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| Top: Madagascar, from left to right, †Aldabrachelys abrupta, †Al. grandidieri, Pyxis planicauda, P. arachnoides, †Astrochelys rogerbouri n. sp., As. yniphora, and As. radiata. Center: Granitic Seychelles (extinct), Aldabra, Al. gigantea. Kehlmaier, Graciá, Ali, Campbell, Chapman, Deepak, ... et Fritz, 2023 Artwork: Michal Rössler; photo of Al. gigantea: Massimo Delfino. |
Abstract
Before humans arrived, giant tortoises occurred on many western Indian Ocean islands. We combined ancient DNA, phylogenetic, ancestral range, and molecular clock analyses with radiocarbon and paleogeographic evidence to decipher their diversity and biogeography. Using a mitogenomic time tree, we propose that the ancestor of the extinct Mascarene tortoises spread from Africa in the Eocene to now-sunken islands northeast of Madagascar. From these islands, the Mascarenes were repeatedly colonized. Another out-of-Africa dispersal (latest Eocene/Oligocene) produced on Madagascar giant, large, and small tortoise species. Two giant and one large species disappeared c. 1000 to 600 years ago, the latter described here as new to science using nuclear and mitochondrial DNA. From Madagascar, the Granitic Seychelles were colonized (Early Pliocene) and from there, repeatedly Aldabra (Late Pleistocene). The Granitic Seychelles populations were eradicated and later reintroduced from Aldabra. Our results underline that integrating ancient DNA data into a multi-evidence framework substantially enhances the knowledge of the past diversity of island faunas.
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| Top: Madagascar, from left to right, †Aldabrachelys abrupta, †Al. grandidieri, Pyxis planicauda, P. arachnoides, †Astrochelys rogerbouri n. sp., As. yniphora, and As. radiata. Center: Granitic Seychelles (extinct), Aldabra, Al. gigantea. Bottom: Mascarenes, from left to right, †Cylindraspis indica (Réunion), †C. inepta, †C. triserrata (both Mauritius), †C. vosmaeri, and †C. peltastes (both Rodrigues). Sizes to scale, corresponding to reported maximum SCLs. Artwork: Michal Rössler; photo of Al. gigantea: Massimo Delfino. |
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| Top: Madagascar, from left to right, †Aldabrachelys abrupta, †Al. grandidieri, Pyxis planicauda, P. arachnoides, †Astrochelys rogerbouri n. sp., As. yniphora, and As. radiata. Center: Granitic Seychelles (extinct), Aldabra, Al. gigantea. Artwork: Michal Rössler; photo of Al. gigantea: Massimo Delfino. |
Astrochelys rogerbouri n. sp.
Diagnosis: A large-sized extinct tortoise species of the genus Astrochelys with an estimated SCL of 50 cm, only known from a single tibia. Astrochelys rogerbouri n. sp. can be separated from the two extant species of the genus by 59 diagnostic mutations in the mitochondrial genome (table S4 and data file S1) and 23 mutations in the reference alignment of four concatenated nuclear loci (HMGB2, HNF1α, R35, and TB73; 3047 bp; table S5 and data file S2). The mitochondrial genome of the new species differs by an uncorrected p distance of approximately 8% from As. radiata and As. yniphora (data file S3); the concatenated nuclear loci of As. rogerbouri n. sp. differ by 0.9 to 1.3% from its two congeners (data file S4).
Etymology: The new species is named for the late Roger Bour (9 July 1947 – 7 March 2020), who was an outstanding herpetologist and expert on western Indian Ocean giant tortoises. Without his pioneering studies and his advice regarding museum specimens, this investigation would not have been possible.
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| Placement of extinct Malagasy and Mascarene giant tortoises in the tortoise phylogeny. The shown maximum likelihood (ML) topology is based on near-complete mitochondrial genomes (15,537 bp) of all genera and species groups of tortoises (Testudinidae). Codes preceding scientific names are DNA Data Bank of Japan (DDJB)/European Nucleotide Archive (ENA)/GenBank accession numbers or museum numbers. Genus names of extinct taxa bear dagger symbols. Numbers at nodes are thorough bootstrap values and posterior probabilities from a Bayesian analysis yielding the same topology. Asterisks indicate maximum support under both approaches. Colored boxes represent different geographic regions. Inset pictures show the past and present diversity of native tortoise species of the western Indian Ocean (extinct species are in gray). Top: Madagascar, from left to right, †Aldabrachelys abrupta, †Al. grandidieri, Pyxis planicauda, P. arachnoides, †Astrochelys rogerbouri n. sp., As. yniphora, and As. radiata. Center: Granitic Seychelles (extinct), Aldabra, Al. gigantea. Bottom: Mascarenes, from left to right, †Cylindraspis indica (Réunion), †C. inepta, †C. triserrata (both Mauritius), †C. vosmaeri, and †C. peltastes (both Rodrigues). Sizes to scale, corresponding to reported maximum SCLs. Artwork: Michal Rössler; photo of Al. gigantea: Massimo Delfino. |
Christian Kehlmaier, Eva Graciá, Jason R. Ali, Patrick D.Campbell, Sandra D. Chapman, V. Deepak, Flora Ihlow, Nour-Eddine Jalil, Laure Pierre-Huyet, Karen E. Samonds, Miguel Vences and Uwe Fritz. 2023. Ancient DNA elucidates the Lost World of western Indian Ocean Giant Tortoises and reveals A New Extinct Species from Madagascar. SCIENCE ADVANCES. 9(2). DOI: 10.1126/sciadv.abq2574
Molecular Archeology: 1200-Year-Old DNA Sequences From Madagascar Lead to the Discovery of an Extinct Tortoise
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