Here’s an enigmatic mammal
that has been aching to be nested in a recognized clade for over 150 years. It should not have been this difficult to nest Hyaenodon (Fig. 1).
Figure 1. Hyaenodon horrid us was the size of a large dog. This carnivorous marsupial was formerly considered a creodont.
It’s interesting to see how
Wikipedia plays down the affinities of Hyaenodon (Laizer and Parieu, 1838; Eocene-Miocene, Figs. 1-3): “a group of extinct carnivorous fossil mammals from Eurasia, North America and Africa…Some species of this genus were among the largest terrestrial carnivorous mammals of their time; others were only of the size of a marten.” The Wiki authors do not place Hyaenodon into the Eutheria nor the Metatheria. They don’t create a family tree for Hyaenodon. Most authors consider Hyaenodon a member of the Creodonta, a clade considered a ‘wastebasket’ by Wikipedia. That clade may have to be revised or deleted in the future, but at present only one creodont has been tested and it has dropped out.
About Creodonts and Carnivorans
Wikipedia reports, “creodonts and carnivorans were once thought to have shared a common ancestor, but given that different teeth are involved in making up the carnassials (both between creodonts and carnivorans and between the main groups of creodonts), this appears to be a case of evolutionary convergence. Creodonta was coined by Edward Drinker Cope in 1875. Cope included the oxyaenids and the viverravid Didymictis but omitted the hyaenodontids. In 1880. he expanded the term to include Miacidae, Arctocyonidae, Leptictidae (now Pseudorhyncocyonidae), Oxyaenidae, Ambloctonidaeand Mesonychidae. Cope originally placed creodonts within the Insectivora. In 1884, however, he regarded them as a basal group from which both carnivorans and insectivorans arose.Hyaenodontidae was not included among the creodonts until 1909. Over time, various groups were removed, and by 1969 it contained, as it does today, only the oxyaenids and the hyaenodontids.”
Figure 2. Hyaenodon skull cast showing in orange the lacrimal – septomaxilla common to most, if not all, metatherians.
When added to the large reptile tree (now 796 taxa)
Hyaenodon (chimaera taxon, based on several specimens and authors) nested with another carnivorous mammal, Thylacinus, the Tasmanian wolf, a basal marsupial. They scored nearly identically.
Unique for marsupials,
Thylacinus had largely cartilaginous epipubic bones with a highly reduced osseous elements. Perhaps that’s why epipubes were never found with Hyaenodon.
Figure 3. Thylacinus, the Tasmanian wolf, recently extinct, was a sister to Hyaenodon. Note the three molars and lack of epipubes.
In both taxa
only three molars were present. That’s one less than in most marsupials and the number typical, but not universal, in placentals. In both taxa the jugal extends nearly to the back of the skull where the jaw joint is. That’s a typical marsupial trait. Likewise, the septomaxilla appears on the snout (Fig. 2), as in the marsupials Vombatus and Vintana. The occiput (Fig. 4) is also very metatherian.
Figure 4. Hyaenodon occiput. Note the strong resemblance to the occiput of Vintana, with the post parietal above the supraoccipital and framed by tabulars and tall squamosals.
This link to Scott (1895, p183)
discusses “M. Gaudrey’s (1878) view as to the marsupial character of the genus [Hyaenodon] is definitely disproved by the abundant material now at command.” Good job, Gaudrey! You’ve been vindicated! (…about 150 years too late, unfortunately).
Figure 6. Subset of the large reptile tree: the marsupials featuring the newest taxon, Hyaenodon.
According to Wikipedia, H. gigas was the size of a bear (est. 1,100 lbs, 500 kgs, 3 m). H. horridus was the size of a large dog (est. 88 lbs, 40 kgs). H. microdon and H. mustelinus were much smaller, about the size of Eomaia, another basal marsupial. There were several mid-sized taxa, too. Hyaenodon leptorhynchus was the type species.
this discovery was made without ever having seen the fossil first hand. The LRT and a computer monitor are all the tools one needs in many cases, such as this one.
Gaudrey A 1878. Les enchaînements du monde animal dans les temps géologiques mammifères tertiaires. F. Savy. (ed.) Paris 28pp. online here.
Laizier L and de Parieu J 1838. Description et determination d’une machoire fossile appartenant a un mammifere jusqu’a pressent inconnu, Hyaenodon leptorhynchus. Comptes-rendus hebdomadaires des séances de l’Académie des Sciences, Paris 7:442.
Scott WM 1895. The osteology of Hyaenodon. Academy Natural Sciences Philadelphia Journal 9:499-536. online here.