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To understand the land snail fauna of a region we must familiarize ourselves with the environment in which they live and which has helped to shape their destinies.
Our area of interest is the Ranomafana National Park in Madagascar's
southeastern State of Fianarantsoa bordering the Indian Ocean. The park is located on the fourth largest island on the planet. It is only exceeded in area by Greenland, New Guinea, and Borneo. It is slightly smaller than Texas or California plus Oregon. This great land mass is only 250 miles east of Africa across the Mozambique Channel.
The land snail fauna of Madagascar is quite distinct from East Africa. The operculates which are poorly represented in Africa are well developed with 177 species in 17 genera of which 170 species or 96% are endemic. The operculate genus Tropidophora has 95 endemic species and only one which is also found in the Comores. In contrast the pulmonate genera which are well represented in Africa, Achatina and Leucotaenius, are sparsely represented in Madagascar.
“On looking at a map of the world we inhabit, we find that its surface is divided between land and water, continents and oceans; each for the most part, thrown together into vast masses, placed under different temperatures, peopled by different races of man, and inhabited be peculiar sorts of animals. Two questions then occur to the mind. What are the causes that have produced this dissimilarity of creatures? And, secondly, is there method in all this amazing diversity?” This was not written yesterday but in 1835. The questions are still with us today. The geologic history of Madagascar and its relationship with Africa play key roles in the development of the unique Madagascar fauna.
There are two aspects to the geologic history of Madagascar - the deposition of the sediments and the migration of the island. A remarkable fact of Madagascar is that it has not been submerged under the sea since before the Cretaceous. It is a mass of Proterozoic rock whose outlines have been less altered than most other parts of the globe. These erosion remnants are seen at the granite domes of the southern highlands, the eroded sandstone domes in the southwest at Isola, and the remarkable limestone spikes of the tsingy or karst topography of the west central.
When the dinosaurs first rose to dominance during the Triassic Period, 200 million years ago, all the land masses were united into a single continent called Pangea. Madagascar was a northeast-southwest trending region, north of Antarctica and Australia, sandwiched between eastern Africa and future India.
During the next 20 million years, to the end of the Triassic (180 million years ago), Pangea was being split into a northern and a southern section. Africa formed part of the southern continent of Gondwanaland (litterally, “forest of the Gondo” from a district in India) along with South America, India, Australia, and Antarctica. Gondwanaland was separated from the northern continent called Laurasia by the Tethys Sea. Today the remains of this sea is the Mediterranean. In Gondwanaland Madagascar was still sandwiched between Africa and India.
Forty-five million years later, at the end of the Jurassic (135 million years ago) in the west South America was starting to separate from Africa. Madagascar and India had separated from Africa. Although joined together Antarctica and Australia had split from Africa and were moving eastward.
By the end of the Cretaceous, 65 million years ago, some major events occurred. The dinosaurs became extinct and early mammals made their first appearance. Madagascar had split from India which was heading north toward Asia. South America and Africa had separated as had Australia from Antarctica. The early mammals of Madagascar were probably the ancestors of the lemurs.
From the end of the Cretaceous to the present, Madagascar has remained almost stationary while Africa has moved north and the Great African Rift has developed. India has collided with Asia and produced the Himalayas. South America has continued its solo westward trek.
What of Madagascar's geologic future? It is believed that it will remain stationary while Eastern Africa will split along the Great Rift Valley to form the Rift Valley Plate and India will continue its northern journey.
What about the land snails? The native land snail fauna found in Madagascar can be separated into two divisions: time and location. From the fossil record the families Ariophantidae and Helixarionidae originated in Europe in the Late Cretaceous. The families Acavidae, Achatinidae, and Urocyclidae originated in Africa in the Pleistocene. From the present location of the non endemic snails (excluding recent introductions) we may have a clue to their ancestry. Of these 20% are also found in India reinforcing the Madagascar-India connection.
The climate is hot and subtropical. The rainy months are February and March with the dry months of May, September, October, and November. The average rainfall ranges from 2500-4000 mm (100-160 inches).
The park was founded in 1991 and comprises 43,500 hectares (169 sq miles) with elevations from 500 to 1500 meters (1625-4875 feet). It is in the mountane rain forest. The park consists of three sites, Talatakely, Vatoharanana, and Vohiparara. Talatakely has the research center and is the most visited by tourists. Vatoharanana is 4 km south of Talatakely and is the most remote. Vohiparara is north of the Ranomafana road.
Talatakely trail system in a low montane eastern rainforest that was selectively logged in 1986-1987 has the most developed trail system. The trails, with many ups and downs, are usually designated by letters and are marked every fifty meters with a sign giving the trail name and the distance in meters from the start of the trail. We are fortunate the trails have been mapped by George Williams.
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Vatoharanana trail system, at 1200 meters, is in a minimum disturbed montane rainforest. Vohiparara Trail System is on the high plateau at the escarpment at1500 meters.