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Leptarctus spp.
Leptarctus spp.

Temporal Range: Miocene

Scientific classification
Kingdom:  Animalia
Phylum:  Chordata
Class:  Mammalia
Order:  Carnivora
Family:  Mustelidae
Subfamily:  †Leptarctinae
Genus:  †Leptarctus Leidy, 1856
  • Leptarctus ancipidens White, 1941
  • Leptarctus mummorum Worth & Baskin, 2009
  • Leptarctus oregonensis Stock, 1930
  • Leptarctus primus Leidy, 1856

The Leptarctinae are an enigmatic subfamily of mustelids present in North America and Eurasia during the Miocene (Arikareean to Hemphillian North American Land Mammal Ages). Their diet and ecology have been particularly controversial. Some workers have suggested they were similar to koalas, whereas others suggested they were crushing omnivores analogous to raccoons.

Journal Reference:
William W. Korth and Jon A. Baskin "A New Species of Leptarctus (Carnivora, Mustelidae) from the Late Clarendonian (Late Miocene) of Kansas," Annals of Carnegie Museum 78(1), (1 May 2009).

Leptarctus mummorum, new species, from the late Clarendonian of northwestern Kansas is described for the unusual mustelid genus Leptarctus Leidy, 1856. It is distinguished from other species of the genus by its larger size, temporal crests meeting along the central axis of the skull, relatively longer rostrum and bullar processes, depression anterior to the orbit, oblique ridge along the orbital wall, upper third premolar (P3) with an accessory internal accessory cusp, and upper fourth premolar (P4) with a greatly enlarged parastyle. It appears that previous suggestions that Leptarctus was strictly herbivorous and arboreal are not supportable based on the morphologies of the skull and dentition.

Journal Reference:
Calede, J., Kehl, W., & Davis, E. (2018). Craniodental morphology and diet of Leptarctus oregonensis (Mammalia, Carnivora, Mustelidae) from the Mascall Formation (Miocene) of central Oregon. Journal of Paleontology, 92(2), 289-304. doi:10.1017/jpa.2017.78

The Leptarctinae are an enigmatic subfamily of mustelids present in North America and Eurasia during the Miocene (Arikareean to Hemphillian North American Land Mammal Ages). Their diet and ecology have been particularly controversial. Some workers have suggested they were similar to koalas, whereas others suggested they were crushing omnivores analogous to raccoons. Leptarctus oregonensis Stock, 1930, a poorly known leptarctine from the early Barstovian, is represented by fragmented cranial elements and isolated teeth from the Mascall Formation of Oregon, and some fairly complete but undescribed material from the Olcott Formation of western Nebraska. Herein, we describe the first well-preserved skull of L. oregonensis from the type formation. Based on this new specimen, we confirm that L. oregonensis is a distinct species from L. primus Leidy, 1856 and L. ancipidens White, 1941 that is characterized by a distinct morphology of its tympanic projections and first upper molars. We are also able to describe intraspecific variation within L. oregonensis coinciding with the geographic distribution of the specimens (Oregon and Nebraska). The most variable characters are concentrated in the morphology of the frontals and the upper fourth premolar. Additional specimens will be needed to settle the debate over sexual dimorphism in this species, but this new specimen suggests that Leptarctus oregonensis, despite being one of the smallest members of the Leptarctinae, was an animal-dominated omnivore with considerable crushing ability.

Extinct weasel relative with confounding skull likely ate meat with a side of veggies

February 22, 2019, American Museum of Natural History

[Image: extinctwease.jpg]
A reconstruction of the skull (upper) and head (lower) of Leptarctus primus, an extinct weasel relative that lived in North America and Asia about 20 million years ago. Credit: AMNH/N. Wong

New research on an extinct weasel relative reveals what it might have eaten when it lived in North America and Asia about 20 million years ago. The oddly shaped skull of Leptarctus primus has long led to conflicting theories about its diet. But the new work, based on biomechanical modeling and published this week in the Journal of Vertebrate Paleontology, shows that Leptarctus was likely a carnivorous predator, with capability for omnivory and a broader diet when prey was scarce, and had a skull that functioned similarly to that of the living American badger.

Leptarctus primus, which lived in the Miocene and was just a little larger than a housecat, has intrigued researchers because of its unusual and extremely robust skull.

"For a mammal, its skull is really strange," said co-author Z. Jack Tseng, a research associate at the American Museum of Natural History and an assistant professor of pathology and anatomical sciences at the Jacobs School of Medicine and Biomedical Science at the University at Buffalo. "It's heavily built—like a tank—with very thick zygomatic cheek bones. The top of its head looks like it's wearing a helmet."

