Saturday, October 15, 2016

Xenungulates: The Strange Ungulates

The Xenungulata, whose name means “strange ungulates”, is an archaic and poorly known group of South American ungulates that were temporally restricted to the Paleocene. The order, which was coined by Paula Couto in 1958, contains a single family (Carodniidae) with five currently recognized species.

Mounted skeleton of Carodnia vierai. Photo by Lily P. Bergqvist from
General Characteristics
As with most Paleocene animals, xenungulate fossils are very scarce. By far the best known member of the group is the species Carodina vierai, which is known from reasonably complete skeletal material. C. vieirai was somewhat tapir-like in its size, build, and presumably in its ecology. It had a complete dentition with procumbunt, laterally flattened incisors and particularly large canines which may have been used as combat weapons against rivals predators. The cheek teeth were low-crowned and well-suited to a browsing diet of mostly leaves, twigs, and fruits. The foot bones are unique among South American ungulates in that they are short and robust, and the digits terminate in broad, flat, unfissured hoof-like unguals*. The limbs are short and somewhat slender, and their anatomy seems to suggest that the animal had a gait similar to that of an African Elephant (Loxodonta africana).

Xenungulates have been considered to be close relatives to both the South American pyrotheres and North American uintatheres based mainly on dental similarities with the two groups; the first and second molars are bilophodont* as they are in advanced pyrotheres, and the third molar is more complex and lophate* as in uintatheres. However, these characteristics are likely the result of convergent evolution rather than a shared heritage. The discovery of the more basal Etayoa in 1987 confirmed this; unlike uinatheres or its later relative Carodnia, it lacked a lophate third molar, thus confirming that this trait evolved separately from uintatheres. Furthermore, since bilophodonty is not present in basal pyrotheres from the early Eocene, we can also conclude that this trait evolved separately in the Xenungulata and the Pyrotheria.

Genera & Species
Etayoa (Villarroel, 1987)
Known only from upper Paleocene Colombia, Etayoa bacatensis is the most basal xenungulate yet discovered. It lacks the distinctive lophate molars observed in the more well-known genus Carodnia.

Notoetayoa (Gelfo, Lopez, Bond, 2008)
Notoetayoa gargantuai is the most recently discovered xenungulate from the middle Paleocene of Colombia. In body size it seems to have been smaller than Carodnia but larger than Etayoa.

Carodnia (Simpson, 1935)
Three species have been described within this genus; C. feruglioi (Paula Couto, 1952), C. cabrerai (Simpson, 1935), and C. vieirai (Simpson, 1935). The former two species are known only from dental remains which differ only in size, which raises the possibility of the two representing different growth stages or perhaps different genders of a single species. The third species, C. vieirai, is known from much more complete dental, cranial, and postcranial material. In fact the skeleton of this animal is among the most complete for any Paleocene mammal. The now invalid genus Ctalecarodnia is a synonym of Carodnia.

A parsimonious tree showing the proposed relationships between members of Xenungulata.
Cropped from Figure 3 in Gelfo et al., 2008.
Bilophodont: when two separate crests form transverse, often ring-shaped ridges on the tooth.
Lophate: describes a cheek tooth with heightened ridges or crests.
Ungual: the distal phalanx from which the claw or hoof grows.

References & Further Reading
Farina, Richard A; Vizcaino, Sergio F; Iuliis, Gerry De. “Megafauna: Giant Beasts of Pleistocene South America”. New York, New York: Columbia University Press, 2012. <Book>

Gelfo JN, Lopez GM, Bond M (2008). “A New Xenungulata (Mammalia) from the Paleocene of Patagonia, Argentina”. Journal of Paleontology 82(2): 329-335 <Full Article>

Rose, Kenneth D. “The Beginning of the Age of Mammals”. John Hopkins University Press, 2006. Simpson GG (1935). “Descriptions of the Oldest Known South American Mammals, from the Rio Chico Formation”. American Museum Novitates 793: 1-25 <Full Article>

Gingerich PD (1985). “South American Mammals in the Paleocene of North America”. pp 123-137 in FG Stehli (ed), The Great American Biotic Interchange <Full Article>

Paula Couto (1952). “Fossil Mammals from the Beginning of the Cenozoic in Brazil”. Bulletin of the American Museum of Natural History 99(6): 355-394 <Full Article>

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