''Tropidostoma'' is a medium-sized

herbivorous A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...

oudenodontid

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typic ...

therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...

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 251.9 Mya. It is the last ...

(

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 terms l ...

) period in

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...

. The first ''Tropidostoma'' fossil was described by

Harry Govier Seeley Harry Govier Seeley (18 February 1839 – 8 January 1909) was a British paleontologist. Early life Seeley was born in London on 18 February 1839, the second son of Richard Hovill Seeley, a goldsmith, and his second wife Mary Govier. When his fat ...

in 1889. Later two subspecies 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 preserved ...

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 Bio ...

in a

biozone In biostratigraphy, biostratigraphic units or biozones are intervals of geological strata that are defined on the basis of their characteristic fossil taxa, as opposed to a lithostratigraphic unit which is defined by the lithological properties ...

of the Karoo Basin known as the ''Tropidostoma'' Assemblage Zone. This

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 The Beaufort Group is the third of the main subdivisions of the Karoo Supergroup in South Africa. It is composed of a lower Adelaide Subgroup and an upper Tarkastad Subgroup. It follows conformably after the Ecca Group and unconformably underli ...

. 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 FRS FRSE (30 November 1866 6 April 1951) was a British- South African doctor and palaeontologist. He qualified as a medical practitioner in 1895 and received his DSc in 1905 from the University of Glasgow. From 1903 to 1910, he ...

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

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

. 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 subspecies 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 unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

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 teeth, as with pigs and walruses, or, in the case of elephants, elongated incisors. Tusks share ...

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 the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...

or individual variation.

Classification

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

clade

Bidentalia Bidentalia is a group of dicynodont therapsids. Bidentalia was one of the first names used to describe dicynodonts; the group was established in 1876, while the name "bidentals" dates back as far as 1845. With the increasing prominence of phyloge ...

. This clade is characterized collectively by their reduced dentition with only their

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates 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. T ...

s being intact. However, many species in this clade 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''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...

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 Dicynodonts Lopingian synapsids of Africa Fossil taxa described in 1876 Lopingian genus first appearances Lopingian genus extinctions Anomodont genera

History of discovery

Description

Classification

See also

References