Craniodental and humeral morphology of a new species of Masrasector (Teratodontinae, Hyaenodonta, Placentalia) from the late Eocene of Egypt and locomotor diversity in hyaenodonts

Abstract

Hyaenodonta is a diverse clade of carnivorous mammals that were part of terrestrial faunas in the Paleogene of Eurasia and North America, but the oldest record for the group is Afro-Arabian, making the record there vital for understanding the evolution of this wide-spread group. Previous studies show an ancient split between two major clades of hyaenodonts that converged in hypercarnivory: Hyainailourinae and Hyaenodontinae. These clades are each supported by cranial characters. Phylogenetic analyses of hyaenodonts also support the monophyly of Teratodontinae, an Afro-Arabian clade of mesocarnivorous to hypercarnivorous hyaenodonts. Unfortunately, the cranial anatomy of teratodontines is poorly known, and aligning the clade with other lineages has been difficult. Here, a new species of the phylogenetically controversial teratodontine Masrasector is described from Locality 41 (latest Priabonian, late Eocene) from the Fayum Depression, Egypt. The hypodigm includes the most complete remains of a Paleogene teratodontine, including largely complete crania, multiple dentaries, and isolated humeri. Standard and "tip-dating" Bayesian analyses of a character-taxon matrix that samples cranial, postcranial, and dental characters support a monophyletic Masrasector within Teratodontinae, which is consistently placed as a close sister group of Hyainailouridae. The cranial morphology of Masrasector provides new support for an expanded Hyainailouroidea (Teratodontinae + Hyainailouridae), particularly characters of the nuchal crest, palate, and basicranium. A discriminant function analysis was performed using measurements of the distal humerus from a diverse sample of extant carnivorans to infer the locomotor habits of Masrasector. Masrasector was assigned to the "terrestrial" locomotor category, a result consistent with the well-defined medial trochlear ridges, and moderately developed supinator crests of the specimens. Masrasector appears to have been a fast-moving terrestrial form with a diverse diet. These specimens considerably improve our understanding of Teratodontinae, an ancient member of the Afro-Arabian mammalian fauna, and our understanding of hyaenodont diversity before the dispersal of Carnivora to the continent near the end of the Paleogene.

Fig 1. Dental nomenclature used in this study.

Upper left M² and lower left M₃ of Proviverra typica (A–E) showing dental terminology and measurements used in this study. (A) Proviverra typica M² in occlusal and (B), buccal views and M₃ in (C), occlusal (D), lingual, and (E), buccal views. Measurements are indicated in italics. Modified from [9].

Fig 2. Humerus measurements for DFA.

Humerus measurements used in the Discriminant Function Analysis. Collected from digital photographs in (A) anterior view, (B) distal view, and (C) posterior view. Abbreviations for each measurement are listed with the anatomical view they were collected from and a description of the measurement. Measurements illustrated on DPC 10831.

Fig 3. Cranium of Masrasector nananubis (DPC 12157).

Cranium of Masrasector nananubis nov. sp. (DPC 12157 with alveoli for P^{1, 2}, complete P³–M³) in (A) dorsal view, (B) ventral view, (C) left lateral view, (D) right lateral view. Rostrum points left in (A), (B), and (C) and right in (D).

Fig 4. Sketch and model of Masrasector nananubis cranium (DPC 12157).

Sketch of cranium Masrasector nananubis sp. nov. (DPC 12157 with alveoli for P^{1, 2}, complete P³–M³) in (A) dorsal view, rostrum points left, (B) ventral view, rostrum points left, (C) left lateral view, rostrum points left, (D) right lateral view, rostrum points right. Dotted lines indicate uncertain sutures or root of sagittal crest. Digital model of cranium in (E) dorsal, (F) ventral, (G) left lateral, and (H) right lateral views. The digital model was generated in Avizo and is available on Morphosource. Abbreviations: bsp, basisphenoid; eam, external auditory meatus; eoc, external occipital crista; exo, exoccipital; exprm, exposed promontorium; fmg, foramen magnum; fov, foramen ovale; hgf, hypoglossal foramen; juf, jugular foramen?; occ, occipital condyle; pcp, paracondylar process; pgf, post glenoid foramen; pgp, postglenoid process; prm, promontorium; smf, stylomastoid foramen; squ, squamosal; th, tympanohyal.

Fig 5. Cranium of Masrasector nananubis (DPC 11990).

