow chamber, as is typical among toxodonts. The mastoid bullae, while relatively small, are swollen into a globular form, and have a large hollow chamber. The palate extends back to just behind the last molar, a feature distinguishing this genus from Morphippus.
Of the vertebral column, twenty-six vertebra are preserved (a few being represented by neural arches only). The atlas and axis are unknown. Five cervicals are present, each with a short, slightly opisthocoelus centrum, and with low weak spines. The foramena for the vertebra artery are usually large. Cervical 3 has a rather slender transverse process, projecting down—and backward. On cervicals 4, 5 and 6, these lateral processes are enlarged into broad lamellae, which reach their maximum of size on the sixth. Cervical 7 has no lamella, simply a slender transverse process. These transverse processes are strikingly like those of Nesodon. The thoracic vertebrae (of which I have complete or in parts 15) have moderately high spines, which resemble those of Adinotherium, not only in the general build, but also in the presence of a foramen for the exit of spinal nerves through each neural arch. These foramena can not be referred to as adaptations, but are special features indicating close relationship with the Nesodontidae. Six lumbar vertebra are present, each having broad depressed centra, and short wide spines. The rest of the column is unknown.
The distal portion of the humerus is preserved, showing the trochlea to be relatively narrow, with a prominent internal phlange for the ulna. The epicondyles are both small. The supratrochlear fossa is moderately deep, the anconeal fossa very deep, a large perforation connecting the two. Of the ulna, only the distal end is preserved, and it is marked by a prominent styloid process, ending in the facet for the pyramidal, this facet continuing uninterruptedly into that for the pisiform. The two ends of the radius are preserved but its length can only be conjectured. The proximal end has a large facet for the humerus; the distal end two facets, for the scaphoid and luna respectively, the two being almost continuous, except as the outline of the shallow depressions is constricted near the middle.
The carpus is preserved in toto and is decidedly weak for an animal of this size, though no more so than is the case of Nesodon and Adinotherium, with which forms the arrangement of the bones agrees in almost every detail. The scaphoid has a broad facet on the upper side for the radius, on the ulna side a narrow band-like facet for the luna, and distally facets for the trapezoid and the magnum, none for the trapezium. The luna is a larger bone with a broad radial facet on the upper side, a narrow facet for the scaphoid on the radial side, a larger one for the pyramidal on the ulnal side, and two broad facets for the magnum and unciform on the lower side. The pyramidal is a low, flattened bone, with a cup-like facet for the ulna on the upper surface, an elongated flat facet for the pisiform on the palmar surface, and below a broad facet occupying the entire lower side for the unciform. The pisiform is shaped like a tiny calcaneum, and, beside the facet for the pyramidal, has a broad contact on the styliform process of the ulna. The trapezium is a flattened nodular bone, resting against the side of the upper end of Mc. II, for which it has a flattened contact surface, but it does not properly articulate with any of the carpals. The trapezoid is a small bone, cuboidal in shape, with the proximal facet for the scaphoid rounded, and with a narrow facet for the support of Mc. II on the distal end. The magnum is a larger bone, articulating proximally with the scaphoid and luna, on the ulnal side with the unciform and distally supporting Mc. III. The unciform is the largest of the carpal bones, articulates proximally on the luna and pyramidal, on the radial side with the magnum, and distally carries Mc. IV, while on the ulnal side there is a facet for the modular vestige of Mc. V.
The metacarpals are longer than those of Adinotherium, and are much more closely pressed together, making a narrower, firmer foot. Three metacarpals are present (beside the modular vestige of Mc. V.). Mc. II and Mc. IV are slightly shorter than Mc. III, but not materially weaker, so that all three would reach the ground when the animal was standing. Though closely packed, the metacarpals are not grown together at any point. Distally each has a narrow trochlea, which carries a median crest on the palmar side only. Under the distal end of each metacarpal, there is a pair of small sesamoid bones.
