Cynelos stenos, Hunt & Yatkola, 2020

Hunt, Robert M. & Yatkola, Jr Daniel A., 2020, A new species of the amphicyonid carnivore Cynelos Jourdan, 1862 from the early Miocene of North America, Geodiversitas 42 (5), pp. 57-67 : 59-64

Cynelos stenos n. sp.

( Figs 3-6 ; Table 1 )

urn:lsid:zoobank.org:act:82B04120-7E1C-4CB2-ACD3-5C773281F85C

“ Cynelos n. sp. II” – Hunt 1998: 211.

Cynelos sp. B – Hunt 2002: 35.

HOLOTYPE. — UNSM 44723 , a cranium and associated mandibles, with both left and right upper and lower dentition (P1-M3: p1-m3); the left P1-p1 each represented by a single alveolus; the left upper and right lower canines; left I2 and left i3. A partial femur and fragments are attributed to the cranial material. Collected by D. A. Yatkola and William Rovnak, University of Nebraska State Museum. Daniel Yatkola discovered UNSM 44723 during his doctoral study of the Miocene stratigraphy and mammalian fauna of northwest Nebraska ( Yatkola 1978).

ETYMOLOGY. — From the Greek “stenos” (σΤΕΝός) (= narrow). The species nomen emphasizes the narrowness of the skull in dorsal view relative to the broader-skulled large contemporary early Miocene amphicyonids Amphicyon galushai Hunt, 2003 and Daphoenodon (Borocyon) robustum ( Peterson, 1910).

TYPE LOCALITY. — UNSM 44723 was found in a basal arkosic sand and gravel channel at the stratotype locality of the Runningwater Formation ( Cook, 1965), Box Butte Co., northwest Nebraska. The site was designated Runningwater Quarry by its discoverer M. F. Skinner of the American Museum’s Frick Laboratory, and was later reopened by UNSM under the direction of Daniel Yatkola. The fauna is conserved in the F: AM and UNSM collections.

GEOLOGICAL AGE. — Early Miocene c. 18 Ma.

DESCRIPTION

Cynelos stenos n. sp. (UNSM 44723) represents the first occurrence in the North American Miocene of a complete skull of the genus with mandibles in articulation. Only two other skulls of Cynelos are known in the early Miocene of North America: the skull of Cynelos idoneus (AMNH 20495) from the late Hemingfordian (He2) of western Nebraska and the skull of Cynelos malasi (OCPC 21791) from the latest Arikareean (Ar4) of southern California. No skulls were found with the Cynelos population sampled from the early Hemingfordian Bridgeport Quarries, western Nebraska, where the genus was represented by over 60 isolated cheek teeth, two maxillae, three mandibles, and numerous postcranials. The skulls attributed to the genus in North America from the early middle Miocene (c. 14.8 to 16 Ma) are those of the large early Barstovian C. sinapius from the Olcott Formation, Sioux County, Nebraska, including one (AMNH 18257: Matthew 1924: figs 22-23) very similar in form to the skull of C. stenos n. sp. except for its much larger size. Such narrow skulls probably characterize

the Miocene species of Cynelos in North America, including the terminal species, C. sinapius.

Cranium ( Figs 3 ; 4 )

The skull is essentially uncrushed preserving its cranial proportions and a nearly intact dentition except for damaged canines and the loss of incisors. It is exceptional in retaining the basicranial region. The sutures of the cranium remain open in the rostral area but due to age have closed elsewhere or are obscured by breakage.

