Beckumia sinemeckelia, Martin & Averianov & Schultz & Schellhorn & Schwermann, 2022
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publication ID |
https://doi.org/ 10.4202/app.00914.2021 |
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persistent identifier |
https://treatment.plazi.org/id/EB57E55D-915B-654A-FFEF-FAEEFEA9F589 |
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treatment provided by |
Felipe |
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scientific name |
Beckumia sinemeckelia |
| status |
sp. nov. |
Beckumia sinemeckelia sp. nov.
Figs. 9 View Fig , 10 View Fig .
Zoobank LSID: urn:lsid:zoobank.org:act:738E1658-6C43-4516-B3C3-2D23D15856B2
Etymology: From Latin sine, without, and Meckel, referring to the absence of Meckel’s groove.
Holotype: WMNM P82301 , left mandible with strongly worn m2–m5 present, other teeth fallen out or broken off postmortem.
Type locality: Fissure filling within the Busche quarry near Balve, Sauerland, North Rhine-Westphalia, Germany.
Type horizon: Barremian–Aptian (Lower Cretaceous).
Material.— Holotype and left lower premolar ( WMNM P82310 ) from the type locality and horizon.
Diagnosis.—Referred to Dryolestidae based on the following derived characters: roots of lower molars extremely disparate with mesial root much larger and stronger than distal root; mandibular border medially higher than laterally, molars unilaterally (labially) hypsodont; lower molars strongly compressed mesio-distally with small unicuspid talonid cusp; mandibular body deep, with s-shaped curvature in dorsal view ( Fig. 9 View Fig ). Dental formula 4i, 1c, 4p, 8m; the first lower incisor (i1) implanted almost horizontally in alveolus. Incisors single-rooted, canine, premolars and molars double-rooted, ultimate molar (m8) single-rooted; coronoid process rising almost vertically; angular process well developed and pointing backwards; well developed masseteric and pterygoid fossae; two mental foramina below p1 and m1; no trace of coronoid or splenial detectable.
Differs from all other known dryolestids by complete reduction of Meckel’s groove (derived). Differs further from Crusafontia cuencana Henkel and Krebs, 1969 , by 50% larger molars and absence of a labial cingulid as evident on m5.
Measurements (in mm).—Preserved length of mandible WMNM P82301: 23.0; height of corpus below ultimate molar: 3.5; m2, length*: 1.0, width*: 1.1; m3*, length*: 1.0, width*: 1.3; m4, length*: 1.2, width*: 1.5. WMNM P82310, lower p4: length: 0.72; width: 0.3 (* as preserved).
Description.—WMNM P82301 is a left mandible ( Fig. 9 View Fig ) with the dorsal part of the coronoid process, the condylar process, and the distal tip of the angular process broken off. Most of the teeth have fallen out postmortem. According to the present alveoli, the dental formula is 4i, 1c, 4p, and 8m ( Fig. 9A View Fig 5 View Fig ). The single roots of the incisors have an oval shape in cross section (long axis of cross section oriented linguo-labially). Judging by the alveoli, i1 was inserted almost horizontally, i2 at 45°, and i3 and i4 at 70° in the mandible. The canine was double-rooted with both roots of equal size. All four premolars had two roots with a round cross section. In p1 and p2, the anterior root was slightly larger than the posterior; in p3 the posterior root was slightly larger; and in p4 both roots were of equal size. The sizes of the alveoli suggest that p4 was the largest, with p3, p1, and p2 being successively smaller. Except for the ultimate, the molars are double-rooted with dryolestid-typical large anterior and tiny posterior root; m8 was single-rooted with only the anterior root present. The m1 and m6–m8 are not present (posterior root of m1 still sitting in the alveolus). The heavily worn m2–m5 are still in place (m5 partly broken), their crowns are almost entirely worn away with the pulp cavities open. The molars are more strongly worn on the labial side, resulting in an oblique occlusal surface. On the lingual side, parts of the enamel-covered crowns are still present. The paraconid (best preserved on m4) is erect and separated from the metaconid by a narrow v-shaped notch. The paraconid and the metaconid taper apically and it can be inferred that they were pointed, although their tips are removed by the oblique wear. The single-cusped talonids are very short. On m5, a small cervical portion of the enamel-covered labial part of the crown is preserved which lacks any trace of a labial cingulid. The extremely worn down molars with extensive transversely oriented striations suggest a senile individual. As typical for dryolestids, the alveolar border is much higher on the lingual than on the labial side which is correlated with a unilateral (labial) hypsodonty of the molars.
