''Tropidostoma'' is a medium-sized

herbivorous A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...

oudenodontid

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivores that typically bore a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, t ...

therapsid that lived during the Late

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

(

Lopingian The Lopingian is the uppermost series/last epoch of the Permian. It is the last epoch of the Paleozoic. The Lopingian was preceded by the Guadalupian and followed by the Early Triassic. The Lopingian is often synonymous with the informal te ...

) period in

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

. The first ''Tropidostoma'' fossil was described by Harry Govier Seeley in 1889. Later two

subspecies In Taxonomy (biology), biological classification, subspecies (: subspecies) is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (Morphology (biology), morpholog ...

were identified. ''Tropidostoma''

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s are an

index fossil Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...

in a biozone of the Karoo Basin known as the ''Tropidostoma'' Assemblage Zone. This biozone is characterized by the presence of this species in association with another

species, '' Endothiodon uniseries''.

History of discovery

The first ''Tropidostoma''

material was found during a field expedition in the Upper

-aged Teekloof Formation of the Beaufort Group. This material was later described by Seeley (1889) in a study in which he described two

s which had been named ''Dicynodon microtrema'' and ''Tropidostoma dunni''. In 1915, several years after Seeley's death, the paleontologist,

Robert Broom Robert Broom Fellow of the Royal Society, FRS FRSE (30 November 1866 6 April 1951) was a British- South African medical doctor and palaeontologist. He qualified as a medical practitioner in 1895 and received his DSc in 1905 from the University ...

, reexamined the same material and discovered that the fossil material in question was of the same

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

. This new single species was renamed ''Tropidostoma microtrema''. Some years later, the name ''T. microtrema'' was changed to ''Tropidostoma dubium'', and ''Tropidostoma dunni'' is now considered to be the type species.

Description

Two

exist among the specimens referred to as ''Tropidostoma dubium'' and ''Tropidostoma dunni'' . ''T. dubium'' is observed to have two

cranial Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Thi ...

morphs, one being robust form with a tall snout and large

tusk Tusks are elongated, continuously growing front teeth that protrude well beyond the mouth of certain mammal species. They are most commonly canine tooth, canine teeth, as with Narwhal, narwhals, chevrotains, musk deer, water deer, muntjac, pigs, ...

s and the other more gracile with a low snout and small or no tusks. The robust and gracile forms are considered to either represent

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

or individual variation.

Classification

''Tropidostoma'' is currently classified as an oudenodontid within the larger

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

Bidentalia. This

is characterized collectively by their reduced

dentition Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiology ...

with only their

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

s being intact. However, many species in this

sporadically lack tusks completely and their

s only hold evidence that they retained their

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. It is the key structural material making up Scale (anatomy), scales, hair, Nail (anatomy), nails, feathers, horn (anatomy), horns, claws, Hoof, hoove ...

ous beaks. Many ''Tropidostoma'' fossils previously collected have been misidentified as other species, such as of '' Oudenodon bainii'' due to their remarkable similarity. In addition, the type fossil of the recently discovered '' Bulbasaurus phylloxyron'' was misidentified as a ''Tropidostoma''

for several years.

References

{{Taxonbar, from=Q7846231 Bidentalia Lopingian synapsids of Africa Fossil taxa described in 1876 Lopingian genus first appearances Lopingian genus extinctions

History of discovery

Description

Classification

See also

References