Chimento and Agnolin 2020: Litopterna interrelations suffers from taxon exclusion

Chimento and Agnolin 2020
discuss interrelationships within and outside the clade Litopterna, famous for including Macrauchenia (Fig. 1), the odd South American ungulate. Strangely the chalicotheres were excluded. In the large reptile tree (LRT, 1721+ taxa, subset Fig. 2), litopterns and chalicotheres nest in the same clade. Phylogenetically, Macrauchenia IS a chalicothere in the LRT.

Figure 1. Macrauchenia museum mount.

Figure 1. Macrauchenia museum mount.

From the abstract
“The Litopterna is an extinct clade of endemic South American ungulates that range from Paleocene up to late Pleistocene times. Because of their unique anatomy, litopterns are of uncertain phylogenetic affinities”

Not so. The LRT (Fig. 2) nests litopterns with high Bootstrap scores between paenungulates and perissodactyls and following chalicotheres. Omitting chalicotheres may be the cause of prior problems.

Figure 1. Subset of the LRT focusing on hoofed mammals.

Figure 1. Subset of the LRT focusing on hoofed mammals.

From the abstract
“However, some nineteenth century authors, considered litopterns as related to perissodactyl ungulates, a hypothesis recently sustained by molecular data. The aim of the present contribution is to include litopterns and other South American related taxa in a comprehensive phylogenetic analysis together with several extant and extinct basal perissodactyl ungulates.”

The Chimento and Agnolin taxon list is far from comprehensive. Many extant taxa were excluded.

Figure 3. From Chimento and Agnolin 2020, colors added. Grayed out taxa are not tested in the LRT. Many of these are based on teeth only.

Figure 3. From Chimento and Agnolin 2020, colors added. Grayed out taxa are not tested in the LRT. Many of these are based on teeth only.

“The analysis resulted in the nesting of litopterns and kin as successive stem-clades of crown Perissodactyla. Further, litopterns are not phylogenetically grouped with any North American basal ungulate, in agreement with some previous proposals”

Notably the authors excluded chalicotheres (Fig. 2) from their taxon list. Some of these are known from North America. Europe, Africa and Asia and they nest with Macrauchenia in the LRT. Phylogenetically, Macrauchenia IS a chalicothere in the LRT. Theosodon is a taxon employed by both cladograms. Macrauchenia is not found in the Chimento and Agnolin cladogram.

Figure. 2. Chalicothere skulls to scale. Lophiodon is an outgroup closer to tapirs.

Figure. 4. Chalicothere skulls to scale. Lophiodon is a traditional chalicothere relative, but nests with marsupials in the LRT.

“Presence of pan-perissodactyls in South America and India indicates that southern continents probably played an important role in the early evolution of hoofed mammals.”

Or not. These continents might have been more of a backwater or refuge for highly derived taxa with earlier, pre-continental-separation origins.


References
Chimento NR and Agnolin FL 2020. Phylogenetic tree of Litopterna and Perissodactyla indicates a complex early history of hoofed mammals. Nature.com/scientific reports.

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