Thrichomys laurentius

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 : 17

Thrichomys laurentius

Nine age groups were described for the sample from Caruaru, identified as T. laurentius ( Fig 1 .A–I). As expected, the wear of occlusal surface of cheekteeth varied considerably during ontogeny. Age 1 shows pre-molar and first molar erupted ( Fig 1 .A), while the second molar begins to erupt at age 2, but its occlusal surface is completely formed only in age 3 ( Figs 1 .B and 1.C). These three ages, before the eruption of third molar, are grouped in Juvenile category. The third molar is erupted at age 4 ( Fig 1 .D) and its occlusal surface is completely formed at age 5 ( Fig 1 .E). These ages are grouped in Sub-adult category. All specimens have basisphenoid-basiocciptal suture obliterated at age 6, marking the Adult. The first signal of wear appears at age 6 in pre-molar, and at age 7, both flexi of pre-molar and first molar leaves the impression of an enamel island, even as the first flexus of second molar ( Fig 1 .G). Senile specimens show severe wear of cheek teeth’s occlusal surface. All flexi looks like an enamel island ( Fig 1 .H) even as the hypoflexi of pre-molar and first molar ( Fig. 1 .I). Most cranial characters observed and the wear of cheekteeth varied during ontogeny. However, some aspects of the skull did not vary, as the extension of the supra-orbital ridge, which always reaches the suture between frontal and squamosal. The anterior palatal foramens are always anterior to the anterior margin of the pre-molars. The mesopterygoid fossa is "V"-shaped; the interpremaxilar foramen is oval, narrow and short; and the incisive foramen is rounded, and does not show a constriction at the suture between pre-maxila and maxilla ( Fig 4 .A). The shape of the hamular process of the pterygoid bones is has a pointed shape. ( Fig 5 .A). Finally, the pre-maxillary portion of the septum of the incisive foramen exceeds half of the length of the foramen ( Fig 4 .A). The nasal bones reach posterior limit of pre-maxilla only at ages 1 to 3. The ventral crest of the infra-orbital foramen develops through ontogeny. The position of ventral crest at age 1 is after the edge of infra-orbital foramen. However, from sub-adult the ventral crest is anterior to the edge of infra-orbital foramen. The post-orbital process of zygomatic arch can be formed by only squamosal, only jugal or a combination of these two bones. The posterior palatal foramens are located at posterior limit of palate. At age 1, the mesopterigoid fossa reaches posterior margin of first molar and at age 9 it extends up to half of the third molar.

Figures

FIGURE 1. Wear pattern of superior cheekteeth of Thrichomys laurentius in the sequence of age groups described from younger (A) to older (I). 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.