Bathyergoides neotertiarius Stromer, 1923

Costa, Laura Bento Da & Senut, Brigitte, 2022, Skeleton of Early Miocene Bathyergoides neotertiarius Stromer, 1923 (Rodentia, Mammalia) from Namibia: behavioural implication, Geodiversitas 44 (10), pp. 291-322 : 294-306

Bathyergoides neotertiarius Stromer, 1923

EMENDED DIAGNOSIS BY Lavocat 1973. — The complete series of the lower cheek teeth consists of five (sic) teeth; there are four upper cheek teeth bearing four crests, the paracone crest usually separated from the protocone, sometimes fused with wear; crest of the metacone always separated. Skull very high, with a very characteristic wedge shape.Posterior area of the skull anteroposteriorly very short. Large infraorbital foramen. Strong upper incisor which reaches the top of the root of the M3. Lower incisor touching the anterior face of the condyle, anteriorly to this one.

HOLOTYPE. — A fragment of right mandible with the m2 and an isolated lower molar.

MATERIAL EXAMINED. — GT 50’06 (Skull and mandible with P4- M3/p4-m3, incisors, partial skeleton), LT 200b’98 (right mandible with p4-m3), LT 50’19 (right mandible with p4-m3) .

DENTAL FORMULA. — 1.0.1.3/1.0.1.3

DESCRIPTION OF SKULL, MANDIBLE AND TEETH

INCISORS ( Fig. 5 )

The left upper incisor was found isolated, while the lower ones are preserved in the mandibles ( Fig. 4 ). The incisors are robust, triangular in transverse section. A thick layer of brown enamel is present on the external surface of the teeth, not extending beyond the first third of the labial surface. The radicular end of the lower incisors terminates close to the base of the articular condyle, well beyond the m3.

The premolars and molars exhibit the same structures. The p4 and P4 are smaller than the molars. The m2 and M2 are the biggest cheek teeth; the m3 and M3 are always smaller than the m2 and M2.

P4-M1-M2-M3 ( Table 1; Fig. 6 )

The upper premolars and molars are wider than long. The teeth are lightly worn, showing the outer groove and forming a crescent-shaped fossa, which disappears with advanced wear. The protocone and hypocone are connected to a small endoloph. The lingual sinus is poorly developed, slit-shaped when the tooth is slightly worn. The anteroloph extends from the protocone and joins the anterior face of the paracone. This crest, fused with the paracone, forms the anterior border of the tooth. The paracone and metacone are centred on the buccal margin of the tooth. Depending on the wear, these cusps tend to fuse, the metacone being more prominent than the paracone. It should be noted that there is a problem concerning the definition of the metacone which will be treated in the discussion.

The posterior part of the cheek teeth looks like the anterior part, the posteroloph forming the posterior wall of the teeth. It extends from the posterior face of the hypocone to the posterobuccal surface of the metacone. During wear, the posteroloph does not join the metacone in the M3; the latter cusp forming the posterobuccal border of the tooth.

p4-m1-m2-m3 ( Table 1; Fig. 7 )

Buccolingually, the p4 is narrower anteriorly than posteriorly. The metaconid projects anteriorly forming the mesial margin of the tooth. The structures are poorly identifiable due to advanced wear. The structure of the p4 is similar to that of the molars. The central furrow is reduced to an islet of enamel and the posterior one is reduced to a small notch on the posterolingual border. The lower molars present three principal crests (metalophulid I, entolophid and posterolophid). The posterior part of the tooth is slightly narrower buccolingually than the anterior one, the posterolophid being shorter than the other two crests. This shortening is more marked in the m3s which are still erupting and are therefore lightly worn. The protoconid and the hypoconid are prominent and connected by a short ectolophid. The narrow buccal V-shaped sinusid is limited by these three structures. It is buccoanteriorly to posterolingually oriented. A short metalophulid I extends from the anterior face of the protoconid to the anterolingual side of the metaconid forming the anterior wall of the tooth. The hypoconid is connected to the prominent entoconid by a short crest: the entolophid. Due to the advanced degree of wear, the hypoconid, entolophid and entoconid are fused together forming the central crest. The posterolophid stretches from the posterior face of the hypoconid to the lingual border of the tooth.

SKULL ( Figs 8 ; 9 )

The fragmentary skull is slightly deformed due to compaction and to fossilization in salty and gypsiferous deposits. It preserves the palate with the tooth rows, the left zygomatic arch and a part of the occipital areas.

Dorsal view

The left zygomatic arch projects strongly laterally, resulting in a great width of the skull with a rounded aspect of the lateral side. It gradually enlarges backwards. Despite the poor preservation of the cranial roof, a marked sagittal crest can be seen.

