Date Published: March 28, 2018
Publisher: Public Library of Science
Author(s): Julian P. Hume, David Martill, Richard Hing, William Oki Wong.
The Pleistocene vertebrate assemblage of Aldabra Atoll has been comparatively well studied. Three Upper Pleistocene fossil localities have been described yielding birds, reptiles and terrestrial molluscs. Those of Bassin Cabri and Bassin Lebine on Ile Picard are undated but must be in excess of 136,000 YBP, whereas Point Hodoul on Malabar Island is circa 100,000 YBP. Aldabra was seemingly completely submerged between deposition of the Ile Picard and Point Hodoul deposits, resulting in local faunal extinctions. Here we present the results of an ongoing study of fossil material collected on Ile Picard in 1987, which reveals a more diverse assemblage than previously realised. Notable discoveries are an Ardeola heron, three Procellariformes, tropic-bird Phaethon, gull Larus, rail Dryolimnas, harrier Circus and owl Tyto, plus evidence of recolonisation of the atoll by some seabirds, rail, harrier, owl, giant tortoises and lizards after the Ile Picard/Point Hodoul submergence event.
Aldabra Atoll in the southwestern Indian Ocean is an isolated, raised atoll, the second largest in the world after Kiribati in the central Pacific Ocean (Figs 1 and 2). Aldabra Atoll is famous for harbouring the densest population of giant tortoises anywhere (>100,000), and is also home to the last surviving flightless bird in the Indian Ocean, the endemic subspecies of the White-throated Rail Dryolimnas cuvieri aldabranus [1–3]. Because of its unique fauna and flora, including over 400 endemic species, Aldabra was designated the first UNESCO World Heritage Site in 1982 . However, the previous introduction of domestic goats Capra aegagrus, domestic cats Felis catus and the Black Rat Rattus rattus resulted in the decline and probable extinction of some terrestrial gastropods and extinction of the endemic Aldabra Brush Warbler Nesillas aldabrana, last recorded in 1983 [5–6].
Aldabra Atoll (9°24′S, 46°22′E) is part of the Aldabra Group, comprising Aldabra Atoll, Assumption Island and the atolls of Astove and Cosmeledo. It is situated 425 km to the northwest of Madagascar and belongs to the outer islands of the Seychelles, situated 1,138 km from the granitic islands to the northeast (Fig 1). Aldabra is the second largest raised coral reef in the world with an elevation of 8 m at its highest point . The atoll is surrounded by a coral reef and is 34 km long and 13 km wide with a surface area of 380 km2, of which 196 km2 comprises a large shallow internal lagoon [13–15]. The lagoon has a very high tidal range for an oceanic atoll, with a mean spring-tide range of 2.74 m , with four main tidal passages that connect the lagoon to the surrounding ocean; the largest, Grande Passe (Main Channel), is a 65 m wide and around 20 m deep channel on the north west coast [15–16]. These tidal channels divide Aldabra into four main islands: Grande Terre (South Island), Malabar (Middle Island), Polymnie Island and Ile Picard (West Island) (Fig 2).
The surface geology of Aldabra Atoll is represented entirely by Pleistocene and Holocene strata; no data are available for earlier geological events. It is complex, reflecting numerous periods of uplift and submergence, summarised in detail in references [8, 10, 13–14]. There are at least three major marine transgressive events represented entirely by carbonate and phosphate deposition. As there is only one dated sequence (Aldabra Limestone), the age of the others is based on their depositional sequence, and are here presented oldest first.
A number of studies provided a petrographic study of the carbonates of Aldabra have concluded that the petrology was generally heterogeneous with considerable lateral and vertical facies diversity; mineralogy and porosity were also extremely diverse [26–28]. The limestone has been subject to much reworking, so that large volumes consist of phosphatic cement sequences with infilled cavities or are internal sediments . The analysed samples were simple bioclastic deposits, cemented by high magnesium calcite or aragonite, with the presence of small amounts of phosphates. The petrology of the Picard calcarenites (calcarenite and calcilutite) contained a mineralogy of aragonite, calcite, low and high magnesium calcite and iron oxides . No information is available for Point Hodoul.
The Bassin Cabri and Bassin Lebine area on Ile Picard at the time of the deposition of the Picard calcarenites was probably well vegetated, but with a moist, low, grassy habitat and an absence of true soil development ; a terrestrial gastropod fauna was abundant. Fossil pollen and spores reveal a vegetational change occurred at the time of the Takamaka deposits, showing that the atoll had an irregular rocky surface with true soil accumulation in pockets. The abundant and diverse gastropod assemblage suggests open, low vegetation habitats subject to periodic flooding, open wood and scrub with leaf litter development, to drier habitats with open scrub . The diversity of vertebrates and terrestrial gastropods of Point Hodoul’s pipe-fill deposits indicate that the atoll was probably well vegetated, with a complex open or dense scrub and scrub forest, generally xerophytic, and broken by open grass areas, perhaps similar to the present-day vegetation, but with a higher rainfall [7, 29].
