Masrasector nananubis Borths and Seiffert, 2017

Borths, Matthew R. & Stevens, Nancy J., 2017, Deciduous dentition and dental eruption of Hyainailouroidea (Hyaenodonta, “ Creodonta, ” Placentalia, Mammalia), Palaeontologia Electronica 20 (3), pp. 1-34 : 14-18

Masrasector nananubis Borths and Seiffert, 2017

Figures 7-9 , Tables 1-2

Referred specimens. DPC 20882, right maxilla fragment with dP 3 and dP 4 (Fayum Depression, Jebel Qatrani Formation, Quarry L-41); DPC 13837, right maxilla fragment with dP 3, dP 4, M 1 (Fayum Depression, Jebel Qatrani Formation, Quarry L-41); DPC 18276, right dentary with dP 2, dP 3, dP 4 (Fayum Depression, Jebel Qatrani Formation, Quarry L-41). Note: These specimens were all found in isolation in Quarry L-41 and are not likely associated.

Description

The upper deciduous dentition of Masrasector nananubis is best represented by the two maxillary fragments we identify here: DPC 20882 and DPC 13837 ( Figure 7 ). Both are right maxillary fragments preserving portions of dP 3 and dP 4. DPC 13837 further preserves M 1, illustrated here to highlight some of the differences between dP 4 and M 1.

The infraorbital foramen is preserved in both specimens superior to the mesial root of dP 3. The parastyle of dP 3 is elongate and buccolingually compressed into a narrow crista that connects with the preparacrista of the paracone. The dP 3 is best preserved on DPC 20882 where the parastyle is mesiodistally subequal in length to the metastyle and is subequal in height to the apex of the metastyle. A thin buccal cingulum connects the parastyle to the metastyle. The paracone is the tallest cusp on dP 3. The apex of the paracone forms an equilateral triangle in buccal view. The metacone is half the height of the paracone and it is closely appressed to the paracone, though it is a distinct cusp with a shallow furrow defining the margins of the metacone where it fuses with the paracone. In occlusal view, the metacone is positioned slightly lingual to the apex of the paracone. The metacone forms a deep carnassial notch at the juncture with the sectorial metastyle. A thin shelf is formed between the metastyle, the metacone, and the buccal cingulum. The protocone is mesiodistally narrow and isolated from the parastyle and metastyle.

The crown of dP 4 is best preserved on DPC 20882 ( Figure 7 ). The parastyle is a distinct cusp that is mesiodistally shorter than the metastyle. As on dP 3 and M 1 (DPC 13837) the parastyle and metastyle of dP 4 are connected by a distinct buccal cingulum. The paracone and metacone are fused at their bases and diverge near the midpoint of their height. The paracone and metacone are subequal in height, are slightly buccolingually compressed and have similar basal diameters. The paracone is proximate to the buccal cingulum with the metacone positioned more lingually. The postmetacrista meets the metastyle at a deep carnassial notch. A wide basin is formed between the sectorial metastyle, the buccal cingulum, and the bases of the paracone and metacone. The buccal cingulum traces a shallow ectoflexus. The protocone is mesially oriented and lingually aligned with the mesial margin of the parastyle. A thin mesial cingulum connects the parastyle to the protocone, but the protocone is not connected to the metastyle. The protocone is a mesiodistally broad cusp that projects beyond the divergence of the paracone and metacone.

Maxillary specimens of Masrasector ( Figure 7 ) were µ-CT scanned, revealing that the developing permanent dentition has not reached the bell stage of differentiation in DPC 20882, a specimen that only preserves dP 3 and dP 4. DPC 13837 ( Figure 7 ) preserves a fully erupted M1 with open roots, and developing portions of P 3 and P 4 that both advanced into the bell phase of tooth differentiation. The paracone of P 3 has coalesced in the crypt, framed by the roots of dP 3. The paracone of P 4 had only just started to coalesce in the crypt, evidence that in Masrasector , P 3 likely erupted before P 4.

