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Isopods living in Indian Caves and the Early Mesozoic Era

In 2009, a colleague from the Indian Acharya Nagarjuna University in Andhra Pradesh, Professor Yenumula Ranga Reddy, sent me some specimens that his students had collected in a nearby cave. Prof Ranga is well known for many publications on calanoid copepods from Indian lakes but had not worked with isopods before so he contacted me. I was delighted to discover that they were a new genus of phreatoicidean isopod hitherto unknown from India, so we decided to write a manuscript on the species.

Nichollsia kashiense, the first phreatoicidean described from India
Figure 1. Nichollsia kashiense Chopra & Tiwari, 1950

The suborder Phreatoicidea has previous Indian records for the genus Nichollsia, which was described in Chopra & Tiwari (1950; Figure 1) and subsquently assigned to a new family Nichollsiidae Tiwari, 1955 (later transferred to the family Hypsimetopidae). This species was found in the northern part of India in wells on the plains of the Ganges river.

The new species was similar to Nichollsia but had a decidedly different shape of the posterior section of the body. We decided to assign it to a new genus Andhracoides to indicate its more southeastern home in Andhra Pradesh (= state). The species was named after the student who collected it, Shabuddin Shaik. The paper was finally completed and published in Zootaxa (Wilson & Ranga Reddy, 2011; Figure 2).

Andhracoides, the second genus described from India
Figure 2. Andhracoides shabuddin

Shabuddin subsequently collected another species in another cave. As part of his Ph.D dissertation work, he described the species with us (Wilson et al., 2015).

While working on his Ph.D, Shabuddin Shaik visited another cave in the northern part of Andhra Pradesh. He found a completely new genus and species that shares many characteristics with the other Indian genera. In December 2022, we published the description of this species Speonastes venkataramani in the Journal of Crustacean Biology (Figure 3).

Speonastes, a new genus of Phreatoicidean Isopod
Figure 3. Speonastes venkataramani from Andhra Pradesh, India. Scale 1 mm

This last species provided clues to the paleobiogeography of the suborder Phreatoicidea. All of these Indian species are now part of the larger family Hypsimetopidae that also has species in Western Australia (Hyperoedesipus & Pilbarophreatoicus) and also in Victoria and Tasmania (Hypsimetopus & Phreatoicoides. All genera are significantly different from one another. They all, however, are similar for lacking the large pleural plates found on the pleon of other phreatoicideans and have an elongated pleotelson. Speonastes, on the other hand, is especially similar to the other phreatoicideans in this regard and has multiple other similarities with the hypogean genus Crenisopus Wilson & Keable (1999). Analysis of all character data and most genera of the suborder Phreatoicidea produced the estimated phylogeny (Figure 4). The numbers on the branches indicate the strength of relationship.

Phylogeny of living Phreatoicean Isopods
Figure 4. Analysis of Phreatoicidean genera using TNT (Goloboff & Catalano, 2016)

Most of the phreatoicideans are in the lower half of the tree. The Hypsimetopidae, the subject of this post, are found in the upper part of the tree with a seemingly close relationship to Crenisopus. The tree is rooted using several non-phreatoicidean outgroups. The classification of this last genus is not certain because other slightly less optimal patterns appeared in the analysis.

To understand what is going on here, we have to dive back in time to the beginning of the Mesozoic era, in the Triassic period around 233 million years ago (mya). Phreatoicideans are known as early as the Paleozoic Middle Carboniferous (around 306 mya) from a fossil called Hesslerella, which looks similar to Eophreatoicus species that live today in Australia's northern territory. During the late Triassic, the world's continents of today were variously joined together into a supercontinent named Pangea (go to Chris Scotese's excellent PALEOMAP website to see the tectonic evolution of the world through time). During this time, Antarctica, India, Australia and New Zealand were joined together as part of Gondwana, which became its own supercontinent during the Jurassic-Cretaceous periods when the uniqueness of the biota of these continents was established. On the right side of Figure 5, I have cut out the eastern section of Gondwana to show the relationship of India to Australia during the late Triassic (233 mya). It shows India and Australia are directly joined by a section known as Greater Northern India, which became the Himalayas when India crashed into the northern supercontinent Laurasia during early Cenozoic.

