Why South America's strangest mammals have joints unlike any other creature on Earth
In short: The superorder Xenarthra, including sloths, anteaters, and armadillos, is defined by unique vertebral joints called xenarthry. These ancient mammals evolved in isolation for millions of years, developing low metabolic rates and specialized teeth that defy standard mammalian classification. Their evolutionary history remains one of the most debated puzzles in biology.
The name Xenarthra, derived from Greek for strange joints, refers to extra articulations in their vertebrae that stiffen and strengthen the spine. This adaptation, which likely aided in digging, is absent in all other placental mammals. While tree sloths have lost these specific articulations to increase spinal flexibility, their fossil ancestors possessed them. Beyond their spines, these animals exhibit a suite of unusual traits: they have single-color vision, fused pelvic bones, and the lowest metabolic rates among all therians. Their reproductive anatomy is also distinct, with females lacking a clear distinction between the uterus and vagina, and males possessing internal testicles.
Their dentition is equally anomalous. Most xenarthrans lack functional enamel and possess teeth that grow continuously throughout their lives. Because their teeth are often reduced, highly simplified, or entirely absent, and lack the standard crown morphology found in other mammals, scientists have struggled to define their evolutionary relationships. Some researchers suggest they evolved from ancestors that had already lost basic dental features, such as enamel and cusped crowns, likely as an adaptation to feeding on social insects. While some groups developed cheek teeth to grind plants, these lacked the durability of typical mammalian enamel, relying instead on harder and softer dentine patterns.
Originating in South America about 60 million years ago during the late Paleocene, xenarthrans diversified during the continent's long period of isolation. Their low metabolic rates may have been a key adaptation, allowing vast numbers of massive herbivores to thrive in resource-poor scrublands. However, their history is marked by the extinction of megafaunal giants—such as glyptodonts and ground sloths—at the end of the Pleistocene. Today, the group is divided into two orders: Cingulata, which includes armadillos, and Pilosa, comprising anteaters and sloths. Their precise place on the mammalian family tree remains contested, with various studies placing them as a sister group to Afrotheria, Boreoeutheria, or as a distinct branch that split from other placentals shortly after the Cretaceous.
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- Summary by Intellect · source: Xenarthra
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