Hippopotamodon erymanthius (Pickford, 2015)

Pickford, M., 2016, Hippopotamodon erymanthius (Suidae, Mammalia) from Mahmutgazi, Denizli-Çal basin, Turkey, Fossil Imprint 72, pp. 183-201 : 186-191

Species Hippopotamodon erymanthius (ROTH et WAGNER, 1854)

Holotype. Lower jaw illustrated by Roth and Wagner (1854: pl. 5, fig. 1). The whereabouts of the fossil are not known, although a cast is reported to be preserved in the NHMUK, London (Van der Made et al. 2013).

Type locality. Pikermi, Greece.

Diagnosis. Large species of Hippopotamodon in which the length of m/3 ranges between 40.5 and 51.6 mm, and M3/ from 38 to 44 mm. Tendency for P1/ and p/1 to be suppressed. Adult males have a prominent supra-canine flange with rugose lateral and dorsal surfaces.

Description

Skull

In the Karlsruhe collection of Hippopotamodon erymanthius from Mahmutgazi, there is a partial skull, and several mandible fragments, including parts of a juvenile maxilla and mandibles.

The most complete skull fragment in the sample is a right maxilla ( Text-figs 2 , 3 ) of a fully adult individual (M1/ deeply worn, M3/ in moderate wear) (SMNK Ma1 MP 8 ). Almost none of the zygomatic or frontal bones is preserved. There is a large supra-canine flange above and behind the level of the canine, which has highly rugose lateral and dorsal surfaces, which indicate that the specimen is likely to represent a male individual. Dorsally, the rugose part of the canine flange is separated from the facial part of the maxilla by a deep, smooth, groove via which passed the tendons of the snout musculature. The area of the origins of the snout musculature are large, that for the levator rostri occupying a large area of the zygomatic process of the maxilla above the facial crest, beneath which is an extensive zone for the depressor rostri and dilatator nasalis lateralis musculature.

In ventral view, the front of the canine flange ends above the canine, and the rear edge terminates opposite the level of the P2/, forming a right angle with the facial process of the maxilla. There is an alveolar ridge in front of the P2/, which curves slightly laterally as it courses anteriorly, and in the preserved part there is no sign of a P1/ alveolus. The leading edge of the zygomatic arch flares laterally at an angle of about 45°, its anterior root being opposite the front of M2/ and the maxillary recess lying opposite the rear loph of the M3/. A small part of the anterior edge of the orbit is preserved above and well behind the level of the M3/.

Mandible

The lower jaw of Hippopotamodon erymanthius from Mahmutgazi (SMNK Ma1 Gips 6 ) shows a long diastema between the canine and the p/2 occupied by a sharp alveolar ridge ( Text-fig. 4 ). There is no sign of the p/1 or of an alveolus for this tooth. There is a short gap between the i/3 and the canine. The rear of the symphysis extends as far back as the level of the front of p/2. The incisor battery is proclive and lies well beneath the occlusal surface of the cheek teeth. The lower canine is short and stubby, and projects laterally and anteriorly only slightly. The left mandible and symphysis from the site (SMNK Ma1 Gips 17 ) shows essentially the same morphology as the previous specimen ( Text-fig. 5 ).

Deciduous teeth

There are two juvenile mandibles and a maxilla in the collection at Karlsruhe. Ma1 MP 5 is a left maxilla containing D3/–D4/ and M1/ ( Text-fig. 6a ), and Ma1 Nr 196 is a right mandible containing d/2–d/4, m/1 and a left mandible with d/4–m/1 ( Text-fig. 6b ).

In occlusal view, the outline of the D3/ is triangular with rounded corners, with a large mesial cusp and a pair of distal cusps forming the rear loph. There are weak mesial distal cingula, as well as small cingular tubercles on the inner face of the mesial cusp. The main cusps are indented by Fürchen.

The D4/ is tetracuspid, like the permanent molars, but the mesial pair of cusps is slightly narrower than the distal pair, which imparts a trapezoidal occlusal outline to the tooth. In the centre-line of the tooth there are small, low, anterior, median and posterior accessory cusplets. The mesial and distal cingula are well formed but do not extend onto the buccal or lingual surface. There is a low tubercle in the lingual end of the median transverse valley. The Fürchen pattern is like that in the permanent molars.

