I was curious about this taxon,
Chelonoidis niger (Quoy and Gaimard 1824; 138cm long male, 88cm long female; Figs. 1, 2), and how it related to a very similar turtle from the other side of the world in the Seychelle Islands, Aldabrachelys gigantea (originally Testudo gigantea, Schweigger 1812; Fig. 3), which entered the LRT earlier here.
Were they sisters? Or not?
And if so, or if not, how did that happen?
There are two hypotheses,
on how the smaller ancestors of these giant turtles found themselves on three different continents and then some became island-bound and grew to become giants.
Hypothesis number 1:
The radiation of this clade of turtles predates the rifting of Africa and India from South America. A little research indicates mainland turtle ancestors (Geochelone – India) on (Chelonoidis chilensis – South America) and Africa (Geochelone sulcata) are much smaller. Only the islanders (e.g. Chelonoidis alburyorum – Bahamas) get this big.
Hypothesis number 2:
Wikipedia reports, “Their ancestors apparently floated across the Atlantic from Africa to South America in the Oligocene. 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.”
After testing
in the large reptile tree (LRT, 1904+ taxa) the Galápagos tortoise, nests between the snapping turtle and the Aldabra giant tortoise, not with the Aldabra giant tortoise. Snapping turtles in North America also give rise to giant sea-going turtles with feet transformed to flippers, as in Archelon.
Seems like
this rafting hypothesis pops up every time someone doesn’t think certain genera go back to the Cretaceous, as they conservatively follow a far from complete fossil record. We’ve seen failures of the rafting hypothesis several times before.
We’ve all heard of lumpers vs. splitters.
Is there a similar competition between oldsters and youngsters? The fossil record is not as complete as the phylogenetic record. Phylogenetic bracketing includes geography and chronology. The LRT splits South American toucans from related African hornbills (Fig. 5) and they can fly.
Occam’s razor
indicates that small turtles were already widely present on South America, Africa and India prior to the splitting of Gondwana. Thereafter the rising and falling of various land bridges gets rid of the need to raft to various islands where gigantism can take place after isolation.
Rafting is chancy for individuals.
At minimum you need one male and one female. Evolution prefers populations. Walking to new lands involves populations. Thereafter isolation and speciation can take place. Other examples include Australian marsupials and Madagascaran lemurs. In the absence of evidence to the contrary, the LRT indicates the turtle ancestors of these island giants had spread world-wide prior to the splitting of Pangaea. The sparse fossil record will just have to catch up someday.
Other island giants:
Geochelone burchardi Tenerife (Canary Islands) giant tortoise, Geochelone vulcanica Gran Canaria (Canary Islands) giant tortoise, Geochelone robusta Malta giant tortoise.
References
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Loveridge A and Williams EE 1957. Revision of the African Tortoises and Turtles of the Suborder Cryptodira. Bull. Mus. Comp. Zool. Harvard 115 (6): 163-557.
Quoy JR and Gaimard JP 1824. Zoologie. In: M.L. de Freycinet: Voyage Autour du Monde, Entrepris par le ministére et conformément aux instructions de s. exc. M. le Vicomte du Bouchage, Secrétaire d’etat au Department de la Marine, Exécuté sur les corvettes de S.M. l’Uranie et la Paris. iv + 712 pp.
Schweigger AF 1812. Prodromus monographiae Cheloniorum. Königsberger Archiv für Naturwissenschaft und Mathematik 1:271-462.
The Pterosaur Heresies 