Unravelling the Evolutionary Relationships of a New Family of Eutherian Mammals

A new family of mammals discovered by Dr. Craig Scott, Curator of Fossil Mammals, provides insights into the evolutionary relationship of eutherian mammals. This research helps better understand the organization of mammalian communities during the late Palaeocene (61.6 – 56 million years ago).

Eutherian mammals bear live young that have been nourished in utero through the placenta. These mammals are an incredibly diverse group, ranging from shrews and bats to whales and humans. Although there are many different kinds of eutherian mammals currently living, the group has a complicated evolutionary history, containing multiple extinct groups.

The extinction of non-avian dinosaurs 66 million years ago was a pivotal event in the evolutionary history of mammals. Ecological niches occupied by dinosaurs in the Late Cretaceous were suddenly available to mammals in the Palaeocene Epoch. Eutherian mammals took advantage of these opportunities, becoming the dominant mammals for the remainder of the Cenozoic Era (66 million years ago to present).

In 2006, Dr. Scott and colleagues described a new genus and species of eutherian mammal from Alberta, Horolodectes sunae.  They compared Horolodectes to a number of eutherian mammals, but could not determine what it might be most closely related to. The molars and the premolars of Horolodectes are very unusual, making it difficult to make meaningful comparisons to other eutherians.

A field photo of one of the localities along the Canadian Pacific Railway tracks where Ferrequitherium sweeti was discovered.]

A field photo of one of the localities along the Canadian Pacific Railway tracks where Ferrequitherium sweeti was discovered.

Dr. Scott has been conducting fieldwork in Glenbow Ranch Provincial Park, near Cochrane, Alberta since 2011. From his work there, he discovered the fossil teeth of a mammal that had molars like those of Horolodectes, but with premolars that were much more similar to those of other eutherians. Dr. Scott determined that the teeth represented a new genus and species of eutherian mammal, and named it Ferrequitherium sweeti. The genus name Ferrequitherium means “iron horse beast,” in reference to the locality’s closeness to the CP rail tracks. The species name sweeti is in honour of the recently deceased Arthur Sweet, a palynologist who worked for the Geological Survey of Canada.

Incomplete left lower jaw of Ferrequitherium sweeti. A) labial view (adjacent to the inside of the cheek). B) lingual view (adjacent to the tongue). C) dorsolabial (biting) view.

Incomplete left lower jaw of Ferrequitherium sweeti. A) labial view (adjacent to the inside of the cheek). B) lingual view (adjacent to the tongue). C) dorsolabial (biting) view.

Dr. Scott studied the teeth of Ferrequitherium, Horolodectes, and several other eutherian mammals in order to unravel their evolutionary relationships. Analysis revealed that Ferrequitherium and Horolodectes are closely related. Based on this research, Dr. Scott has named a new family of eutherian mammals, the Horolodectidae.

Individual upper molars of Ferrequitherium sweeti photographed from multiple angles for analysis.

Individual upper molars of Ferrequitherium sweeti photographed from multiple angles for analysis.

The abrupt appearance of Horolodectidae in the fossil record adds to a growing body of evidence that mammalian communities in the Western Interior of North America were undergoing significant and complex reorganization during the late Palaeocene, and may have included one or more immigration events. While Dr. Scott’s research supports a close evolutionary relationship between Ferrequitherium and Horolodectes, the broader relationship of the Horolodectidae remains unclear and requires further research.

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