We investigate the cranial, mandibular, and dental ontogeny of the pacarana Dinomys branickii, the only extant species of the rodent family Dinomyidae. A bivariate allometric study (via reduced major axis), including 21 variables from the skull and 6 from the lower jaw on samples varying from 21 to 32 specimens, was produced. Results of the analyses indicate fast growth in the rostrum and palatal length but isometric growth in nasal width, rostral height, and length of the upper diastema. Positive allometry is also present in mandibular length, in retromolar length of the lower jaw, and, especially, in height of the articular process. Negative allometry is found in the length and width of the tympanic bulla, braincase width, width of the occipital condyles, and occipital height. We document for the 1st time the presence of a 3rd premolar (dP3), represented by a tiny, rooted element, and which is lost in early postnatal ontogeny. Replacement of dP4/dp4 occurs after eruption of M3/m3. The complete eruption of P4/p4 happens when M3/m3 are fully functional. Ontogenetic changes in the postnatal ontogeny of the skull of D. branickii are mostly related to the development of the occlusal system and action of associated muscles. This is evidenced by rapid growth of the facial region, a deepening and strengthening of the temporal and occipital regions and a widening of the posterior portion of the palate which is related to the eruption and replacement of molariform teeth. Some observed that craniomandibular trends in the ontogeny of D. branickii are reflected in the size of Ctenohystrica taxa, with small species of this lineage showing features similar to those of juveniles of D. branickii and large species resembling adults of the pacarana.
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