User:Ankylosaur Enthusiast/sandbox5: Difference between revisions

Line 36: Line 36:

==Classification==

==Classification==

In their 1997 paper, Karen Black and Michael Archer were able to assign ”Silvabestius” to Zygomaturinae based on the following synapomorphies: a parastyle on the third upper premolar, and a ventral alisphenoid tympanic process. They also regarded it as the most plesiomorphic zygomaturine known with the possible exception of Raemeotherium. Their results support the idea first proposed by Stirton et al. (1967) that zygomaturines evolved from a diprotodontine-like ancestor. The tricuspid premolar of ”S. michaelbirti” is intermediate between the simple, bicuspid premolar of diprotodontines and the quadricuspid premolar of S. johnnilandi and other zygomaturines.

[[File:Reconstruction_of_Nimbadon_lavarackorum_mother_and_juvenile_-_journal.pone.0048213.g002.png|thumb|left|Reconstruction of the closely related ”Nimbadon lavarackorum”.]]

In their 1997 paper describing ”Silvabestius”, Karen Black and Michael Archer regarded the taxon to be the most plesiomorphic zygomaturine known with the possible exception of ”[[Raemeotherium]]”.

<ref name=”Black1999″>{{cite journal|first1=K. H.|last1=Black |first2=B. S.|last2=Mackness |year=1999 |title=Diversity and relationships of diprotodontoid marsupials |journal=Australian Mammalogy |volume=21 |pages=20–21}}</ref>

<ref name=”Black1999″>{{cite journal|first1=K. H.|last1=Black |first2=B. S.|last2=Mackness |year=1999 |title=Diversity and relationships of diprotodontoid marsupials |journal=Australian Mammalogy |volume=21 |pages=20–21}}</ref>

Extinct genus of marsupials

Silvabestius is an extinct genus of diprotodontid marsupial which inhabited Australia during the Late Oligocene. Its fossils have been found from various sites at the Riversleigh World Heritage Area (north-western Queensland). Two species are currently known, S. johnnilandi and S. michaelbirti. Two well preserved Silvabestius skulls were found close together, believed to be from a mother and child. It represents the most plesiomorphic zygomaturine to date.

Discovery and naming

The fossils of Silvabestius were discovered in various locations across the Riversleigh World Heritage Area of north-western Queensland. The fossils were described in 1997 as a distinct genus of primitive zygomaturine diprotodontid. The type species is S. johnnilandi, described on the basis of two perfectly preserved crania from the Late Oligocene VIP Site. These skulls were found close together and are thought to represent a mother and child. The second species, S. michaelbirti, was named in the same publication based on a relatively complete skull from the similarly aged Hiatus Site. Additionally, a maxillary fragment from Site D, originally described by Tedford (1967) as belonging to a palorchestine, was referred to Silvabestius sp.^[1][2]

The generic name is a combination of the Latin words for “forest” (silva) and “beast” (bestia). This name was chosen to reference its inferred habitat.^[1]

Species

• Silvabestius johnnilandi^[1]

The type species of the genus, S. johnnilandi lived during the Late Oligocene and was described by Karen Black and Michael Archer in 1997. Its remains were found in 1989 from a site at the Riversleigh WHA. The species name was chosen to honour Professor John Niland, for their strong support of the Riversleigh Project and helping to collect fossils at Riversleigh.

• Silvabestius michaelbirti^[1]

S. michaelbirti is the smallest of the two species. It is known from a single skull that was discovered at Riversleigh in 1992. It was named after the former Vice Chancellor of the University of New South Wales Professor Michael Birt, who assisted in the collection and preparation of specimens at Riversleigh.

Description

The first upper incisor of Silvabestius is large and curved, while the second, and especially third, are small. Both species retain upper canines, a feature not seen in any other diprotodontid except for Neohelos. However, the canines of S. johnnilandi are limited to vestigial dental alveoli. The third upper premolar of S. johnnilandi has four main cusps: the parametacone, protocone, parastyle, and hypocone. In S. michaelbirti, meanwhile, there are only three as it lacks the hypocone. Viewed from the top of the tooth (occlusal view), the parametacone has a pyramid-like appearance, with steep faces. This cusp is positioned more towards the centre of the premolar in S. michaelbirti, whereas in S. johnnilandi it is more posterior. The protocone is lingually (towards the tongue) opposite to the parametacone, and varies in size depending upon species. Both species possess a small parastyle, a feature that sets it apart from all other zygomaturines. In addition, they also lack a deep trench that separates the parastyle from the base of the parametacone. The first upper molar is subrectangular, while the second to fourth molars are trapezoidal. These molars are also low-crowned, lacking a well-developed lingual margin. S. michaelbirti has steeper sloping surfaces on the protolophs and metalophs compared to the type species.^[1]

It shares an anterobuccal blade on the parametacone in the upper premolar, molar gradient does not appreciably increase posteriorly and lack of well-developed buccal cingulum or metastyle with Nimbadon 

The lower incisors are procumbent and lanceolate in shape. The lower premolar is longer than it is wide and tapers towards the front of the tooth. It possesses a protoconid with a short, bow-like blade. This blade extends from behind the apex of the protoconid and ends in a small cuspid on the posterior cingulid. In front of the protoconid, a poorly developed crest ends in a slight swelling at the front tooth margin. Like the upper molars, the lower molars are bilophodont (two-ridged). The relative size of the protolophid and hypolophid differs, with the two being subequal on the second molar and the hypolophid smaller on the third and fourth molars. <— needs to be proofread and fixed

Classification

In their 1997 paper, Karen Black and Michael Archer were able to assign Silvabestius to Zygomaturinae based on the following synapomorphies: a parastyle on the third upper premolar, and a ventral alisphenoid tympanic process. They also regarded it as the most plesiomorphic zygomaturine known with the possible exception of Raemeotherium. Their results support the idea first proposed by Stirton et al. (1967) that zygomaturines evolved from a diprotodontine-like ancestor. The tricuspid premolar of S. michaelbirti is intermediate between the simple, bicuspid premolar of diprotodontines and the quadricuspid premolar of S. johnnilandi and other zygomaturines.

^[3]

Paleobiology

Silvabestius is known from several Late Oligocene-aged deposits at the Riversleigh World Heritage Area. S. johnnilandi is known from the VIP Site, whereas S. michaelbirti stems from the Hiatus site. A third possible species was discovered at Site D.^[1] During the Oligocene, Riversleigh would have been covered in woodland or open forest.^[4] These forests would have been dominated by trees or shrubs from the family Casuarinaceae.^[5] Patches of rainforest may have also been present, specifically around pools or along watercourses.^[6] Silvabestius lived alongside other diprotodontids like Neohelos tirarensis and Ngapakaldia bonythoni; and the palorchestid Propalorchestes ponticulus.^[7]

References