Brachyodus, DEPERET, 1895

Brachyodus onoideus ( GERVAIS, 1859)

Text-figs 2–13, 19

H o l o t y p e. Musée d’Orléans 05 (2016-0-3-75, 314), right mandible fragment with p/4–m/3. (Cast in MNHN).

T y p e l o c a l i t y. Neuville-aux-Bois, France.

C o r r e l a t i o n. MN 3b.

D e s c r i p t i o n a n d c o m p a r i s o n. Coll. Sinturet FY 3416 is a moderately worn right m/3 of Brachyodus onoideus from La Fuye, France. The crown is normally developed on its buccal side, but on the lingual side the entoconid is considerably larger than usual and it projects markedly lingually, thereby adding ca. 25 % to the breadth of the tooth. The posthypocristid is also greatly enlarged, both mesio-distally and bucco-lingually, such that it extends to the lingual margin of the tooth (arrow in Text-fig. 2a 1). In lingual aspect, the base of the entoconid is seen to extend slightly beneath the rest of the cervix ( Text-fig. 2a 5) which, in normally developed teeth of Brachyodus , is usually relatively straight from mesial to distal as in two specimens from La Guimardière (Coll. Gain 20585a; Text-fig. 2b 3) and Les Beilleaux (Coll. Sinturet Bx 3235; Text-fig. 19d). The hypoconid and entoconid in FY 3416 are separated by a shallow valley, as is usual in Brachyodus ( Text-fig. 2b 1) but, unusual for the genus, the preentocristid does not fuse with the prehypocristid. The protoconid and metaconid are of the habitual morphology for Brachyodus , as is the hypoconulid (talonid) but they are slightly twisted with respect to the rest of the crown.

The roots of the tooth reflect the crown dimensions, such that the root supporting the entoconid is much enlarged ( Text-fig. 2a2) in comparison with those that support the other cuspids. Comparison of the root system of FY 3416 with those of other m/3s of Brachyodus , indicate that the entoconidal root has been displaced lingually leaving a wide space between the two roots that support the second lophid. Another specimen in the Sinturet collection (# 65; Text-fig. 3) shows the normal condition of the two roots, which are subequal in dimensions and are close together.

I n t e r p r e t a t i o n. The basic structural elements of the crown of the m/3 from La Fuye are the same as those in other specimens of Brachyodus , comprising five cusps arranged into three lophids, accompanied by a well-developed mesial cingulum ( Text-fig. 2a 3). The protoconid and metaconid comprise the mesial lophid, the hypoconid and entoconid form the second lophid and the hypoconulid forms the centrally placed talonid at the rear of the tooth.

What differs in the tooth from La Fuye is the great enlargement of the posthypocristid (arrows in Text-fig. 2a 1) and the entoconid relative to the normal dimensions of the other cusps, with the result that the mesial lophid is slightly twisted with respect to the second lophid, and the hypoconulid is displaced slightly lingually from its usual orientation in Brachyodus . As a result, the pattern of wear in this tooth is somewhat different from what it is in other specimens of the genus. A further consequence of the anomaly was an increase in the dimensions of the root that supports the entoconid (compare Text-figs 2a2, 3b) accompanied by its displacement lingually.

A left M3/ from La Fuye (Musée du Savignéen 98-2, 37; Text-fig. 4a) has a normally developed crown, but the wear pattern differs widely from the usual situation in Brachyodus as shown in Musée du Savignéen FS 862 ( Text-fig. 4b). The protocone is markedly more worn than the paracone, metacone and metaconule, and there is a deep gully worn into the mesial part of the protocone and lingual cingulum.