Viewed from above, the cranium is quite narrow compared to the larger contemporary amphicyonid species (Amphicyon galushai, Daphoenodon [B.] robustum) that possess much broader skulls. Relative to skull length, the rostrum is short and rather constricted at the level of the P2. Behind this constriction the palate broadens to include the crushing dentition comprising the prominent molars and carnassials. At the posterior palatal border, the nasal cavity opens at the nasal choanal aperture into a ventrally open narrow nasopharyngeal fossa (9 cm in length, 2 cm in width) that continues to the basicranial region.The considerable breadth of both the basicranium and palate contrast with the intervening narrow interorbital area. Within the broad basicranium the auditory regions have been preserved with minimal damage. Despite loss of the ventral floor, the auditory bulla retains a capsular flask-like form with short bony external auditory meatus that differs from the more plesiomorphic condition of the bulla in the skull of Cynelos lemanensis from St.- Gérand, France ( Hunt & Stepleton 2015: fig. 8). The C. stenos n. sp. bulla was not large, 2 cm in width, and only slightly inflated.

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In profile the skull has a short, deep rostrum expanded at the canines that slopes gradually upward to the forehead to form a broad frontal region housing inflated frontal sinuses at the level of the postorbital processes. From the frontals the skull roof narrows posteriorly, forming a tall thin sagittal crest 12 cm in length and 5 mm in width that reaches its maximum height (4 cm) above the braincase. The crest then tapers to its termination at the inion. The braincase volume is not large and suggests the less developed brain described by Radinsky (1980). The greater part of the skull (½ rds) lies behind orbits that are set low below the frontal region: here what seems an exceptionally narrow and deep interorbital area intervenes between the orbits and the braincase – much of this interval was occupied by the large temporal and pterygoid musculature essential to the feeding mechanics of this species.

Mandibles ( Fig. 5 )

The conjoined mandibles preserve nearly the complete lower dentition; the dentaries are firmly interdigitated at the mandibular symphysis to form an incipient ankylosis in this mature individual. The jaws are elongate (length, c. 23 cm: articular condyle to canine) in keeping with the length of the skull and are not especially deep below the teeth (4.3 cm below m2). Behind the rather shallow masseteric fossae are wide articular condyles, set low at the back of the mandible nearly at the level of the toothrow, indicating nearly simultaneous occlusion of carnassials and molars (P4-M3, p4-m3) at jaw closure.

Upper Dentition ( Fig. 6 ; Table 1 )

The dentition is fully erupted indicating from its moderate wear a mature adult; the canines and slender rostrum suggest a female. The right canine alveolus (L × W, 21.1 × 13.9 mm) retained the root. The damaged left canine (L × W, 18.2 × 12.5 mm, measured at the base of the enamel) is broken off at the tip but has mesial and distal thin enamel ridges. All incisors were lost except the left I1 (L × W, 6.3 × 4.1 mm); however, the I3 alveoli are large relative to those for I1-I2 showing that I3 was much larger than the small I1-I2.

The P1-P3 are reduced and separated by diastemata. The left P1 is represented by a small circular alveolus and is separated from P2 by 5 mm. The low P2 (L × W, 11.1 × 4.5 mm) has two roots, a single apical cusp with a mesial and more extended distal slope and short distal shelf; the P2-P3 diastema is 9.2 mm. P3 has the same form as P2 but is larger (L × W, 12.8 × 6.2 mm) with a wider distal shelf and a sloping mesial face; the P3-P4 diastema is 3.2 mm. The P4 is short relative to the molars; feeding on hard material has blunted the once-sharper paracone and metastylar blade. The small low protocone does not protrude far lingually and is slightly retracted. A thin enamel ridge runs from the P4 paracone to a weak parastyle and there is a weak labial cingulum. The M1-M2 are moderately worn teeth forming a formidable crushing platform together with the small M3. The M1 paracone is larger and although worn was slightly taller than the metacone. A cingulum occupies the labial margin and extends a short distance to the mesial base of the paracone and distal base of the metacone. The cingulum is weak to absent on the mesial and distal sides of the tooth but expands to form a prominent lingual cingulum. The M1 protocone was closer to the mesial border of the tooth and is situated at the lingual apex of the protocone basin. A thin straight mesial enamel ridge extends from the protocone to a weak paraconule at the base of the paracone. A thin slightly curved distal ridge extends from the protocone to the base of the metacone but without a metaconule: the mesial and distal ridges enclose the protocone basin. The M2 is somewhat smaller than M1 – its protocone forms the apex of an arcuate crest but, where the protocone of M1 is nearer the mesial margin of the tooth, the M2 protocone is more centrally situated. The M2 metacone is slightly reduced relative to the paracone. On M2 the straight mesial and curved distal ridges are weaker without evident conules. The M2 protocone is worn as in M1 but where in M1 the mesial ridge ended at the base of the paracone, in M2 the mesial ridge reached the mesial border of the tooth to join the cingulum; there is a weak swelling along the ridge. In occlusal view M1 and M2 both extend lingually to form an enamel platform, each tooth surrounded by a prominent lingual cingulum. The small M3 contacted the distal margin of M2; a small low paracone and much reduced metacone are aligned along its labial side. A shallow basin bordered mesially by a weak ridge lies between protocone and paracone; the distal ridge is absent. The small lingual extension of M3 is surrounded by a marked cingulum.