Cranium of Masrasector nananubis sp. nov. (DPC 11990 with P²–M³) in (A) dorsal view, rostrum points left, (B) ventral view, rostrum points left.

Fig 6. Sketch and digital model of Masrasector nananubis cranium (DPC 11990).

Sketch of cranium of Masrasector nananubis sp. nov. (DPC 11990 with P²–M³) in (A) dorsal view, rostrum points left, (B) ventral view, rostrum points left. Digital model in (C) dorsal, and (D) ventral view is an isosurface rendering of the specimen generated using Avizo and is available on Morphosource.

Fig 7. Right maxilla (DPC 13285) and rostrum (DPC 8276) of Masrasector nananubis.

Right maxilla of Masrasector nananubis sp. nov. (DPC 13285 with P²–M³) in (A) occlusal view, buccal side to top and mesial to right, (B) lateral/buccal view, mesial to right, (C) lingual/medial view, mesial to left, (D) rostrum of Masrasector nananubis sp. nov. (DPC 8276), rostrum points right, left dentition in occlusal view and right dentition in buccal view.

Fig 8. Right dentary of Masrasector nananubis sp. nov., (CGM 83736 holotype).

DPC 11383 with C, alveoli for P₁, P₂–M₃ in (A) occlusal view, buccal to top, mesial to left, (B) lingual view, mesial to left, (C) buccal view, mesial to right. Digital model in (D) occlusal, (E) lingual, and (F) buccal view is an isosurface rendering of the specimen generated using Avizo and is available on Morphosource.

Fig 9. Right dentary of Masrasector nananubis sp. nov. (DPC 9274).

DPC 9274 with P₁, alveoli for P₂, P₃–P₄, alveoli for M₁, M₂–M₃ in (A) occlusal view, buccal to top, mesial to left, (B) lingual view, mesial to left, (C) buccal view, mesial to right. Digital model in (D) occlusal, (E) lingual, and (F) buccal view is an isosurface rendering of the specimen generated using Avizo and is available on Morphosource.

Fig 10. Right dentaries of Masrasector nananubis sp. nov. (DPC 11359 and DPC 15742).

DPC 11359 with P₂–P₄, M₃ in (A) occlusal view, buccal to top, mesial to left, (B) lingual view, mesial to left. Portions of M₁–M₂ are preserved but fragmentary, (C) buccal view, mesial to right. DPC 15742 with P₂–M₃ in (D) occlusal view, buccal to top, mesial to left, (E) lingual view, mesial to left, (F), buccal view, mesial to right.

Fig 11. Right dentaries of Masrasector nananubis sp. nov. (DPC 12330 and DPC 12524A).

DPC 12330 with P₃–M₁, M₂ in (A) occlusal view, buccal to top, mesial to left, (B) lingual view, mesial to left, (C) buccal view, mesial to right. DPC 12524A with alveoli for P_{2, 3}, P₄–M₃ in (D) occlusal view, buccal to top, mesial to left, (E) lingual view, mesial to left, (F) buccal view, mesial to right.

Fig 12. Left dentaries of Masrasector nananubis sp. nov. (DPC 10358 and DPC 15211).

DPC 10358 with dP_{3, 4}, M_{1, 2} in (A) occlusal view, buccal to top, mesial to right, (B) lingual view, mesial to right, (C) buccal view, mesial to left. Digital model of DPC 15211 generated in Avizo with C, P₁–M₃ in (D) occlusal view, buccal to top, mesial to right, (E) lingual view, mesial to right, (F) buccal view, mesial to left. Model is available on Morphosource.

Fig 13. Left humerus specimens referred to Masrasector.

DPC 10831 in (A) anterior view, (B), posterior view, (C), distal view, medial to left. DPC 15436 in (D) anterior view, (E) posterior view, (F) distal view, medial to left. DPC 11670 in (G) anterior view, (H) posterior view, (I) distal view, medial to left. Digital models were generated in Avizo and are available on Morphosource.

Fig 14. Comparative sample of distal humerus specimens of Hyaenodonta and Carnivora.

