Apterodon macrognathus Andrews, 1904

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 : 3-4

Apterodon macrognathus Andrews, 1904

Figures 1-2 , Tables 1-2

Referred specimens. DPC 4126, right maxilla fragment with dP 4 (Fayum Depression, Jebel Qatrani Formation, Quarry M); DPC 8217, left dentary fragment with dP 3, dP 4, M 1 (Fayum Depression, Jebel Qatrani Formation, Quarry M). Note: These specimens were not found in association.

Description

In the maxilla of Apterodon macrognathus (DPC 4126; Figure 1 ) the infraorbital foramen is superior and slightly anterior to the mesial root of dP 4. The parastyle is mesiodistally short and divided by a buccolingually compressed ridge that continues the preparacrista onto the parastyle. The tall paracone is crenulated and buccolingually compressed and rises to an acute, slightly distallydirected peak. The metacone is individuated from the paracone and rises over half the height of the paracone after the cusps diverge. The mesiodistal length of the paracone base is subequal to the mesiodistal length of the metacone. The metacone forms a distinct carnassial notch with the buccolingually compressed and mesiodistally short metastyle. The metastyle rises to a distinct apex that is approximately one-third the height of the metacone. The buccal margin of the tooth is traced by a distinct, beaded buccal cingulum that connects the parastyle to the metastyle, dipping ventrally at the distobuccal face of the paracone. The protocone is mesiodistally broad. The lingual point of the protocone is a distinct cusp that projects to the same height as the apex of the metastyle. The trigon basin is deep and defined by a wide lingual cingulum. The alveoli of dP 3 are also preserved, indicating the root that supported the protocone of dP 3 was closely packed with the distal root of dP 3.

In the dentary of Apterodon macrognathus (DPC 8217, Figure 2 ) dP 3 is a buccolingually compressed tooth with a prominent protoconid and small, but distinct talonid. The preprotocristid curves distally to the apex of the protoconid. The postprotocristid slopes more gently to the cristid obliqua. The entoconid is a prominent, sectorial cusp along the buccal margin of the talonid. Distinct buccal and lingual cingula rim the entire double-rooted tooth. A mesiodistally elongate protoconid dominates the outline of dP 4. The paraconid is low and mesiodistally short. The carnassial notch dives to the same level as the cristid obliqua. As on dP 3, the protoconid points slightly distally. The pre- and postprotocristid are buccolingually compressed and blade-like. The postprotocristid forms a deep notch at the cristid obliqua. The entoconid rises from the notch to a point taller than the paraconid and nearly half the height of the protoconid. There is no indication of a metaconid on dP 4. The µ-CT of the dentary of Apterodon macrognathus ( Figure 2 ) indicates the protoconid of M 2 was formed before the germs of either P 4 or P 3 began to coalesce.

Remarks

Apterodontines combine a key morphological feature of hypercarnivores, the reduction or absence of metaconids on the lower molars, with features of more generalist carnivores such as well-developed talonids on the lower molars, divergent metacones and paracones, and mesiodistally abbreviated metastyles on the upper molars ( Van Valkenburgh, 2007) complicating dietary and phylogenetic inferences (Van Valen 1966; Szalay, 1967; Grohé et al., 2012). Recent phylogenetic studies (Borths et al., 2016; Borths and Seiffert, 2017) incorporate morphological features of the cranial vault ( Solé et al., 2015), suggesting Apterodontinae as the sister clade of Hyainailourinae . Like the hyainailourines Paroxyaena and Leakitherium , the dP 4 of Apterodon macrognathus ( Figure 1 ) exhibits a buccolingually compressed paracone that is distally directed. A. macrognathus also shares with Leakitherium and Paroxyaena a mesiodistally short metastyle on dP 4, crenulated enamel, and a lobe-like protocone. In contrast, the dP 4 of the teratodontines Metasinopa and Masrasector exhibit extensive metastyles, smooth and thin enamel, and buccolingually narrow protocones, morphology shared with Pakakali , a hyaenodont from the late Oligocene of the Rukwa Rift Basin (Borths and Stevens, 2017). These observations, in conjunction with the phylogenetic analysis conducted here to test the utility of deciduous dental characters, support the close relationship between Apterodontines and Hyainailourines in the clade Hyainailouridae .

Figures

FIGURE 1. Apterodon macrognathus left maxilla fragment with dP4 and alveoli of dP3 (DPC 4126) from Quarry M (early Oligocene) in the Fayum Depression, Egypt in 1) occlusal view; 2) buccal view; and 3) lingual view. Scale bar is 10 mm.

FIGURE 2. Apterodon macrognathus left dentary with dP3, dP4, M1 and M2 in crypt (DPC 8217) from Quarry M (early Oligocene) in the Fayum Depression, Egypt. Original specimen figured with digital model based on µ-CT scans reconstructed in Avizo. Mandibular corpus 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.