The answer to whether an armadillo is a marsupial is no. Despite both groups having ancient origins in the Southern Hemisphere, armadillos belong to a completely different branch of the mammalian family tree. They are classified as placental mammals, meaning they carry their developing young inside the uterus for a prolonged period, which is the opposite of the marsupial reproductive strategy.
The Armadillo’s True Identity
The armadillo belongs to the mammalian Superorder Xenarthra, a group that also includes sloths and anteaters. This classification makes them placental mammals, or Eutherians. Within Xenarthra, armadillos are further categorized into the Order Cingulata, which is distinguished by their bony armor plating.
The Xenarthra superorder is one of four major lineages of placental mammals, having diverged from other groups approximately 100 million years ago. This distinct evolutionary path, primarily developed during South America’s long period of isolation, sets them apart from the Metatheria, the group containing all marsupials. The 21 living species of armadillo are the only surviving members of the ancient Cingulata order, which once included the giant, extinct glyptodonts.
Defining the Marsupial
Marsupials belong to the infraclass Metatheria, a group defined by a reproductive strategy markedly different from placental mammals. The primary characteristic is a very short gestation period, typically lasting between 12 and 33 days. This rapid development inside the uterus is possible because the young are born in an extremely underdeveloped, embryonic state.
The tiny, blind, and mostly naked newborn must then complete a strenuous journey immediately after birth, crawling from the birth canal to a nipple, often located within a specialized pouch. This pouch, or marsupium, provides the necessary shelter and prolonged milk supply for the young to complete their development, which can take many months. Marsupial females also possess a unique reproductive anatomy, featuring two uteri and two lateral vaginas, which is entirely unlike the single reproductive tract of placental mammals.
Unique Traits of Xenarthrans
Skeletal Features
Armadillos, sloths, and anteaters are grouped under Xenarthra, a name that literally means “strange joints,” reflecting their defining skeletal feature. The xenarthrous articulations are extra connections found in the lumbar and posterior thoracic vertebrae. These additional zygapophyses strengthen the spine, providing a rigid lower back that is particularly useful for the powerful digging motions employed by armadillos and anteaters.
Metabolism
Members of this superorder share the characteristic of having an exceptionally low metabolic rate compared to other mammals of similar size. Some species burn calories at less than half the expected rate, which contributes to their lower body temperatures. This reduced metabolism allows them to subsist on low-energy food sources, such as insects or leaves.
Dental Structure
The dental structure of Xenarthrans is highly unusual among mammals, as they entirely lack incisors and canine teeth. Armadillos possess simple, peg-like cheekteeth that are homodont, meaning they are all the same shape. Furthermore, these teeth lack the enamel layer common to most mammalian teeth, and they are open-rooted, meaning they grow continuously throughout the animal’s life to compensate for wear.