Freshwater Sponges of the Order Spongillida have a long fossil record, with isolated spicules known from the Permo-Carboniferous of the Massif Central of France and the Saar-Nahe Basin of southwest Germany, and the oldest identifiable species, Spongilla purbeckensis, coming from the Jurassic of England. Despite this, fossils of Freshwater Sponges are extremely rare, due to the fragile nature of their siliceous skeletons. Most fossils assignable to the Spongillida are simple spicules, which cannot be placed within a family, genus, or species, with more detailed classification requiring the discovery of gemmuloscleres, spicules which produce gemmules, an asexual reproductive stage. The oldest known fossil gemmule attributable to the Spongillida is Palaeospongilla chubutensis from the Lower Cretaceous of Patagonia.
Spicules of Freshwater Sponges have been recorded from several sites within the Cretaceous-Palaeocene Deccan Traps deposits of India, although most of these have been impossible to identify, although in 2021 a new species of Freshwater Sponge, Palaeocorvospongilla cretacea, was described from the Terminal Cretaceous Deccan intertrappean lacustrine deposits of the Malwa Group of Madhya Pradesh.
In a paper published in the journal Acta Palaeontologica Polonica on 8 March 2023, Bandana Samant of the Department of Geology at Rashtrasant Tukadoji Maharaj Nagpur University, Roberto Pronzato of the Dipartimento di Scienze della Terra, dell’Ambiente e della Vita at the Università di Genova, Dhananjay Mahendrakumar Mohabey of the Geological Survey of India, Tiziana Cubeddu and Giacinta Angela Stocchino of the Dipartimento di Medicina Veterinaria at the Università di Sassari, Krutika Jangale, Pranay Thalal, and Anup Dhobale, also of the Department of Geology at Rashtrasant Tukadoji Maharaj Nagpur University, and Renata Manconi, also of the Dipartimento di Medicina Veterinaria at the Università di Sassari, describe a new species of Freshwater Sponge from the Deccan infratrappean deposits of Naskal in Telangana State, India.
The Deccan Traps are a series of Late Cretaceous-Early Palaeocene volcanogenic sediments covering an area of 500 000 km² across southern, western, and central India. As well as the numerous lava flows of these deposits, the sequence also includes infratrappean and intertrappean sediments (that is to say sediments covered over by lava flows, or between lava flows), which provide a fossil record of many organisms across the Cretaceous-Palaeocene boundary. The Naskal site is located in the Ranga Reddy District of Telangana State, in south-central India. This sequence is less than 3 m deep, and has a lateral extent of less than 15 m, and exposes sediments laid down between flows 3 and 4 of the Deccan Traps sequence, which have been dated to between 66.136 and 66.056 million years before the present, slightly below the Cretaceous-Palaeocene Boundary, at 66.043 million years ago.
Within the total Naskal exposure, Sponge spicules were found only at one site, Naskal GSI Quarry, within a portion of the exposure 120 mm thick. The spicule-yielding section includes exposures of (bottom-to-top) black to grey cherty limestone, hard yellowish shaly mudstone, loose shaly to carbonate mudstone to marlstone, white mudstone, and dark clay with sandy lenses. No spicules were recovered from the Naskal B exposure, which is only 7 m from the Naskal GSI Quarry exposure.
The Naskal exposure is noted for its Mammal fauna, but also includes Fish, Anurans, Squamates, a Sphenodontian, Turtles, and Crocodilians. Pollen recovered from Naskal contain a mixture of forms which are known in both Cretaceous and Palaeocene deposits, as well as forms that are otherwise exclusively Cretaceous or exclusively Palaeocene.
Samples were obtained by acid-washing bulk sediment samples that sieving the remnants, and examining the filtered samples under light and scanning electron microscopes.
The specimens are placed in a new genus and genus, and given the name Longibirotula antiqua, where 'Longibirotula' refers to the long shaft of the birotules (a type of spicule with wheel-shaped ends), assumed to be gemmuloscleres, which would have produced gemmules, which in turn would have acted as a resting stage for these Sponges, and 'antiqua' means 'old'. The distinctive birotules of this species and long and slender, reaching 47-76 μm in length, straight or slightly curved, and have scattered spines, which very in their density and number.
Longibirotula antiqua also has two types of monaxial megascleres (large unbranching spicules), slim long, microspiny to smooth oxeas (needle shaped spicules) reaching 142-425 μm in length, and shorter, stouter acanthoxeas (spiny spicules) measuring 71-105 μm in length.
The spicules of the Sponge are associated with diatoms of the genus Aulacoseira, which implies a eutrophic (nutrient rich) environment, possibly caused by volcanic material entering the lake. The presence of gemmuloscleres birotules suggests that the lake was only sporadically a suitable environment for the Sponges, possibly due to seasonal variations in water level. Gemmulation, timed to match seasonal conditions, is considered to be a key evolutionary strategy for Freshwater Sponges, the the gemmules forming asexual propagative agents which can survive periods of adverse conditions and disperse to colonize new environments.
Gemmules (often used as the diagnostic feature for Freshwater Sponge species) for Longibirotula antiqua have not been found in the Naskal deposits, but the distinctive combination of long slim oxeas, shorter acanthoxeas and long gemmuloscleres birotules, and an absence of microscleres (a smaller class of Sponge spicules), leads Samant et al. to conclude that the material is sufficiently unique to be described as a new species.
Fossil Freshwater Sponges of the Eocene-Miocene genus Ephydatia have gemmuloscleres birotules, as do members of the modern genera Anheteromeyenia, Corvoheteromeyenia, Corvomeyenia, Heteromeyenia, Racekiela, and Umborotula. The birotules of Longibirotula antiqua most closely resemble those of Ephydatia and Heteromeyenia, giving Samant et al. confidence that these are in fact gemmuloscleres.
The smooth to microspiny long oxeas of Longibirotula antiqua resemble those of the extant genus Heterorotula. The short acanthoxeas are less similar, but still close enough that Samant et al. suspect that like Heterorotula, Longibirotula antiqua may have used these shorter spicules to form a gemmular cage, protecting its resting stage. The genus Heterorotula has a broadly Gondwanan distribution, with living species known from Australia, New Zealand, New Caledonia, and the Americas, as well as a fossil record in subequatorial Brazil (although it is absent from the Afrotropical Region), which likely suggests that Longibirotula antiqua was derived from Freshwater Sponges from India rather than Eurasia. The similarity of Longibirotula antiqua to modern members of the group underlines the structurally conservative nature of Freshwater Sponges, as well as the success of a morphology and lifestyle which has apparently been able to persist with little change for tens of millions of years.
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