Ventral view

The tympanic bullae are ovoid and not prominent. The left paracondylar process is preserved and shows a strong posterior development, which terminates at the level of the occipital condyle. The triangular mastoid process is well developed. The tooth rows are parallel (probably partially due to compaction), on both sides bordering a thin palatine bone. Close to the M2s, a round and deep foramen occurs in the middle of the palatine bone.

Lateral view

The left zygomatic arch is massive, thickened (tall) in its middle part. The maximum height is reached at the junction of the zygomatic bone and the temporal bone. The frontal process of the zygomatic bone extends above the end of the zygomatic process of the temporal bone. The inferior part of the anterior end of the zygomatic arch extends well below the zygomatic process of the temporal bone. The height of the arch decreases backwards towards the temporal bone. Despite the compaction, it is possible to estimate the proportions of the skull: the posterior part of the skull being shorter than the palatal one (1.74cm against 1.2 cm).

Occipital view

In occipital view, the skull is wider than tall. The preservation of the occipital surface does not permit precise description of the basioccipital, the exoccipital and the supraoccipital bones. However, we observe that the occipital bones join at the level of the supraoccipital to form a prominent sagittal crest. The foramen magnum is ovoid, facing backwards, flanked at its base by the two rounded occipital condyles.

MANDIBLE ( Fig. 10 )

The right hemi-mandible is well-preserved, except for the coronoid process, which is broken at its base, and the ascending ramus which is missing a slight fragment at its apex. The less well-preserved left hemi-mandible is missing the gonial area, the ascending ramus and the upper part of the coronoid process.

Buccal view

The hemi-mandibles are robust. In the right one, the masseteric fossa is large and deep. Despite the fact that the coronoid process is not entirely preserved, the remaining part suggests that it was higher than the articular condyle on the left mandible, agreeing with the morphology observed on other specimens of the species. The articular condyle is low. The mental foramen is deep and ovoid, located below the anterior part of the m1. The enamel extends onto the base of the lateral surface of the incisor.

Dorsal view

The horizontal ramus projects strongly laterally, at the level of the front of the m2, before curving gently backwards. Broken in its anterior part, the articular condyle is triangular in shape, transversely compressed and anterobucally/posterolingually elongated. The radicular end of the incisor extends beyond the back of the m3. The cheek tooth row is oriented in the plane of the incisor. The premolars and molars are in the same plane as in LT 50’19 and LT 200b’98 ( Figs 10 ; 11 ), which show a slight difference in the orientation of the m2 and m3, certainly due to the stage of wear.

It is not possible to describe the lingual view of the mandible due to the state of preservation of the bones.

DESCRIPTION OF POSTCRANIAL ELEMENTS

HUMERUS ( Fig. 12 )

The right humerus and the proximal end of the left one are preserved. The bone is short and rather massive.

Anterior view

The tuberculum majus and tuberculum minus are poorly developed, the former extending proximally up to the top of the caput humeri. The tuberculum minor is lower than the tuberculum major. The posterior border of the tuberculum major joins distally the deltoid crest, which is strongly developed and extends as far as the two lower thirds of the bone. The crista tuberculi minoris is marked, descending distally as far as the middle of the humeral shaft. The teres minor tuberosity is weakly salient. The sulcus intertubercularis is shallow and broad. On the distal epiphysis, the epicondylus medialis is developed, with an inclination of 135° to the shaft axis. The trochlea humeri is mediolaterally short, bordered by a medial lip and slightly orientated downward. The distal part of the medial lip of the trochlea and the distal part of the capitulum are roughly at the same level. The capitulum is ovoid. The epicondylus lateralis is poorly developed and located above the capitulum. From the epicondylus lateralis extends the susepicondylar crest, forming a convex lateral border at its base, in contrast to the concave medial border of the humerus.

The fossa radialis is deep, large and perforated, connecting to the olecranon fossa. The fossa coronoidea is poorly marked. There is no sus-epitrochlear foramen.

Posterior view

The caput humeri is rounded and proximomedially to distolaterally oblique. The posterior humeral shaft is reinforced by a crest which provides insertion for the m. triceps brachii. The olecranon fossa is ovoid and wide and bordered by the medial and lateral pillars; the medial one being the wider.

Distally, the lateral and medial lips of the trochlea are parallel, inclined along the distomedial/proximolateral axis. The fossa olecrani is broad, perforated and delimited by two blunt, low crests.

