| Azygonyx | |
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| Genus: | †Azygonyx Gingerich 1989 |
| Type species | |
| Azygonyx gunnelli (Gingerich & Gunnell 1979) | |
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Azygonyx was a smalltillodont mammal, likely the size of a cat to raccoon, that lived in North America during thePaleocene andEocene in the early part of theCenozoic Era. The only fossils that have been recovered are from theWillwood andFort Union Formations in theBighorn Basin of Wyoming, United States, and date to theClarkforkian toWasatchian, about 56 to 50 million years ago.[1] Fifty-six collections that have been recovered thus far include the remains ofAzygonyx.Azygonyx survived thePaleocene Eocene Thermal Maximum along with other mammals likePhenacodus andEctocion, both of which were ground-dwelling mammals.[2]Azygonyx probably was a generalist terrestrial mammal that may have roamed around the ground, but was also capable of climbing trees.[1]
The genus name comes from the "a-" meaning absent, "zygos-" meaning joining, and "onyx" meaning claw referencing the unjoined claw-like incisors.[1]
Azygonyx is placed in the suborderTillodontia, an extinct group of mammals characterized by rodent-like incisors, clawed feet, and an elongated rostrum and mandibular symphysis.Azygonyx belongs to the familyEsthonychidae, but differs from other esthonyids by having an unfused mandibular symphysis.[1]
Five species ofAzygonyx are currently known and includes the type speciesAzygonyx gunnelli (Gingerich and Gunnell, 1979) and the additional speciesAzygonyx ancylion (Gingerich and Gunnell, 1979),Azygonyx grangeri (Simpson, 1937),Azygonyx latidens (Simpson, 1937), andAzygonyx xenicus (Gingerich and Gunnell, 1979). The genus has a relatively problematic taxonomic history, as the type speciesAzygonyx gunelli and other species of the genus are alternatively namedEsthonyx by Lucas and Schoch (1998).[3]
The type species comes fromPark County in northwestern Wyoming, and is composed of teeth, cranial, and postcranial bones. The bone fragments were broken and scattered suggesting a taphonomic pathway that includes trampling before burial. The holotype specimen (UM 83874) from Wyoming, USA consists of upper and lower dentition with dentary bone. Post cranial remains include the scapula, ulna, radius, ankle bones, and others.[1]
No complete skeleton ofAzygonyx has been recovered, making the exact appearance and body size of the animal relatively difficult to determine. Compared to other tillodonts,Azygonyx was relatively small, as indicated by an ulna length of about 50 millimetres (2.0 in) and a mandible about 60 millimetres (2.4 in).[1]
The upper dentition ofAzygonyx includes three incisors (I1–I3), one canine (C1), premolars (P2–P4), and molars (M1–M3), and the lower dentition includes two incisors (I1–I2), one canine (C1), premolars (P2–P4), and molars (M1–M3).[1] A lower first incisor has not actually been recovered, but is believed to be present due to the available space in the lower jaw. A lower third incisor was likely extremely reduced or absent. The incisors ofAzygonyx are tall and nearly vertical. The anterior teeth, the incisors and canines, were crowded. The second incisor has a massive oval crown. Lower premolars have smooth enamel set in a relatively deep jaw and the third premolar crown is large and bulbous.[1]
Tillodonts are considered to be generalists and some may have beenrhizophagous.[1] The large and laterally compressed claws and shallow trochlea of the astragalus suggests that the mode of life ofAzygonyx wasscansorial, or adapted for climbing.[1] In contrast, bones of the foot suggest a somewhat cursorial or fossorial lifestyle. Worn enamel on the teeth additionally suggestAzygonyx stripped vegetation.Azygonyx likely spent a lot of time in trees, but also occasionally searched for food on the ground or dug for roots.[2]
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