The d/2 is triangular in buccal view with the mesial and distal cusplets about half the height of the main cusp, and it is sectorial in occlusal view. It is narrower mesially than distally. The two roots are splayed apart anteriorly and posteriorly.

The d/3 is a larger version of the d/2.

The d/ 4 in contrast is molariform, with six cusplets arranged in three lophids, as is usual in artiodactyls. The root arrangement is the usual one found in suids, with splayed mesial and distal roots supporting the anterior and posterior lophids, and a single buccal root under the protoconid of the second lophid.

Permanent teeth

The upper right central incisor from Mahmutgazi (SMNK Ma1 Nr 176 ) is unworn and shows a prominent cusplet on the lingual part of the tooth ( Text-fig. 7a ). This tubercle is an enlarged bead of the lingual cingulum. The cusplet is confluent with the rest of the lingual cingulum and walls off a capacious lingual fossa. The labial side of the crown is bulbous. There is a shallow slit apically, and the distal edge of the crown is beaded, markedly so near the cervix.

SMNK Ma1 MP 6, is a worn right I3 / with a swollen lingual cingulum interrupted distally by a slit ( Text-fig. 7b ).

The upper premolars and molars of Hippopotamodon erymanthius were described by Pickford (2015). The Mahmutgazi specimens are typical of the species ( Text-fig. 8 ). Although the evidence is weak, the only available maxilla suggests that the P1/ was suppressed, which indicates a derived species of the genus. The P2/ has a diminutive disto-lingual tubercle, as in material from Pikermi. The P3/ possesses a thick lingual cingulum, closing off a small mesial fovea, and a capacious distal lingual basin. The rear of the tooth is broad, due to the well-developed disto-lingual cusp. The P4/ is tricuspid, with a pair of sagittal cusplets in the valley between the protocone on the one hand and the paracone-metacone on the other. The incision between the paracone and metacone is relatively shallow, and becomes obsolete in medium wear.

The upper third molars from Mahmutgazi have a short lingually positioned talon ( Text-fig. 9a, b ), while the lower third molars have a double-cusped talonid, with the lingual cuspid usually smaller and lower than the buccal one ( Text-fig. 9c, d, e ).

The central and second lower incisors are extremely tall, and even the i/3 is elongated and barely separated from the canine ( Text-figs 10 , 11 ). The incisors are arranged such that they comprise a continuous occlusal surface which wears almost flat. The canine extends only slightly above this occlusal surface, and is functionally linked to the same dental battery.

The anterior lower premolars are sectorial with the mesial and distal tubercles about half the height of the main cusp. The p/4 has a clear inner cusp and a large posterior cusplet ( Text-figs 12 , 13 ). The p/1 is absent in all three mandibles that preserve the requisite part of the jaw, which indicates a derived species of the genus ( Text-figs 4 , 5 ).

Post-cranial bones

The only suid post-cranial element in the Mahmutgazi collection in Karlsruhe is a proximal left radio-ulna ( Ma1 Gips 12 ) ( Text-fig. 14 ). The radius is solidly fused to the ulna, even though the sutures between the bones are clearly visible.

The olecranon process is relatively short and medio-laterally robust (broken caudally). In the radius, the articular surface for the distal epiphysis of the humerus shows a broad medial basin, a relatively prominent central groove and a deep but small lateral basin. The articular part of the medial edge of the sigmoid notch in the ulna is expansive. The corresponding part on the lateral side is less expansive. The sigmoid notch extends over more than a semi-circle, all of which indicates an elbow in which joint movements were constrained to the para-sagittal plane.

Metric analysis of the teeth of Hippopotamodon erymanthius from Mahmutgazi, Turkey

The dimensions of the dental sample of Hippopotamodon from Mahmutgazi ( Tab. 2) are smaller than any of the specimens from Luberon, France, the type locality of Hippopotamodon major (basal MN 13), and are smaller on average than the mean of the Pikermi (MN 12) and Samos (MN 11 + MN 12 or MN 13) samples of Hippopotamodon erymanthius ( Text-fig. 15 ). They are similar to the collection from Akkaşdağı, Turkey, ( Text-fig. 16 ) correlated to MN 12 by Liu et al. (2005), and to the large sample from Maragheh (= Maragha) (Iran) correlated to MN 11 by Pickford (2015). Overall the Mahmutgazi specimens are similar in dimensions to the comprehensive sample from Dorn-Dürkheim 1, Germany (Van der Made 1997), which is correlated to MN 11, but are on average smaller than the material from Kalimantsi (Kotopoulos et al. 2001), although there is overlap in the ranges of variation of the fossils from the two localities.

