Bonisicyon
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| Bonisicyon Temporal range: Messinian, | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Carnivora |
| Family: | †Amphicyonidae |
| Subfamily: | †Amphicyoninae |
| Genus: | †Bonisicyon Werdelin and Simpson, 2009 |
| Type species | |
| †Bonisicyon illacabo Werdelin and Simpson, 2009 | |
Bonisicyon is an extinct genus of carnivoran mammals, belonging to the family Amphicyonidae ("bear dogs"). It is the last-surviving member of its family, living in East Africa during the end of the Miocene epoch. Known only from a damaged mandible and isolated teeth from the Nawata Formation, and possibly also the Lukeino Formation, its closer taxonomic affinities are unclear. It is notable for both its small size, and its unique dentition.[1]
The first published material belonging to this genus includes a broken mandibular ramus with a damaged m1 and alveoli for m2 and m3, as well as roots for p4, from the Upper Nawata Formation in Lothagam, Kenya.[2] Bonisicyon was described on basis of material found at Gona, Ethiopia, with a right m1 (HMD1/P11) serving as holotype. Other described material includes an isolated left m2 and a damaged right M2. A M1 from Lemudong'o, Kenya, originally referred to the ailurid Simocyon, has also been tentatively attributed to the genus.[1] A P4 discovered at the Kenyan Lukeino Formation has also been proposed to belong to an amphicyonid, as both its age and dimensions are similar to Bonisicyon.[3]
The genus is named in honour of Dr Louis de Bonis, whereas illacabo is the Afar word for (as in "the end of the story").[1]
Description
Bonisicyon is a rather small amphicyonid. Its body mass has been estimated at 33 kg based on tooth size, though this is likely an overestimation. The relatively slender mandible, and similarities in tooth size to a coyote, suggest that its mass was closer to 20 kg than 30 kg.[4]
The robust m1 is relatively wide at the base of its crown, and possesses an entirely mesiodistal carnassial shear. The tall hypoconid is formed into an elongated crest, and is effectively part of the carnassial shear, only separated from the trigonid by a narrow postvallid notch, which is morphologically similar to the carnassial notch. The trigonid consists of a well-developed paraconid and protoconid, and a reduced metaconid, which is only evidenced as a bulge on the lingual side of the protoconid. The almost entirely mesiodistal carnassial shear only has a very slight angle between paraconid and protoconid. All these features suggest that Bonisicyon was a hypercarnivore. However, it lacks several features usually associated with such a diet, such as taller trigonid cuspids and a shortened talonid. Indeed, the trigonid is relatively short and its cuspids blunt. Therefore, its dentition possesses a peculiar mix of hypercarnivorous and mesocarnivorous features, although the narrowness of the tooth means it lacks a proper crushing basin, therefore pointing towards hypercarnivory.[1][4]
Other notable features include the paraconid, which is much lower and mesodistally shorter than the tall protoconid, and possesses a very short cristid. The well-developed preprotocristid is expanded slightly dorsally roughly midway between the tooth's apex and the carnassial notch. The postprotocristid is much more vertically oriented in comparison, meeting the talonid at the posterior carnassial notch. The talonid is about half as long as the trigonid, and is dominated by the hypocinid, which is about ⅔ the height of the protoconid and almost reaches the length of the directly distally located protoconid. Elongated into a cristid, it runs the entire length of the talonid and ends in a nearly vertical face dropping to the distal end of the tooth. The talonids lingual part consists of a lingual cristid, which is separated from the hypoconid by a narrow, shallow groove. The two roots are about equally long mesiodistally, although the posterior is perhaps slightly longer. It lacks a basal cingulum, possessing only a modest out-bulging of the enamel. Both the m2 and M1 are very similar to other small and medium-sized amphicyonids.[1]
The m1 of Bonisicyon shares several features with that of Afrocyon, such as the relatively short talonid and reduced metaconid. However, Afrocyon differs in its very tall and relatively transversely thin mandibular ramus and elongated m2. Bonisicyon lacks the peculiar m1 of Myacyon dojambir, and numerous features differentiate it from the similarly sized ?M. peignei. Among these are its taller, more bulbous, but less trenchant paraconid, lower and blunter protoconid and reduced metaconid. In both size and shape, it is most similar to Cynelos/Hecubides euryodon. Their M1 is almost indistinguishable, and their m2 are very similar, although C. euryodon has a less developed metaconid and a more cusp-like hypoconid. Their m1 too shares many similarities, although most of these are shared, primitive characteristics. In comparison, Bonisicyon possesses a much more developed hypoconid and paraconid, a reduced metaconid and a distinct, sharp notch between protoconid and hypoconid.
