Bonisicyon illacabo, Werdelin & Simpson, 2009

Werdelin, Lars & Simpson, Scott W., 2009, The last amphicyonid (Mammalia, Carnivora) in Africa, Geodiversitas 31 (4), pp. 775-787 : 777-780

Bonisicyon illacabo n. sp. ( Fig. 1 A-F)

Amphicyonidae sp. B – Werdelin 2003.

Simocyon sp. — Howell & García 2007: 124.

HOLOTYPE. — HMD1 /P11 from Gona , Ethiopia, right m1. Fig. 1 A-C herein ( NME).

ETYMOLOGY. — From the Afar illacabo , end (as in “the end of the story”), a noun in apposition. The last syllable is stressed.

GEOGRAPHICAL RANGE. — Eastern Africa ( Ethiopia, Kenya).

STRATIGRAPHIC RANGE. — Latest Miocene, c. 6.5-5.3 Ma (Messinian; ~MN13 equivalent).

LOCALITIES. — Hamadi Das (HMD), Escarpment (ESC), and?Asbole Dora (ABD), Gona ( Ethiopia), Upper Nawata Formation, Lothagam ( Kenya),?Lemudong’o

( Kenya). MATERIAL EXAMINED. — Gona (Afar Autonomous Region,

Ethiopia): HMD1 /P11, isolated right m1 (holotype) ; ESC2 /P224, isolated left m2 ; ABD1/P35, damaged right M2. Lothagam (Turkana District, Kenya): KNM-LT 23944, a partial right horizontal ramus with roots of p4, a broken m1, the alveolus for m2 and the anterior part of the alveolus for m3 ; Lemudong’o (Narok District, Kenya): KNM-NK 45780, isolated left M1 .

DIAGNOSIS. — Amphicyonidae of small size; carnassial shear on m1 entirely mesiodistal; m1 hypoconid formed into elongated crest, separated from trigonid by a narrow postvallid notch and effectively a part of the carnassial shear; m1 metaconid in evidence only as a bulge on the lingual side of the protoconid; m1 relatively wide and bulbous at the base of the crown; m2 broad and short.

DESCRIPTION

Material referred to Bonisicyon illacabo n. sp.

HMD1/P11, right m1 ( Fig. 1 A-C). This specimen is a very well preserved right m1. There is some sediment clinging to the specimen in places, but otherwise the preservation is pristine. The tooth is robust, with a trigonid that is about twice the length of the talonid.The mesial root is broken about 5 mm below the crown, but otherwise the tooth is complete. It is nearly unworn, with only some blunting of the apices of the main cusps. The widest part of the tooth is just mesial to the apex of the hypoconid. Mesial to this point the tooth is nearly equally wide until just mesial to the base of the metaconid remnant, then it tapers to the carnassial notch, where there is an in-turning on the lingual side, then is equally wide until just mesial to the paraconid apex, mesial to which it tapers rapidly but smoothly to the mesialmost point of the tooth. Distal to the widest point the tooth tapers gradually in width to the distal end of the hypoconid, then tapers abruptly but smoothly to the distal end of the tooth.

The trigonid consists of a well-developed paraconid and protoconid, and a metaconid that has been reduced to little more than a bump on the lingual face of the protoconid. The mesial face of the paraconid slopes almost vertically down to the mesialmost point of the tooth, which is just dorsal to the crown/root juncture. The paraconid cristid is very short and the paraconid is much lower than the protoconid and even lower than the point where the (almost entirely reduced) metaconid apex joins with the protoconid.The carnassial notch is acute but not very deep. The protoconid is tall and mesiodistally longer than the paraconid. The preprotocristid is well developed and is slightly expanded dorsally about midway between the apex of the tooth and the carnassial notch. The postprotocristid is much more vertically oriented than the preprotocristid and meets the talonid at what can only be termed a posterior carnassial notch. Carnassial shear is almost entirely mesiodistal, with only a very slight angle between paraconid and protoconid. As noted, the metaconid is almost entirely reduced. It can best be discerned in its distal part, where it is partially separated from the protoconid by a shallow groove. No cristids are developed on the metaconid.

The talonid is entirely dominated by the hypoconid, which is almost as long as the protoconid and located directly distal to it. It is elongated into a cristid that runs the entire length of the talonid, ending with a nearly vertical face that drops to the distal end of the tooth. The hypoconid is about ⅔ the height of the protoconid. The lingual part of the talonid consists of a lingual cristid that includes what are probably poorly developed entoconid and entoconulid. This cristid is separated from the hypoconid by a narrow, shallow groove.

The anterior root is broken, as noted. At the crown level the two roots are about equally long mesiodistally, with the posterior perhaps slightly longer. There is no basal cingulum, merely a modest out-bulging of the enamel.

Lm1: 18.9; Wm1: 9.7; Ltm1: 12.0.