Dental occlusion, maleruption and life h i s t o r y. Despite the anomalous enlargement, the slight anterior displacement and minor anticlockwise twist of the entoconid, the m/3 from La Fuye shows almost normal occlusal wear on its other main cusps, with large dentine lakes developed on the protoconid, hypoconid and hypoconulid, and a smaller one on the metaconid ( Text-fig. 2a 1) (the last facet is somewhat damaged by post-fossilisation breakage). The entoconid, in contrast, has only a small dentine lake at its apex, indicating that it was not in occlusal antagonism with the mesostyle of its corresponding upper third molar, or that cusp-to-cusp contact was minimal or very occasional. In normally developed m/3s, the apex of the entoconid would be as deeply worn as that of the metaconid if not slightly more deeply worn ( Text-figs 2b 1, 3a).

A significant difference in wear pattern is that, in the specimen from La Fuye, the dentine lake developed on the junction of the posthypocristid with the prehypoconulid cristid is much enlarged when compared to the corresponding part of a normally developed tooth (red arrows in Text-fig. 2). Indeed this part of the tooth from La Fuye appears to have been occluding with the mesostyle of the M3/, instead of the entoconid as would have been the case in a normally developed crown. This suggests that the tooth was not parallel to the other cheek teeth, but was twisted anticlockwise by about 20°, thereby indicating a mild degree of maleruption.

The stage of wear in FY 3416 indicates that the individual survived to fully adult status, but died prior to senility. The protoconid, hypoconid and hypoconulid are worn to about one third of the height of the cusps, suggesting that it survived for a significant period (several years) following the complete eruption of the third molars. Thus, it is inferred that the dental anomaly did not adversely affect the life history of the individual concerned.

In the context of this paper, a left M3/ (Musée du Savignéen 98-2, 37) also from La Fuye, is of interest because, even though it has a normally developed crown, its wear pattern is anomalous, in that the protocone and the mesial part of the lingual cingulum are far more deeply worn than the other cusps ( Text-fig. 4a). The metaconule is also more deeply worn than is usual in this genus. This wear pattern is markedly divergent from the normal wear pattern observed in other third molars of Brachyodus from France, that are at a comparable stage of wear ( Text-fig. 4b).

O t h e r f o s s i l s o f B r a c h y o d u s f r o m L a

F u y e. In the Gagnaison collection (FY4; Text-fig. 5) there is a damaged mandibular symphysis from La Fuye of a male individual of Brachyodus onoideus. The root and a small part of the crown of the right second incisor are preserved, and on the left side of the jaw, parts of the alveoli of i/1 and i/2 are present. In posterior view the anterior ends of the mandibular canals are visible on both sides, and on the lateral surfaces of the symphysis, beneath the diastema, there are shallow fossae, bordered anteriorly by the swelling that houses the roots of the i/2s. Damage to the superior margin of the bone has removed any evidence concerning the presence or absence of the i/3. The specimen represents a male individual.

Coll. Gagnaison FY 3 ( Text-fig. 6), is a right i/2 (female) of Brachyodus onoideus from La Fuye. The apex is slightly worn where it contacted the upper central incisor, and there are grooves on the root beneath gingival level; shallow ones close to cervix worn into the roots during the life of the individual, and deeper, more irregular ones further down the root due to post-mortem damage. The crown is labio-lingually compressed and the mesial and distal edges are sub-parallel. The root is conical, tapering to a point and slightly curved from cervix to apex. The apical part is moderately bent towards the distal side.

Coll. Gagnaison FY 2 ( Text-fig. 7), is a right i/2 (male) of Brachyodus onoideus from La Fuye. Its dimensions are appreciably smaller than those of the corresponding tooth in the symphysis described above (FY 4). The main wear facet is on the anterior and apical part of the crown, due to contact with the upper central incisor. The distal crest is prominent apically but fades out towards cervix. The root is robust and shows a longitudinal groove along its posterior side close to where the i/3 root would lie ( Text-fig. 7a). There is a narrow and shallow horizontal groove worn into the root immediately beneath the cervix that was developed during the lifetime of the individual. The latter groove is especially evident on the mesial side of the tooth ( Text-fig. 7e).

