First Approach to the Paleobiology of Extinct Prospaniomys (Rodentia, Hystricognathi, Octodontoidea) Through Head Muscle Reconstruction and the Study of Craniomandibular Shape Variation

Abstract

†Prospaniomys is a basal octodontoid recorded in the early Miocene in Patagonia (Argentina; Colhuehuapian SALMA). Nearly complete cranial and mandibular remains known for this genus provide a unique opportunity to explore its paleobiology. For this, masticatory muscles were reconstructed and craniomandibular shape variation assessed. While such reconstruction indicates that most masticatory muscles would have presented moderate development, both the masseter lateralis and posterior muscles were poorly developed. In contrast, we found that the temporalis muscle was well developed, while conspicuous postorbital constriction, postorbital processes, and superior temporal lines revealed a substantial orbital portion of this muscle. According to geometric morphometric results, craniomandibular shape was interpreted as generalized. Features such as shortened palate, narrower bizygomatic width, orthodont incisors, enlarged incisive foramina, and a shallow jaw could be linked to epigean habits. The moderate development of auditory bullae in Prospaniomys suggests that it is unlikely that it may have lived in extreme arid environments. Additionally, based on its generalized dental morphology, an omnivorous or generalized herbivorous diet that may have included leaves, fruit, and potentially animal matter was inferred. By the early Miocene, Patagonia experienced the initial expansion stage of arid-adapted vegetation, with grasses present in low amounts and abundant forests. Generalized habits and soft and non-abrasive diet suggest that Prospaniomys was possibly associated with more closed environments. Morphology alone cannot be used as an environmental proxy, but it could undoubtedly contribute to the interpretations based on data provided by paleobotanical and geological frameworks in studies on the evolution of environments.

Appendix 1

Definition of landmarks used in this study (numbers as in Fig. 3). Lateral view of cranium: 1, anterior lower end of premaxillary bone (on sagittal plane); 2, anterior upper end of premaxillary bone (on sagittal plane); 3, anterior end of suture between nasal and premaxillary bones; 4, anterior end of nasal bone; 5, junction of sutures among premaxillary and frontal bones, and dorsal margin of cranium; 6, junction of sutures among premaxillary, maxillary, and frontal bones; 7, anterior end of masseteric fossa of rostrum; 8, antero-ventral border of incisor alveolus; 9, junction between maxillar-premaxillar suture and ventral margin of rostrum; 10, most anterior point of zygomatic arch; 11, junction between lacrimal and frontal bones on antero-dorsal margin of orbit; 12, junction of sutures among maxillary, lacrimal, and frontal bones; 13, junction between jugal and lacrimal bones on anterior margin of orbit; 14, dorsal junction between maxillary and jugal bones; 15, dorsal junction between jugal and squamosal bones; 16, ventral junction between maxillary and jugal bones; 17, posterior tip of zygomatic arch; 18, postero-dorsal end of cranial glenoid fossa; 19, junction of squamosal, frontal, and parietal bones; 20, junction between frontal-squamosal suture and dorsal margin of skull; 21, junction of squamosal, parietal, and occipital bones; 22, junction of squamosal, occipital, and tympanic bones; 23, most dorsal point of external auditory meatus; 24, anterior end of auditory bulla; 25, posterior end of auditory bulla; 26, tip of paraoccipital process; 27, most posterior point of skull. Ventral view of cranium: 1, lateral edge of upper incisor; 2, medial edge of upper incisor; 3, junction between maxillary-premaxillary suture and lateral margin of incisive foramen; 4 and 5, extremities of incisive foramen; 6, intersection between margins of rostrum and zygomatic arch; 7 and 8, maximum length of ventral root of zygomatic arch; 9, lateral junction between maxillary and jugal bones; 10, posterior tip of zygomatic arch; 11 and 12, anterior and posterior ends of glenoid fossa, at their mid-point; 13 and 14, anterior and posterior ends of tooth row; 15, junction between maxillary and palatine bones in sagittal plane; 16, posterior (midsagittal) tip of palate; 17, most ventral point of foramen magnum; 18 and 19, anterior and posterior ends of auditory bulla. Lateral view of mandible: 1, anterodorsal border of incisor alveolus; 2, extreme of diastema invagination; 3, anterior end of mandibular tooth row; 4, anterior end of base of coronoid process; 5, maximum curvature of incisura mandibulae; 6, anterior edge of condylar process; 7, posterior-most edge of postcondyloid process; 8, maximum curvature of curve between postcondyloid process and angular process; 9, dorsal-most point on ventral border of mandibular corpus; 10, posterior extremity of mandibular symphysis; 11, anteroventral border of incisor alveolus.