Charles Darwin, who was in South America as part of the
second voyaging expedition of HMS Beagle, was one of the first to collect
Toxodon fossils. In his book covering the expedition,
The Voyage of the Beagle. Darwin wrote, "November 26th – I set out on my return in a direct line for Montevideo. Having heard of some giant's bones at a neighbouring farm-house on the Sarandis, a small stream entering the Rio Negro, I rode there accompanied by my host, and purchased for the value of eighteen pence the head of the
Toxodon." The skull had been propped up against a fence and been used as target practice for throwing stones by local children, who had knocked out its teeth. Since Darwin discovered that the fossils of similar mammals of South America were different from those in Europe, he invoked many debates about the evolution and natural selection of animals. In his own words, Darwin wrote down in his journal,
Toxodon and its
type species,
T. platensis, were described in 1837 by
Richard Owen based on remains collected by Darwin, in a paper titled "
A description of the cranium of the Toxodon platensis, a gigantic extinct mammiferous species, referrible by its dentition to the Rodentia, but with affinities to the Pachydermata and the herbivorous Cetacea", reflecting the many unusual characteristics of its anatomy.
Evolution Toxodon is a member of
Notoungulata, a group of
South American native ungulates that had been part of the fauna of South America since the
Paleocene, over 60 million years ago, and had evolved in isolation in South America, prior to the arrival of living ungulates in South America around 2.5 million years ago as part of the
Great American Interchange. Notoungulates are morphologically diverse, including forms morphologically distant from
Toxodon such as rodent and rabbit-like forms. This finding has been corroborated by an analysis of mitochondrial DNA extracted from a
Macrauchenia fossil, which yielded a date of 66 million years ago for the time of the split from perissodactyls.
Toxodon belongs to
Toxodontidae, a large bodied group of notoungulates which first appeared in the
Late Oligocene (
Deseadan), ~28-23 million years ago, and underwent a great radiation during the
Miocene epoch (~23-5.3 million years ago), when they reached their apex of diversity. The diversity of toxodontids, along with other notoungulates began to decline from around the
Pliocene onwards, By the
Late Pleistocene (
Lujanian), the once great diversity of notoungulates had declined to only a few of species of toxodontids (belong to the genera
Toxodon,
Mixotoxodon,
Trigodonops and
Piauhytherium, the last possibly being a synonym of
Trigodonops) with all other notoungulate families having become extinct. }}
Species There has not been a recent taxonomic revision of the genus
Toxodon, leaving the number of valid species uncertain. The species
Toxodon chapalmalensis is known from the
Pliocene (
Montehermosan-
Chapadmalalan) of Argentina, while
Toxodon platensis, the
type species, is known from the
Pleistocene. The validity of other potential species like
Toxodon darwini Burmeister, 1866, and
Toxodon ensenadensis Ameghino, 1887 from the Early Pleistocene of Argentina is uncertain, and the species
Toxodon gezi C. Ameghino, 1917 and
Toxodon aguirrei Ameghino, 1917 have been considered junior synonyms of
Toxodon platensis by recent authors. Some recent authors have argued that
Toxodon gracilis Gervais and Ameghino, 1880, should be recognised as a distinct species from the Pleistocene of the Pampas significantly smaller than
T. platensis, with these authors suggesting that
T. platensis and
T. gracilis represent the only valid species of
Toxodon in the Pleistocene of the Pampas region. Other authors have argued that all Pleistocene
Toxodon species should be considered synonymous with
T. platensis. == Description ==