Comparative sample of distal humerus specimens of Hyaenodonta (A–E) and Carnivora (F–J) in anterior view (all left specimens except E, which has been flipped). (A) Arfia shoshoniensis, AMNH 15515, (B) Hyaenodon horridus, AMNH 1381, (C) Apterodon langebadreae, BNHM M85318, (D) “Pterodon” africanus, BNHM M8886, (E) Megistotherium osteothlastes, BNHM UB20577, (F) Potos flavus (kinkajou), Arboreal, AMNH 46513, (G) Lontra canadensis, northern river otter, Semiaquatic, MCZ 62635, (H) Galerella sanguineus, slender mongoose, Scansorial, AMNH 187750, (I) Urocyon cinereoargenteus, gray fox, Terrestrial, MCZ 64169, (J) Ursus americanus, black bear, Terrestrial, MCZ 59938. Specimens not to scale. Black horizontal lines = 10 mm.

Fig 15. Standard Bayesian consensus tree.

“Allcompat” consensus (majority rule plus compatible groups) tree. B# corresponds to the node to the right of the label and are used in the results and discussion sections. Posterior probabilities (PP) are placed in the center of the black circle over each node.

Fig 16. Tip-dating consensus tree.

“Allcompat” (majority rule plus compatible groups) consensus derived from the post-burnin sample of trees recovered by tip-dating Bayesian analysis. T# corresponds to S3 Table and the results and discussion section. Posterior probability (PP) shown in italics to the right or below relevant node. Divergence dates represent median divergence date for clades and taxa. The vertical line over each geological age range (see S2 Table for sources of age information) is the median estimated age for the taxon based on the tip-dating analysis. Branch colors correspond to legend for % change/Ma along branches. More rapidly changing branches include more red and more slowly changing branches include more blue. Most rapidly evolving lineages are near the base of Hyaenodonta. Continental area of origin for each taxon is shown by the shading the proposed age range: black, Afro-Arabia; grey, Europe; checked, Asia or India; white, North America. Aq., Aquitanian; Bart., Bartonian; Burd., Burdigalian; Dan., Danian; La., Langhian; Late K, Late Cretaceous; Maa., Maastrichtian; Me., Messinian; Pria., Priabonian; Sel., Selanian; Ser., Serravallian; Tha., Thanetian; Torto., Tortonian.

Fig 17. Parsimony consensus trees.

Consensus trees of 1262 most parsimonious trees (MPTs), each with 1066 steps, a consistency index score (CI) of 0.181, and a retention index score (RI) of 0.617. P# next to each node is used in the discussion of the consensus trees. A, strict consensus tree with Bootstrap support great than 50% indicated next to the relevant node; B, Adams consensus tree with an agreement subtree for all MPTs indicated in red. The agreement subtree shows relationships recovered in all MPTs. Bremer support values are shown next to relevant nodes. Major clades discussed in this study are enclosed with the clade name by rounded boxes.

Fig 18. Locomotor categorization of hyaenodont humeri on DF1 and DF2.

The results of discriminant function analysis for DF1 (explains 52.7% of variance) plotted against DF2 (explains 23.0% of variance). The analysis is based on 20 linear measurements (see Fig 2) collected for 155 carnivoran specimens (55 species) and 12 hyaenodont specimens (8 species). Numbers correspond to specimen numbers in Table 5, S4 and S5 Tables. Refer to Table 6 and S5 Table for classification of the hyaenodontan sample. Refer to Table 7 for measurements loadings. Each point represents a specimen, not a species mean. Green plus, Arboreal carnivoran specimens; Yellow X, Scansorial carnivoran specimens; Black diamond, Terrestrial carnivoran specimens; Red squares, Fossorial carnivoran specimens; Blue triangles, Semiaquatic carnivoran specimens; Red dots, hyaenodontans with unassigned locomotor behaviors.

Fig 19. Locomotor categorization of hyaenodont humeri on DF1 and DF3.

The results of discriminant function analysis for DF1 (explains 52.7% of variance) plotted against DF3 (explains 18.0% of variance). The analysis is based on 20 linear measurements (see Fig 2) collected for 155 carnivoran specimens (55 species) and 12 hyaenodont specimens (8 species). Numbers correspond to specimen numbers in Table 5, S4 and S5 Tables. Refer to Table 6 for classification of the hyaenodontan sample. Refer to Table 7 for measurements loadings. Each point represents a specimen, not a species mean. Green plus, Arboreal carnivoran specimens; Yellow X, Scansorial carnivoran specimens; Black diamond, Terrestrial carnivoran specimens; Red squares, Fossorial carnivoran specimens; Blue triangles, Semiaquatic carnivoran specimens; Red dots, hyaenodontans with unassigned locomotor behaviors.