A suitable ancestor
for the odd mammal clade Xenarthra has been long sought. As it turns out, that ancestor (Fig. 2) has been hiding in plain sight for several decades.
When Barylambda was added to the large reptile tree (LRT; Fig. 3) it nested as a sister to Orycteropus and basal to the Xenarthra (sloths, anteaters and armadillos). That should not come as any big surprise, considering how workers have portrayed and described Barylambda as capable of assuming a tripodal posture, like a ground sloth, with its robust tail.
Often compared to ground sloths, Barylambda (Patterson 1933; originally Titanoides, renamed by Patterson 1937; CNHM P-14637, a partial skeleton; late Paleocene, 55 mya) now nests with aardvarks, sloths, anteaters and armadillos. Not sure why this relationship has been overlooked or missed for over 75 years. Certainly many have seen the similarity in overall build. Wikipedia describes current uncertain relationships between Xenarthra and other mammal clades. In any case, it’s good to find yet another ancestor for a former enigma clade using a verifiable analysis.
Note the robust build
of Barylambda, the deep tail chevrons, the tiny premaxillary teeth. the out-turned ilia, the low jugals, the lack of large canine-like canine teeth (in females), all traits that point to aardvark and edentate morphologies. No other tested taxa nest closer. Here Barylambda nests next to, but not with, the basal condylarths in the LRT, Onychodectes. Ectoconus and Pantolambda.
Wikipedia describes Barylambda as a genus of herbivorous pantodont mammal the size of a pony, but with five-toed plantigrade hands and feet. The unguals (claws) were not sharp, but small and rounded. Three species are known. We should look for a smaller sister to Barylambda with a longer rostrum in the Paleocene for more direct ancestors to anteaters and armadillos.
The LRT (now 801 taxa) does not recover the same
cladogram topology as O’Leary et al. 2013, which reported, “Our tree suggests that Placentalia first split into Xenarthra and Epitheria.” I don’t find Barylambda in their taxon list. More criticism of that recent paper on mammal relationships can be found here.
A final thought
Barylambda is a big taxon. Evolutionary novelty most often occurs among small taxa. We might expect that Barylambda and Orycterpus AND the members of the Xenarthra are all descendant from a smaller ancestor in the early Paleocene. Or… perhaps more likely, the novelty we see in Orycteropus AND members of the Xenarthra are based on several smaller descendants of Barylambda, some of them armored, starting in the mid Paleocene when Barylambda was probably common and widespread, rather than rare at its genesis.
O’Leary, MA et al. 2013. The placental mammal ancestor and the post-K-Pg radiation of placentals. Science 339:662-667. abstract
Patterson B 1933. A new species of the amblypod Titanoides from western Colorado. American Journal of Science, 25:4 15-425.
Patterson B 1937. A new genus, Barylambda, for Titanoides faberi, Paleocene amblypod. Geological Series, Field Museum of Natural History, 6:229-231. online
Simons EL 1960. The Paleocene Pantodonta. Transactions of the American Philosophical Society, New Series 50(6):1-81