Musée du Savignéen 98-2, 49 ( Text-fig. 8), is a left mandible fragment of Brachyodus onoideus from La Fuye with damaged p/4 and roots of p/2–p/3. The p/4 is deeply worn and is missing part of the protoconid. It has a strongly developed preprotoconid cristid that reaches mesiolingually where it terminates against the mesial cingulum. The postprotoconid cristid is deeply worn but was directed distolingually where it fuses with a raised part of the lingual cingulum. There is a clear lingual cingulum extending between the ends of the pre- and postprotoconid cristids, and the enamel is lightly wrinkled.

Musée du Savignéen 36 ( Text-fig. 9), is a deeply worn right p/4 of Brachyodus onoideus from La Fuye. The lingual cingulum is prominent and rises posteriorly to join the postprotoconid cristid as in the p/4 described above (Musée du Savignéen 98-2, 49). The main difference between these teeth is the slightly greater breadth of Musée du Savignéen 36. There are two robust roots, the anterior one conical and tapering apically, the distal one comprised of two coalescent columns that give the root an 8-shaped section with the apices separated from each other ( Text-fig. 9c).

Coll. Guével FY 82 ( Text-fig. 10), a right d/3 of Brachyodus onoideus from La Fuye, is moderately worn. Its overall morphology recalls that of a permanent p/4, but the enamel is thinner, and the distal complex behind the protoconid is more extensive, with a prominent, tall entoconid and a lower hypoconid forming a transverse complex that is separated from the protoconid by a transverse depression. The pre- and postprotoconid cristids are sharp and terminate at the lingual side of the crown, the preprotoconid cristid ending at a minute metaconid. The lingual cingulum extends along the entire length of the tooth rising to meet the postprotoconid cristid and curling round the base of the entoconid to join the distal cingulum. Buccally the cingulum is weaker but it extends mesially and eventually joins the lingual cingulum at the mesiolingual corner of the tooth.

There are three upper fourth premolars of Brachyodus onoideus from La Fuye (Musée du Savignéen 51 ( Text-fig. 11a); 98-1, 50 Levé, I81 ( Text-fig. 11b); and sans # ( Text-fig. 11c)). The most notable aspect of these teeth is the enormous degree of variation that they express. Musée du Savignéen 51 has a weak preprotocrista that terminates before reaching the mesial cingulum, and there is no sign of a metaconule. In contrast, Musée du Savignéen 98-1, 50 Levé, I81, and Musée du Savignéen sans #, each possess a well-developed preprotocrista that reaches the mesial cingulum as well as a prominent metaconule. In Musée du Savignéen 98-1, 50 Levé, I81, the metaconule is mesio-distally compressed and narrow, whereas in Musée du Savignéen sans # the metaconule is large and circular. There are also differences in the strength of the postprotocrista, which is well developed in the first two specimens, but very weak in the third one with the large metaconule. The barrel on the buccal surface of the metaconid is strong in the first tooth, but weak in the third one. All three teeth are almost completely surrounded by a cingulum, with minor weakening of the structure on the buccal side.

The second specimen (Musée du Savignéen 98-1, 50 Levé I81) is distinctly smaller than the other two ( Tab. 1), which raises the possibility that it belongs to Brachyodus intermedius MAYET, 1908, rather than to Brachyodus onoideus (see metric analysis).

Coll. Sinturet FY 3106 ( Text-fig. 12), is a left maxilla fragment of Brachyodus onoideus containing the M2/ and the anterior half of M3/. The M2/ is deeply worn to the stage that the protocone and paraconule are no longer distinguishable from each other. The paracone, metacone and metaconule in contrast, are only moderately worn, which suggests that the heavy wear on the protocone may be anomalous. The parastyle and mesostyle are voluminous, but the metastyle is barely distinguishable at the end of the postmetacrista. The protocone and metacone each have a strongly developed barrel extending from the apex of the cusp towards the cervix, fading out as it goes. The metaconule has clear pre- and post-cristae that are oriented mesio-buccally and disto-buccally respectively, terminating in the midline of the crown. The mesial, lingual and distal cingula are narrow and continuous, and the buccal cingula are present but weak at the bases of the paracone and metacone. The enamel is finely wrinkled.