All the phalanges are very short and weak, with the articular surfaces cut under obliquely, suggesting that the toes are bent upward. Distally each toe ends in a slender cleft ungual phalanx, suggesting a claw-like hoof. Rhynchippus clearly walked in a semidigitigrade manner, the weight coming principally on the metapodials, the foot as a whole resembling that of a dog.
The pelvis is unidentified. Though the bones are of about the same weight, the hind limb is longer than that of Adinotherium. The femur has a rounded head on a short but distinct neck, with the pit for the round ligament on the posterior margin of the head. The narrow digital fossa is deep. The greater trochanter is rugose and strong, but does not rise quite to the height of the head. The lesser trochanter makes a strong shelf-like process well below the head, while the third trochanter is a prominent process about the middle of the shaft, on the posterior side. The shaft is broad and flattened above, but narrows and becomes circular in section below. The condyles are relatively small, the inner being the smaller and more convex, while the outer is broader and less convex.
The tibia and fibula are a little longer than the femur, fused proximally, free distally, as in toxodonts. The proximal end of the tibia is too weathered to permit detailed description. However the upper end is wide and flattened from front to back. Distally the bone narrows until the lower part of the shaft is circular in section, the distal end enlarging again in the neighborhood of the articulation. The fibula has a broad facet for the inner side of the astragulus, and on the distal end a flattened slightly concave facet for the calcaneum.
The calcaneum is longer than in Nesodontidae. It is, however, heavy and stout, the tuber broadening slightly toward the free end, on the plantar side of which there is a tendinous sulcus as in Nesodon. The fibular facet is wide, rectangular, and convex. Of the astragular facets, the sustentacular extends well back onto the tuber, and the ectal is the usual ovoid surface. Distally there is a broad slightly concave facet for the cuboid, and external to this a narrow surface for contact on the navicular. Except in length, this bone is very like that of Adinotherium. The astragulus and rest of the tarsal bones are wanting. Parts of the metatarsals and a phalanx indicate that the hind foot is of the same tridactyl type as the front, differing only in that the median digit seems to be relatively a little heavier.
Ameghino described three species of Rhynchippus, R. equinus, R. pumulis, and R. medianus.
Rhynchippus equinus Ameghino
R. equinus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 463.
This species is the dominant one in the Deseado from the Chico del Chubut, west of Puerto Pisser, no less than fourteen individuals being represented in our collection, three by skulls, and one by the major part of a skeleton which was found associated with the skull of Leontinia, but was determined as belonging to this species, by the duplication of the radius with a specimen having the radius and lower jaws associated. The description of the generic characters is largely based on this skeleton. The three species are differentiated largely by their size, R. equinus being the largest, but as compared with R. pumulis it is not only larger but much heavier built.
Fig. 53. Dorsal view of skull—
½ natural size.
The skull has been described under the generic discussion. In young individuals, the furrow in the incisors and canine is marked as a groove, later as a pit, and still later is lacking altogether, as is the case in the specimen figured, for all three of my more complete specimens are old individuals, as is also Ameghino’s type. All the incisors show the cingulum near the base of the enamel. On account of the development of the cingulum on the inner anterior corner of the premolars, these teeth are broadened out and overhang the palate in a marked degree, and are also much wider than long. Molar 1 is intermediate in character between the premolars and the succeeding molars, being only slightly rhomboidal in outline, while in the last two molars the anterior external corner is markedly prolonged.
Fig. 54. Left upper
dentition,
old individual—
½ natural size.
Fig. 55. Right lower
dentition of
R. equinus;
A, of a young individual;
B, of an old individual—
½ natural size.
