Stephanorhinus kirchbergensis

Lobachev, Y. V., Shpansky, А. V., Bondarev, A. A., Lobachev, A. Y., Vasiliev, S. K., Klementev, A. M., Grebnev, I. E. & Silaev, V. I., 2021, New findings of Stephanorhinus kirchbergensis in Siberia, Palaeontologia Electronica (a 14) 24 (1), pp. 1-42 : 26-28

Stephanorhinus kirchbergensis Coelodonta antiquitatis

1. The proximal articular surface has a wide notch in the The notch is either very weak or absent. latero-plantar side.

2. The dorso-lateral angle is not very acute and only slightly The dorso-lateral angle always well pronounced and elongated. Correlatively with this, the width of the markedly elongated in the lateral direction. The proximal proximal articular surface is approximately the same as articular surface is more extended latero-medially.

the length.

3. The process talocaudalis (which is used for connection The process talocaudalis is placed at the middle of the with a cubic bone) is displaced to the plantar edge of the lateral side.

lateral side.

4. The dorso-medial angle has a convex shape.

5. The joint, on the distal side, has a well pronounced indentation in the proximal direction between facets for connection with III cuneiform and I cuneiform.

The dorso-medial angle is oblique.

This indentation is no such pronounced.

6. The III cuneiform facet has a well pronounced gap from This gap is no such pronounced. the dorso-lateral edge of the bone.

the plant tissues. In this stage, a brief but intense stress concentrated on a small area of surface is produced. Only the serrated edge of the ectoloph participates in this phase. The shape of the ectoloph curves follows the shape of curves in lophids of the lower teeth and the dorsal buccal edge of lophids in the lower teeth. This leads to the occurrence of polished facets on the buccal edge of lophids, which are at an acute angle to the occlusal surface ( Figure 24.1 ). In the second phase (the crushing phase), there is an interaction of the main part of the occlusal surface for both upper and lower teeth, with a translocation in the oral-aboral direction ( Figure 24.5 ). The features of this masticatory apparatus in Stephanorhinus kirchbergensis were developed from a juvenile age. A characteristic facet is observed on the milk teeth of the juvenile mandible NSMLL-12 ( Figure 2). In brachyodont teeth, both phases are well developed, while in hypsodont teeth, the first phase is rudimentary and the second phase proves to be the main one, representing the crushing under high pressure and large side amplitude. As a result, the occlusal surface of the upper and lower teeth of Coelodonta antiquitatis is flat ( Figure 24.2, 24.4, 24.6 ).

The next adaptations are caused by different chewing forces for brachyodont and hypsodont types of teeth. A large proportion of herbaceous plants in the diet of Coelodonta antiquitatis increases the force required for its milling. Mandibles of large herbivores are forced to move laterally with a large amplitude, when the animal is required to create a maximum force to break the plant tissue, which has in its structure long, longitudinal, durable, and elastic fibers (as in grass). If the feeding objects have a mosaic structure, as does the leaf of a shrub or young tree, movements back and forth are enough to break it (Sanson, 2006). The increase in forces in herbivores is reached by relevant morphological adaptations of the mandible. In this case, the relative height and thickness of the horizontal corpus and the relative height of the mandibular condyle above the level of the tooth row are increased. All of the mandibles studied appear more slender than C. antiquitatis according to characteristics listed above. It can be seen on Figure 25 , where indices of molars relative length ( Figure 25 ; Ind. 1), horizontal corpus relative height ( Figure 25 ; Ind. 2-7) and thickness ( Figure 25 ; Ind. 8-12) for S. kirchbergensis are lower than those for C. antiquitatis .

The adaptation to a more selective manner of feeding is presented by a very narrow, spoon-like mandibular symphysis area and shortened viscerocranium, because of the reduced diastema ( Table 4, Figure 3 , Figure 8.1 , occlusal view). In Coelodonta antiquitatis , the area of mandibular symphysis is flat, wide, and relatively long ( Table 4). Another result of this adaptation may be a less wide opening of the mouth. In Stephanorhinus kirchbergensis , this is caused by the more acute angle of rise in the vertical ramus of the mandible ( Figures 7 , 10 ). In C. antiquitatis , the vertical ramus rises slightly, which allows the mouth to open wider. Another adaptation to a more selective manner of feeding could be the high body position and slender structure of the S. kirchbergensis postcraniun. This is well illustrated by the morphometric data of the material presented ( Tables 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, Figures 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ). The structure of the occipital region of the cranium allowed feeding on the vegetation at a higher level. Distinctive features of C. antiquitatis are the adaptations for gathering food from the ground: shortened limbs, a strongly overhanging occipital crest that prevents lifting the head high, and an elongated viscerocranium caused by the shifting of orbits in the caudal direction.

Figures

FIGURE 3. Stephanorhinus kirchbergensis; Chumysh River at Kytmanovo (Kytmanovo District, Altay Territory, southeast Western Siberia); the horizontal right body of a mandible NSMLL-10, occlusal and buccal views. Scale bar ruled in centimeters.

