Molecular cladistics divides eutherian mammals into four supraordinal clades. Xenarthra likely has a basal position and may influence interpretations of character transformations [
1‐
4]. Phylogenetics have stimulated evolutionary studies as on the placental system [
5‐
9]. Distinct placental types have been recognized with regard to invasiveness, shape and internal organization [
6,
8]. However, only about 1% of eutherian species have been investigated, varying on ordinal and family levels [
10]. Attention to rather exotic species such as armadillos were drawn from the middle of 20
th century on [
11,
12], indicating that placental characters vary within Xenarthra. Armadillos are known for haemochorial, villous placentas [
11‐
14] similar to the human [
15], indicating that they could be used as animal models [
16]. Only the nine-banded armadillo
Dasypus novemcinctus from North America is well studied in regard to placental development [
11‐
14,
17‐
20]. Data show a peculiar condition in that the developing villi entered pre-existing maternal blood sinuses and enlarged them while leaving the maternal vessels endothelium largely intact [
11,
13,
14]. The sinuses were supplied by derivatives of the spiral arteries that were passing in endometrial septae and opened into the intervillous space [
11,
13,
14]. However, data for other armadillo species are sparse [
21‐
23]. We investigated armadillos from Brazil and Argentina, the large hairy armadillo
Chaetophractus villosus desmarest, 1904, the small hairy armadillo
Chaetophractus vellerosus gray, 1865, and the six-banded armadillo
Euphractus sexcinctus, linnaeus, 1758
. The species studied here exhibit litter sizes of 1 to 3 young and gestation periods of 60 to 70 days and inhabit pampas, cerrado and chaco vegetations [
24‐
28].