In different individuals, the lower jaw varies greatly in height, but this seems to me to be individual and sex variation. The three incisors and the canine are subequal in size, closely crowded, and each with a small cingulum near the base of the enamel.
| Measurements of Skull, Specimen 3191 | |
|---|---|
| Skull, length from front of nasal to back of lambdoidal crest | 211 mm. |
| Skull, width across zygomatic arches | 172 mm. |
| Skull, width across postorbital processes | 78 mm. |
| Skull, height above molar 2 | 84 mm. |
| Dentition, from upper inc. 1 to molar 3 | 140 mm. |
| Dentition, from lower inc. 1 to molar 3 | 136 mm. |
| Mandible, height under molar 2 | 35 mm. |
| Mandible, height under the articulation | 118 mm. |
The atlas and axis are wanting, and the characteristics of the other cervicals have been given under the generic description.
| Measurements Specimen 3291 | ||
|---|---|---|
| Cervical 3, | length of centrum | 18 mm. |
| Cervical 5, | length of centrum | 18 mm. |
| Cervical 6, | length of centrum | 21 mm. |
| Cervical 6, | height from base of centrum to top of spine | 74 mm. |
| Cervical 7, | length of centrum | 21 mm. |
| Cervical 7, | height from base of centrum to top of spine | 55 mm. |
Fig. 56. Cervicals. 5, 6 and 7—
½ natural size.
Fig. 57. Dorsal 6 and 7—
½ natural size.
The first three or four of the thoracic vertebrae are represented only by their neural arches and spines. There were at least fifteen in the series, for I have that number represented, but more probably the number was sixteen as in the case in Adinotherium. Typical vertebrae measure:
| Specimen 3291 | ||
|---|---|---|
| Thoracic 3, | length of centrum | 21 mm. |
| Thoracic 3, | height from base of centrum to spine | 56 mm. |
| Thoracic 6, | length of centrum | 22 mm. |
| Thoracic 6, | height from base of centrum to spine | 79 mm. |
| Thoracic 14, | length of centrum | 26 mm. |
| Thoracic 14, | height from base of centrum to spine | 65 mm. |
In my series, there are six lumbars, which is one more than is credited to Adinotherium, though in that genus the number has not been definitely fixed. Typical lumbars measure as follows:
| Lumbar 2, | length of centrum | 29 mm. |
| Lumbar 2, | height from base of centrum to spine | 65 mm. |
| Lumbar 4, | length of centrum | 31 mm. |
| Lumbar 4, | height from base of centrum to spine | 57 mm. |
There is nothing to represent the sacrum or caudals.
Only the lower half of the humerus is preserved and that with specimen 3191, and it measures:
| Humerus, | diameter of shaft | 22 mm. |
| Humerus, | greatest width of distal end | 46 mm. |
| Humerus, | width of trochlea | 36 mm. |
Fig. 58. Humerus—
½ natural size.
Fig. 59. Right front foot—
½ natural size.
The distal ends of the radius and ulna are preserved in specimen 3291, as they were found in association with the carpus.
| Radius, | diameter of the distal end | 29 mm. |
| Ulna, | diameter just above styloid process | 21 mm. |
| Ulna, | diameter of styloid process | 11 mm. |
The carpus is carefully drawn, from which the various measurements may be obtained. There is a tendency for the two rows of carpals to alternate, but this is not advanced to any considerable degree. The trapezium is entirely isolated from the other carpals, and lies as a flattened scale, on the side of the upper end of Mc. II.
The metacarpals are closely crowded together, making a compact foot with very little freedom of motion in its upper part. The three carpals are of nearly equal length, though the third is slightly heavier and longer than the others, but there is no tendency toward a further reduction of the toes.
Fig. 60. Digit 4 of Rhynchippus
from the side—
½ natural size.
Fig. 61. Pair of sesamoids
under Mc. IV—
½ natural size.
| Metacarpus II, | length | 67 mm. |
| Metacarpus III, | length | 74 mm. |
| Metacarpus IV, | length | 69 mm. |
Under each metacarpal are two small sesamoid bones which lie either side of the low crest of the metacarpus. The toes are all short, with flattened articular ends, which are cut under in a very oblique manner. The second phalanx is much shorter than the others, while the distal, or ungual phalanx, is the longest and highest of the three. Each ungual phalanx is cleft by a deep narrow notch, much more suggestive of a claw than a hoof. The phalanges of all the toes are subequal in size, so that the measurements of the middle digit are given.
| First phalanx of digit III, | length | 12 mm. |
| Second phalanx of digit III, | length | 8 mm. |
| Third phalanx of digit III, | length | 16 mm. |
As preserved, the femur is crushed, and the distal end of the rotular trochlea is weathered off, but all the other characters are well preserved. The femur is slender, with a small rounded head. The greater trochanter is heavy but does not project above the head, the lesser is small but well marked; and the third is usually far down the shaft. Of the two condyles the inner is the smaller and more convex.