Around 273 species of plants, 19 of which are endemic, have been recorded on Aldabra [29–30]. The vegetation has been divided into zones, with the most important being: Pemphis scrub (Pemphis acidula), which forms almost pure stands up to 6 m tall at lower levels and mixes with Sideroxylon and other scrub species at higher levels; Mixed scrub and scrub woodland which consists of a large diversity of evergreen species, especially Pandanus; and Mangroves that occur within the lagoon and occupy a total area of around 800 ha [30–32]. However, due to absence of terrestrial sediment and considerable flushing due to high tidal ranges, the tallest mangrove trees attain a height of only 10 m, and are generally much shorter . Other vegetation components are: Broadleaf forest comprising Pisonia, Thespesia and Calophyllum (Takamaka); Scaevola coastal scrub of coastal and inland areas; and Sporobolus grassland, Sclerodactylon tussock grassland and Tortoise turf which are heavily grazed by giant tortoises. Tortoise turf is a distinct association of grasses, sedges and herbs [2, 30]. Because they occur in the areas of the highest tortoise concentration and are subject to intense tortoise grazing, more than half of these Tortoise turf species are genetically dwarfed and consequently has evolved highly specialised growth strategies, e.g. flowers and fruits produced at the base of the plants .
The avian vertebrate fauna comprises 12 endemic terrestrial birds and 14 breeding natives [3, 33–34] (Table 1), and numerous migrant and vagrant species, notably waders, occur, as comprehensively listed by Safford & Hawkins  and SRBC . No native terrestrial mammals occur, but five bats (three endemic) are present [35–37]. Reptiles make up the remaining vertebrate fauna, which consists of the largest and densest population of giant tortoises, Aldabrachelys gigantea anywhere , but unlike in the Pleistocene assemblage, when Aldabran lizard diversity was high, only three species (one endemic) now inhabit the atoll [38–39] (Table 2).
A number of alien species have been introduced to Aldabra Atoll since its settlement by humans in 1880, which include the accidental introduction of black rat Rattus rattus (circa 1890) and some lizards, and the intentional introduction of goat Capra aegagrus (circa 1878) and cat Felis catus (circa 1892) . The mammalian introductions have been particularly detrimental to the endemic fauna of Aldabra.
All fossil material mentioned is accessioned to the collection of the United States National Museum (USNM) except where stated otherwise. A large amount of fossil material had been prepared prior to this study, with the remaining fossil samples still in matrix. These were submerged in 10% acetic acid to free the fossiliferous material, and once this was completed the acetic acid was neutralised in water. No additional preparation was undertaken on the majority of the specimens, but the more delicate specimens were coated with Paraloid B-72 to consolidate them and prevent damage. Fossil material was compared with Aldabra fossil specimens, held at the Palaeontology division of the Department of Earth Sciences, Natural History Museum, London (NHMUK), which were collected in 1969 by Dr. J. D. Taylor. All measurements were made using a dial calipers and rounded to the nearest 0.1 mm. Much of the fossil material used in this analysis is fragmentary, so obtaining measurements was sometimes problematic. Osteological abbreviations used in the text are R (right), L (left), p (proximal) and d (distal). Bird nomenclature follows Safford & Hawkins . Unless otherwise stated, all fossil material is bulk numbered USNM 381095.
Analysis of the Bassin Cabri and Bassin Lebine fossil localities reveals a more diverse faunal composition in the Upper Pleistocene than previously documented, especially among sea birds. During deposition of the Picard calcarenites, as evidenced at Bassin Cabri, Aldabra at 136,000+ YBP, Aldabra was an extensive, prograding carbonate sand cay that developed behind an active reef, rose to a probable maximum of 2 m above sea level, and was heavily vegetated but lacked true soil development [7, 13]. A probable shallow dune field occurred at the western end, in which standing bodies of fresh water occurred between the dunes . The island was colonised by terrestrial snails, including Tropidophora, which is an inhabitant of drier, more open areas, an Oplurus iguanid lizard, giant tortoises, an endemic population of gadfly petrels Pterodroma kurodai, and a terrestrial duck Aldabranus cabri [7, 9, 11–12]. This duck is known from just two fragmentary bones collected at Bassin Cabri, with no new material recovered in this study. Alongside the above-named fauna was alarge Pterodroma, a Puffinus shearwater, an Oceanodroma storm petrel, a flightless White-throated Rail D. cuvieri and possibly resident/endemic populations of a Circus harrier and a Tyto barn owl. Sea and shore birds included Red-tailed Tropicbird P. rubricauda, a Larus gull and a possible Ardeola pond heron. Giant tortoise A. gigantea and at least two terrestrial lizards, Trachylepis and Oplurus, were also present, the last-named were previously only known from Point Hodoul. Crocodilian teeth have been reported from this deposit, and used to be thought to represent Nile Crocodile Crocodylus niloticus [11, 39], but all have now been referred to the endemic horned crocodile Aldabrachampsus dilophus .