Medial view

The caput humeri does not strongly project posteriorly above the humeral shaft. The deltoid crest is salient and flat on the anteromedial surface.

Lateral view

The posterior part of the diaphysis is flat but its anterior part is salient at midshaft level due to the development of the deltoid crest.

Proximal view:

The caput humeri is globular. The tuberculum majus is more salient anterolaterally than the tuberculum minus anteromedially. The sulcus intertubercularis is wide and shallow.

Distal view

The capitulum is mediolaterally elongated, its anterior part being slightly higher than the medial lip of the trochlea. The lateral lip of the trochlea is posteriorly prominent. The epicondylus medialis is mediolaterally oriented, slightly oblique posteriorly. The epicondylus lateralis is poorly expressed.

ULNA ( Fig. 13 )

The right ulna is almost complete (> 3.88 cm), missing only the styloid process and the most proximal part of the olecranon. The shaft is rather massive (only the width can be securely measured (0.28 cm )). The left ulna preserves only the olecranon and the anconeal process.

Anterior view

The olecranon process is elongated, broadened in its proximal part. The incisura trochlearis is proximodistally elongated and proximolaterally to distomedially oblique. Its proximal part is wider than the distal part. The incisura radialis faces anterolaterally, and is not laterally salient. The ulnar tuberosity is elongated, with a central depression for the insertion of the m. brachialis. The shaft is straight and thickens distally. The crista supinatora is well marked and extends from the base of the incisura radialis to the distal end of the bone.

Lateral view

The bone exhibits a sigmoid curvature. The elongated olecranon (length = 0.7mm) is bent forward. The anconeal and coronoid processes which limit the greater sigmoid cavity project equally anteriorly. The greater sigmoid cavity is semicircular. The incisura radialis is triangular with the apex pointing distally. A long and wide gutter is present on the lateral part, extending from the proximal part of the incisura trochelaris to the middle of the shaft.

Medial view

The medial fossa of the olecranon is deep and large, extending to the level of the ulnar tuberosity. The anconeal fossa is deep and extends as far as the middle part of the ulnae.

Posterior view

The proximal end of the incisura trochlearis is strongly laterally salient, whereas the lower medial one is weakly medially developed.

RADIUS ( Fig. 14 )

The left radius only preserves its distal two thirds and the right is trapped within sediment. It is not possible to extract the right radius because of its fragility, but we can observe that it is almost complete despite the break. The right proximal epiphysis is preserved, flattened antero-posteriorly with a long neck and an elongated tuberositas radii. The caput radii is ovoid.

The radial shaft, which is more flattened in its proximal part, is rather thin and clearly widens at the distal epiphysis level, suggesting a reinforcement of the carpal area. On the ulnar side, two long crests run down and are separated by a long well-marked groove which broadens distally. These crests would give insertion to the abductor pollicis longus and m. digitorum profundus. The distal epiphysis of the radius is robust, the poorly salient processus styloideus being lower than the facies articularis carpea, which is not deep. Because of the poor preservation, it is not possible to confirm the existence of two articular surfaces for the lunate and the scaphoid. Despite its poor preservation the epiphysis appears triangular. A few carpal bones are preserved in the matrix, but are displaced and are not easily cleanable because of their fragility and will be part of another paper dealing with the hand of the species.

FEMUR ( Fig. 15 )

Despite the crushing, the proximal end and a fragment of diaphysis of the left femoral shaft, and a fragment of the right femoral shaft are preserved. The bone is massive and compressed anteroposteriorly. The left femoral condyles are fragmentary. The description is based on the left femur which is the better preserved.

Anterior view

The tuberculus majus (greater trochanter) is broken proximally, but the preserved part suggests that it is lower than the proximal part of the femoral head. The femoral neck is short and the proximal notch shallow. The third trochanter is elongated and weakly salient.

Posterior view

The fossa trochanterica (trochanteric fossa) is deep and broad. The femoral head is rounded. The neck is short and wide. The prominent crista intertrochanterica (posterior intertrochanteric crest) is well developed.The tuberculus minus (lesser trochanter) is broken proximally, and posteromedially oriented. The intertrochanteric crest forms an obtuse angle before joining the tuberculus minus (lesser trochanter).

Medial view

The neck and femoral head project anteriorly. The fovea capitis cannot be seen due to the poor preservation of the specimen.

Lateral view

The tuberculus majus (greater trochanter) projects slightly anteriorly. The third trochanter is represented by a slightly developed crest-shaped tuberosity. Its anterior surface is concave, while its posterior one is slightly convex.