Lower Dentition ( Fig. 5 ; Table 1 )

The incisors in this individual are damaged; however, only i2 and i3 were present in life – the i1s failed to erupt due to marked narrowing of the symphyseal region. The heavily worn left i3 (L × W, 6.8 × 5.4 mm) is the only surviving incisor; only the broken root of the left i2 remains. The right i2-i3 alveoli are present and show that the root of i3 extended beneath the root of i2 due to crowding of the incisors in the symphysis – Dehm (1950: 24) also observed this in C. hel- bingi. The well-worn right canine is 16.4 mm in length, 11.5 mm in width measured at the base of the enamel. Relative to their evident length, the mandibles are rather gracile and not deep even below the molars. The p1-p3 are much reduced compared to p4 and are separated by diastemata: 14.6 mm between canine and p1; 3.4 mm between p1-p2; 9.2 mm between p2-p3; 3.5 mm between p3-p4. The p4 is much larger than p3, and wide distally where it abuts the mesial face of m1. The p2-p3 do not show wear and only the tip of the principal cusp of p4 is slightly worn in contrast to the worn m1-m2 where processing of hard gristle and bone is evident in this mature individual.

In youth the paraconid and protoconid of the m1 may have ‘sheared’ for a time against the paracone/metastylar blade of P4 but in this individual the m1 protoconid and paraconid have become blunted from wear, as has the tip of the metaconid. The tall M1 paracone deeply grooved the distolabial corner of the m1 protoconid while the prominent m1 hypoconid occluded between the M1 paracone and metacone when entering the protocone basin. This latter action akin to a mortar-in-pestle created the mesial part of the m1-m3 crushing platform.

The m2 trigonid occluded between M1 and M2 where its robust protoconid-metaconid and small mesial basin abraded the distolabial corner of M1, and the mesial face of the M2 paracone. The low m2 hypoconid occluded against the M2 paracone, then as the bite closed, the broad m2 talonid contacted the M2 lingual platform. The slightly elevated mesial face of m3 with weak protoconid occluded with the distal face of the M2 metacone; the distal m3 with low hypoconid abraded the low platform comprising the entire M3. The occlusion of m2-m3 with the distal margin of M1 and with M2-M3 created the distal half of the crushing surface. The pronounced wear on P4-M1-M2 and m1-m2 of C. stenos n. sp. indicates that food processing was concentrated on these teeth. In this individual the orientation of wear surfaces on cusps of P4-M2 and m1-m2 when occluded suggests that this type of wear results primarily from crushing hard items such as bone rather than by direct tooth-on-tooth contact.