Fig. 62. Right femur
posterior side—
½ natural size.
Fig. 63. Right tibia and fibula
posterior side—
½ natural size.
Fig. 64. Calcaneum—
½ natural size.
| Specimen 3291 | ||
|---|---|---|
| Femur, | greatest length | 202 mm. |
| Femur, | diameter of head | 26 mm. |
| Femur, | width across head and greater trochanter | 62 mm. |
| Femur, | width of internal condyle | 16 mm. |
| Femur, | width of external condyle | 24 mm. |
The tibia is much flattened at the upper end and tapers to nearly circular in section in the distal portion of the shaft. It is fused proximally to the fibula, but free distally.
| Tibia, | length | 222 mm. |
| Tibia, | greatest width proximally | 51 mm. |
| Tibia, | least diameter of the shaft | 24 mm. |
| Tibia, | diameter of distal articular end | 23 mm. |
The fibula is a very slender bone, with the distal end swollen and heavy. As it rises from the carpus it is so twisted that it unites with the upper end of the tibia almost on the posterior surface.
| Fibula, | diameter of the articular end | 17 mm. |
| Fibula, | least diameter of the shaft | 10 mm. |
The calcaneum is of moderate length, and very stout, resembling that of Adinotherium, except that it is longer.
| Calcaneum, | length | 64 mm. |
| Calcaneum, | width | 28 mm. |
Of the hind foot there is preserved only the distal portions of two metatarsals, which are about the same size and character as those of the front foot, and a phalanx of the first row, also similar to the same one of the front foot.
[Figure 65] gives a restoration of the animal based on the bones described in the foregoing pages. The animal in all features turns out a typical toxodont, adapted, by the cropping teeth, and the broad-faced premolars and molars, for a grazing animal, but its advancement in adapting itself to feeding on grass has not proceeded very far, as is indicated by the shortness of the molars. The legs are longer than in the other families of the toxodonts which would signify that it had developed some speed, but the feet have progressed toward the modification of the hoofs into claws, indicating a foot more like that of a dog, in which the weight is not carried on the ungual phalanges, but rather on the ball of the foot, or bases of the metapodials.
Fig. 65. (Half tone.) Restoration of Rhynchippus equinus,
Ameghino—⅙ natural size.
I should not feel that this group was the ancestral one to later groups of toxodonts, but it seems rather to represent a line which terminates in the Deseado or very little later, not having run up into the Santa Cruz. The line of ancestry for the toxodonts is rather through Leontinidae.
Rhynchippus pumulis Ameghino
R. pumulis Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 464.
Fig. 66. R. pumulis—½ natural size;
A, top of skull; B, upper dentition,
after Ameghino.
We found no specimens of this species, but Ameghino has described a complete skull, a figure of which is reproduced here. It indicates a smaller lighter built animal, differing from R. equinus not only in small size, but also in having a relatively longer and narrower head. The individual is a rather old one, so that the pits in inc. 1 and 2 have disappeared, as is also the case with the cingulum on the ant. int. corners of the premolars. Ameghino gives the following measurements in his description.
| Skull, length over all | 155 mm. |
| Upper dentition, from inc. 1 to m. 3 | 80 mm. |
Rhynchippus medianus Ameghino
R. medianus Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 375.
This third species is intermediate in size between the two foregoing. No figure is given, but the following measurements give the size:
| Upper molar 2, length, 17 mm., width | 11 mm. |
| Length of lower molars 1 to 3 | 40 mm. |
| Height of mandible under m. 2 | 24 mm. |
These figures would indicate a form about 15% larger than R. pumulis, and 35% smaller than R. equinus.