Xenarthra

The xenarthrans (Xenarthra, Greek "unusual joints") or edentulous (Edentata) are a superorder of exclusively American placental mammals that include anteaters, armadillos, and sloths.

The name Xenarthra alludes to the fact that the vertebral joints at the lumbar level are different from those of any other mammal.

Features

The xenarthrans are a group that includes very heterogeneous forms. There are three basic morphological types; the anteaters, with an elongated body and a very long snout, dense fur, and terrestrial or arboreal habits; the sloths, with a compact body, short snout, dense fur and arboreal; and the armadillos, with a broad and depressed body covered on the back with a shell, with little hair and terrestrial habits.

The only external character they have in common is large, robust claw-like nails, often sickle-shaped, adapted for various functions, both in terrestrial (digging) and arboreal (clinging to branches) forms.

Teething

Xenarthrans have peculiar teeth due to a specialization in their diet. The anteaters lack teeth, while the sloths and armadillos have teeth with a more or less pronounced degree of reduction, without roots or enamel and with continuous growth; the anterior teeth are absent and the posterior ones are rows of cylindrical knobs. The reduction or loss of teeth as a consequence of the adoption of a diet based on soft invertebrates, especially ants and termites (myrmecophagy) has occurred independently at least five times in mammals: echidnas (monotremes), Myrmecobius (marsupial anteater), pangolins (folidotus) and aardvarks (tubulidentates), as well as xenarthrans. This habit is to be expected since the mammalian trunk was initially insectivorous.

Spine

The vertebral column has special characteristics; bradypodids have 6 to 10 cervical vertebrae, an exceptional condition in mammals (7 is the normal number in the rest of the orders), which is only found again in sirenians. On the other hand, the last dorsal vertebrae and the lumbar vertebrae have additional anterior and posterior processes that give rise to special joints called xenarthral joints, which give the group its name.

Homeothermy

Xenarthrans are imperfect homeotherms and their body temperature varies depending on the ambient temperature; in sloths from 28 to 35 °C, in Myrmecophaga 32-35 °C and in Dasypus 32-36 °C.

History

This group of mammals is characteristic of the Neotropical region and well known in the fossil record of South America, due to its wide temporal extension and that the fossil records in the literature show findings of them in relatively close times, that is, It is a group that has a high frequency of records, to such an extent that no fossiliferous continental stratigraphic unit of the Cenozoic is found that does not contain traces of xenarthrans.

Especially in Argentina and its surroundings, many traces have been found, since this is a geographical area in which a large number of remains have been found and not only of this group of mammals, but of many others, which show a great variation in age, that is, due to its climatic conditions, it was always an excellent place to stay and due to terrestrial conditions, fossils are easily preserved.

Almost all taxa in this group of mammals are recorded in South America from the late Paleocene to the present, some other taxa are recorded in North America from the late Miocene to the present, and several records also exist from Antarctica that can be considered within the group of xenarthros in the Eocene epoch.

From the dates from which fossil records are found, it can be assumed or demonstrated that the xenarthros are a group that originated in South America, evolved and expanded its territory until it reached other borders; The representatives of the xenarthros in a time in which South America was practically isolated (part of the Cenozoic in which it was considered as an island continent) adapted to exploit a great diversity of environments, which facilitated their evolution, diversification and great distribution.

From the late Miocene, in which a passage from South America to North America is assumed, shortly after the formation of the Isthmus of Panama occurred throughout America at the beginning of the Pleistocene, xenarthros can be recorded in North America as specimens of South American origin. During these times there was the greatest diversification of the group, which dates from more than 160 recognized fossil genera, of which more than 400 species can be found, during almost the entire Cenozoic. And the passage from South America to Antarctica could have taken place at that time when Antarctica was close to or very close to the south of Argentina.

Taxonomy

The xenarthrans are a group with many more fossil than extant forms; 208 extinct genera are known and only 12 extant.

In the past, members of this group were classified together with pangolins (order Pholidota) and aardvarks (order Tubulidentata) in the order Edentata (toothless), which has now been abandoned, since it was not monophyletic; however, some authors continue to use the name Edentata, but strictly speaking, as a synonym for Xenarthra.

This superorder is divided into the order Pilosa, which contains the suborders Vermilingua and Folivora, and the order Cingulata.

• Order Pilosa

Suborden Vermilingua - bears hormigueros

Family Cyclopedidae

Family Myrmecophagidae

Suborden Folivora - lazy

Family Bradypodidae

Family Megalonychidae

Family Megatheriidae †

Family Mylodontidae †

• Order Cingulata - armadillos

Family Pampatheriidae †

Glyptodontidae Family †

Dasypodidae Family

Phylogeny

The phylogenetic relationships of the extant xenarthran groups are as follows according to genetic analyses:

Xenarthra is recognized as one of four clades within Placentalia. The relationships between the four cohorts (Xenarthra, Afrotheria, Laurasiatheria, and Euarchontoglires) and the identity of the placental root are still somewhat controversial.

Genetic studies favor the hypothesis that Afrotheria and Xenarthra comprise sister taxa at the base of the mammalian placental radiation, suggesting an ancient Gonduan clade of placental mammals called the Atlantogenata. Xenarthra diverged from the sister group Afrotheria in the middle from the Cretaceous 105-120 million years ago when Africa and South America separated in great masses. Despite phylogenetic studies there is nowhere where xenarthrans and aphroterians coexist.