The morphology of the mesial loph of the M3/ is like that of the M2/ described immediately above. Being less worn, it is possible to see that the protocone and paraconule are closely appressed against each other, being separated by a narrow groove which would be obliterated with a medium amount of wear. The barrel on the buccal surface of the paracone is well developed, as is the cingulum at the base of the cusp.

Coll. Chevrier 557 ( Text-fig. 13), a left M3/ of Brachyodus onoideus from La Fuye, is lightly worn, with small wear facets on the parastyle and paraconule, but none on the other cusps and styles. As a consequence the specimen preserves all the morphological details of the crown in pristine condition. The roots are missing but in radicular view, it is possible to discern the distal extension of the root beneath the metacone ( Text-fig. 13e) which reveals that the tooth is a third molar, and not a second molar (which lacks such an extension).

The protocone and paraconule are separated from each other distally by quite a broad groove, but they coaslesce along the preprotocrista. The postprotocrista is low and weak, but the postparaconule crista is clear and descends towards the median transverse valley, but not encroaching into the valley. The preparaconule crista descends mesio-buccally, reaching the mesial cingulum close to the parastyle. The paracone has a buccal barrel that fades out towards the cervix and the barrel on the metacone is weakly expressed. The parastyle and mesostyle are well developed and bulbous at their bases, but are pinched in where they approach the paracone and metacone. The metaconule has clear pre- and postcristae which lead towards the midline of the crown. The metacone is angled in such a way that the postmetacrista is almost in the midline of the tooth, whereas the premetacrista is close to the buccal edge of the tooth, near the mesostyle. There is a strongly developed mesial, lingual and distal cingulum, which contrasts with the narrower cingulum on the buccal side of the crown. The enamel is wrinkled.

M e t r i c a n a l y s i s. In his unpublished PhD Thesis, Dineur (1982, sometimes cited as 1981) listed measurements (lingual length, buccal length, mesial breadth and distal breadth) of the teeth of Brachyodus . The main article dealing with the dimensional variation in Brachyodus from France was published by Dineur and Ginsburg (1986). The measurements of the M3/s used in the latter paper correspond to the lingual length and mesial breadth ( Text-fig. 14). Concerning the lower third molars, Dineur and Ginsburg (1986) were unable to decide to which species four of the specimens belonged, and they proposed two explanations; that they were either small specimens of B. onoideus, or large specimens of B. intermedius. They appear not to have considered the possibility of a third taxon in the sample.

Because of the dimensional and morphological diversity of the dental sample of Brachyodus from La Fuye , it was decided to perform a metric analysis of available fossils from the French Faluns, in order to determine the species to which the teeth belong ( Text-figs 15–18). Most of the teeth plot within the range of metric variation of Brachyodus onoideus, but two specimens (a P4/ and an m/3) are somewhat smaller than the others and possibly represent the species Brachyodus intermedius or a species intermediate in dimensions between B. onoideus and B. intermedius. The M3/ with the peculiar wear pattern from La Fuye, plots within the range of metric variation of the larger of the two named species, Brachyodus onoideus.

It is noted that the bivariate plots ( Text-figs 15–18) suggest that there are possibly four size groups (species?) of Brachyodus in the Faluns, one of which is larger than Brachyodus onoideus, but not yet formally diagnosed due to inadequacies of the fossil record (no suitable holotype available), the other is intermediate in dimensions between B. onoideus and B. intermedius. Alternatively, but perhaps less likely, is the hypothesis that the four metric groups could represent two species in which there was a large body size difference between males and females.

The balance of the metric evidence is that the anomalous tooth from La Fuye (Coll. Sinturet FY 3416) and a small P4/ (Musée du Savignéen 98-1, 51) belong to the size group that is intermediate in dimensions between Brachyodus onoideus and Brachyodus intermedius. The upper third molar with a peculiar wear pattern (Musée du Savignéen 98-2, 37) and the other teeth from the site plot into the range of variation of Brachyodus onoideus.