<P> The family Rhinocerotidae consists of only four extant genera: Ceratotherium (White rhinoceros), Dicerorhinus (Sumatran rhinoceros), Diceros (Black rhinoceros) and Rhinoceros (Indian and Javan rhinoceros). The living species fall into three categories . The two African species, the white rhinoceros and the black rhinoceros, belong to the tribe Dicerotini, which originated in the middle Miocene, about 14.2 million years ago . The species diverged during the early Pliocene (about 5 million years ago). The main difference between black and white rhinos is the shape of their mouths--white rhinos have broad flat lips for grazing, whereas black rhinos have long pointed lips for eating foliage . There are two living Rhinocerotini species, the Indian rhinoceros and the Javan rhinoceros, which diverged from one another about 10 million years ago . The Sumatran rhinoceros is the only surviving representative of the most primitive group, the Dicerorhinini, which emerged in the Miocene (about 20 million years ago). </P> <P> A subspecific hybrid white rhino (Ceratotherium s . simum × C. s . cottoni) was bred at the Dvůr Králové Zoo (Zoological Garden Dvur Kralove nad Labem) in the Czech Republic in 1977 . Interspecific hybridisation of black and white rhinoceros has also been confirmed . </P> <P> While the black rhinoceros has 84 chromosomes (diploid number, 2N, per cell), all other rhinoceros species have 82 chromosomes . However, chromosomal polymorphism might lead to varying chromosome counts . For instance, in a study there were three northern white rhinoceroses with 81 chromosomes . </P> <P> There are two subspecies of white rhinoceros: the southern white rhinoceros (Ceratotherium simum simum) and the northern white rhinoceros (Ceratotherium simum cottoni). As of 2013, the southern subspecies has a wild population of 20,405--making them the most abundant rhino subspecies in the world . However, the northern subspecies is critically endangered, with as few as three known individuals left in captivity . There is no conclusive explanation of the name white rhinoceros . A popular idea that "white" is a distortion of either the Afrikaans word wyd or the Dutch word wijd (or its other possible spellings whyde, weit, etc.,) meaning wide and referring to the rhino's square lips is not supported by linguistic studies . </P>

Name the most endangered types of rhino species in africa