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Arazamys castiglionii
Taxonomy
Arazamys castiglionii was named by Rinderknecht et al. (2011). Its type specimen is MNHN 2521, a skull (Posterior region of the skull. Complete dental series except for the left P4 and part of the right incisor. Well preserved Atlas), and it is a 3D body fossil. It is the type species of Arazamys.
Synonymy list
| Year | Name and author |
|---|---|
| 2011 | Arazamys castiglionii Rinderknecht et al. p. 170 figs. 2, 3d, 4c, 6 |
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If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.
†Arazamys castiglionii Rinderknecht et al. 2011
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Diagnosis
| Reference | Diagnosis | |
|---|---|---|
| A. Rinderknecht et al. 2011 | Skull.—Only the rear half of the skull has been preserved, which includes much of the frontal and parietal bones all solidly fused together. Two parasagittal crests are visible in dorsal view and as in all representatives of the family Dinomyidae are remarkable for their great development. Although the most posterior region of the skull has not been preserved, we assume that these 2 crests joined each other, forming a short sagittal crest (see Fig. 2a). In posterior view the well-developed occipital condyles can be seen. The foramen magnum is subcircular and lies perpendicular to the nuchal plane. A pair of extra articular extensions called paracondyles or accessory condyles (Ameghino 1916; Burmeister 1885) are positioned in the nuchal plane at the external lateral side of each of the occipital condyles (Fig. 3). In posterior view they form 2 well-defined articular sheets of slightly convex subcircular outline, connected to the occipital condyles through a flat surface. These unique structures do not exist in any other group of mammals and are of great systematic importance because their presence is a diagnostic feature of the Dinomyidae (including D. branickii) and clearly separates them from other giant rodents such as the Hydrochoeridae, Neoepiblemidae, and ‘‘Heptaxodontidae’’ (Ameghino 1916; Horovitz et al. 2006; Mones 1997; Negri and Ferigolo 1999; Rinderknecht and Blanco 2008). Compared with other Dinomyidae (D. branickii, Josephoartigasia, Drytomomys, Telicomys, Tetrastylus, Eumegamysops praependens, MACN 4006, and MACN 4007) from which these structures are also known, in Arazamys they are more rounded and with a larger surface. Above each of the paracondyles is a depression for the dorsal edge of the atlas. Although the function of the paracondyles is uncertain (Ameghino 1916; Mones 1997), the existence of these accessory articular facets, in conjunction with the development of a marked occipital fossa for the atlas, indicates that the articulation of the 1st vertebra with the skull was extremely solid. The auditory bullae are small in proportion to the size of the skull (Fig. 2b), kidney-shaped and not inflated, as character- istically observed in Eumegamyinae (in contrast with the condition present in the remaining subfamilies that possess inflated auditory bullae). The meatus acusticus externus forms a short tube of circular cross section (Fig. 4c). As in some Eumegamyinae, an excavated pit or cavity is located anterior to the meatus acusticus externus. Despite an exhaustive search in the classical anatomical literature, we were unable to find a description or a suggested nomenclature for this particular cavity or any similar structure. Faced with this dilemma we propose here to name it ectotympanic cavity, considering that a more inclusive review of the mammalian ear region might confirm the uniqueness of this anatomical structure. A detailed description of the cavity is offered in the next section. Below this ectotympanic cavity an accessory ventral opening is observed. The foramen stylomastoideum is located just below the meatus acusticus externus and is notable for its great development. Under the foramen stylomastoideum a canal that joins this foramen with a small circular groove is observed that we interpret as an attachment surface for articulation of the tympanohyal bone of the hyoid arch. The groove corresponds to the vagina processus hyoidei described by Van der Klaauw (1931) in several mammals (Fig. 4).
Due to the imperfect state of preservation of the mastoid bone, it is not possible to determine whether a secondary foramen stylomastoideum was present. Among the Dinomyi- dae some forms such as Dinomys, Tetrastylus, and possibly Drytomomys do present this secondary foramen (Figs. 4 and 5). As in all members of the subfamily Eumegamyinae, the foramen jugular (5 foramen metoticum) of Arazamys is located in a retrotympanic fossa (Fernandez de Alvarez 1958; Pascual 1967). Dentition.—The dentition of Arazamys has been preserved almost completely except for the left P4 and part of the right incisor (Figs. 6 and 7). As frequently happens in the Dinomyidae, the grinding teeth in Arazamys are small relative to the estimated size of the skull and the great development of the incisors (Rinderknecht and Blanco 2008). The robust incisors, from which only the anterior enamel band is retained, are pale orange in color, lack marked grooves, and present tiny striations that can be seen along the entire length of each tooth. The teeth are curved in lateral view, but the enamel band itself is flat. All the molars of the cheek-tooth series are similar in size (with P4 the shortest and M3 the longest of the series), hypselodont, and consist of 5 lophs or transverse sheets of dentine surrounded by a thin band of enamel—almost imperceptible in the back of each loph—and separated by a narrow layer of cement. Although the union pattern between the lophs varies with each tooth, when it occurs it is always at he lingual side. The occlusal surface on each tooth is almost flat with the labial and lingual borders equally worn. The P4 is triangular in occlusal view and composed of 5 transverse lophs with marked curvature. The 2 anterior lophs remain free, and the 3 posterior ones are joined together along the lingual side. As in other representatives of the Eumega- myinae, the 1st loph has a transverse width significantly narrower than the rest of the lophs in the tooth (Fig. 6). M1 and M2 possess a conformation similar to that of P4, with 2 free anterior lophs and 3 posterior ones joined lingually. These teeth differ from P4 by presenting less curved lophs, and the 1st loph of each tooth is less reduced in transverse width. M3 is composed of 3 free anterior lophs and 2 smaller posterior lophs that also are united lingually. Atlas.—The atlas is the only preserved postcranial bone and is characterized by its robustness and the presence of an extra pair of articular facets for the paracondyles (Fig. 8). The neural spine has not been preserved, but a detailed study of this region suggests that it was very small. In frontal view the neural canal presents a subelliptical shape that becomes an inverted bell in posterior view. On both sides the maximum transverse width is achieved in its dorsal portion. Given the fragile state of the bone, it has not been possible to remove the sediments that fill this cavity. The transverse processes are not preserved. The paracondylar articular facets are well devel- oped and extend laterally. They are concave, subtriangular, anteriorly projected, and separated by a tenuous ridge from the occipital facets. The articular facets for the occipital condyles are concave and less elongated longitudinally than in their transverse axis. The articular facets for the axis are nearly flat. The transverse foramina are covered with matrix, precluding their description. The degree of fusion of the sutures of the atlas indicates that the specimen was an adult at the time of death (Fig. 8). |
Measurements
No measurements are available
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| Source: f = family, subc = subclass, c = class, subp = subphylum | |||||
| References: Hendy et al. 2009, Carroll 1988, Ji et al. 2002, Lillegraven 1979, Nowak 1991 | |||||
Age range: Late/Upper Miocene or 11.60800 to 5.33300 Ma
Collections: one only
| Time interval | Ma | Country or state | Original ID and collection number |
|---|---|---|---|
| Late/Upper Miocene | Uruguay (San Jose) | Arazamys castiglionii (152542) |