nasal and anterior maxilla are missing in Trigonostylops (Fig. 1). Nevertheless this taxon nests between Meniscotherium and Brachycurus, basal to astrapotheres in the large reptile tree (LRT, 1970+ taxa; Fig. 3).
Simpson 1933 imagined
a possible cat-like rostrum due to missing facial bones (Fig. 1).
MacPhee et al. 2021
labeled Trigonostylops an ungulate. They wrote, “In 1933 George G. Simpson described a remarkably complete skull of Trigonostylops, an Eocene South American native ungulate (SANU) whose relationships were, in his mind, quite uncertain. Although some authorities, such as Florentino Ameghino and William B. Scott, thought that a case could be made for regarding Trigonostylops as an astrapothere, Simpson took a different position, emphasizing what would now be regarded as autapomorphies. He pointed out a number of features of the skull of Trigonostylops that he thought were not represented in other major clades of SANUs,
and regarded these as evidence of its phyletic uniqueness.”
‘Phyletic uniqueness’ is an oxymoron. Phylogenies, by definition, are based on similarities and relationships. In any case, Trigonostylops is not close to ungulates in the LRT. MacPhee et al 2021 omitted too many pertinent taxa and added unrelated marsupials they thought were related because vertebrate paleontology textbooks said so and other workers imagined so.
“Simpson’s classification was not favored by most later authors, and in recent decades trigonostylopids have been almost universally assigned to Astrapotheria.”
In LRT Trigonostylops also nests with Astrapotherium and kin, confirming earlier workers.
“Overall, we found that this new assessment strengthened the placement of Trigonostylops within a monophyletic group that includes Astrapotherium and Astraponotus, to the exclusion of other SANU clades.” SANU = South American Native Ungulate = misnomer
taxon exclusion marred the phylogenetic analysis of MacPhee et al. 2021. The authors mistakenly included several marsupials they thought were notoungulate placentals, but to their credit, they did nest Trigonostylops with Astraponutus and Astrapotherium (Fig. 2). The only resolution in their cladogram was in the unrelated perissodactyls (3 taxa), liptoterns (4 taxa) and the marsupials they believed were members of the Notoungulata, an invalid clade in the LRT due to the polyphyly of its traditional membership.
Adding taxa solves most problems like this.
You can study your favorite taxon until exhaustion, but then you’re only halfway there. Every focused study needs to be combined with a wider view of related taxa. Pterosaur workers don’t get this simple concept. Neither do whale, turtle, shark, placoderm, archosaur, lepidosaur, archosauriform and dinosaur workers. Be the first paleontologist you know to cover all the bases. Grow your own cladogram. Don’t borrow one.
Ameghino F 1897. Les mamiferes crétacés de l´Argentine. Boletín Instituto Geográfico Argentino:18: 405–521.
MacPhee RDW, et al. (5 co-authors) 2021. Cranial Morphology and Phylogenetic Relationships of Trigonostylops wortmani, an Eocene South American Native Ungulate. Bulletin of the American Museum of Natural History 449(1), 1-183. https://doi.org/10.1206/0003-0090.449.1.1
Simpson GG 1933. Structure and affinities of Trigonostylops. American Museum Novitates 608: 1–28.