An exceptionally well-preserved skeleton of Thomashuxleya externa (Mammalia, Notoungulata), from the Eocene of Patagonia, Argentina

Abstract

We describe one of the oldest notoungulate skeletons with associated craniodental and postcranial elements: Thomashuxleya externa (Isotemnidae) from Cañadón Vaca in Patagonia, Argentina (Vacan subage of the Casamayoran SALMA, middle Eocene). We provide body mass estimates given by different elements of the skeleton, describe the bone histology, and study its phylogenetic position. We note differences in the scapulae, humerii, ulnae, and radii of the new specimen in comparison with other specimens previously referred to this taxon. We estimate a body mass of 84 ± 24.2 kg, showing that notoungulates had acquired a large body mass by the middle Eocene. Bone histology shows that the new specimen was skeletally mature. The new material supports the placement of Thomashuxleya as an early, divergent member of Toxodontia. Among placentals, our phylogenetic analysis of a combined DNA, collagen, and morphology matrix favor only a limited number of possible phylogenetic relationships, but cannot yet arbitrate between potential affinities with Afrotheria or Laurasiatheria. With no constraint, maximum parsimony supports Thomashuxleya and Carodnia with Afrotheria. With Notoungulata and Litopterna constrained as monophyletic (including Macrauchenia and Toxodon known for collagens), these clades are reconstructed on the stem to Euungulata (i.e., Perissodactyla and Artiodactyla). Unconstrained, Bayesian analysis weakly supports the possibility that Thomashuxleya is a stem xenarthran; with Notoungulata and Litopterna constrained as monophyletic, the two clades are recovered as sister to Perissodactyla. Anatomical data sampled thus far for Thomashuxleya, combined with collagen amino acids for Pleistocene meridiungulates, substantially limit the number of possible affinities for endemic South American species among mammals, although ambiguity still remains.

Improvements in the fossil record may largely resolve current conflicts between morphological and molecular estimates of mammal phylogeny

“…B 285: 20181632 available for mammals, comprising 4541 characters scored for 46 extant and 40 fossil taxa. We modified the matrix by first merging the character scores for the fossil notoungulate Thomashuxleya externa with those from a more recent study [27], and then deleting 407 constant characters. This left a total of 4134 characters: 1170 cranial, 1311 dental, 890 postcranial and 763 soft tissue.…”

First skeleton of the notoungulate mammal Notostylops murinus and palaeobiology of Eocene Notostylopidae

“…This remain (MLP 82-V-1-8) is the most complete known fossil of Notostylops and represents one of the few basal notoungulates with dental and postcranial remains found in direct association. Other basal notoungulate with postcrania associated are the isotemnids Thomashuxleya, Anisotemnus and Pleurostylodon (Simpson 1967;Shockey & Flynn 2007;Carrillo & Asher 2017) and the oldfieldthomasiids Colbertia (Bond 1981;Bergqvist et al 2007) and Allalmeia (Rusconi 1946a,b;Lorente et al 2014).…”

The Neogene Record of Northern South American Native Ungulates

“…The close relationship of Litopterna with Perissodactyla is also supported by mitogenomic data (Westbury et al, 2017). Carrillo and Asher (2017) combined amino acid, collagen sequences, and morphological characters into a dataset including 182 fossil and living taxa in order to evaluate the relationship of Notoungulata and other SANUs within placentals. Their results yielded a limited number of possible phylogenetic relationships, but did not arbitrate between potential affinities with Afrotheria and Laurasiatheria.…”