Credits: Scotese (2014); Mesquite module Cartographer (Maddison & Maddison, 2017)
Figure 5. Geographic plot of hypsimetopid phylogeny and eastern Gondwana during the Triassic

The other thing to notice are large areas along the Antarctic region that were below sea level but might have had fresh water because they were not in direct communication with the sea. India around the same time was known to have substantial fresh water bodies internal to this region. As the continents began to separate during the Jurassic and Cretaceous, they opened great rift valleys, similar to the African rift valley of today. These rift valleys opened freshwater

Credit: Scotese (2014)
Figure 6. Eastern Gondwana during middle Jurassic 170 mya

corridors between between the western and eastern Australian regions, here shown in Scotese's (2014) GoogleEarth plot of the same region during the middle Jurassic 170 mya.

On the left side of the Figure 5, I have shown only the hypsimetopid part of the phylogeny in Figure 4 with Crenisopus included. The "Root" is the branch leading from all phreatoicideans to the pruned subtree. The key observation is that Speonectes is on a lower branch relative to the Australian and Indian hypsimetopids and, at the same time, is adjacent to Crenisopus. The phylogeny also shows a geographic relationship between an earlier western component consisting of western Australia and India and the later emerging eastern component of the tree consisting of Tasmania and Victoria. This pattern implicates a general diversification of the Hypsimetopidae across the lower span of Australia during the early Mesozoic. A study of the fossils Protamphisopus and Hesslerella is planned to elucidate the transition of the Phreatoicidea from lagoonal estuaries to freshwater during the Triassic-Jurassic periods. Interestingly, freshwater Protamphisopus fossils are found in the Australian state of New South Wales (Sydney brick pits, Triassic; Talbragar Fish Bed, Jurassic) and in Antarctica (Jurassic) suggesting that the eastern Gondawana freshwater corridor was open, at least intermittently, for millions of years.


References


Chopra, B. 1947. First record of occurrence in India of the ancient sub-order Phreatoicoidea (Crustacea: Isopoda). Proceedings of the Indian Science Congress, 34: 176.

Chopra, B. & Tiwari, K.K. 1950. On a new genus of phreatoicid isopod from wells in Banaras. Records of the Indian Museum (Calcutta), 47: 277-289, pls. 217-220.

Goloboff, P.A. & Catalano, S.A. 2016. TNT version 1.5, including a full implementation of phylogenetic morphometrics. Cladistics, 32: 221-238. [https://doi.org/10.1111/cla.12160]

Maddison, D.R. & Maddison, W.P. 2017. Cartographer, a Mesquite package for plotting geographic data, ver. 1.5. The Mesquite Project Team, British Columbia & Oregon, http://mesquiteproject.org/packages/cartographer, accessed on 10 Sept 2022.

Scotese, C.R. 2014. The Jurassic and Triassic, Maps 32–42. In:

Atlas of Jurassic paleogeographic maps, PALEOMAP Atlas for

ArcGIS. PALEOMAP Project, Evanston, IL, USA [https://doi.

org/10.13140/2.1.4850.4321].

Wilson, G.D.F. & Keable, S.J. 1999. A new genus of phreatoicidean isopod (Crustacea) from the north Kimberley region, Western Australia. Zoological Journal of the Linnean Society, 126: 51-79. [https://doi.org/10.1111/j.1096-3642.1999.tb00607.x]

Wilson, G.D.F. & Ranga Reddy, Y. 2011. Andhracoides shabuddin gen. nov., sp. nov., a new phreatoicidean isopod (Crustacea, Hypsimetopidae) from hypogean aquatic habitats in Andhra Pradesh, India. Zootaxa, 2869: 37–53. [https://doi.org/10.11646/zootaxa.2869.1.2]

Wilson, G.D.F., Shaik, S. & Ranga Reddy, Y. 2015. A new species of Andhracoides Wilson and Ranga Reddy, 2011 (Isopoda: Hypsimetopidae) from Belum Cave, Andhra Pradesh, India, with a phylogenetic review of the family. Journal of Crustacean Biology, 35: 216-240. [https://doi.org/10.1163/1937240X-00002333]

Wilson, G.D.F. & Shaik, S. 2022. Evidence for Early Mesozoic diversification of Hypsimetopidae Nicholls, 1943 (Isopoda), with the description of a new genus from Andhra Pradesh and notes on threats to Indian cave environments. Journal of Crustacean Biology, 42: 1-24. [https://doi.org/10.1093/jcbiol/ruac052]

Tiwari, K.K. 1955. Nichollsidae, a new family of Phreatoicoidea (Crustacea: Isopoda). Records of the Indian Museum (Calcutta), 53: 293-295.

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