ESC2/P224, isolated left m2 ( Fig. 1 D-F). This tooth has been subjected to considerable chemical weathering, and hence the preservation is less favorable than that of the previous specimen. The specimen is coated with white residue of this process and is considerably pitted. Although the cuspids appear relatively unworn, it is not possible to state with certainty whether this is the case or whether some moderate wear is present.

The m2 is a rounded rectangle in occlusal view, with the distal end tapering more than the mesial, which is cut-off in a slightly oblique line from buccal (mesialmost) to lingual. The trigonid consists of a protoconid and metaconid with no hint of of a paraconid. The protoconid is the tallest trigonid cuspid and has distinct pre- and postcristids. The metaconid is not developed into a cuspid, but forms a low crest along the lingual side of the tooth. The enamel bulges basally on the buccal side below the protoconid but not on the lingual side below the metaconid. The area between the two trigonid cuspids forms a shallow valley but no basin.

The buccal bulge of the enamel continues on to the talonid beneath the hypoconid. The latter is large and crest-like and separated from the protoconid by a shallow valley but no distinct postvallid notch. There is no entoconid and the lingual half of the talonid is formed into a shallow, nearly flat basin.

As in the case of the m1 the anterior root is broken and the posterior nearly complete. The posterior root appears to have been much the larger of the two.

Lm2: 12.4; Wm2: 9.4.

KNM-LT 23944, a partial right mandibular corpus with broken m1 (after Werdelin 2003: fig. 2): the ramus is long and relatively slender. It is both deepest and thickest beneath the m1 talonid. The ramus is broken anteriorly at the anterior end of the anterior root of p4 and posteriorly just at the point where the horizontal ramus begins to ascend to the coronoid process. The latter break continues posteroventrally and the ventral half of the ramus is about 30 mm longer than the dorsal half. Posterior to m2 the ramus becomes noticeably thinner. The anterior end of the masseteric fossa can be felt by tactile inspection rather than seen and lies posterior to the tooth row. The fourth lower premolar was a relatively short and probably quite slender tooth. The lower carnassial is short, with the talonid making up about one third of the total length of the tooth. The trigonid is relatively short given the size of the ramus. Neither trigonid cuspid can be clearly distinguished because of a strong, nearly horizontal wear facet that runs from the mesial end of the trigonid to the distal end of the talonid. The talonid has a single, well-developed and probably trenchant cuspid. There is no buccal cingulum, only a slight marginal swelling at the talonid. The m2 was, to judge from the alveoli, a broad tooth of about 14 mm length. The m3 alveolus is much smaller than either alveolus of m2.

Material referred to cf. Bonisicyon illacabo n. sp.

KNM-NK 45780, isolated left M1 ( Howell & García 2007: fig. 3). The crown of this complete tooth is a triangle that is pinched in the center, about level with the buccal end of the preprotocrista. The trigon has three cusps. The largest is the paracone, which is a substantial, pyramidal tooth with distinct pre- and postparacristae. The metacone, though still a substantial cusp, is more crest-like. It has a short premetacrista and a longer postmetacrista. These two cusps are bounded mesially, buccally, and distally by a cingulum that is broad mesially and tapers gradually to the distal end, where the postmetacrista nearly reaches the distal margin of the tooth. The protocone is a low crescent with substantial pre- and postprotocristae. The space between the trigon cusps is deep but not formed into a flat basin. Lingually, the protocone has crenelated enamel and descends gradually to a broad cingulum that is largest centrally and tapers both mesially and distally.

LM1: 13.4; WM1: 16.9 (after Howell & García 2007).

ABD-1-P35, damaged right M2 ( Fig. 1G ). The tooth is missing the parastyle and a part of the paracone and paracrista. Much of the morphology of the preserved parts of the tooth is obscured by white weathering residue clinging to the occlusal surface. In occlusal outline the tooth as preserved is rectangular. It might be expected that the parastyle would have extended mesially beyond the mesial cingulum. The general relief of the tooth is low. It has three trigon cusps, a low paracone, a very low metacone, and a low protocone that forms a blunt ridge together with the pre- and postprotocristae. The metastyle is well developed and is surrounded by a cingulum. The crown buccal to the protocone forms a broad shelf with low-lying area followed buccally by a broad buccal cingulum.

WM2 at level of protocone: c. 11.

Figures

FIG. 1. — Molars of Bonisicyon illacabo n. gen, n. sp.: A-C, HMD-1-P11 (holotype), right m1, Hamadi Das, Gona, Ethiopia; D-F, ESC- 2-P224, left m2, Escarpment, Gona, Ethiopia; G, ABD-1-P35, damaged right M2, Asbole Dora, Gona, Ethiopia; A, D, lingual views; B, E, buccal views; C, F, G, occlusal views. Scale bar: 10 mm.