Betonnia undetermined

Clemens, William A., 2019, Puercolestes and Betonnia (Cimolestidae, Mammalia) from the early Paleocene (Puercan 3 Interval Zone) of northeastern Montana, U. S. A., PaleoBios 36, pp. 1-17 : 6-10

BETONNIA SP. CF. BE. TSOSIA

FIGS. 5–8

Referred specimens —Garbani Channel Complex localities. P4: UCMP V72129, UCMP 281370; UCMP V73080, UCMP 281372; UCMP V74122, UCMP 281371; UCMP V75194, UCMP 281369. M1: UCMP V73080, UCMP 218909; UCMP V74122, UCMP nos. 218911, 218912, 281341; UCMP V99438, UCMP nos. 218904, 218914. M2: UCMP V73080, UCMP nos. 218903, 218908; UCMP V74122, UCMP nos. 218920, 218921; UCMP V75193, UCMP nos. 218917, 218918; UCMP V99438, UCMP nos. 218906, 218907, 218915, 218916. M3: UCMP V99438, UCMP 281368.

Basis for association —Samples of P4s and upper molars of Betonnia sp. cf. Be. tsosia from the GCC consist of 21 isolated teeth. Identification of their positions in the dental arcade is based on the assumption that Be. sp. cf. Be. tsosia is a cimolestid. Patterns of morphological differentiation distinguishing the P4s and upper molars of Pu. simpsoni and latest Cretaceous cimolestids (see Lillegraven 1969, Fox 2015, 2016) provided the basis for determination of their positions in the dental arcade. Dimensions of the P4s and upper molars of Be. sp. cf. Be.

Mesial Distal Length Width Width 3.10–3.55 P4 OR 2.89 (1) (3)

Mean 3.31

2.27–2.74 3.28–3.88 3.61–4.13 M1 OR (5) (6) (6)

Mean 2.59 3.59 3.86

2.23–2.76 3.15–4.02 3.67–4.29 M2 OR (8) (8) (10)

Mean 2.53 3.67 3.98

M3 OR 2.54 (1) 3.81 (1) 3.32 (1)

tsosia are given in Table 2.

Description

P4 —The parastylar lobe is salient and situated directly mesial to the paracone ( Fig. 5). On the available P4s, this lobe is damaged, but what remains suggests a parastyle was present. Closely approximated on the distal slope of the paracone is a smaller but distinct metacone. A narrow ectocingulum is present above the metacone. Lingually, the paraconule and metaconule are distinct cusps. The postparaconule and premetaconule cristae delimit the shallow trigon basin. A narrow preparaconule crista extends to the parastylar lobe. Similarly, the postmetaconule crista extends to the distobuccal corner of the crown. The protocone is not as high as the paracone. Pre- and postcingula are narrow, of approximately the same buccolingual extent, and almost meet across the lingual side of the protocone. Like the molars, the crown is supported by three roots.

M1 — The large parastylar lobe is directed mesially from the paracone and carries a low but distinct parastyle, which is rarely connected to the paracone by a weak preparacrista ( Fig. 6). Above the paracone, a low ridge extends along the buccal side of the narrow ectocingulum. A shallow ectoflexus separates the mesial section of the ectocingulum from the broader ectocingulum on the metastylar lobe. The paracone is higher and larger than the metacone. Their bases are merged to approximately half the height of the paracone. The paraconule is larger than the metaconule and set farther lingually. Low postparaconule and premetaconule cristae extend toward the bases of the paracone and metacone respectively. The preparaconule crista extends buccally to the parastylar lobe. The postmetaconule crista is broad, flaring distally, and ends dorsal to the apex of the metacone. The talon basin is narrow. The mesiodistal length of the crown measured across the conules is approximately half the total length of the crown. Lingually the base of the protocone is expanded. Both width and length of the pre- and postcingula are variable. On two of the six M1s in the sample, the cingula are continuous across the lingual side of the protocone. On the others, the precingulum does not extend as far lingually as the postcingulum.

M2 —Differing from M1, buccal to the paracone the ectocingulum is slightly more convex while the distal part of the ectocingulum is larger and forms a distinct metastylar lobe projecting buccally ( Fig. 7). This results in a more pronounced ectoflexus. The conules are distinct with the paraconule somewhat larger and closer to the protocone than the metaconule. On one tooth, UCMP 218915, two small conules are present in the position of the paraconule. On one of the ten M2s in the sample, the pre- and postcingula are continuous across the lingual side of the protocone. On the others the precingulum does not extend as far lingually as the postcingulum.

M3 —The parastylar lobe is large and extends mesiobuccally ( Fig. 8). A narrow ectocingulum extends across the paracone and ends dorsal to the apex of the metacone. The paracone appears to have been slightly larger than the metacone, but their apices are worn on the one available M3 (UCMP 281368). Forming a trough from the centrocrista to the buccal side of the protocone, the talon basin is narrow mesiodistally, but wide buccolabially. The paraconule is closer to the protocone than the metaconule. A shelf-like preparaconule crista extends to the parastyle. By contrast, the postmetaconule crista is not as broad and shorter ending above the metacone. The precingulum extends from dorsal to the paraconule to the mesial side of the protocone. The postcingulum is shorter but extends farther lingually than the precingulum.

Remarks

To date, only three eutherians with little modified tribosphenic upper dentitions of similar size have been found in Pu3 faunal assemblages of the GCC.They are the subzalambdont (sensu Fox 2015) Betonnia sp. cf. Be. tsosia and Puercolestes sp. cf. Pu. simpsoni , and Prodiacodon crustulum Novacek, 1977 . P4s and upper molars of Be. sp. cf. Be. tsosia , and Pr. crustulum can be distinguished from those of Pu. sp. cf. Pu. simpsoni by the presence of pre- and postcingula. On P4s and upper molars of Be. sp. cf. Be. tsosia , the pre- and postcingula are buccolingually elongate and relatively narrow. On three of the 21 available P4s and upper molars they meet on the lingual side of the protocone. By contrast, on P4s and upper molars of Pr. crustulum , which is interpreted to be a leptictid ( Clemens 2015), the pre- and postcingula tend to increase in width lingually. On some of the postcingula there is a small but distinct hypocone. On only one of the 21 M1 and M2s of Pr. crustulum are the pre- and postcingula continuous across the the lingual side of the protocone.

Size of its M2 and other cheek teeth was one of the characters cited in the diagnosis of Be. tsosia . Because of the small available samples, only limited direct comparisons can be made. The only dimension of an upper postcanine of Be. tsosia reported by Williamson et al. (2011) is the distal width of an M2, 2.50 mm. Further, Williamson et al. (2011) made specific comparisons with Scollardius propalaeoryctes n. comb Fox, 2015 from Alberta and Chacopterygus minutus Williamson et al., 2011 from New Mexico. The distal width of the single available M2 of S. propalaeoryctes reported by Lillegraven (1969) is 3.90 mm. The distal width of the single available upper molar, an M1 (NMMNH P-47234) of Ch. minutus is 3.10 mm.

The available dimensions of M2s of Be. sp. cf. Be. tsosia, Pu. sp. cf. Pu. simpsoni , and Pr. crustulum from Montana as well as Pu. simpsoni and Be. tsosia from New Mexico are compared in Table 3. If the one available M2 of Be. tsosia is not a small outlier, the upper molars of this species appear to have been distinctly smaller than those of the other species. Stressing that the samples are woefully small, the available data suggests that the dentition of Be. sp. cf. Be. tsosia was larger than that of Be. tsosia . Additional material might indicate that this difference in size warrants recognition of a new species of Betonnia in Montana.