Strikingly, Leptarctus primus has two parallel ridges that line the top of the head (other carnivorans typically have a single central ridge or have smooth skulls). For many years, paleontologists have debated the ecological niche of Leptarctus based on conflicting interpretations of the strong parallel skull ridges, distinctive skull shape, and the shape of its teeth and chewing wear. Previous interpretations of their feeding lifestyle ranged widely, across virtually every type of known feeding behavior in carnivorans (dogs, cats, hyaenas, bears, seals, and weasels and their relatives), including herbivore, carnivore, insectivore, and omnivore. But because of the lack of quantitative research into how Leptarctus skulls functioned, the question of their diets remained unanswered.

[Image: 1-extinctwease.jpg]
A reconstruction of the head of Leptarctus primus attacking the Miocene rodent Cupidinimus. Credit: AMNH/N. Wong

In this study, led by Alixandra Prybyla, who was a student in the Museum's National Science Foundation Summer Research Experience for Undergraduates program while at Columbia University, the researchers took an engineering approach. The team compared an almost complete fossil skull of Leptarctus primus with 18 species of modern carnivorans with known diets as well as to other fossil species, using bite simulations based on CT scans of the skulls and virtual modeling of feeding mechanics.

According to John Flynn, study co-author, research team leader, and Frick curator of fossil mammals in the Museum's Division of Paleontology, "Traditional methods of studying skull, tooth, and skeleton anatomy are still essential for understanding how fossil species lived. But high-resolution X-ray CT images and sophisticated computerized engineering modeling tools have completely transformed our ability to accurately reconstruct feeding habits in extinct animals."

They found that among the other species analyzed, the skull of Leptarctus is mechanically most similar to the skull of the American badger. Despite some differences in their skull appearances, the computer simulations indicate that the badger is the best living biomechanical analog for understanding the dietary lifestyle of Leptarctus. Based upon those comparisons, the team determined that it was primarily a carnivore and an active predator, but that it could also have been an omnivore feeding on a wider range of plant and insect foods when necessary.

[Image: 2-extinctwease.jpg]
Digital model of the skull of Leptarctus primus, showing reconstructed jaw muscle groups in red, yellow, and pink. The virtual muscles were activated in bite simulations to test the biomechanical capability of this extinct weasel relative. Credit: J. Tseng

"It was probably hunting down prey and taking in whatever it had access to most of the time," Tseng said.

Prybyla added: "This complete skull of Leptarctus represents an untapped wellspring of information on the history of ancient relatives of weasels, otters, badgers, and skunks. It's wonderful what one specimen can illuminate for researchers. To spearhead a project of this magnitude as an undergraduate student was extremely empowering."

The researchers will conduct future studies using similar engineering modeling tools to look at variations in skull feeding mechanics among other species in the leptarctine group, to determine how many different types of feeding adaptations may have existed among these unusual extinct predators.

Journal Reference:
Alixandra N. Prybyla et al, Biomechanical simulations of Leptarctus primus (Leptarctinae, Carnivora), and new evidence for a badger-like feeding capability, Journal of Vertebrate Paleontology (2019).DOI: 10.1080/02724634.2018.1531290

Variations in craniodental morphology have been correlated to feeding adaptations in living organisms and used as proxies for paleodiet reconstruction. Within the mammalian order Carnivora, the Miocene fossil musteloid Leptarctushas been variably interpreted as a carnivore, frugivore, herbivore, omnivore, or insectivore based on morphological comparisons with extant species. Here, we perform the first simulation of cranial biomechanics in Leptarctus primus, aiming to identify a living analogue using biomechanical capability rather than qualitative morphology. Finite element models (FEMs) of 18 extant carnivorans and two extinct outgroup taxa were used to compare known diet-biomechanics relationships with the biomechanical properties of L. primus FEMs within a phylogenetic context. Multivariate analyses of simulated bite efficiency and skull stiffness values indicate that L. primus is most similar overall to Taxidea taxus(American badger) in unilateral bite simulations. Based on biomechanical predictions, we postulate that L. primus resembled the American badger in its feeding ecology more closely than any other taxon tested and thus conclude that L. primus was dominantly a carnivore with an auxiliary feeding capability of omnivory. We also compared the L. primus FEMs with the potentially synonymous Hypsoparia bozemanensis to determine a possible range of feeding capabilities. We observed an increase in mechanical efficiency with a deepening of the zygomae of H. bozemanensis, a trait previously used to differentiate it from L. primus. Ongoing work to expand the database of cranial biomechanical simulation data across Carnivoramorpha should help to further clarify evolutionary patterns of skull biomechanical specializations in musteloids and other carnivorans.
[Image: wildcat10-CougarHuntingDeer.jpg]
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