The multiple subspecies of theGalápagos tortoise (C. niger) are among the largestextant terrestrial chelonians. Giant members of the genus, such asLutz's giant tortoise (C. lutzae) were also present in mainland South America and the West Indies during thePleistocene, and the latter into theHolocene.[3]
They were formerly assigned toGeochelone, but a 2006 genetic analysis indicated that they were actually most closely related tohingeback tortoises.[4] However, a more recent genetic analysis ofmtDNA has found that they are actually most closely related to the lineage containingCentrochelys andGeochelone.[5] Their ancestors apparently floated across theAtlantic fromAfrica toSouth America in theOligocene.[4] This crossing was made possible by their ability to float with their heads up and to survive up to six months without food or water.[4] Based onmtDNA analysis, the extantChelonoidis members can be divided into two lineages, with one containing thered-footed tortoise (C. carbonarius) andyellow-footed tortoise (C. denticulatus), and the other containing theChaco tortoise (C. chilensis) and theGalapagos tortoises (C. niger). The now-extinct West Indian radiation is thought to group with the Chaco and Galapagos tortoises but is significantlybasal to both, and was a rather evolutionary distinct lineage, having diverged well before any of the modern species in the genus did and only 7 mya after the divergence ofChelonoidis from African tortoises.[6]
The names of several species names in the genus have often been misspelled, beginning in the 1980s whenChelonoidis was elevated to genus and mistakenly treated as feminine, an error recognized and fixed in 2017.[2]
Presently,Chelonoidis are distributed throughout most ofSouth America, as well as most of theGalápagos; the genus extended north intoCentral America during thePleistocene and most of theWest Indies up to the lateHolocene.C. carbonarius andC. denticulatus are presently found on some of theLesser Antillean islands, but the provenance of these individuals is unknown, and they could have been introduced byAmerindians duringpre-Columbian times.
A 2017 study found that some of these species such as those fromHispaniola were specialists adapted to dry, open habitats such asHispaniolan dry forests and had a major role in shaping them; following a decrease in the extent of these ecosystems after the end of thePleistocene, these tortoises were restricted torefugia habitats up until their extinction.[9]
A 2021 study identified two distinct genetic lineages within the Bahaman tortoise,C. alburyorum, but also sometimes found the remains of members of both lineages on the same island, even though the lineages would have only separated with geographic isolation. This indicates that the early inhabitants of the West Indies were successfully transporting the large tortoises across islands, presumably for the purpose of consumption, and thus causing the remains geographically-isolated lineages to co-occur on the same islands.[5]
Note that the genus nameChelonoidis is masculine under the rules of theICZN, and adjectival species names must agree in gender; the species names below are displayed in keeping with this rule, and may differ from how they commonly appear in publications.[10]
^abOlson, Storrs L; David, Normand (1 January 2014). "The gender of the tortoise genus Chelonoidis Fitzinger, 1835 (Testudines: Testudinidae)".Proceedings of the Biological Society of Washington.126 (4):393–394.doi:10.2988/0006-324X-126.4.393.S2CID83706022.
