Order II. Edentata

Chapter VIII. Edentata—Ganodonta

Terrestrial, partly subterranean, or arboreal creatures of quite small to gigantic size (some extinct genera), with frequently a covering of scales or bony scutes. Limbs clawed. Teeth either totally absent or, if present, imperfect in structure, being without enamel, and not forming a complete series; incisors and canines being as a rule absent. Teats axillary, pectoral, or inguinal. Retia mirabilia very common in the extremities.

To this group the name of Bruta was given by Linnaeus, but then it included not only the families which we now place in the modern order Edentata, but also the Elephant and the genus Trichechus. Mr. Thomas has proposed to change the name into Paratheria, which name is suggestive of what he and some others think concerning the systematic position of the group, i.e. that it is not to be placed in the Eutherian group of mammals at all, but represents a separate twig which has arisen with the Eutheria from a low mammalian stock. This view can hardly be accepted if the Ganodonta—which will be treated of presently—be really ancestral Edentates, for they are not in any way a Prototherian mammalian group, so far as their remains enable us to judge.

The Edentata contain the Sloths, Ant-bears, Armadillos, Manis and Orycteropus, among living forms. The great Ground-Sloths, Megatherium, etc., and Armadillos, Glyptodon, etc., represent the extinct forms.

The name that has been applied to this group is inappropriate inasmuch as many Edentates have teeth. It is, however, by a number of small tooth-characters that the order can be defined. Thus if teeth are present they are simple in structure, without enamel in the adult condition, though a rudimentary enamel-organ has been discovered in an Armadillo. The teeth, moreover, are not found in the anterior part of the mouth, and they grow from persistent pulps; neither is there much differentiation among them. It is not possible, however, to speak of the Edentates as quite homodont, since in Orycteropus there are large cheek-teeth; but there is at any rate not a marked heterodonty in that or in any other Edentate. It used to be said that the Edentates were monophyodont. But the Armadillo Tatusia was subsequently found to possess a second suppressed dentition, and after this discovery Mr. Thomas proved that Orycteropus is also diphyodont. Since then other Armadillos have been shown to be diphyodont; and the whole group therefore, so far as concerns those members that have teeth, may in all probability be regarded as typically mammalian in this respect.

These characters are slender enough, but there seem to be no others by means of which the members of this order can be satisfactorily linked together. The fact is, that we have here a polymorphic order which contains in all probability representatives of at least two separate orders. We have at present a very few, and these perhaps highly modified, descendants of a large and diverse group of mammals. For convenience' sake they will be all treated of under the head of Edentata.

Although for the probable reasons already stated it is a hard matter to frame such a definition as will include all existing Edentates, it is easy enough to define two groups in this heterogeneous order; to define one group we should say, rather, and then to regard the leavings as forming another not so easily definable a group.

The perfectly-definable group is that which includes the American Anteaters, the Armadillos, and the Sloths. In all these creatures, which may certainly be regarded as representing on their own account as many family types, there are a number of important and highly-characteristic anatomical features which they share in common. So exceedingly different are these three types in general appearance and (correlated with that) in way of life that these common characters acquire increased importance.

Fig. 89.—Great Anteater (Myrmecophaga jubata). A, Side view of twelfth and thirteenth thoracic vertebrae. B, Posterior surface of second lumbar vertebra. C, Anterior surface of third lumbar vertebra, × ⅔. az, Anterior zygapophysis; az, az, az, additional anterior, articular facets; cc, facet for capitulum of rib; m, metapophysis; pz, posterior zygapophysis; pz, pz, pz, additional posterior articular facets; t, transverse process; tc, facet for articulation of tubercle of rib. (From Flower's Osteology.)

The first of these characters is the series of additional zygapophyses on the posterior dorsal and lumbar vertebrae; these are very clear in the Anteaters and Armadillos; less clear, but still obviously represented, in the Sloths. In the second place, they all possess a clavicle, rudimentary, it is true, in the Great Ant-bear, but still present. Thirdly, the testes are abdominal throughout life, a character which they share with such lowly-organised animals as the Monotremata and the Whales. Finally, and this is by no means a matter to be overlooked, not only are all the existing members of this group American in range, but there is no evidence to prove that they have ever existed elsewhere. No European or Old-World representatives have as yet been discovered which can be referred to the Anteater, Armadillo, or Sloth type with certainty.

Of these American forms, which will be treated of first, the Armadillos are further apart from either Sloths or Anteaters than the last two are from each other. The name Xenarthra has been suggested for the American Edentates with "abnormal" vertebral articulations; the corresponding Nomarthra includes the Old-World forms.

Fig. 90.—Right scapula and clavicle of Two-toed Sloth (Choloepus hoffmanni). × 1⅔. a, Acromion; af, prescapular fossa; c, coracoid; cl, clavicle; csf, coraco-scapular foramen; gc, glenoid cavity; pf, postscapular fossa. (From Flower's Osteology.)

Between the Sloths and Anteaters the extinct Megatherium and some of its allies are to a certain extent intermediate. But it may be pointed out in the first place that there are certain important resemblances between the living forms. In both, retia mirabilia are developed in the tail (in spite of its reduction in the Sloths) and in the limbs. But, as is well known, retia are also found in other mammals far removed in the series from these under consideration. The reproductive organs generally are very similar, and they have both a dome-shaped and deciduate placenta. The latter character they share with the Armadillos and with the Aard Vark; Manis having a non-deciduate placenta which is, like that of the Carnivora, zonary in form. The Edentates, at any rate the American forms, have a double vena cava posterior and no azygos vein. This condition is also met with among Whales.

Osteologically the Sloths and Anteaters are united by the fact that the coracoid becomes fused with the coracoid border of the scapula, thus forming a foramen; the importance of this character is, however, discounted by its occurrence in three genera of Cebidae.

The above facts embody the views of Sir William Flower.

A subsequent study of the brain and of the muscles of these animals has led to results not entirely in harmony with these views.

Dr. Elliot Smith is of opinion, after an exhaustive study of the Edentate brain, that in this region of the body the present group shows very decided points of likeness to the Carnivora; that is, so far as concerns the Anteaters. On the other hand, Orycteropus is as distinctly comparable with a primitive Ungulate type, such as is exemplified by Moschus. "If the brain of Orycteropus," he remarks, "were given to an anatomist acquainted with all the other variations of the mammalian type of brain, there is probably only one feature which would lead him to hesitate in describing it as an exceedingly simple Ungulate brain. That one feature is the high degree of macrosmatism. Manis, on the other hand, does not come especially near to Orycteropus. The brain of Manis conforms to a simple type of architecture, which agrees in many points with both those of Orycteropus and the American Edentates; there is not sufficient evidence to show which type it really favours." Elliot Smith would, in fact, agree with Max Weber that it is better, if a division is to be made, to divide the group into three orders:—the Xenarthra (Sloths, Anteaters, and Armadillos), Tubulidentata (Orycteropus), and Squamata (Manis), instead of into Xenarthra and Nomarthra.

Messrs. Windle and Parsons are disposed to see in muscular similarities reasons for uniting Manis with the American Edentates, though they confess to being unable to place Orycteropus; in this animal, they say, "we are more struck by the generalised mammalian arrangement of its muscles than by any special Edentate characters. There are, however, two muscles in Orycteropus which show peculiarities not found elsewhere than in the Edentates";—the triceps, which has more than one scapular head, and the tibialis posticus, which is double. They conclude that Orycteropus "presents some feeble claims to be taken into the order."

We shall here adopt the following divisions.