TIBIA AND FIBULA ( Figs 16 ; 17 )

The proximal ends of the right tibia and fibula are preserved. The left tibia is almost complete, only the tibial plateau is missing. The left fibular shaft is preserved.

Anterior view

The tibia is lateromedially compressed, with an acerate tibial crest. The proximal end is laterally curved and the anterior tuberosity rectangular.

Posterior view

The fibula is fused with the fragmentary lateral condyle in the right specimen, and does not permit detailed description due to its poor preservation. There is no intercondylar eminence but it could be due to the state of the specimen. Posteromedially There is an elongated tuberosity at the side, where the m. popliteus inserts, which is salient distally. A slightly concave surface is present above the process, being the insertion of the m. soleus. These structures are separated from an elongated posteromedial gutter by a marked crest, which extends for one third of the bone. In this deep posteromedial gutter inserts the m. tibialis posterior.

Medial view

The medial surface is flat. The m. popliteus tuberosity is wide and convex, distally salient and posteromedially oriented. Above this protrusion, its posteromedial surface is slightly concave, where the m. soleus inserts.

Lateral view

The anterior tuberosity is convex. The tibial fossa for the insertion of a strong m. tibialis anterior is deep and broad. The crest separating the fossae of the m. tibialis anterior and the m. tibialis posterior is directed slightly backwards in its proximal part and extends downwards, disappearing progressively on the shaft.

Proximal view

The triangular tibial plateau is lateromedially compressed. The lateral condyle is developed and strongly posterolaterally projecting. Due to the poor preservation, it is not possible to describe precisely the groove, which does not seem to be deep. Because of the thinness of the tibia, unusual among rodents, it was difficult to determine the different muscular insertions; this is why we add an explicative figure ( Fig. 17 ).

The left fibula shows a marked and elongated gutter on its anterior side. Despite the lack of its most proximal part, we observe that it is flared, corresponding to the junction with the tibial plateau.

CALCANEUM ( Fig. 18 )

Only the left calcaneum is preserved, in connection with part of the astragalus which covers the proximal part of the dorsal face.

Dorsal view

The anterior part of the facies articularis talaris posterior is elongated anteroposteriorly. The facies articularis talaris media is slightly concave, and distolaterally to proximomedially oblique. The two facies articularis are separated by a shallow groove. The facies articularis cuboidea is oblique proximomedially/distolaterally. The processus peronealis is laterally projecting and elongated.

Plantar view

The shaft of tuber calci is mediolaterally compressed, and widens proximally in the tuber calci which is proximally convex. The sustentaculum tali is strongly medially projecting, forming almost a right angle with the tuber calci. Its lateral border is distolaterally to proximomedially oriented, parallel to the axis of the facies articularis cuboidea. The groove of the sustentaculum tali is deep. The sulcus tendinis musculus peroneus longi is wide and deep.

Lateral view

The lateral surface is flat, slightly concave under the facies articularis tali posterior. The ovoid processus peronealis is slightly oblique distodorsally to proximoprantarly. It is divided into two parts by a deep, narrow groove. The plantar tubercle is weak.

Medial view

Only the distal part of the medial face is visible. The plantar border is slightly concave. The sustentaculum tali is thin and proximodorsally/distoplantarly oblique. The groove of the sustentaculum tali is very weak.

Proximal view

The tuber calci is convex and ovoid. It is tall and oriented dorsolaterally to medioplantarly.

Distal view

The processus peronealis is obliquely plantarly oriented. The facies articularis cuboidea is taller than wide, concave and shows a notch in its medial side. The groove between the sustentaculum tali and the facies articularis cuboidea is deep.

Only the external part of the trochlea tali of the astragalus is preserved, articulated with the left calcaneum ( Fig. 18 ). The upper border of the trochlea tali is rounded and wide. The facies malleolaris lateralis is not prominent, slightly concave, fused with the processus lateralis tali. This small processus is triangular and slightly distolaterally projecting. The facies articularis calcanea posterior is wide.

APPENDICULAR SKELETON ( Figs 19-22 )

Parts of the ribs and the vertebral column are preserved but are not complete comprising part of the rib cage, the cervical vertebrae and parts of the dorsal, lumbar and caudal vertebrae. Part of the rib cage is preserved, partly crushed and does not permit a detailed description. The ribs seem to be thick. The cervical vertebrae are compact (but not fused as is sometimes the case in burrowing rodents) suggesting a shortened neck, the usual feature in fossorial mammals. The atlas is poorly preserved but one of the superior articular facets is preserved and deep. On the axis, the odontoid process is stout and slightly ventrally inclined.