The mandibular body is slender and has a slightly curved ventral border. There are two mental foramina, one at the position of the posterior root of p2 and one slightly smaller at the position of the posterior root of m1; both foramina are oval in longitudinal direction and sitting just below the alveolar border ( Fig. 9A View Fig 2 View Fig ). The external border of the mandibular body is convex with foveal bone surface (especially in the anterior part in front of the molars). The masseteric fossa is well developed with a strong anterior crest and shelf-like ventral border ( Fig. 9A View Fig 2 View Fig ). The coronoid process rises steeply (about 80° to the alveolar line) but its dorsal half (or more) is missing ( Fig. 9A View Fig 2 View Fig ). There is no trace of a coronoid facet. The angular process is well developed and pointing posteriorly (distal end is broken off) ( Fig. 9A View Fig 2 View Fig , A 4 View Fig ).
The lingual (internal) surface of the mandibular body is flat ( Fig. 9A View Fig 4 View Fig ). A large symphysis extends from the tip of the mandible to the posterior root of p2 with a strongly pitted surface. Above the symphysis, the mandibular surface is slightly concave and bears two tiny foramina below the canine. From the posterior end of the symphysis to the position of m3 a ventral edge extends along the mandibular body. The pterygoid fossa ( Fig. 9A View Fig 4 View Fig ) is well developed with a pronounced ventral edge. It bears a high oval mandibular foramen. Meckel’s groove is completely reduced and no traces of a splenial bone is detectable.
In dorsal view ( Fig. 9A View Fig 1 View Fig ), the mandibular body is somewhat bent with a slight labial curvature in the region of the anterior-most molars and the premolars which is exaggerated by a crack in that area.
Left p4: WMNM P82309 ( Fig. 10 View Fig ) has been identified as a lower p4 based on similarity to the p4 of Crusafontia cuencana ( Henkel and Krebs 1969: fig. 1c–e; Martin 1998: fig. 4; Figs. 11 View Fig and 12 View Fig ). The crown of p4 is formed by a large main cusp (protoconid) and a mesial and distal cuspule. The main cusp is compressed labio-lingually and slightly bent lingually, and bears an apical wear facet with exposed dentin that continues on the lingual side ( Fig. 10A View Fig 1 View Fig ). The protoconid has a convex mesial side; the lingual side is slightly concave; and the distal side is straight. On the lingual side, the enamel is corroded centrally near the crown base. The mesial cuspule ( Fig. 10A View Fig 1 View Fig , A 3 –A View Fig 5 View Fig ) sits high on the mesial side of the protoconid, at about half of the main cusp’s height. It is separated from the protoconid by a pronounced notch and bears a round apical wear facet. The distal cuspule ( Fig.10A View Fig 1 View Fig , A 3 View Fig , A 5 View Fig , A 6 View Fig ) sits much lower at the base of the protoconid. It is somewhat larger than the mesial cuspule and bears an oval, slightly lingually inclined apical wear facet. The distal cuspule is separated from the protoconid by a rounded notch. The premolar has two roots of which the distal root is slightly larger than the mesial root. Both roots are broken off, exposing the pulp cavities; the mesial root has a round and the distal root an oval (slightly labio-lingually compressed) cross section ( Fig. 10A View Fig 2 View Fig ).
Remarks.—The attribution of WMNM P82301 to Dryolestidae is unquestionable because the molar trigonids are strongly compressed mesio-distally, have a small lingually positioned talonid, and possess extremely uneven roots, which all are derived dryolestid characters. In lingual view, the molars are reminiscent to those of Crusafontia cuencana , showing a slender and pointed paraconid. They differ from the molars of C. cuencana by the absence of a buccal cingulid (evident on m5 of WMNM P82301; the respective area is worn off in the other molars) and a 50% larger size. The molars of WMNM P82301 are also twice in size of the molars of Minutolestes submersus gen. et sp. nov. Dryolestids have a diphyodont mode of tooth replacement, and permanent molars are morphologically clearly distinct from deciduous premolars ( Martin 1997). Therefore, it can be ruled out that the small molariform teeth attributed to Minutolestes submersus gen. et sp. nov. derive from juvenile Beckumia individuals. The complete reduction of Meckel’s groove in the mandible from Balve is unique among Dryolestidae ; in C. cuencana Meckel’s groove is short and reduced to the distal portion, but still clearly discernible ( Henkel and Krebs 1969; Figs. 12 View Fig and 13 View Fig ). In Mf N Uña 2 (paratype specimen), a crack is present in that area which artificially exaggerates Meckel’s groove, but in MfN Uña 1 (holotype specimen) it is preserved undisturbed. In both C. cuencana specimens (MfN Uña 1 and MfN Uña 2), the medial side of the mandibular body bears a longitudinal depression which is rather a postmortem phenomenon (diagenetic compression of the bone) than a further remnant of Meckel’s groove. Minutolestes submersus gen. et sp. nov. and C. cuencana lack a rough surface area that can be interpreted to be for the coronoid bone and any trace for accommodation of a splenial on the inner side of the mandible. By comparison, the coronoid facet and a possible splenial” facet are still present in Late Jurassic dryolestids e.g., Krebs 1969, 1971; Martin 1995, 1999). Like the mandible of C. cuencana, WMNM P 82301 is rather slender for a dryolestid mandible. Size-wise the mandible of WMNM P82301 (preserved length 23 mm; corpus height below last molar 3.5 mm) corresponds to the adult paratype mandible of C. cuencana from Uña (preserved length 19.5 mm, portion anterior of posterior alveolus of canine missing; corpus height below last molar 3.7 mm ( Henkel and Krebs 1969).
