Bustylus marandati ( Crochet and Sigé, 1983 )

Bast, Eric De, Sigé, Bernard & Smith, Thierry, 2012, Diversity of the adapisoriculid mammals from the early Palaeocene of Hainin, Belgium, Acta Palaeontologica Polonica 57 (1), pp. 35-52 : 40-42

Bustylus marandati ( Crochet and Sigé, 1983)

Figs. 3, 4 ; Table 2.

Type material: Holotype: IRSNB M1974 (N2−03), right M2 . Paratypes: IRSNB M1990 (Q2−39), left P4 ; IRSNB M1975 (N2−02), left M1 ; IRSNB M1984 ( R1−68 ), left M3 ; IRSNB M1991 (N2−22), left jaw fragment with p4, talonid of m1 and alveoli of p3 ; IRSNB M1992 (Q2−05), left m1 ; IRSNB M1993 ( R1−27 ), left m2 ; IRSNB M1994 (Q2−42), left m3 .

Type locality: Hainin , Hainaut province, Belgium .

Type horizon: Early Palaeocene, Hainin Formation, Mons Basin.

Referred material.—Right M2; Q2−06, right M3; Q2−29, right m1 or m2; R1−89, posterolabial part of M1 or M2; R1−88, lingual part of left M3; Q1−13, lingual part of right dp4 or m1; talonid of right m1; Q2−15, trigonid of right m2; Q2−18, talonid of left m2; Q1−14, trigonid of left m3; R1−19, trigonid of left m2 or m3; R1−29, trigonid of m2 or m3.

Emended diagnosis.—Differs from B. cernaysi and B. folieae in its smaller size (30% smaller than B. cernaysi and 15% smaller than B. folieae ). Upper molars differ from those of B. cernaysi in their less developed stylar cusps and narrower lingual region, from B. folieae in a transversely less wide M1 and from both species in the presence of longer and more marked internal crests. Lower molars differ from those of B. cernaysi and B. folieae in the presence of a proportionally narrower m1 with a particularly long and narrow trigonid, and from B. folieae in the presence of a smaller precingulid and less compressed trigonid on m2. p4 differs from those of B. cernaysi and B. folieae in the presence of a slightly smaller paraconid and a less individualised metaconid.

Measurements.—See Table 2.

Description.—The P4 ( Fig. 3D) is strongly developed transversely. The stylar shelf is reduced, only developed posteriorly, and bears one cusp in the position of the metastyle. The parastyle is broken at the base, but seems less developed. The postmetacrista is directed posterolabially and slightly curved. The paracone is high, while the metacone is reduced to a small bulge on the crest linking the paracone to the metastyle. There is neither a paracingulum nor a metacingulum. The lingual region of the tooth is more developed than its labial part. The protofossa is large and shallow. No metaconule is visible, and the paraconule is barely noticeable as a protuberance on the preprotocrista. The postparaconule crista is the only visible internal crest, and climbs up on to the paracone. The protocone is small and shifted anteriorly.

The M1 ( Fig. 3A) is relatively long and narrow compared to M2 and shows a moderate degree of transverse development. The stylar shelf is wide and shows three cusps: the parastyle, stylocone and mesostyle. The ectoflexus is relatively deep and rather symmetrical. The parastylar lobe points labially, whereas the metastylar lobe is directed posterolabially. The preparacrista and postmetacrista are relatively long and oblique to the anteroposterior axis of the tooth. The paracone and metacone are well separated and linked by a rectilinear centrocrista. The paracingulum is relatively wide, whereas the metacingulum is very narrow, short and terminates at the posterolingual base of the metacone. The lingual part of the tooth is more developed transversely than its labial part. The protofossa is deep, relatively small and well delimited. The metaconule, and even more so the paraconule, are well developed and carry sharp internal cristae. The protocone is slightly procline and smaller than the paracone and metacone.

The M2 ( Fig. 3B) is relatively similar to M1, but much more developed transversely. The prepara− and postmetacristae are more parallel to the transverse axis of the tooth and the stylar shelf is wider.

The M3 ( Fig. 3C) is similar in size to M2. The parastylar lobe is even more developed than on M2, whereas the metastylar lobe is almost absent. The metacone is moderately reduced. The protofossa is slightly smaller than on M2.

Specimen IRSNB M1991 ( Fig. 3H) is a dentary preserving p 4 in situ, as well as the alveoli for p3. The p3 appears slightly shorter than p4; both p3 and p4 are biradiculate. Relatively short diastemata are present in front of and behind p3.

