Sloth genomics paper 2025 re-describes their already known variety

According to Boscaini A et al 2025,
citing McKenna, Wyss and Flynn 2006, the question, ‘what is a crown clade sloth?‘ was defined as “the most recent common ancestor of Choelepus and Bradypus, plus all its descendants.”

Unfortunately, since these two are the most derived sloths in the large reptile tree (LRT, 2338 taxa) this definition describes only these two extant tree sloths plus their most recent common ancestors, the larger terrestrial sloth Hapalops (Fig 2) and the larger aquatic sloth, Thalassocnus (Fig 1).

By definition that makes Megatherium and its ancestors, Mylodon and Paramylodon outgroup taxa to the sloth clade as defined above.

Perhaps a better definition is needed without the assumption of Cope’s Rule, because sloths started out BIG and evolved smaller through time = phylogenetic miniaturization.

In the large reptile tree
(LRT, 2338 taxa) the last common ancestor of all the above named sloths is Glyptodon (Fig 2), a only glyptodont tested in the LRT. And before that Barylambda (Fig 2), a taxon universally omitted from academic sloth studies.

Phys.org Publicity
“The team’s analyses indicate the earliest sloths would have likely been small ground dwellers, about the size of a great Dane.”

“Likely been” is confessed speculation. Better to use a cladogram that includes all competing candidates, like the LRT (Fig 2). Let the cladogram tell you what the patterns.

“At various points throughout their evolutionary history, sloths adopted a semi-arboreal lifestyle.”

Again, better to document this with a cladogram that pinpoints dichtomys.

“Not all of them stayed in the trees, however. The largest sloths, including Megatherium and Mylodon, likely evolved from a tree-adapted sloth that ultimately decided to stay firmly planted on the ground.”

This hypothesis is not confirmed by the LRT (Fig 2).

SciNews Publicity
“In the new study, Dr. Narducci and her colleagues analyzed ancient DNA and compared more than 400 fossils from 17 natural history museums to figure out how and why extinct sloths got so big. The authors combined information about the shape of fossils with DNA from living and extinct species to create a sloth tree of life that traced the sloth lineage all the way back to their origin more than 35 million years ago.”

“Because the paleontologists were specifically interested in the evolution of size, they collected data for the final analytical ingredient by measuring hundreds of museum fossils, which they used to estimate sloth weight.”

“The largest sloths, including Megatherium and Mylodon, likely evolved from a tree-adapted sloth that ultimately decided to stay firmly planted on the ground.”

The large, big-boned outgroup taxon in the LRT, Barylambda (Fig 2) was once again overlooked.

According to the team, size differences among sloths has been primarily influenced by the types of habitats they lived in and, by extension, climate change.“

No doubt. But I think we all knew that earlier.

In counterpoint, the Editor’s Summary of the paper reported,
“Boscaini et al. looked across the evolutionary history of sloths and reveal that the  ancestral groups were terrestrial and large, with smaller species being derived and convergent. — Sacha Vignieri”

That hypothesis is not reflected in the publicity. Just the opposite.

Taking a middle road, the Boscaini et al abstract emphasizes size rates:
“we show that sloth body mass evolved with major lifestyle shifts and that most terrestrial lineages reached their largest sizes through slower evolutionary rates compared with extant arboreal forms. Size disparity increased during the late Cenozoic climatic cooling,”

One small fact gleaned from the publicity:
“Some ground sloths also had little pebble-like osteoderms embedded in their skin.”

That armor reflects their common origin with armored glyptodonts (Fig 2).
Follow that clue!

Lessons for today:
1. Don’t omit pertinent taxa, like glyptodonts and Barylambda.
2. Don’t use genomics in paleontology. Use phenomics = trait analysis.

References
Boscaini A et al 2025. The emergence and demise of giant sloths. Science (2025). DOI: 10.1126/science.adu0704. www.science.org/doi/10.1126/science.adu0704|
Ameghino F 1887. Enumeración sistemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia. Boletín del Museo de La Plata 1: 1–26.
Cuvier G 1796. Notice sur le squelette d’une très-grande espèce de quadrupède unconnue jusqu’a présent, trouvé a Paraguay, et déposé au cabinet d’histoire
naturelle de Madrid. — Magasin Enciclopedique: ou Journal des Sciences, des Lettres et des Arts 1: 303–310, 2: 227–228; Paris.
McKenna MC, Wyss AR and Flynn JJ 2006. Paleogene pseudoglyptodont xenarthrans from central Chile and Argentine Patagonia. American Museum Novitates 3536: 1-18.
de Muizon C and McDonald HG 1995. An aquatic sloth from the Pliocene of Peru. Nature. 375 (6528): 224–227.

wiki/Hapalops
wiki/Megatherium
wiki/Thalassocnus

Publicity
Phys.org scientists-figured-extinct-giant-ground.html
SciNews: New Research Sheds Light on Emergence and Demise of Giant Sloths

Source: https://pterosaurheresies.wordpress.com/2025/06/08/sloth-genomics-paper-2025-re-describes-their-already-known-variety/