Concerning Turkish samples of Microstonyx major (which according to Liu et al. (2005), includes Microstonyx erymanthius), the authors wrote that “we usually deal with faunal lists or fragmentary and isolated specimens which preclude a direct comparison with the Akkaşdağı form”. This concern can now be partly addressed (see measurements and descriptions of Turkish suids from many localities published by Pickford (2015)), and it is clear that the cranial and dental suid specimens from Akkaşdağı and Mahmutgazi are metrically closely similar to each other, suggesting similar chronological and/or palaeoecological contexts. The fact that both of these localities have yielded material that is on average smaller than the samples from Pikermi and Samos (Greece) suggests either that the latter two localities are not the same age as Mahmutgazi and Akkaşdağı, or that their palaeoenvironments may have differed, or that they comprise multiple levels (the case for Samos).

Specimens of Hippopotamodon erymanthius from Pikermi, Greece, in the SMNK

The SMNK possesses two dentognathic specimens (containing 12 teeth) of Hippopotamodon erymanthius from Pikermi, Greece ( Tab. 3 ). This sample augments the large amount of material that has been described from Pikermi (now standing at 358 teeth) ( Pickford 2015).

The snout ( SMNK MP3) is interesting, as it probably represents a young adult female individual, with no sign of a supra-canine flange, only a bowing out of the maxilla behind the level of the canine ( Text-fig. 17 ). The left and right I2 /s are in their crypts, but show the serrated postcrista typical of the genus, and the right P2/–M2/ are fully erupted. The only teeth showing light wear at the apices of the cusps are the M1/ and the P3/.

The P4/ shows well-formed sagittal cusplets attached to the buccal cusps and strong mesial and distal cingula ( Text-fig. 17b ).

The second specimen of Hippopotamodon erymanthius in the Karlsruhe collection is a left mandible fragment with moderately worn m/1–m/2 and the front lophid of m/3. Measurements of the teeth are provided below ( Tab. 3 ).

Sexual dimorphism in Hippopotamodon erymanthius

The fossil record of Hippopotamodon erymanthius is now comprehensive enough to indicate that the species was sexually dimorphic in the area of the upper canine. There seems to be no bimodality in dental dimensions, save for the fact that males seem to have a slightly larger upper canines than females. The incisors and cheek teeth appear to be metrically unimodal. This finding agrees with the discussion by Liu et al. (2005).

The most obvious sexual character is the supra-canine flange, which is large in males and is adorned with rugose lateral and dorsal surfaces, almost like the rugose surface of cauliflowers. Females show a bowing out of the maxilla behind the upper canine, but there is little or no development of a flange ( Liu et al. (2005) thought that females also have supra-canine flanges). In males, the supra-canine flange extends from above the canine backwards to the level of the P2/, corresponding to the diastema between the canine and P2/. In the Mahmutgazi specimen, the flange is 7 cm long and adds ca. 3 cm to the breadth of the snout on each side. The contrast between males and females is well shown in the Karlsruhe collection by a male specimen from Mahmutgazi and a female individual from Pikermi. The presence of a large supra-canine flange in males suggests that it served as a weapon during male-on-male combat, or perhaps for protection during such encounters. The canines in Hippopotamodon erymanthius are reduced in dimensions and probably played little if any role in intraspecific combat. The canine flanges in contrast are strong in males, and were probably covered in thick skin, perhaps with bristles emerging from them, in which case pushing, shoving and butting the opponent with the side of the snout was probably the main form of combat between males during the rutting period.

Figures

Text-fig. 2. SMNK Ma1 MP8, right maxilla of Hippopotamodon erymanthius from Mahmutgazi, Turkey. A) stereo occlusal view, B) stereo dorsal view (scale 10 cm).

Text-fig. 3. SMNK Ma1 MP 8, right maxilla and parts of the zygomatic and frontal bones of Hippopotamodon erymanthius from Mahmutgazi, Turkey. A) lingual view, B) lateral view (arrow shows the leading edge of the orbit) (scale 10 cm).