The lower deciduous dentition of Masrasector nananubis was described by Borths and Seiffert (2017), though that previous study did not detail the development of the permanent dentition revealed by µ-CT. The dentary specimen DPC 18276 ( Figure 8 ) preserves the crowns of the deciduous canine, dP 2, dP 3, dP 4, and the alveoli of M 1. The developing buds of P 2, P 3, and M 2 are preserved in their crypts. Of these coalescing permanent teeth, M 2 is most advanced, with the enamel bells of the trigonid and talonid fully formed; it likely would have erupted before the other developing dentition. The P 2 is framed by the roots of dP 2. The crown of P2 is oriented oblique to the body of the dentary. A tiny portion of the differentiated enamel of the P 3 paraconid is visible between the roots of dP 3. The dentary specimen DPC 10358 ( Figure 9 ) is from a developmentally older individual than DPC 18276 ( Figure 8 ). DPC 10358 preserves a fully erupted M 2, though the roots are not fully differentiated, the crown of P 2 is nearly completely developed and it is oriented subparallel to the mandibular corpus, and P 3 is advanced into the enamel bell phase. Distal to M 2, a dense mass is visible in the µ-CT scans, interpreted as the early differentiation of M 3. Neither Masrasector dentary specimen preserves any portion of the differentiated tooth bud of P 4. Taken together, these specimens suggest the following order of permanent dental eruption in Masrasector nananubis : M 1, M 2, P 2, P 3, M 3, and P 4.

Remarks

Masrasector nananubis is much smaller than the other hyainailouroids discussed in this study. Whereas the metacone is immediately distal to the dP 3 paracone in Anasinopa , Dissopsalis , and Leakitherium , the dP 3 metacone in Masrasector nananubis is slightly lingually offset, a feature shared with Metasinopa . The metacone is lingually offset from the paracone even more dramatically on the dP 4 of M. nananubis , a feature also shared with the teratodontines Brychotherium , Anasinopa , and Metasinopa , but not Dissopsalis . The lingually projecting protocone of the dP 4 in M. nananubis frames a broad trigon basin, a feature also shared with Brychotherium , Anasinopa , and Metasinopa , but not with Dissopsalis (which exhibits a dP 4 protocone that projects mesially and is not framed by the bases of both the metacone and paracone). This trigon basin diversity within Teratodontinae may reflect dietary differences within the clade, with Masrasector acting as a mesocarnivore and Dissopsalis acting as a hypercarnivore, though Dissopsalis retains some mesocarnivorous dental traits that help distinguish the taxon from the hyainailourids.

Figures

FIGURE 7. Masrasector nananubis left maxillae from Locality 41 (latest Eocene), Fayum Depression, Egypt. Original specimens figured with digital models based on µ-CT scans reconstructed in Avizo. Maxilla rendered transparent to reveal developing dentition. Grey, deciduous dentition. Yellow, permanent dentition. DPC 20882 is a left maxilla fragment that preserves dP3 and dP4 and no indication of developing permanent dentition. 1) specimen occlusal view; 2) model occlusal view; 3) specimen buccal view; 4) model buccal view; 5) specimen lingual view; 6) model lingual view. DPC 13837 is a left maxilla fragment that preserves erupted dP3, dP4, and M1 and P3 and P4 developing in crypts. 7) specimen occlusal view; 8) model occlusal view; 9) specimen buccal view; 10) model buccal view; 11) specimen lingual view; 12) model lingual view. Scale bar is 10 mm.

FIGURE 8. Masrasector nananubis right dentary with erupted dC, dP2, dP3, and dP4, M1 alveoli and P2, P3, and M2 in crypts (DPC 18276) from Locality 41 (latest Eocene), Fayum Depression, Egypt. Original specimen figured with digital model based on µ-CT scans reconstructed in Avizo. Dentary rendered transparent to reveal developing dentition. Grey, deciduous dentition. Yellow, permanent dentition. 1) specimen occlusal view; 2) model occlusal view; 3) specimen lingual view; 4) model lingual view; 5) specimen buccal view; 6) model buccal view. Scale bar is 10 mm.

FIGURE 9. Masrasector nananubis left dentary with erupted dP3, dP4, M1 and M2 and P2, P3, and M3 in crypts (DPC 10358) from Locality 41 (latest Eocene), Fayum Depression, Egypt. Original specimen figured with digital model based on µ-CT scans reconstructed in Avizo. Dentary rendered transparent to reveal developing dentition. Grey, deciduous dentition. Yellow, permanent dentition. 1) specimen occlusal view; 2) model occlusal view; 3) specimen lingual view; 4) model lingual view; 5) specimen buccal view; 6) model buccal view. Scale bar is 10 mm.