Friday 16 November 2018

Boa blanchardensis: A dwarf Boid Snake from the Pleistocene of from Marie-Galante Island, Lesser Antilles.

The islands of the Caribbean are one of the world’s biodiversity hotspots, with large number of speciation events having occurred as organisms reach new islands and adapt to their new environments. The islands have also seen several extinction events, particularly as various waves of Humans have reached the islands in the Holocene, with the effect that many endemic species are now found on isolated islands with no close relatives on any nearby landmasses. One such group are the Boid Snakes, which have an origin in South America, but with species found on Saint Lucia and Dominica in the central part of the Lesser Antilles island chain. The presence of these Snakes in the central Lesser Antilles clearly suggests that these Snakes were once present on the islands to the south, and historical records of large Snakes, presumably Boids, are known from two of these islands, Martinique and Saint Vincent. Fossil Boid Snake remains have also been found on islands to the north of Dominica, with Holocene remains known from Antigua, and Pleistocene fossils from Marie-Galante in the Guadeloupe island group, though these last specimens have never been formally described.

In a paper published in the Journal of Vertebrate Paleontology on 29 May 2018, Corentin Bochaton of the Max Planck Institute for the Science of Human History, the Laboratoire Archéozoologie et Archéobotanique: Sociétés, Pratiques et Environnements and the Institut de Systématique, Évolution, Biodiversité at the Muséum national d’Histoire naturelle, and Salvador Bailon, also of the Laboratoire Archéozoologie et Archéobotanique: Sociétés, Pratiques et Environnements and of the Laboratoire Histoire naturelle de l’Homme préhistorique at the Muséum national d’Histoire naturelle, formally describe and name the Boid material from Marie-Galante Island.

The material comprises 297 specimens from three cave systems, sixteen from the Cadet 2 Cave, four from the Cadet 3 cave, and 277 from Blanchard Cave. This material dates from between 34 000 and 15 000 years before the present, and includes both vertebrae and cranial material. Bochaton and Bailon name the species Boa blanchardensis, meaning ‘from Blanchard’ in reference to Blanchard Cave. The specimens were formerly suggested to have been the remains of Snakes killed by Birds of Prey and subsequently dropped into the caves, but the remains show no signs of predation injuries or digestion, so instead Bochaton and Bailon suggest that they may be the remains of Snakes that were feeding on Bats in the caves, behaviour known from Boids in South America. Based upon comparison with skeletons of the living Boa constrictor, the examined specimens of Boa blanchardensis are estimated to have been between 73 and 139 cm in length, yet developmentally they appear to have been adults, strongly suggesting that this species was an example of island dwarfism (species shrinking on small islands, due to a lack of resources, and an absence of predators that require size to resist).

Boa blanchardensis, cranial and mandibular elements from Blanchard Cave. (A)–(B) Left quadrate in (A) anterior and (B) posterior views. (C) Right supratemporal in dorsal view. (D) Right exoccipital in posterolateral view. (E)–(F) Right dentary in (E) lateral and (F) medial views. (G)–(H) Left compound bone in (G) lateral and (H) medial views. Abbreviations: a. f., anterior foramen; a. p., anterior part; a. s. f., anterior supracondylar fossa; c. c., cephalic condyle; co. p., coronoid process; f. p., foramen of the posterior supracondylar fossa; f. i. f., foramina of the infraglenoid fossa; g. f., glenoid fossa; i. c., imprint of the cephalic condyle; j. f., jugular foramen; l. p., lateral process; m. c., mandibular condyle; m. f., mental foramen; M. g., Meckel’s groove; ma. f., mandibular fossa; me. p., medial process; o. c., occipital condyle; o. cr., occipital crest; o. e., opisthotic-exoccipital eave; p. c., prearticular crest; p. s. f., posterior supracondylar fossa; po. p., posterior part; r. p., retroarticular process; s. c., surangular crest; s. f., small foramen; s. p., stapedial process; su. f., surangular foramina. Salvador Bailon in Bochaton & Bailon (2018). 

Most extinctions of island endemics in the Caribbean are associated with the arrival of Humans in the area, but Boa blanchardensis appears to have died out in the Late Pleistocene, long before any Human explorers reached the area, which requires an explanation, particularly as the island also produces remains of a similar-sized Colubrid Snake, which did persist until the first Humans settled on Marie-Galante Island, with the youngest specimens of this species being found at pre-Columbian archaeological sites. At the end of the Pleistocene Marie-Galante Island would have been larger than today, and well-forested, though with a slightly drier climate, overall a good environment for Snakes. Bochaton and Bailon hypothesise that the difference between Boa blanchardensis and the Marie-Galante Colubrid may have been its environment and diet. Colubrids are ground dwelling, generalist hunters that will take a variety of prey, whereas Boids are tree-dwelling Snakes, which often target Birds and their nests. In the Pleistocene, as now, the Lesser Antilles were prone to hurricanes, which can potentially do great damage to forests and tree-dwelling Birds, which causes Bochaton and Bailon to speculate that a large hurricane hitting the island at the wrong time could have wiped out local Bird populations, depriving Boa blanchardensis of its most important prey, subsequently leading to the extinction of the species.

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