The adult paratype mandible (MfN Uña 2) is about 50% larger than the juvenile holotype mandible (MfN Uña 1), but the teeth are of comparable dimensions. The holotype and paratype specimens of C. cuencana have three mental foramina ( Henkel and Krebs 1969; Krebs 1971; Figs. 11A View Fig 3 View Fig and 12A View Fig 2 View Fig ) whereas there are only two in WMNM P82301.
Premolar WMNM P82310 closely resembles a left p4 from Galve attributed to Crusafontia cuencana (Galve Th 21) ( Martin 1998: fig. 4), both in morphology and size (Galve Th 21, length: 0.73 mm, width 0.37 mm), and fits very well the dimensions of mandible WMNM P82301. In dryolestids, wear on the premolars occurs on the disto-lingual side rather than on the labial side as in most other mammals. This wear pattern regularly occurs in connection with pronounced apical wear which is typical for dryolestids ( Martin 1999; Schultz and Martin 2011).
We offer a new interpretation for the premolar and molar count in the holotype specimen of Crusafontia cuencana (Mf N Uña 1) based on µCT analysis ( Fig. 11 View Fig ). In the original description, Henkel and Krebs (1969) interpreted the dental formula as 4i, 1c, 4p, 7m, with single-rooted p1 and p2 (only represented by alveoli). According to our re-interpretation the dental formula is 4i, 1c, 4p, 6m, with dP4 still in place (“m1” of Henkel and Krebs 1969) ( Fig. 11A View Fig 6 View Fig ). This tooth differs from the molars by a more elongated crown (145% of width compared to 133–126% in m1–m5), which is a typical trait of dryolestid deciduous premolars ( Martin 1997). The shape of the two alveoli immediately behind the canine indicates that they belong to a double-rooted premolar (p1) and not to two single-rooted teeth as suggested by Henkel and Krebs (1969). Judging by the size of the alveoli, p1 was larger than p2, which is the smallest of the premolars. This is in accordance with the observation that p2 generally is the smallest premolar in dryolestids (e.g., Martin 1999). The stage of eruption of the premolars, p 2 in process of eruption, p3 fully erupted, and dp4 still in place, is also in congruence with the earlier observed alternating mode of tooth replacement in dryolestids with p4 being the last premolar to erupt ( Martin 1997). This new interpretation of premolar positions in the juvenile C. cuencana holotype specimen resolves the discrepancy to the premolar count of the adult paratype specimen (Mf N Uña 2, Fig. 12 View Fig ) noticed by Henkel and Krebs (1969). According to Henkel and Krebs (1969) MfN Uña 2 possessed four double-rooted lower premolars which is in accordance with our analysis.
The mandible of the Beckumia sinemeckelia gen. et sp. nov. type specimen (WMNM P82301) is somewhat larger than that of Crusafontia cuencana and its molars are of double size. The strongly worn molars indicate that it is from an old individual. The ultimate molar (m8) has a single root, and is directly in front of the coronoid process. Both features are adult characteristics. The small and single root of m8 suggests that m8 is the last tooth to erupt. This interpretation is based on the observation that the confluence of roots or a reduction to a single root of the ultimate molar is a typical character of the adult growth stage in other Mesozoic mammaliaforms for which the growth series are known ( Schultz et al. 2019; Panciroli et al. 2019). Also, the last molar of the toothrow shifts anteriorly relative to the coronoid process during the growth of mammaliaforms ( Schultz et al. 2019: fig. 4). In a growth series, the younger individuals tend to have their last molar positioned medial to the anterior edge of the coronoid process at the time of eruption; the larger and older adult specimens usually have the ultimate molar located directly in front of the coronoid process.
Stratigraphic and geographic range.— Type locality and horizon only.
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