Drawida Michaelsen 1900

Time of colonisation of Drawida in the Indian peninsula Gondwanaland was an undisturbed supercontinent in the Permian period (approximately 300–180 Ma) and the present day peninsular India being a part of the Gondwanan landmass was positioned between Africa and Madagascar on the west and Australia and Antarctica on the east and south (Colbert 1977). During the Early Cretaceous period (about 145 Ma) the Indian peninsula separated from the Gondwanaland and moved northwards where it collided with the Tibetan plate (Asia) during the early part of the Tertiary (Early Eocene: 56 Ma) (Srivastava & Mehrotra 2010). But various authors proposed different time period for the beginning of the separation of the Indian peninsula in the Mesozoic era. Its long journey from the southern hemisphere to the northern hemisphere is based on various lines of evidence, particularly of geophysics and geology (Colbert 1977). According to Colbert (1977), India has travelled about 8,000 km from the southern latitude to equatorial zone and then to reach its present position and passing through all the phases of consequent global climatic and tectonic changes. However, Chatterjee & Bajpai (2016) communicated that this land mass has travelled about 9,000 km to reach the present position. Nevertheless, the vegetation of the Indian subcontinent underwent significant changes in comparison to other continents as it crossed various palaeoclimatic belts after its breakup from the Gondwanaland (Briggs 2003). Data from the various streams of studies such as stratigraphy, sedimentology, palaeontology, geochronology, oceanic microplate formation, magnetic lineation, palaeomagnetism, palaeogeography reconstruction, faunal affinities etc. provide different timings for the India-Asia impingement/collision (Jain 2014; Chatterjee & Bajpai 2016). Hence, the estimation of timing of the collision of the Indian Plate with the Asian landmass is one of the highly controversial topics (Jain 2014; Chatterjee & Bajpai 2016) as it is estimated anywhere between 65 and 35 Ma (Jain 2014), and the most widely accepted date is 55 Ma (Corlett 2007). Current models favour either a direct continent to continent collision between the Indian landmass and Asia (Yang et al. 2015; Chatterjee & Bajpai 2016) or first collision involving Indian Arc and followed by final suturing of Indian craton with the Asian plate (Aitchison et al. 2007; Chatterjee & Bajpai 2016). Recent studies especially of the various fossil taxa such as wasps, ants, snakes and mammals, collected from the Lower and Early Eocene Cambay Formation of Vastan (Gujarat state) in western India showed affinities with contemporaneous taxa from Europe, southeast Asia and Australia (Rana et al. 2008; Rage et al. 2008; Rust et al. 2010) but lacking any African or Malagasy connection (Rust et al. 2010). The presence of these taxa in the Vastan gives the clue that the final joining of the two landmasses happened in the Early Eocene. But African elements are still present in India in the form of the frog family Nasikabatrachidae from the Western Ghats which is closely related to the endemic Seychellian family Sooglossidae (Biju & Bossuyt 2003) and the presence of sister genera of cichlid fishes in southern India and Madagascar (Rust et al. 2010; Lal 2016). Aitchison et al. (2007) opined that the final suturing of the two land masses happened in the Eocene-Oligocene boundary 35 Ma. According to Aitchison et al. (2007), initial collisions happened at the Neo-Tethys Seabed and this made many intraoceanic island arcs. Taxa with high dispersal ability might have established in the peninsula either through the island arcs or through various dispersal methods which we speak in the case of modern day animal distributions (see Ali et al. 2021). The amalgamation of these landmasses had a deep effect on the paleogeography, climate, oceanography, and most importantly, a significant interchange between many taxa of plants and animals from the neighboring China and south-east Asian regions into India and out of India through the present day north-eastern India (Biju & Bossuyt 2003; Rust et al. 2010; Srivastava & Mehrotra 2010; Chetterjee & Bajpai 2016; Puri et al. 2016). After the collision with Eurasia, the fauna of the Western Ghats underwent drastic changes due to their migrations between Southeast Asia and the Indian plate (Sudhikumar et al. 2010). According to Chatterjee & Bajpai (2016) this ‘Great Indo-Eurasian Interchange’ was a paleogeographic landmark event that led to the origin of the modern day orders of mammals. Present day biota of India harbour taxa that reflect this geological history (Rust et al. 2010). As per Srivastava & Mehrotra (2010), before the joining of the two landmasses (in Palaeocene) many numbers of plant families such as Arecaceae, Clusiaceae, Fabaceae, Bombacaceae, Myrtaceae, Sterculiaceae, Lauraceae, Moraceae and a few genera like Nypa and Calophyllum, etc. were common to Southeast Asia and