DE BAST ET AL.—ADAPISORICULID MAMMALS FROM PALAEOCENE OF BELGIUM 41

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The dentary is moderately high (slightly higher than the crown of p4) and has a dental foramen under the anterior root of m1.

The p4 ( Fig. 3H) is premolariform. The trigonid is much longer than the talonid, but of similar width. The paraconid is small and situated at the anterolingual base of the protoconid. There is no precingulid, and the protoconid is the highest cusp. The reduced metaconid is partially fused to the protoconid and located slightly posterior to it. The postmetacristid is well developed. The talonid has two cusps, corresponding to the entoconid and hypoconid, respectively. A poorly developed crista obliqua is visible, but delimits no basin.

The m1 ( Fig. 3E) is narrow, with little height difference between the trigonid and talonid. The trigonid is narrower than the talonid and rather long. The trigonid cusps are sharp, but have a robust base. The precingulid is moderately developed. The paraconid, the smallest cusp of the trigonid, is reduced, points lingually and anteriorly, and is aligned with the metaconid along the anteroposterior axis of the tooth. The slightly lingually protruding metaconid is lower than the protoconid. The latter is located anterior to the metaconid, resulting in an oblique and almost vertical posterior wall of the trigonid. The postmetacristid is relatively well marked. The hypoconid is the most voluminous cusp of the talonid and is shifted labially. The three talonid cusps are equidistant, and the talonid basin is deep. The crista obliqua reaches the trigonid lingual to the protocristid notch and extends up towards the tip of the metaconid. The hypoflexid is narrow and deep, and the tooth is slightly narrower (labiolingually) in this area.

The m2 ( Fig. 3F) is morphologically similar to m1, but more robust, distinctly wider, and slightly longer. The trigonid is markedly compressed anteroposteriorly. The hypoconulid is slightly closer to the entoconid than to the hypoconid.

http://dx.doi.org/10.4202/app.2010.0115

The m3 ( Fig. 3G) is very similar to m2. The talonid is slightly narrower than the trigonid, while the hypoconulid is bigger than on m1 and m2, and shifted posteriorly.

Discussion.—The lower teeth referred here to Bustylus marandati conform in their size, morphology and occlusal pattern to those of Adapisoriculidae . Their affinity with the family Adapisoriculidae was discussed by Gheerbrant (1991). The upper molars share with other species of Bustylus the presence of a mesostyle and a rectilinear centrocrista, a rather short protocone, and internal crests of the conules less marked than in Afrodon .

The upper molars of Bustylus marandati differ from those of Bustylus cernaysi in their smaller size (70%), more crestiform stylar cusps, a shorter lingual region, less transverse development and more marked internal crests. The M1 (IRSNB M1273) of Bustylus sp. from Dormaal, Belgium ( Smith 1997) is very similar to that of B. marandati , but differs from B. marandati in its slightly smaller size (90%), a lesser degree of transverse development, more individualised stylar cusps, a shallower and less asymmetrical ectoflexus, and a shorter postmetacrista. B. marandati is the best known species as regards the upper molars, and a tentative reconstruction of the jugal teeth is proposed in Fig. 4 . Among the specimens initially attributed to B. marandati by Crochet and Sigé (1983), the M3 IRSNB M1976 is here transferred to Bustylus folieae sp. nov. (see discussion of that species). The p4 is morphologically very similar to that of B. cernaysi , but is slightly less molariform, with a smaller paraconid and a less individualised metaconid. The lower molars are very similar to those of other species of Adapisoriculidae . The talonid is much deeper than in Afrodon , but similar to other species of Bustylus . The hypoconid is larger and located more labially than in all species of Afrodon . The m1 (IRSNB M1274) of Bustylus sp. ( Smith 1997) is remarkably similar in size and morphology to that of B. marandati . The trigonid of the figured m1 is unusually narrow and somewhat resembles the shape of a dp4, except for the relatively steep paraconid. The m1 talonid preserved in the jaw fragment (IRSNB M1991) is slightly wider, and the possibility of the isolated m1 being a dp4 can not be excluded. However, the narrowness of m1 seems compatible with the relatively small degree of transverse development of M1, and we therefore here regard the isolated tooth as an m1.

Stratigraphic and geographic range.—Known only from the early Palaeocene of the Mons Basin, Belgium.

Figures

Fig. 4. Occlusal sketch of the reconstructed upper tooth row, from P4 to M3, of Bustylus marandati (Crochet and Sigé, 1983), the best known species from Hainin as regards the upper dentition.