Text-fig. 4. SMNK Ma1 Gips 6, mandible of Hippopotamodon erymanthius lacking the ascending rami, from Mahmutgazi, Turkey. A) stereo occlusal view, B) right lateral view (scale 10 cm).

Text-fig. 5. SMNK Ma1 Gips 17, left mandible and symphysis of Hippopotamodon erymanthius lacking the ascending ramus, from Mahmutgazi, Turkey. A) left lateral view, B) stereo occlusal view (note the d/2 between the p/2 and p/3 (scale 10 cm).

Text-fig. 6. Juvenile specimens of Hippopotamodon erymanthius from Mahmutgazi, Turkey. A) Ma1 MP 5, left maxilla containing D3/–D4/ and M1/ (A1 – stereo occlusal, A2 – lingual, A3 – buccal views), B) Ma Nr 196, right mandible containing d/2–d/4 and m/1 (B1 – stereo occlusal, B2 – lingual, B3 – buccal views) (scale 5 cm).

Text-fig. 7. Upper incisors of Hippopotamodon erymanthius from Mahmutgazi, Turkey. A) SMNK Ma1 Nr 176, right I1/ (A1 – distal, A2 – stereo lingual, A3 – mesial, A4 – labial views), B) SMNK Ma1 MP6, right I3/, stereo lingual view (scale 10 mm).

Text-fig. 8. Stereo occlusal views of upper premolars of Hippopotamodon erymanthius from Mahmutgazi, Turkey. A) SMNK Ma1 Nr 180, left P3/, B) SMNK Ma1 Nr 190, left P4/ (scale 10 mm).

Text-fig. 9. Stereo occlusal views of upper and lower molars of Hippopotamodon erymanthius from Mahmutgazi, Turkey. A) Ma1 Nr 181, deeply worn right M3/, B) Ma1 Nr 190, deeply worn left M2/–M3/, C) Ma1 Nr 131, left m/3 in fragment of mandible, D) Ma1 Nr 176, left m/3, E) Ma1 Nr 145, right m/2–m/3 in mandible fragment (scale 5 cm).

Text-fig. 10. The lower incisor-canine battery of Hippopotamodon erymanthius from Mahmutgazi, Turkey. A) SMNK Ma1 Gips 6 (A1 – oblique stereo anterior view, A2 – anterior view); B) SMNK Ma1 Gips 17, oblique stereo anterior view (scales 10 mm).

Text-fig. 11. SMNK Ma1 200, mandibular symphysis of Hippopotamodon erymanthius with all incisors and canines from Mahmutgazi, Turkey. A) stereo occlusal view, B) stereo ventral view, C) left lateral view, D) right lateral view (scale 5 cm).

Text-fig. 12. Left mandible of Hippopotamodon erymanthius, SMNK Ma1 Gips 2, from Mahmutgazi, Turkey, containing lightly worn p/2–m/3. A) buccal view, B) stereo occlusal view, C) lingual view (scale 5 cm).

Text-fig. 13. SMNK Ma1 Gips 13, right mandible of Hippopotamodon erymanthius from Mahmutgazi, Turkey, with worn p/3–m/3. A) lingual view, B) stereo occlusal view, C) buccal view. This mandible may represent the same individual as the maxilla Ma1 MP 8 (scale 5 cm).

Text-fig. 14. SMNK Ma1 Gips 12, proximal left radio-ulna of Hippopotamodon erymanthius from Mahmutgazi, Turkey. A) medial, B) stereo cranial, C) lateral views (scale 5 cm).

Text-fig. 15. Bivariate plots of upper and lower fourth premolars and third molars of Hippopotamodon erymanthius and Hippopotamodon major (stars: Luberon, dots: Pikermi and open squares: Mahmutgazi).

Text-fig. 16. Metric comparisons of upper and lower fourth premolars and third molars of Hippopotamodon erymanthius from Mahmutgazi (open squares) and Akkaşdaği (dots), Turkey. The two samples are metrically closely similar.

Text-fig. 17. SMNK MP 3, probable female snout of Hippopotamodon erymanthius from Pikermi. A) stereo occlusal view of snout, B) close-up stereo occlusal view of right upper cheek tooth row (scales 5 cm and 10 mm).