Dissopsalis pyroclasticus Barry, 1988

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 : 12-13

Dissopsalis pyroclasticus Barry, 1988

Figure 6 , Table 1

Referred specimens. KNM-FT 3357, left maxilla fragment with dP 3 and dP 4 (Fort Ternan, Kenya).

Description

The maxillary specimen, KNM-FT 3357 ( Figure 6 ), is referred to Dissopsalis based on comparisons with other material referred to Dissopsalis from Fort Ternan, including KNM-FT 3562, a close size match to KNM-FT 3357. It is possible this represents the upper deciduous dentition of Anasinopa , but based on morphological comparisons detailed below we identify the Fort Ternan specimen as a maxillary fragment of Dissopsalis , and refer the slightly smaller KNM-RU 2385 ( Figure 4 ) to Anasinopa .

Only fragmentary portions of the lateral margin of the maxilla are preserved on KNM-FT 3357, including the inferior margin of the infraorbital foramen superior to the mesial root of dP 3. A single alveolus of P 1 is preserved at the rostral-most portion of the specimen. Based on the fragmentary preservation of this alveolus, it is difficult to determine whether this tooth had a single root in the single alveolus, or if this is the distal of two roots. Both alveoli of dP 2 are preserved. The distal alveolus of that tooth position exhibits a larger circumference than does the mesial root, crowding the mesial margin of dP 3. The morphology of this distal alveolus indicates dP 2 and dP 3 would have been closely packed with the metastyle of dP 2 braced buccally by the parastyle of dP 3.

The entire crown of dP 3 is preserved. The mesiodistal length of the parastyle and metastyle are subequal. The parastyle and metastyle are mesiodistally aligned and are intersected at a perpendicular angle by the protocone. The parastyle, metastyle and protocone are distinct cusps, but each is relatively low compared to the paracone, which is the tallest cusp on the tooth. The mesiodistal length of the paracone is subequal to the height of the paracone, making the cusp appear long and low in buccal view. The preparacrista is twice the length of the postparacrista, which intersects the metacone. The metacone is distinct, only half the height of the paracone, and exhibits greater convexity along the buccal surface than the lingual surface. The metacone forms a shallow carnassial notch with the short, low metastyle. The parastyle and metastyle are connected to the large protocone, defining the mesial and distal margins of the wide trigon basin. The protocone on dP 3 is distally oriented, defining a relatively narrow space between the lingual aspects of dP 3 and dP 4.

In Dissopsalis (KNM-FT 3357, Figure 6 ) dP 4 is mesiodistally only slightly longer than dP 3, though the relative proportions of the cusps are very different with the distinct parastyle mesiodistally shorter than the metastyle and the protocone shifted mesially so that it is lingual to the paracone. The small, cusp-like parastyle connects the distinct buccal cingulum of dP 4 to the paraconule. The paracone and metacone are fused for about half of their height, then the apices diverge, with both oriented perpendicular to the palate. The paracone and metacone are subequal in mesiodistal length, and the metacone is slightly taller than the paracone. Both the metacone and paracone are buccolingually compressed. The distal margin of the metacone is the most compressed and sectorial portion of these cusps. The metacone forms a distinct, though shallow, carnassial notch where it intersects the metastyle. The metastyle is lower than the metacone and comparable in height to the parastyle. The protocone is a mesiodistally broad cusp that is subequal in height to the parastyle. The protocone frames the lingual margin of a deep trigon basin that is buccally defined by the paracone, mesially defined by a distinct paraconule, and distally defined by a distinct metaconule.

Remarks

Dissopsalis is one of the last teratodontines documented in the fossil record ( Solé et al., 2015) and is also one of the most hypercarnivorous, as reflected by the elongate metastyle and buccolingually compressed metacone on dP 4 in KNM-FT 3357 ( Figure 6 ). The dP 3 of Dissopsalis shares with Masrasector , Metasinopa , Anasinopa , and Pakakali a low metacone and a mesiodistally elongate metastyle. The dP 3 of Dissopsalis and Anasinopa are distinct from Masrasector and from the other hyainailouroid dP 3 specimens considered in this study by having a mesiodistally elongate paracone with a low buccal profile and a mesiodistally elongate metacone. The carnassial notch of dP 3 in Dissopsalis also differs from all other hyainailouroids by being relatively shallow rather than defined by a deep notch that almost divides the metastyle and metacone down to the buccal cingulum.

In teratodontines the M 1 and M 2 paracones are subequal to or lower than the metacones, leaving the metacone as the lead piercing cusp on the molars. In Dissopsalis the dP 4 anticipates the morphology of the adult molars in exhibiting a slightly taller metacone than paracone, morphology that is shared with the dP 4 of Masrasector and Anasinopa . Unlike Anasinopa , the bases of the paracone and metacone on dP 4 in Dissopsalis are partially fused, also anticipating the more fused paracone and metacone found on the molars of Dissopsalis . The dP 4 of Dissopsalis differs from the dP 4 of the teratodontines Brychotherium , Masrasector , and Metasinopa , in having a constricted space between the metastyle, metacone, and buccal cingulum. Overall, the more exaggerated hypercarnivorous characters evident on the deciduous dentition of Dissopsalis reveal dietary adaptations that are important for interpreting the biology of one of the latest surviving hyaenodonts in Afro-Arabia.

Figures

FIGURE 4. Anasinopa leakeyi left maxilla cast with dP2, dP3 and dP4 (KNM-RU 2385) from Rusinga Island (early Miocene), Kenya in 1) occlusal view; 2) buccal view; and 3) lingual view. Scale bar is 10 mm.

FIGURE 6. Dissopsalis pyroclasticus left maxilla with dP3 and dP4 (KNM-FT 3357) from Fort Ternan (early Miocene), Kenya in 1) occlusal view; 2) buccal view; and 3) lingual view. Scale bar is 10 mm.