Raranimus dashankouensis, Liu & Rubidge & Li, 2009
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publication ID |
https://doi.org/ 10.4202/app.2008.0071 |
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persistent identifier |
https://treatment.plazi.org/id/03EB87A7-5949-7537-5F36-F9BE208CFCCF |
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treatment provided by |
Felipe |
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scientific name |
Raranimus dashankouensis |
| status |
gen. et sp. nov. |
Raranimus dashankouensis gen. et sp. nov.
Figs. 1 View Fig , 2 View Fig .
Etymology: Specific name from Dashankou,the name of the fossil locality. Holotype: IVPP V15424.
Type locality: Dashankou Locality , Yumen, Gansu Province, China . Type horizon: Xidagou Formation, Middle Permian ( Li et al. 2004).
Diagnosis.—A plesiomorphic therapsid characterised by: choana short, with the posterior margin lying at the level of the first pair of canines; long facial process of septomaxilla; presence of one precanine and two functional linguo−labially compressed canines on maxilla; six incisors.
Description.—The specimen consists of a well preserved though slightly laterally crushed, slender snout (length of 100 mm, height of 65 mm, width between canines 33 mm) with a marginal tooth series comprising incisors, precanines, canines and the roots of three postcanines ( Figs. 1B View Fig , 2 View Fig ). Recurved and slender incisor teeth and the presence of serrations on the posterior edge of the second canine suggest that it belonged to a large predator with the complete skull probably exceeding 16 cm in length.
A large oval external naris ( Figs. 1A View Fig , 2 View Fig ) is positioned close to the anterior margin of the snout. The dorsal process of the premaxilla makes up most of the internarial bar, and terminates posteriorly beyond the posterior margins of the external nares where it is overlapped by the nasals. Paired nasals extend backwards from the posterodorsal margin of the external naris to meet the prefrontal posteriorly, and ventrally form a long sutural contact with the maxilla and septomaxilla. The latter bone comprises the floor of the external naris with its posterodorsal process wedged between the maxilla and the nasal and extending further posteriorly on the snout than the dorsal process of the premaxilla. While the posteriormost extent of the maxilla is not preserved, it contacts the nasal dorsally and the prefrontal posterodorsally. Anteriorly the vertical suture between the maxilla and the premaxilla descends from the front of the external naris to a point between the third and fourth incisor, and continues posteriorly along the ventral edge labial to the incisors before turning medially to reach the choana in front of the precanine. In lateral view the ventral margin of the maxilla turns sharply downwards forming a notch between the last incisor and canine. Bone sculpturing is present on the snout with small pits on the anterior surface of the premaxilla and radial striations converging on the concave area above the root of the canine on the maxilla, while longitudinal striations occur on the rest of the snout.
In palatal view the premaxilla forms the anterior and most of the lateral margin of the choana up to the level of the precanine, while being anteroventrally overlain by the anterior process of the vomer as in dinocephalians. Long, thin and edentulous paired vomers form the medial border of the choana. Their ventral surface is flat with the anterior section being slightly ventrally convex and the lateral edges of the posterior interchoanal portion forming weak ridges. The choanae are short, extending from the level of the fourth incisor to that of the first canine, a character unknown in other therapsids. Only the anterior part of the left palatine is preserved. It underlies the maxilla, possibly contacts the vomer medially, and extends anteriorly to the level of the first postcanine. No palatine teeth are evident and only the anterior portions of the pterygoids are present.
Six incisors were present on each premaxilla. Those with preserved crowns show them to be similar in size, recurved and unserrated, and therefore resembling the morphology of those of most theriodont therapsids ( Fig. 1B View Fig ). A diastema is present between the last incisor and the first canine on the left side and the last incisor and precanine on the right. Two recurved canines, ovoid in cross section, are present in each maxilla. The complete left second canine (c 2 in Fig. 2B View Fig ) is considered to be newly erupted as it only partially occupies its alveolus. No serrations are preserved on the first canine, but they do exist on the posterior ridge of the right second canine. A small replacement tooth, lingual to the left first canine indicates that the two canines are not simply replacements of one another, but functioned simultaneously. This makes Raranimus the only therapsid with two functional canines, a condition reminiscent of the caniniform teeth seen in the large predatory sphenacodontids ( Romer and Price 1940; Reisz 1986). These canines, despite being doubled as in basal synapsids, have a more derived therapsid morphology in being quite slen− der and compressed linguo−labially, rather than having the massiveness seen in similarly sized sphenacodontids.
A small precanine with fine serrations on its anterior ridge is present in the maxilla anterior to the right first canine ( Fig. 1B View Fig ) and is reminiscent of the small precanine teeth known in Dimetrodon ( Romer and Price 1940) and Tetraceratops ( Laurin and Reisz 1996) . Roots of three postcanines are preserved in the left maxilla but the rest of this bone is missing. Judging by root diameter, the postcanines vary in size but are all much smaller than the canines.
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