Thrichomys pachyurus

Neves, Antonio Carlos Da S. A. & Pessôa, Leila Maria, 2011, Morphological distinction of species of Thrichomys (Rodentia: Echimyidae) through ontogeny of cranial and dental characters, Zootaxa 2804, pp. 15-24 : 18

Thrichomys pachyurus

Eight ages were described for sample from Barão de Melgaço, identified as T. pachyurus ( Fig 2 .A–H). The level of wear of cheekteeth varies according to age. The pre-molar and first molar are already erupted with occlusal surfaces completely formed at age 1 ( Fig 2 .A). The second molar starts to erupt at age 2, but its occlusal surface is not completely formed ( Fig 2 .B). Until age 3, the specimens are grouped in Juvenile category. The sub-adults specimens at ages 4, 5 and 6 shows the third molar erupted and first signals of wear in the occlusal surface of both first and second molars ( Fig 2 .E). The first flexi of pre-molar has the format of island only at age 6 ( Fig 2 .F). At same age, the first flexus of second and third molar are isolated from hipoflexus. The adults at age 7, which shows the basisphenoid-basiocciptal suture obliterated, have only second flexus of third molar opened to margin ( Fig 2 .G). At age 8, all flexi are enamel island, even as the hipoflexus of pre-molar ( Fig 2 .H). The most cranial characters examined also vary through ontogeny, but some of them are stabilized throughout ontogeny, such as jugal, which were always dorsum-ventrally wide and with transverse ridges along its entire length. The supra-orbital edge always has a crest shape. The vomer is visible in ventral view. The interpremaxilar foramen of T. pachyurus is oval, short and narrow. The incisive foramen of T. pachyurus has a strong contrition in the suture between the maxilla and premaxilla ( Fig 4 .B). The ventral crest of the infra-orbital foramen, poorly developed, is located above the edge of the infra-orbital foramen. The hamular process is large in tip and keeps a rectangular shape ( Fig 5 .B). The anterior palatal foramens, separated by the well developed median crest of the palate, are located above the pre-molars, near to incisive foramen. The mesopterygoid fossa has a "U"-shape. The nasal bones reach the posterior limit of pre-maxilla at ages 2, 7 and 8. Only at age 1 the supra-orbital crest does not extend to suture between frontal and squamosal. The post-orbital process of zygomatic arch is formed mainly by squamosal in the most ages, but at ages 8 and 4 the participation of jugal is equal to squamosal. At age 6 only jugal forms that process. Juveniles show the pre-maxillary portion of septum of incisive foramen longer than half of length of foramen. In sub-adults and adults this is half the length of the incisive foramen. The palate reaches or exceeds the posterior margin of the last molar. The posterior palatal foramens are located at posterior limit of palate. The mesopterygoid fossa extends to medial portion of the last molar.

Figures

FIGURE 2. Wear pattern of superior cheekteeth of Thrichomys pachyurus in the sequence of age groups described from younger (A) to older (H). Scale bar is 1 mm.

FIGURE 4. The shapes of interpremaxilar foramina (above) and incisive foramina of: (A) Thrichomys laurentius, (B) T. pachyurus and (C) T. inermis. Scale bar is 5 mm.

FIGURE 5. The shapes of the hamular processes of: (A) Thrichomys laurentius, (B) T. pachyurus and (C) T. inermis. Scale bar is 1 mm.