A new genus and species of cymothoidan isopod that lived during the Early Cretaceous epoch has been identified from two well-preserved specimens found in Lebanon. Stemming from a freshwater lake environment, this isopod provides an unconventional perspective on the evolutionary origin of living cave- and groundwater-dwelling cymothoidans.
Paleoenvironmental habitat reconstruction for Dysopodus gezei (foreground): a Barremian freshwater lake in the region of present-day Bkassine, Lebanon. Image credit: Aldrich Hezekiah.
Dysopodus gezei lived in shallow freshwater lakes in what is now Lebanon around 125 million years ago (Early Cretaceous epoch).
This creature had an elongated body, slightly more than twice as long as wide (total length is between 1.8 and 2.5 cm).
It was type of isopod, an order of crustaceans that includes both aquatic and land-dwelling species.
“Isopoda is a diverse group of malacostracan crustaceans that comprises more than 10,000 described living species,” said Dr. Mario Schädel, a paleontologist with the Senckenberg Centre for Human Evolution and Palaeoenvironment at the University of Tübingen, and his colleagues.
“Most living species are found in various marine environments ranging from the deep sea to sandy beaches and rocky shores.”
“Isopoda can be considered a primarily marine group, with the most recent common ancestor of all isopods most likely being marine.”
“However, there are also many isopod species that live outside of the marine realm.”
“A species-rich ingroup of Isopoda — Oniscidea — houses more than 3,800 species, of which most live in various fully terrestrial habitats.”
“Apart from marine and terrestrial environments, isopods also inhabit brackish and freshwater environments, with around 1,000 described species living in freshwater.”
“Isopods have colonized freshwater habitats multiple independent times, resulting in a variety of distinct species being present in freshwater habitats, ranging from old and, in some cases, very species-rich groups to single phylogenetically isolated species.”
Dysopodus gezei bore a strong resemblance to living non-parasitic lineages of the Cirolanidae, a group of isopods within the suborder Cymothoida.
“Cymothoida is a group of isopods that includes scavengers, predators, micropredators and parasites,” the paleontologists said.
“Among those, micropredatory and parasitic species likely form a natural group.”
“Within Cymothoida, many freshwater species are parasites that likely entered freshwater habitats along with their hosts, which are either fishes or crustaceans.”
“Among representatives of Cymothoida (cymothoidans — not to be confused with cymothoids), there are many that are neither micro-predators nor parasites throughout their life.”
“These are often referred to as Cirolanidae, a group of morphologically distinct isopods.”
Dysopodus gezei, holotype. Image credit: Schädel et al., doi: 10.1098/rsos.241512.
The two specimens of Dysopodus gezei were excavated in the years 2003 and 2023 in Lebanon.
“The specimens were found in the Lebanese dysodiles of Jdeidet Bkassine,” the researchers said.
“These layers correspond to finely laminated organic-rich sediments cropping out at five mining localities within the Grès du Liban Alloformation: one in the north of Lebanon, one in the center and three localities in the south of Lebanon, among which is the Jdeidet Bkassine outcrop.”
“All evidence points to a series of small, shallow lakes and/or swamps located in proximity to volcanic edifices.”
The scarcity of living cirolanid freshwater species highlights the importance of the discovery of Dysopodus gezei as an extinct species with support for a true freshwater paleohabitat, not only for the evolutionary history of above-ground but also below-ground freshwater species.
“The discoveries of the new fossils represent a rare find of fossil isopods from a freshwater habitat,” the scientists said.
“This puts the origin of extant non-parasitic freshwater cymothoidans into a new perspective.”
“While this find does not disprove the colonization of cave and groundwater habitats through a disconnection of subterranean species by a regressing coastline, the presence of freshwater cymothoidans in the eastern Tethyan region during the Early Cretaceous shines a different light on the origin of the living freshwater fauna.”
“With the potential to preserve fine morphological details, additional specimens of this species could provide more morphological details that then could be used to draw more precise conclusions about the relationship of the Cretaceous freshwater species with the extant cave and groundwater fauna.”
The findings were published in the April 2025 issue of the journal Royal Society Open Science.
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Mario Schädel et al. 2025. A 125 million-year-old freshwater isopod shines new light on the origin of subterranean freshwater species. R. Soc. Open Sci 12 (4): 241512; doi: 10.1098/rsos.241512