Food Processing

With increasing age, molars and also carnassials of this amphicyonine beardog seem to function more in crushing than in shearing or slicing. Crushing occurred as carnassials as well as molars came into near simultaneous occlusion with jaw closure because of the placement of the articular condyles at the level of the toothrow. Often in living larger carnivores with slicing carnassials, rotation of the mandible about its long axis makes possible a close registration of P4 with m1 for cutting meat during jaw closure. However, ankylosis at the mandibular symphysis in C. stenos n. sp. prevented this axial rotation and precise carnassial registration so that shear has been subordinated to a crushing role. The crushing function becomes obvious when the skull and mandibles are articulated: both left and right upper and lower cheek teeth occlude without requiring ipsilateral-contralateral shifting of the lower jaws during mastication. Whether grinding by the molars (as opposed to crushing) occurred during food processing is doubtful due to a limited capability of the mandibular articular condyles to shift from side-to-side within the glenoid fossae of the skull.

COMPARISON

Cynelos stenos n. sp. ( Table 1 ) is distinguished from the smaller European species of the genus, C. crassidens (Filhol, 1874), C. rugosidens (Schlosser, 1899), and C. schlosseri ( Dehm, 1950), and the North American C. malasi Hunt and Stepleton, 2015 by its much larger overall size of skull, mandibles, and dentition. Cynelos stenos n. sp. is more similar in size to the European C. lemanensis and C. helbingi but differs from the former by much larger cheek teeth (P4, M1, M2, see Hunt & Stepleton 2015: figs 9-11; m1, m2, Table 3 ) including cheek teeth of the C. lemanensis population from Ulm-Westtangente ( Peigné & Heizmann 2003). P4, M1, M2 of C. stenos n. sp. exceed in size all but the largest individuals of the Bavarian C. helbingi ( Hunt & Stepleton 2015: figs 9-11) as do m1 and m2 ( Tables 1 , 2 ; Figs 9 , 10 ). Consequently, C. stenos n. sp. has evolved a broader molar platform (comprising occlusion of M1, M2, M3 with m2, m3, and the m1 talonid) for crushing hard foods than is seen in C. helbingi and in C. lemanensis. All younger North American species of Cynelos from the late Hemingfordian (C. idoneus Matthew, 1924, basilar length of skull, 33-34 cm) and early Barstovian (C. sinapius Matthew, 1902, basilar length, 39-44 cm) are larger carnivores than C. stenos n. sp. (basilar length, 29 cm) and both species continue the further expansion of the molar platform.

Figures

FIG. 3. — Cynelos stenos n. sp., UNSM 44723, Runningwater Formation, Runningwater Quarry (early Hemingfordian), Box Butte Co., Nebraska. Cranium in right lateral view (A) and in ventral view (B). Scale bars: 5 cm.

FIG. 4. — Cynelos stenos n. sp., UNSM 44723,Runningwater Formation, Runningwater Quarry (early Hemingfordian), Box Butte Co., Nebraska. Cranium in dorsal view. Note the narrow cranium, constricted rostrum, broad frontal region, and tall thin sagittal crest above the small braincase. Scale bar: 5 cm.

FIG. 5. — Cynelos stenos n. sp., UNSM 44723, Runningwater Formation, Runningwater Quarry (early Hemingfordian), Box Butte Co., Nebraska. Right and left associated mandibles. Scale bar: 5 cm.

FIG. 6. — Cynelos stenos n. sp., UNSM 44723, Runningwater Formation, Runningwater Quarry (early Hemingfordian), Box Butte Co., Nebraska. Palatal view of the cranium with right P2-M3 (broken P1) and left P3-M3, partial P2, and P1 alveolus. Plant roots have eroded the enamel on the right M1-M2. Scale bar: 3 cm.

FIG. 9. — Bivariate graph of m1 dimensions (in mm) of Cynelos helbingi (Dehm, 1950) (Wintershof-West, southern Germany) relative to m1 of Cynelos stenos n. sp. (UNSM 44723, western Nebraska). Cynelos helbingi measurements from Dehm (1950).

FIG. 10. — Bivariate graph of m2 dimensions (in mm) in Cynelos helbingi (Dehm, 1950) (Wintershof-West, southern Germany) relative to m2 of Cynelos stenos n. sp. (UNSM 44723, western Nebraska). Cynelos helbingi data from Dehm (1950). Note the larger broad m2 in C. stenos n. sp.