Indian peninsula. Recently Su et al. (2020) discovered highly diverse humid subtropical fossil vegetation assemblage of Middle Eocene (ca. 47 Mya) at Jiangla Village in Bangor Basin of Tibet. They calculated that these were from a forest of around ca. 1500 m elevation at east-west trending valley under a monsoonal climate. Up to this time, two extinct Poeciloneuron species of the Clusiaceae family (now Calophyllaceae) are known from India and among these, P. preindicum from the Late Oligocene of Makum Coalfield of Assam (Srivastava & Mehrotra 2013) and P. palaeoindicum from the Middle Miocene Warkali (=Varkala) beds of southern Kerala in the south-west corner of India (Srivastava & Awasthi 1996). From the above extinct species, we can assume that once this genus was wide spread in India up to the warm and humid Middle Miocene (Middle Miocene climatic optima 17–11 Ma) (Zachos et al. 2001). Currently no living representative of this genus is found in northeast Indian region and the genus has a lone living endemic representative, Poeciloneuron indicum in the evergreen forests of the Western Ghats mountain ranges of southwest India (Srivastava & Mehrotra 2013). Southern portion of Western Ghats is considered as a ‘forest refugia’ of the once widespread tropical wet evergreen forests vegetation that existed during the Palaeogene without underwent much change subsequently during the geological past (Gimaret-Carpentier 2003; Prasad et al. 2009). Based on the fossil evidence, it is realized that during the middle Miocene time, much of India was also covered with humid forest and that was continuous with the forests of Southeast Asia (Karanth 2003). The reason for the extinction of Poeciloneuron species in northeast India is not known, but according to Srivastava and Mehrotra (2010) and Puri et al. (2016) this is the time when the plant families such as Melastomataceae and Gluta species of the Anarcadiaceae, etc. entered India from Laurasia. Colonisation of these species, the competition and the changes in the climatic conditions would be the reason for the extinction of Poeciloneuron in the north-east region. Srivastava & Mehrotra (2010) stated that above mentioned families and Gluta species (family Anacardiaceae) were not only absent in north-east India but also in whole India during the entire Palaeogene (66–23 Ma). By the middle Miocene, family Dipterocarpaceae (Shorea, Hopea) and Gluta species become dominant in the flora of northeast India from where it spread to the northwest, west and south India (Ghosh & Taneja 1961; Awasthi 1992; Guleria 1992; Mehrotra et al. 1999; Biswas 2008). Even though, the recent discovery of large volume of dammar-type resin and anatomy of the wood in the Cambay Shale reveals the presence of Dipterocarpaceae family trees in India from the Early Eocene (Rust et al. 2010). By citing various authors Rajkumar et al. (2017) stated that India and Sri Lanka form the western most boundaries of the plant family Dipterocarpaceae and are widely distributed in the rainforests of these regions. Sri Lanka drifted away in Miocene from the mainland India (Biswas 2008). That means, many of the Dipterocarpaceae family species reached Sri Lanka before its separation from the Indian mainland in the Miocene. From this, it is clear that the spread of Southeast Asian biota started moving the northeast Indian region before the Miocene. Since earthworms are mainly associated with the vegetation type and they are rather poor natural dispersers (Anderson et al. 2017), we hypothesize that the ancestral colonizing stock of the Drawdia species would have entered India through the present day north-eastern corner of India in or just earlier to Middle Miocene (during Burdigalian age: ca. 16 Ma) along with the spread of plant family Melastomataceae and Gluta species and with abundant growth of species of the Dipterocarpaceae family. The probable route of its colonisation There are a number of hypotheses about the occurrence of various wet zone species in Peninsular India and its disjunct distribution in the peninsula, Northeast India and Sri Lanka. According to James (2004), in general, the outlines of earthworm distributions were set by geological and biological evolution namely diversification, dispersion and extinction. The major proposed hypothetical dispersal routes of various taxa to Western Ghats are viz., i) the Vindhya-Satpura ranges (Hora 1949), ii) the Eastern Ghats ranges (Abdulali 1949), iii) through Brij region (Dilger 1952) and iv) through Aravalli range (Mani 1974). Karanth (2003) stated that wet-zone species could have taken multiple routes to reach the Western Ghats. No native or endemic Drawida species are known to live in the Western Himalaya (Paliwal & Julka 2005; Kumari et al. 2021; Narayanan et al. 2023a, 2024a), Aravalli ranges or Brij areas and the latter two regions are now typical arid landscapes. Most of the moniligastrids prefer soils with high soil moisture (Julka 1993) but Gates (1972) implied th