Metasinopa sp.

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 : 18-21

Metasinopa sp.

Figures 10-11 , Tables 1-2

Referred specimens. DPC 10199, associated cranial fragments with dP 3, dP 4, M 1 (Fayum Depression, Jebel Qatrani Formation, Quarry V); DPC 4544, P 2, dP 4, M 1 –M 3 (Fayum Depression, Jebel Qatrani Formation, Quarry V). Note: The specimens were found in the same quarry, but are not likely associated with the same individual.

Description

The enamel on both dP 3 and dP 4 is smooth and thin (DPC 10199; Figure 10 ). The parastyle of dP 3 is elongate and buccolingually compressed. The paracone of dP 3 is the largest cusp on the tooth, with a paracone basal diameter twice the size of the basal diameter of the buccolingually compressed metacone. The apex of the metacone is lightly worn and did not much exceed the maximum height of the metastyle. A very deep carnassial notch separates the metastyle from the metacone. The mesiodistal width of the protocone is subequal in diameter to the paracone. The protocone is isolated from the metastyle, but connected to the parastyle by a thin cingulum.

The dP 4 exhibits a reduced parastyle compared to the dP 3 and an elongate metastyle. The mesiodistal length of the dP 4 metastyle is subequal to the mesiodistal diameter of the paracone and metacone base. Along the buccal margin of the metastyle is a shallow ectoflexus traced by a slight buccal cingulum. The apices of the paracone and metacone are not preserved, but the bases of these cusps indicate the paracone was conical and the metacone was more buccolingually compressed with a blade-like postmetacrista. The protocone of dP 4 projects mesially and is nearly aligned with the mesial edge of the parastyle. The trigon basin is mesiodistally broad and framed by a distinct paraconule, metaconule and protocone.

In contrast to dP 4, the mesiodistal length of the metacone of M 1 is significantly longer than the base of the paracone, and the paracone is reduced to a mesially directed cusp that is lower than the metacone. The ectoflexus of M 1 is deeper than the ectoflexus of dP 4, and the buccal cingulum is thin to absent.

A dentary referable to Metasinopa (DPC 4544; Figure 11 ) was also discovered at Quarry V. The buccal portions of the dentary were prepared away, exposing the buccal faces P 2 and M 3. On the buccal aspect of the dentary, mental foramina are present inferior to the alveoli of both P 1 and P 2. The alveoli of P 1 indicate that the tooth had two roots and was aligned with the base of canine. The alveoli of dP 3 are also preserved. The dP 4 is buccolingually narrow compared with M 1, and the paraconid of dP 4 is aligned with the protocone. This contrasts with the more lingually oriented paracone of M 1. A low metaconid is retained on dp4, a cusp also present, and at nearly the same absolute size, on each of the molars. The talonid of dP 4 is closed lingually by a complete entocristid that connects to the base of the metaconid. The hypoconid is large and well developed, and the hypoconulid is a distinct distal cusp on the talonid. There is substantial wear on the protoconid and paraconid of dP 4.

The µ-CT scans reveal that all of the permanent dentition were present in DPC 4544 ( Figure 11 ). A permanent incisor with three distinct tines is present mesial to the deciduous canine. The specimen does not preserve the complete incisor row, making it difficult to determine the precise incisor locus represented in this specimen. The incisor root is open, though the crown is complete. The permanent canine is developing in the crypt, lingual to the root of the deciduous canine. The crown of P 2, both paraconid and talonid, and portions of the roots are fully developed and the apex of the tooth is preserved superior to the alveolar margin. The enamel crown of P 3 is almost entirely differentiated in the crypt and the mesial root is partially coalesced, though portions of the talonid have not fully developed. The apex of P 3 protrudes nearly to the alveolar margin. The crown and talonid of P 4 are almost fully developed, though no portions of the roots have differentiated and the roots of dP 4 are complete. Both M 1 and M 2 are erupted and M 3 is in the crypt with the trigonid and talonid fully developed, though the roots are undifferentiated. The crypt is recessed in the dentary and there is not enough space in the tooth row to fully accommodate the crown of M 3, evidence that the body of the dentary had yet to elongate to accommodate the final lower molar. Given the development of each permanent tooth, the inferred eruption sequence for Metasinopa was likely: M 1, M 2, P 2, P 3, M 3, and P 4, then the canine with the mesial incisor erupting around the time of P 2.

Remarks

Metasinopa has been assigned several species that range from the late Eocene through the Miocene of Afro-Arabia. The diagnoses of these species are in need of close examination (Lewis and Morlo, 2010), but we refer specimens from the early Oligocene of the Jebel Qatrani formation to Metasinopa based on comparisons with the lower dentition preserved in Metasinopa fraasi (Osborn, 1909) . Slightly larger than Masrasector , Metasinopa exhibits more hypercarnivorous dental features than does Masrasector , including a reduced dP 4 metaconid, more mesiodistally extensive metastyles on dP 3 and dP 4, and a taller, more buccolingually compressed metacone on M 1. Metasinopa shares with Masrasector a slightly lingually placed metacone on dP 3 and dP 4 that divides the protocone from the metastyle, and a tall dP 4 protocone that projects beyond the point of divergence between the metacone and paracone. Metasinopa shares with Brychotherium a deep ectoflexus on dP 3 and dP 4, not present in Masrasector or in the hyainailourids.

The dP 4 of Metasinopa is relatively more buccolingually compressed than the dP 4 of Masrasector with a narrower talonid relative to the trigonid. The dP 4 metaconid of Metasinopa is relatively smaller than that of Masrasector , with former rising only slightly above the alveolar margin.

Figures

FIGURE 10. Metasinopa sp. left maxilla fragments with dP3, dP4, and M1 (DPC 10199) from Quarry V (early Oligocene), Fayum Depression, Egypt in 1) occlusal view; 2) buccal view; and 3) lingual view. Note that there is matrix around the roots of the specimens that has been cropped to make the dentition easier to view. Scale bar is 10 mm.

FIGURE 11. Metasinopa sp. right dentary with dC, dP4, M1, M2 and I, C, P2, P3, P4 M3 in crypts (DPC 4544) from Quarry V (early Oligocene), 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; and 6) model buccal view. Scale bar is 10 mm.