FIGURE 7. Stephanorhinus kirchbergensis; Biysk (Altay Territory, southeast Western Siberia); the horizontal right body of a mandible GR PC-1164, occlusal and buccal views. Scale bar ruled in centimeters.

FIGURE 8. Stephanorhinus kirchbergensis; Biysk (Altay Territory, southeast Western Siberia); the mandible GR PC-1165, 1, occlusal view. 2, posterior view. 3, buccal view. Scale bar ruled in centimeters.

FIGURE 10. Stephanorhinus kirchbergensis; Tobol Horizon level (Middle Pleistocene); at Asino (Asino District, Tomsk Province, southeast Western Siberia); first left upper molar M1 PM TSU 1/396, 1. buccal view. 2, occlusal view. KB MAN K-397 the horizontal left body of a mandible of the Tobol Horizon level (Middle Pleistocene); Ob River at Kindal village (Kargasok District, Tomsk Province, southeast Western Siberia). 3, buccal view. 4, occlusal view. 5, occlusal view р3 – m3.

FIGURE 16. Stephanorhinus kirchbergensis; Chumysh River at Kytmanovo (Kytmanovo District, Altay Territory, southeast Western Siberia). The left ulna, NSMLL-109. 1, medial view. 2, anterior view. 3, lateral view. The right astragalus, NSMLL-107. 4, dorsal view. 5, plantar view. Scale bar ruled in centimeters.

FIGURE 17. Stephanorhinus kirchbergensis; Chumysh River at Kytmanovo (Kytmanovo District, Altay Territory, southeast Western Siberia). The right radius, IAE CHU- 108. 1, anterior view. 2, lateral view. 3, proximal view. 4, distal view. The left tibia, NSMLL-110. 5, distal view. 6, proximal view. 7, anterior view. 8, posterior view. Scale bar ruled in centimeters.

FIGURE 18. Stephanorhinus kirchbergensis; Chumysh River at Kytmanovo (Kytmanovo District, Altay Territory, southeast Western Siberia). The left Mc II, NSMLL-102. 1, dorsal view. 2, lateral view. 3, volar view. 4, proximal view. Ob River at Taradanovo Village (Suzun District, Novosibirsk Province, southeast Western Siberia). The left Mc IV, IAE TRD-17. 5, dorsal view. 6, medial view. 7, volar view. 8, proximal view. Scale bar ruled in centimeters.

FIGURE 19. Stephanorhinus kirchbergensis; Bijsk (Altay Territori, southeast Western Siberia). The left Mc II, GR PC- 203. 1, dorsal view. 2, lateral view. 3, volar view. 4, proximal view. The right Mc II, GR PC-214, 5. dorsal view. 6, lateral view. 7, volar view. 8, proximal view. Scale bar ruled in centimeters.

FIGURE 20. Stephanorhinus kirchbergensis; Ob River at Taradanovo village (Suzun District, Novosibirsk Province, southeast Western Siberia). The left Mc IV, IAE TRD-18. 1, dorsal view. 2, medial view. 3, volar view. 4, proximal view. The left Mt IV, IAE TRD-2. 5, dorsal view. 6, medial view. 7, plantar view. 8, proximal view. Scale bar ruled in centimeters.

FIGURE 21. Stephanorhinus kirchbergensis; Tobol Horizon level (Middle Pleistocene), Ob River at Krasniy Yar (Tomsk Province, southeast Western Siberia). The right Mc III, PM TSU 5/5197. 1, lateral view. 2, dorsal view. The left Mc III, PM TSU 5/2723. 3, lateral view. 4, dorsal view. Scale bar ruled in centimeters.

FIGURE 22. The navicular: Stephanorhinus kirchbergensis, Tobol Horizon level (Middle Pleistocene), Ob River at Krasniy Yar (Tomsk Province, southeast Western Siberia). 1, PM TSU 5/2538. 2. PM TSU 5/3063. Coelodonta antiquitatis, Karginian Horizon, Ob River at Krasniy Yar (Tomsk Province, southeast Western Siberia), 3. PM TSU 5/3815. а, proximal view. b, distal view. c, dorsal-medial view.

FIGURE 23. Stephanorhinus kirchbergensis; Chumysh River at Kytmanovo (Kytmanovo District, Altay Territory, southeast Western Siberia). The left Mt II, NSMLL-101. 1, dorsal view. 2, lateral view. 3, plantar view. 4, proximal view. The left Mt IV, NSMLL-105. 5, dorsal view. 6, medial view. 7, plantar view. 8, proximal view. Scale bar ruled in centimeters.

FIGURE 24. The specifics of the wear of the occlusal surfaces of the lower and upper teeth of Stephanorhinus kirchbergensis (1,3,5) and Coelodonta antiquitatis (2,4,6). Arrows indicate the morphological differences associated with a different biomechanical masticatory system for the browser and the grazer. Scale bar ruled in centimeters.

FIGURE 25. Indexes of the mandible of Stephanorhinus kirchbergensis and Coelodonta antiquitatis.