Raranimus
Encyclopedia
Raranimus is an extinct genus
of therapsid of the Middle Permian. It was described in 2009 from a partial skull found in 1998 from the Dashankou locality of the Xidagou Formation
, outcropping in the Qilian Mountains of Gansu
, China
. The genus is the most basal
known member of the paraphyletic order
Therapsida, from which the class
Mammalia is a descendant taxon.
. The skull consists of a well preserved rostrum
. The teeth suggest a carnivorous lifestyle for Raranimus, 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 derived theriodont
s. The presence of two linguo−labially compressed canines is a diagnostic feature of Raranimus. The presence of two functional canines is characteristic of sphenacodontids, and this condition is seen in no other therapsid other than Rananimus. 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 synapsids Dimetrodon
and Tetraceratops
.
In the palate region of the skull, the anterior process of the vomer
ventrally overlies the premaxilla
at the anterior margin of the choana
. This overlap is also seen in dinocephalia
ns. However, unlike any other therapsid, the choanae are short and extend only from the level of the fourth incisor back to the first canine.
. If Tetraceratops is a therapsid, as has recently been proposed, it would be the oldest and most basal one known, surpassing Raranimus in age by several million years. However, later studies have questioned the placement of Tetraceratops within Therapsida, and the 2009 phylogenetic analysis using Raranimus places the genus outside of the clade.
Raranimus occurs in strata that were deposited during the early Roadian
stage of the Middle Permian. The sphenacodontids were most diverse before the Roadian in the Early 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 called Olson’s Extinction.
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 rapid apomorphy accumulation sometime during the gap, while the other proposes that therapsids evolved gradually over the course of up to 35 Ma. Only recently have remains of basal therapsids such as Raranimus been found from China that occur during Olson's Extinction. Other therapsids that are known to have existed during the gap include Sinophoneus and Stenocybus
.
Below is a cladogram
modified from Liu et al., 2009 depicting the phylogenetic relationships of Raranimus with therapsids that occur in Olson's Gap highlighted:
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of therapsid of the Middle Permian. It was described in 2009 from a partial skull found in 1998 from the Dashankou locality of the Xidagou Formation
Xidagou formation
-Therapsida:-Reptilia:...
, outcropping in the Qilian Mountains of Gansu
Gansu
' is a province located in the northwest of the People's Republic of China.It lies between the Tibetan and Huangtu plateaus, and borders Mongolia, Inner Mongolia, and Ningxia to the north, Xinjiang and Qinghai to the west, Sichuan to the south, and Shaanxi to the east...
, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
. The genus is the most basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
known member of the paraphyletic order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Therapsida, from which the class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
Mammalia is a descendant taxon.
Description
Raranimus shares a number of features with later therapsids and ancestral sphenacodontidsSphenacodontidae
Sphenacodontidae is a family of small to large, advanced, carnivorous, Late Pennsylvanian to middle Permian pelycosaurs. Primitive forms were generally small in size , but during the later part of the early Permian these animals grew progressively larger , to become the top predators of their...
. The skull consists of a well preserved rostrum
Rostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....
. The teeth suggest a carnivorous lifestyle for Raranimus, 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 derived theriodont
Theriodont
Theriodonts , are a major group of therapsids. They can be defined in traditional, Linnaean terms, in which case they are a suborder of mammal-like reptiles that lived from the Middle Permian to the Middle Cretaceous, or in Cladistic terms, in which case they include not only the traditional...
s. The presence of two linguo−labially compressed canines is a diagnostic feature of Raranimus. The presence of two functional canines is characteristic of sphenacodontids, and this condition is seen in no other therapsid other than Rananimus. 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 synapsids Dimetrodon
Dimetrodon
Dimetrodon was a predatory synapsid genus that flourished during the Permian period, living between 280–265 million years ago ....
and Tetraceratops
Tetraceratops
Tetraceratops insignis was a lizard-like synapsid. It lived during the Early Permian period. According to a recent, controversial report, T. insignis is the first known therapsid. Other scientists, on the other hand, say that it is a more primitive species of synapsid, possibly an unusual,...
.
In the palate region of the skull, the anterior process of the vomer
Vomer
The vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.-Biology:...
ventrally overlies the premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
at the anterior margin of the choana
Choana
Choana is the posterior nasal aperture.The choanae are separated by the vomer.- Boundaries :It is the opening between the nasal cavity and the nasopharynx....
. This overlap is also seen in dinocephalia
Dinocephalia
Dinocephalia are a clade of large early therapsids that flourished during the Middle Permian, but became extinct leaving no descendants.-Description:...
ns. However, unlike any other therapsid, the choanae are short and extend only from the level of the fourth incisor back to the first canine.
Phylogenetics
According to a phylogenetic analysis conducted along with its initial description, Raranimus is considered to be the basalmost therapsid. There has been some controversy as to whether Tetraceratops is a therapsid or a more basal pelycosaurPelycosaur
The pelycosaurs are an informal grouping composed of basal or primitive Late Paleozoic synapsid amniotes. Some species were quite large and could grow up to 3 meters or more, although most species were much smaller...
. If Tetraceratops is a therapsid, as has recently been proposed, it would be the oldest and most basal one known, surpassing Raranimus in age by several million years. However, later studies have questioned the placement of Tetraceratops within Therapsida, and the 2009 phylogenetic analysis using Raranimus places the genus outside of the clade.
Raranimus occurs in strata that were deposited during the early Roadian
Roadian
In the geologic timescale, the Roadian is an age or stage of the Permian. It is the earliest or lower of three subdivisions of the Guadalupian epoch or series. The Roadian lasted between 270.6 ± 0.7 and 268.0 ± 0.7 million years ago...
stage of the Middle Permian. The sphenacodontids were most diverse before the Roadian in the Early 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 called Olson’s Extinction.
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 rapid apomorphy accumulation sometime during the gap, while the other proposes that therapsids evolved gradually over the course of up to 35 Ma. Only recently have remains of basal therapsids such as Raranimus been found from China that occur during Olson's Extinction. Other therapsids that are known to have existed during the gap include Sinophoneus and Stenocybus
Stenocybus
Stenocybus is an extinct genus of dinocephalian therapsid....
.
Below is a cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
modified from Liu et al., 2009 depicting the phylogenetic relationships of Raranimus with therapsids that occur in Olson's Gap highlighted: