Drawida, Michaelsen, 1900

Narayanan, S. Prasanth, Kumari, Shailja, Kurien, Vijo T., Thomas, A. P., Paliwal, R. & Julka, J. M., 2025, On the biogeography of earthworms of the primitive genus Drawida Michaelsen, 1900 (Clitellata: Moniligastridae) in the Indian subcontinent, Zootaxa 5589 (1), pp. 127-141 : 129-134

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 that Drawida can survive in dry soils as well. Even though, Aravalli and Brij regions still lacks any endemic Drawida species. If they have used Brij or Aravalli path to enter the Indian peninsula, northern parts of the Western Ghats should hold substantial number of Drawida species. But only one species Drawida impertusa Stephenson, 1920 has been described from the northern side and it is now considered as a peregrine within its native range ( Blakemore 2012; Narayanan et al. 2023a, 2024a). Being a peregrine, it cannot be considered as an example for this kind of analysis. Same is the case with respect to Vindhya-Satpura hill ranges. Two species of Drawida , viz., D. willsi Michaelsen, 1907 and D. calebi Gates, 1945b are described from Central India. But these two are widespread endemic species ( Narayanan et al. 2023a,b). Thus , as in the case of D. impertusa , we cannot consider these species as an example for Drawida in Vindhya-Satpura hills. At the same instance, it is also important to note that the forests of the Vindhya-Satpura ranges are not explored well with regards to earthworms. The remaining possible pathway is the Eastern Ghats. Unlike the Western Ghats , the Eastern Ghats are more like a discontinuous range of hills along the eastern coast of Peninsular India. It starts from Mahanadi Basin in Odisha in the north, southwards through Andhra Pradesh to Central Tamil Nadu, where the Eastern Ghats turn southwest to meet the Western Ghats in the Nilgiri Hills , with an average elevation of 600 m and the highest peak in Biligirirangan Hills (up to 1816 m) ( Srinivasan & Prashanth 2006; Kehimkar 2008; Nayaka et al. 2013). Much of the region is covered with dry deciduous forest, yet it supports pockets of various tropical forests such as moist deciduous, evergreen and semi-evergreen forests too ( Kehimkar 2008; Nayaka et al. 2013). Karanth (2003) stated that based on the fossil evidence much of India was covered with humid forest and that was continuous with the forests of Southeast Asia in the middle Miocene. Various authors pointed out that current discontinuity of some species might be representative of a relic of former continuous distribution ( Karanth 2003). Hendrix et al. (2008) communicated that on smaller spatiotemporal scales, changing climate and ecological process affect earthworm distributions. If Drawida species were wide spread in the middle Miocene time in Peninsular India, many relic endemic species should be present at Brij region , Vindhya-Satupura ranges and especially in the northern Western Ghats. From the complete lack of native or endemic species of Drawida in the above mentioned regions, we assume that ancestral Drawida species have used the Eastern Ghats hill ranges to trickle into the southern peninsula and finally into the Western Ghats through Nilgiris ( Fig. 1 ). The distribution of Drawida species with primitive characters, D. scandens Rao, 1921 , and a native of the Eastern Ghats , Drawida caenosa Gates, 1945a , at Coimbatore forest division also lends support to the dispersal of Drawida from the northeast to the southern Peninsula through the Eastern Ghats corridor. The Toba volcano in northern Sumatra erupted about 74,000 years ago, which was the largest explosive eruption of the Quaternary ( Williams et al. 2009; Petraglia et al. 2012). Study of Williams et al. (2009) show that the Toba eruption steered to extended drought and deforestation in India, probably lasting for 1,000 –2,000 years. He also stated that the carbon isotope evidence from fossil soils found immediately beneath and above the Toba ash in central India demonstrates a major isochronous change in vegetation from forest before the eruption to open woodland or grassland thereafter. This volcanic event was an important reason why even today there are no luxuriant forests in places like eastern Andhra Pradesh and north eastern Tamil Nadu that were worst affected by the Toba eruption ( Lal 2016). This incident would have severely affected many endemic Drawida species of the Eastern Ghats , even though the remnant moist forests of the isolated Eastern Ghats hill groups and the eastern coastal plains still has relic population of many endemic Drawida species, such as D. aculeata Gates, 1945 , D. caenosa Gates, 1945 , D. exilis Gates, 1945 as well as comparatively widespread species like D. limella Gates, 1934 , D. calebi Gates, 1945 , D. lennora Gates, 1945 , D. willsi Michaelsen, 1907 ( Gates 1945a,b; Julka 1976; Julka et al. 1989; Halder & Dhani 2005; Mandal et al. 2011; Narayanan et al. 2023a,b, 2024a). Even in further south endemic species such as D. ferina Gates, 1945 , D. mysorensis Gates, 1945 have been described from the plain region like Bangalore in southern portion of the peninsular India ( Gates 1945a), and Drawida nandiensis Stephenson, 1924 from the isolated Nandi Hills which is located within the plains ( Stephenson 1924). Nilgiris, where the Eastern Ghats join the Western Ghats has high diversity of Drawida species compared to the other isolated Eastern Ghats hills in the eastern portion. Nilgiris alone holds 14 (50%) out of the 28 Drawida species recorded from the entire Tamil Nadu state ( Kathireswari et al. 2005, 2008; Narayanan et al. 2023a, 2024a).

Citing various authors Karanth (2015) stated that the Cretaceous volcanism and latitudinal shift that the drifting Indian plate experienced during its northward migration might have caused large-scale extinctions of Gondwanan biota in the Indian peninsula, at the same instance many of the Gondwanan elements did overcome this extinction (e.g. precursors of many present day endemic genera of family Acanthodrilidae , Megascolex Templeton, 1844 and Notoscolex Fletcher, 1886 species in earthworms). Studies showed that there are drastic changes in the pattern behaviour of the Indian monsoon since the late Miocene and this in turn drove significant changes in the fauna and flora including marine biota, terrestrial vegetation and vertebrate animals ( Gupta 2010). This further triggered the dispersal and colonization of various floral and faunal elements through the present day northeastern India. Variation in the monsoon and extinctions of Gondwanan elements would have made biotic impoverishment (as indicated by Biswas (2008) in the case of Sri Lankan diversification of various fauna) of earthworms in the Western Ghats, and this would have helped colonisation of Drawida from other areas to occupy the vacuum of niches created by the extinction. According to Prasad et al. (2009), in distant past, the geological records of the Western Ghats provide evidence of more or less consistent wet and humid climate. This has created high diversity of plants and various climax vegetation, organic rich soil and spatial pattern, climatic condition and high monsoon rainfall, all these might have been conducive for the proliferation of Drawida . This helped them to radiate into various species (in situ diversification) and one lineage could successfully separate out to a new genus Moniligaster in the Western Ghats, especially in the southern portion. There are currently 274 earthworm species known to exists in the Western Ghats ( Narayanan et al. 2020, 2023a,c, 2024b, c). Southern Western Ghats is dominated by megascolecid species (especially Megascolex and Notoscolex ) ( Narayanan et al. 2020) and are considered to be of Gondwanan origin. Now along with megascolecids, moniligastrid species become a dominant genus with high diversity in the southern Western Ghats ( Narayanan et al. 2020). The colonisation of Drawida would have eliminated many previous native species or genera through competition. While discussing the radiation of toads Van Bocxlaer et al. (2009) stated that the high taxonomic level endemism in Western Ghats and Sri Lanka is due to the geological isolation (result of Cretaceous isolation), subsequent invasion and radiation of the new elements colonised India after its suturing with the Laurasian mainland. This major biogeographic factor also ‘diluted’ the composition of previously isolated Indian biota ( Van Bocxlaer et al. 2009; Sudhikumar et al. 2010). However, it seems that unlike toads, Drawida failed to colonise Sri Lanka.At the same instance, except Travoscolides Gates, 1940 species and the native peregrine Octochaetona beatrix ( Beddard, 1902) , native species of the family Acanthodrilidae are absent in the Western Ghats south of Palakkad (Palghat) Gap ( Narayanan et al. 2016a,b, 2023a). This would be due to the already existing Gondwanan elements such as Megascolex and Notoscolex in that region. Diversification of Drawida is found in the southern Western Ghats, where the monsoon begins first and lasts for much prolonged period of time than the northern regions. At this point southern Western Ghats is dominated by megascolecid and moniligastrid earthworms ( Narayanan et al. 2020). The absence of evolutionarily advanced pheretimoid earthworms in the Western Ghats also would have helped the colonisation of the Drawida species in the Western Ghats. At present a few exotic invasive species of pheretimoid earthworms are naturalised in the Western Ghats. Their effects on the distribution of the native moniligastrid species should be studied in detail. Yuan et al. (2019) recently studied the dispersal and diversification of earthworms related to paleogeographical events in the Hengduan Mountains of China. They stated that the 151 out of 171 species found at the Hengduan Mountains are of the family Megascolecidae and 83% among these are of the pheretimoid genera Amynthas Kinberg, 1867 and Metaphire Sims & Easton, 1972 . Furthermore, Yuan et al. (2019) pointed out that the ancestor of Megascolecidae in the Hengduan Mountains arrived and diversified around 44.42 Ma, during the period Eocene to Oligocene from Indochina Peninsula to China subsequent to Himalayan tectonic movement. At the same instance they also stated that the dispersion of Moniligastridae species to this region is unclear to them. We hypothesise that before the uplift of the Himalayan Mountains, the Hengduan Mountainous region had the Moniligastridae species, but they have been marginalised by the dispersion of the highly advanced pheretimoid group.

On the occurrence of Drawida in Sri Lanka

Barring Gates (1972), workers like Easton (1981), Blakemore (2012), Anderson et al. (2017), Narayanan et al. (2021a) mentioned that Drawida species are present in the island country Sri Lanka. In 1972, Gates opined that the records of two dubious Drawida species of Sri Lanka could be attributed to introduction. The species reported are D. friderici ( Michaelsen, 1897) and D. pellucida bournei ( Michaelsen, 1897) ( Narayanan et al. 2021a) . Sri Lanka drifted away in the Miocene from the mainland India ( Biswas 2008). The Western Ghats and Sri Lanka have similar levels of endemism among various taxa, though the fauna of these wet zones have been found to be quite distinctive and numerous species form endemic clades (Bossyut et al. 2004; Gunawardene et al. 2007; Biswas 2008). The fauna and flora of lowland dry forest of northern Sri Lanka seems more similar to the nearby southern India, probably because both get backs their connection in many times in the Pliestocene epoch due to low sea level as part of a series of glacial periods, which enabled multiple colonisations (Bossyut et al. 2004; Gunawardene et al. 2007). Among many groups of taxa such as bryophytes, trees, earthworms, odonates, land snails, reptiles and amphibians show higher levels of endemicity than butterflies, birds and mammals, therefore likelihood of dispersal ability must have got reduced during the sea level rise and habitat specialisation could be related to the levels of endemism among taxa ( Gunawardene et al. 2007; Narayanan et al. 2021a).

Earthworms being rather poor natural dispersers ( Anderson et al. 2017), it may not be able to colonise large areas in short period of time. Hence, they are excellent subject for biogeographical studies, due to their narrow ecological requirements and primitiveness ( Novo et al. 2011). We presume that the initial colonisation of Drawida into the present day northeast Indian region happened in or just before the middle Miocene and by the time Sri Lanka drifted away from India, and hence the chance for colonising Sri Lanka by Drawida might not have happened. Recent studies indicate that Sri Lankan fauna is derived from evolutionary diverse ancestral stock from the Indian mainland mostly before the Pleistocene epoch and underwent in-situ radiation within the island ( Bossuyt et al. 2004; Biswas, 2008). That led to high number of endemic species in island nation. If Drawida were there in Sri Lanka during the Miocene, certainly it could have diverged into multiple species at this island due to the endemic radiation, as happened in many other taxa in Sri Lanka ( Bossuyt et al. 2004). As stated before, the Drawida species showed endemic radiation in the Western Ghats Mountains and gave birth to its sister genus Moniligaster . While describing Moniligaster deshayesi Perrier, 1872 , the type genus of the moniligastrid family, Perrier (1872) mentioned the type locality as Ceylon ( Sri Lanka) without giving any other information. After the initial description of M. deshayesi , it has not been reported from Sri Lankan region ( Narayanan et al. 2021a), but it is widely distributed in the extreme southern portion of the Western Ghats ( Narayanan et al. 2016a, 2022, 2023a; Sathrumithra et al. 2018). Thus Narayanan et al. (2022) presumed that the type specimen of M. deshayesi probably originated from the southern portion of the Western Ghats in Kerala state. Besides M. deshayesi , another 13 valid species have been described from various areas of Western Ghats ( Stephenson 1915; Gates 1940b; Jamieson 1977; Narayanan et al. 2021b, 2022, 2023a, d) but remained unreported from Sri Lanka. Given that Moniligaster is an apomorph sister group of Drawida ( Jamieson 1977) , more Drawida species should be present on the island, however this is not the case. Due to the island’s Miocene separation from mainland India, Drawida was unable to colonize the area. Thus , Sri Lanka theoretically should not have any native endemic Drawida species. At the same instance, two Drawida species viz., D. friderici ( Michaelsen, 1897) and D. pellucida bournei ( Michaelsen, 1897) have been described from Sri Lanka and are apparent endemic to the island ( Narayanan et al. 2021a). Apart from these, one more species ( Moniligaster (now Drawida ) pauli Michaelsen, 1897) was described from Sri Lanka, but later it is synonymised with the Drawida pellucida bournei ( Narayanan et al. 2021a) . As of now D. p. bournei is recorded from Bentota , Candy (now Kandy), Colombo museum garden, Kaniya (now Kanniya), Peradeniya ( Michaelsen 1910), Trincomali (Trincomalee), Vakvalla (now Wakwella) ( Michaelsen 1897, 1899, 1900, 1910) in Sri Lanka which span from Eastern , Central , Western and Southern provinces ( Narayanan et al. 2021a). Four more subspecies of Drawida pellucida were described from the Indian mainland, of which three are from the Western Ghats (D. p. pallida Michalsen, 1910, D. p. pellucida ( Bourne, 1894) , D. p. raoi Stephenson, 1926) and one is from Eastern Himalaya (D. p. stewarti Stephenson, 1914) ( Jayaram 1949; Narayanan et al. 2023a). Hence , we assume that this subspecies (D. p. bournei) would have been carried to the island from India, but it is yet to be reported from the mainland India. D. friderici is described from Trincomali (= Trincomalee) of the Eastern Province of Sri Lanka ( Michaelsen 1897; Narayanan et al. 2021a). After the initial description no information is available on this species. Hence , as stated by Gates (1972), we believe that this species also would have been carried to Sri Lanka from the mainland India in the past. It may be noted that, as in the case of D. p. bournei, this species also has not been recorded from the Peninsular India. As stated by Daniels (1992), it is important to note that humans have played a significant role in eliminating local populations of species, fragmenting them over their ranges or even extending their ranges by introducing them into new geographical areas. Another scenario for Drawida ’s colonization to Sri Lanka would have occurred during the glacial phases of the Pliestocene era, when the island regained its connection to mainland India because of low sea levels. Due to this, fauna and flora of lowland dry regions of southern India and northern Sri Lanka are found more similar (Bossyut et al. 2004; Gunawardene et al. 2007). We hypothesize that, if Drawida arrived in Sri Lanka during the Pleistocene it should have shown some radiation with a number of species in the island.

Figures

FIGURE 1. Probable route of Drawida colonisation to the Western Ghats.