Résumé
Combining morphological and molecular characters through Bayesian
total-evidence dating allows inferring the phylogenetic and timescale
framework of both extant and fossil taxa, while accounting for the
stochasticity and incompleteness of the fossil record. Such an integrative
approach is particularly needed when dealing with clades such as sloths
(Mammalia: Folivora), for which developmental and biomechanical studies
have shown high levels of morphological convergence whereas molecular data
can only account for a limited percentage of their total species richness.
Here, we propose an alternative hypothesis of sloth evolution that
emphasizes the pervasiveness of morphological convergence and the
importance of considering the fossil record and an adequate taxon sampling
in both phylogenetic and biogeographic inferences. Regardless of different
clock models and morphological datasets, the extant sloth Bradypus is
consistently recovered as a megatherioid, and Choloepus as a mylodontoid,
in agreement with molecular-only analyses. The recently extinct Caribbean
sloths (Megalocnoidea) are found to be a monophyletic sister-clade of
Megatherioidea, in contrast to previous phylogenetic hypotheses. Our
results contradict previous morphological analyses and further support the
polyphyly of “Megalonychidae”, whose members were found in five different
clades. Regardless of taxon sampling and clock models, the Caribbean
colonization of sloths is compatible with the exhumation of islands along
Aves Ridge and its geological time frame. Overall, our total-evidence
analysis illustrates the difficulty of positioning highly incomplete
fossils, although a robust phylogenetic framework was recovered by an a
posteriori removal of taxa with high percentages of missing characters.
Elimination of these taxa improved topological resolution by reducing
polytomies and increasing node support. However, it introduced a
systematic and geographic bias because most of these incomplete specimens
are from northern South America. This is evident in biogeographic
reconstructions, which suggest Patagonia as the area of origin of many
clades when taxa are underrepresented, but Amazonia and/or Central and
Southern Andes when all taxa are included. More generally, our analyses
demonstrate the instability of topology and divergence time estimates when
using different morphological datasets and clock models, and thus caution
against making macroevolutionary inferences when node support is weak or
when uncertainties in the fossil record are not considered.