Biseridens

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Biseridens
Temporal range:
Ma
↓
Life illustration of Biseridens qilianicus
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade:
Therapsida
Suborder: †
Anomodontia
Genus: †Biseridens
Li and Cheng, 1997
Species:
†B. qilianicus
Binomial name
†Biseridens qilianicus
Li and Cheng, 1997

Biseridens ("two rows of teeth") is an

dicynodonts.[1]

Among

anomodontia in which Biseridens belong has one of the largest population sizes, highest levels of diversity, and longest stratigraphic range (Middle Permian to the Triassic, possibly into the Cretaceous), as well as being one of the only clades known from all the continents.[2][3] Primarily understood from the recently discovered, most-well preserved specimen, Biseridens is most notably distinguishable as an anomodont due to its short snout, dorsally elevated zygomatic arch, and presence of a septomaxilla that distinctly lacks a facial exposure between the nasal and maxilla.[1] The features that distinguish Biseridens from other anomodonts include the presence of heterodont dentition, or differentiated teeth rows in which different teeth have distinct morphology (ex. precanines, canines, molars, etc.), small toothlike projections, or denticles, located on the palatine and pterygoid, articulation between the opisthotic bone and the tabular bones on the posterior surface of the skull, the absence of the mandibular foramen on the lower jaw, and a pterygoid in which the transverse flange of the pterygoid has a laterally extending process but lacks posterior ramus.[1]

Etymology

Biseri- is Latin for "double rows"; -dens originated from Greek "odons" for "teeth"

Geology and paleoenvironment

At a

anterior-posterior movement of jaw for slicing.[4]

On the basis of converging faunal evidence and comparison to other regions, the Xidagou Formation has been considered to be Middle Permian[6] and home to the oldest therapsid fauna, suggesting that anomodonts had a Laurasian rather than a Gondwanan origin.[7]

Discovery

Until the more recent discovery of a new specimen, Biseridens was primarily known from fragmented skull and jaw material discovered in the 1990s in the upper part of the upper Xidagou Formation of Gansu, China.[8] The specimen included an incomplete skull with the posterior portion of the left ramus of the lower jaw and the anterior part of the lower jaws including well-preserved teeth.[8] The more recently discovered specimen includes a nearly complete skull (lacking the occiput) with lower jaws, found with a set of 14 articulated vertebrae. This new specimen was preserved in mudrock and also discovered in the upper part of Xidagou Formation in Dashankau, Yumen in Gansu.[1]

Description

Biseridens is known mostly from

zygomatic arches
, similar to but not as developed as those of later dicynodonts. The skull is estimated to be at least 17 centimetres (6.7 in) long, relatively small compared to other basal anomodonts.

Dentition

There are several diagnostic features that characterize this specimen as a Biseridens. Related to its heterodont dentition, Biseridens is distinct from other anomodonts because of a differentiated tooth row that includes two rows of teeth on both the jaws, precanine teeth on the dentary and premaxilla, and a broad spread of teeth on the pterygoid and palatine. There are also denticles on the vomer, palatine, and pterygoid. The postcanines possess oval cross-sections and have grinding surfaces, and the canines have a basal diameter of 10 mm. There are no serrations on the marginal teeth, which aligns Biseridens within the anomodont clade.[1]

Palate

The paired pterygoid of Biseridens is the most distinctly visible bone on its palate. It contains an anterior process, a quadrate ramus and a ventromedial process. There is a prominent posterolaterally curving transverse curving process. Similar to other basal therapsids, this process reaches much lower than the level of the palate.[1]

Skull

Regarding its skull structure, the features that distinguish Biseridens from other anomodonts include having an intertemporal region that is wider than the interorbital region, and a

temporal fenestra larger than the orbits. Additionally, the mandibular fenestra is absent from the lower jaw, the transverse flange of the pterygoid extends laterally, and there is contact between the tabular and opisthotic, or the posterior part of the otic capsule. There is a concavity on the posterior region of the postorbital bone as well as a lateral process on the postorbital that fails to reach the zygomatic arch. Characteristic of other basal therapsids, the prominent maxilla constitutes the majority of the snout and the nasal bones are the longest bones of the skull roof.[1]

Holotype skull of Biseridens qilianicus on display at the Paleozoological Museum of China.


Classification

Biseridens belong to the clade

synapsids, one of the major branches of amniotes that gave rise to mammals
.

The holotype and paratype of Biseridens were initially referred to Eotitanosuchia, a former

suborder of therapsids that included various groups of early therapsids (although the range inclusion varied between authors). The genus was named from the Latin biseri- for "double rows" and -dens ("teeth") to refer to the paired double rows of cheek teeth in each jaw, while the species was named after the Qilian mountains where the fossils were discovered. They also assigned Biseridens to a distinct new family of eotitanosuchians, the Biseridensidae, although they did not provide a diagnosis for this family outside of the genus itself.[8]

Although relatively primitive, a number of shared traits (

synapomorphies) ally Biseridens with anomodonts including the shortened snout, raised zygomatic arch and exclusion of the septomaxilla between the maxilla and nasals. However, it retains a number of primitive traits that exclude it from the more derived anomodonts, including the differentiated tooth row, palatal teeth, contact between tabular and opisthotic; lateral process of transverse flange of pterygoid free of posterior ramus and absence of mandibular foramen. Several cladistic analyses indicate that Biseridens is the most basal anomodont known, including that of Liu and colleagues (2009) shown below,[1] as well as those of Cisneros and colleagues (2011)[9] and Kammerer and colleagues (2013):[10]

The most recent analysis performed by Liu et al. in 2009 indicates a significantly stable placement of Biseridens as the most basal anomodont based on a matrix study performed on 75 cranial characters over 15 synapsid genera and conclusions of primitive and derived features. Based on the geographic discoveries of Biseridans, the most basal anomodont, Raranimus, the most basal therapsid, with basal dinocephalians, bolosaurids, and dissorophids, there is now strong evidence that the oldest therapsids resided in China.[1]

Therapsida 

See also

References

  1. ^
    PMID 19640887
    .
  2. PMID 12803915
    .
  3. ^ King G (1990). The dicynodonts, A study in palaeobiology. Vol. 91. London: Chapman and Hall.
  4. ^ a b c "Palaeos Vertebrates Therapsida: Anomodontia". palaeos.com. Retrieved 2020-03-04.
  5. ^ King, Gillian (1988). Anomodontia. Encyclopedia of paleoherpetology. Stuttgart, Germany: Gustav Fischer Verlag.
  6. ISSN 2296-701X
    .
  7. doi:10.4202/app.2008.0071
    .
  8. ^ a b c Li J, Cheng Z (1997). "First discovery of eotitanosuchian (Therapsida, Synapsida) of China" (PDF). Vertebrata PalAsiatica. 35 (4): 268–282.
  9. S2CID 8178585
    .
  10. PMID 23741307
    .
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