The sacrum was not found; it is thus not possible to estimate the number of sacral vertebrae and to determine whether there was fusion with the lumbar vertebrae. The number and length of the caudal vertebrae suggests that the tail was not strongly reduced in length, unlike the case in extant subterranean mammals.

Figures

FIG. 5 — Upper Incisor and mandible of GT 50’06: A-D, left upper incisor; A, labial view; B, mesial view; C, lingual view; D, distal view. E-G, mandible; E, right lateral view; F, inferior view; G, occlusal view. Scale bars: 1 cm.

FIG. 4 — List of measurements taken in this study and used for the index calculations.Bathyergus janetta Thomas and Schwann,1904,specimen TM 39332: A, dorsal view of the skull;B, ventral view of the skull;C, dorsal view of the mandible; D, occlusal view of the left cheek teeth raw;E, anterior view of the left humerus; F, anterior view of the left ulna; G, medial view of the left ulna. Abbreviations:SL, length of the skull; SW, width of the skull; CRL, length of the cheek teeth raw; ML, length of the hemimandible; HL, length of the humerus; DW, length of the humeral diaphysis; DEW, width of the distal epiphysis; PEW, width of the proximal epiphysis; UL, length of the ulna; DuW, width of the ulna diaphysis; OPL, length of the olecranon process; TL, length of the tooth; TW, width of the tooth. Scale bars: 1 cm.

FIG. 6. — Right upper cheek tooth row of GT 50’06: A, P4-M3, B, detail of the posterior part of the M3 (occlusal view) with the separation between the posteroloph and the metacone (in red). Scale bar: 1 mm.

FIG. 7. — Right lower cheek tooth row of GT 50’06 (p4-m3, occlusal view). Scale bar: 1 mm.

FIG. 8. — Views of the skull GT 50’06 before cleaning. Scale bar: 1 cm.

FIG. 9. — Skull of GT 50’06: A, dorsal view; B, ventral view; C, left lateral view; D, occipital view. Scale bar: 1 cm.

FIG. 10. — Mandible of GT 50’06: A, ventral view; B, dorsal view; C, left buccal view; D, right buccal view. Scale bar: 1 cm.

FIG. 11. — Lingual view of A, LT 200b’98; B, LT 50’19, specimens of Bathyergoides neotertiarius. Scale bar: 1cm.

FIG. 12. — Humeri of GT 50’06: A-D, Right humerus; A, anterior view; B, lateral view; C, posterior view; D, medial view. E-H, Left humerus; E, anterior view; F, lateral view; G, posterior view; H, medial view. I-J, Proximal (I) and distal (J) view of the right humerus. K, proximal view of the left humerus. Scale bar: 1 cm.

FIG. 13. — Ulnae of GT 50’06: A-D, Right ulna; A, anterior view; B, lateral view; C, posterior view; D, medial view. E-F, Olecranon process of the left ulna; E, anterior view; F, lateral view; G, posterior view; H, medial view. Scale bar: 1 cm.

FIG. 14. — Radius of GT 50’06: A-D, Left radius: A, lateral view; B, medial view; C, anterior view; D, posterior view. E-F, Right radius and a fragment of the right ulna. E, medial view; F, anterior view. Scale bar: 1 cm.

FIG. 15. — Femora of GT 50’06: A-D, Proximal epiphysis of the left femur;A, anterior view; B, lateral view;C, posterior view;D, medial view. E-H,Shaft of the right femur; E, anterior view; F, lateral view; G, posterior view; H, medial view. I-L, Shaft of the left femur; I, anterior view; J, lateral view; K, posterior view; L, medial view. Scale bar: 1cm.

FIG. 16. — Tibiae of GT 50’06: A-D, Left tibia (A, anterior view; B, lateral view; C, posterior view; D, medial view); E-H, proximal epiphysis of the right tibia (E, anterior view; F, lateral view; G, posterior view; H, medial view;) I, left fibula. Scale bar: 1 cm.

FIG. 17. — Muscular insertions on the left tibia GT 50’06. Scale bars: 1 cm.

FIG. 18. — Left calcaneum of GT 50’06: A, dorsal view; B, lateral view; C, plantar view; D, medial view; E, distal view; F, Proximal view. Scale bar: 1 cm.

FIG. 19. — Rib cage GT 50’06. Scale bar: 1 cm.

FIG. 20. — Cervical vertebrae of GT 50’06: A, Ventral view; B, Dorsal view. Scale bar: 1 cm.

FIG. 21. — Lumbar vertebrae of GT 50’06 (dorsal view). Scale bar: 1 cm.

FIG. 22. — Caudal vertebrae of GT 50’06 (dorsal view).