| Raranimus | |
|---|---|
 | |
| Holotype skull from the sides | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Genus: | †Raranimus Liuet al.,2009 |
| Type species | |
| †Raranimus dashankouensis Liuet al., 2009 | |
Raranimus is an extinctgenus oftherapsids of theMiddle Permian. It was described in 2009 from a partial skull found in 1998 from the Dashankou locality of theQingtoushan Formation, outcropping in theQilian Mountains ofGansu,China. The genus is the mostbasal known member of theclade Therapsida, to which the laterMammalia belong.[1]
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Raranimus shares a number of features with later therapsids and ancestralSphenacodontia. The skull consists of a well preservedrostrum. The teeth suggest a carnivorous lifestyle forRaranimus, as the incisors are recurved and the second canines are serrated on their posterior edges. The incisors are morphologically similar to those seen in more derivedtheriodonts. The presence of two linguo−labially compressed canines is a diagnostic feature ofRaranimus. The presence of two functional canines is characteristic of sphenacodontids, and this condition is seen in no other therapsid other thanRananimus. However, the slender, compressed shape of these canines is a derived characteristic of therapsids, with the canines of similarly sized sphenacodontids being more massively built. The precanines are small and anteriorly serrated, similar to what is seen in the synapsidsDimetrodon andTetraceratops.[2][3]
In the palate region of the skull, the anterior process of thevomer ventrally overlies thepremaxilla at the anterior margin of thechoana. This overlap is also seen indinocephalians. However, unlike any other therapsid, the choanae are short and extend only from the level of the fourth incisor back to the first canine.[1]
According to a phylogenetic analysis conducted along with its initial description,Raranimus is considered to be the basalmost therapsid.[1] There has been some controversy as to whetherTetraceratops is a therapsid or a more basalpelycosaur. IfTetraceratops is a therapsid, as has recently been proposed, it would be the oldest and most basal one known, surpassingRaranimus in age by several million years.[3] However, later studies have questioned the placement ofTetraceratops within Therapsida, and the 2009 phylogenetic analysis usingRaranimus places the genus outside of the clade.[1][4][5]
Raranimus occurs in strata that were deposited during the earlyRoadian stage of theMiddle Permian.[6] The sphenacodontids were most diverse before the Roadian in theEarly Permian, yet therapsids did not appear as a diverse group until near the Roadian-Wordian boundary. This has left a morphological and temporal gap in the fossil record during which the origin of therapsids must have occurred[7] calledOlson’s Extinction.[8][9]
With the general absence of therapsid remains from Olson's Extinction, different hypotheses have developed in order to explain the group's origins and initial diversification. One theory suggests that therapsids diversified quickly through rapidapomorphy accumulation sometime during the gap, while the other proposes that therapsids evolved gradually over the course of up to 35 Ma.[7][10] Only recently have remains of basal therapsids such asRaranimus been found from China that occur during Olson's Extinction. Other therapsids that are known to have existed during the gap includeSinophoneus andStenocybus.[11]
Below is acladogram modified from Liuet al., 2009 depicting the phylogenetic relationships ofRaranimus with therapsids that occur in Olson's Gap highlighted:[1]
| Synapsida | Taxa occurring in "Olson's Gap" |
