Fossil Isopods (notes of an accidental paleontologist)

When I was at the Australian Museum, paleontologist Greg Edgecombe was a good friend; we got on well because we shared a strong interest in systematics and phylogeny. One day, he walked into my office and said that we needed to work on a fossil isopod kept in the Museum's collection. I didn't know much about working on fossils but I was excited to compare ancient isopod species with modern ones to understand the phylogeny of the whole group. We revised Protamphisopus wianamattensis Chilton, 1918

and included it in my taxonomic database (DELTA) for all phreatoicidean isopods. This allowed us to compare it to all extant phreatoicideans, which showed that it was not stem group but crown group Phreatoicidea (Wilson & Edgecombe, 2003). Although the precise position of Protamphisopus has changed with the addition of more data and taxa in the database, the general outlines of phylogenetic relationships of the phreatoicideans remained the same.

Subsequently, I collaborated with Chinese scientists on another species of this genus (Fu et al., 2010).


Fred Schram, a crustacean paleontologist, heard that I was working on isopod fossils and asked me to describe a strange sphaeromatoid that Daniele Guinot sent to him (Guinot et al., 2005). Since that fossil was strongly three dimensional, I experimented with stereoscopic microimaging.

Greg later dumped a large rock on my bench sent to him by other two Australian paleontologists that was astoundingly covered with many cirolanoid* isopods. Several months chipping away at the rock to expose the fossils resulted in another publication (Wilson, Paterson & Kear 2011). This work exposed the mess that continues in Mesozoic cirolanoid* taxonomy. Every new species often gets a new genus name, which is not rational: monotypic genera tell you nothing about relationships and do not improve the systematics of a group. Additionally, using modern generic names for earlier fossil taxa is also not rational owing to the incompleteness of fossil information. No consensus seems to exist on the correct approach for naming new taxa in the face of incomplete information (aka fossils). I based the genus name of this species on the abundance of characters shared with another Mesozoic species in Brunnagea Polz, 2005. This was subsequently dismissed by later publications, one of which only considered an existing diagnosis and not all available information. Similarly, naming fossil taxa after modern taxa (e.g., Cirolana) suffers from incomplete fossil information, given that modern taxa are based on features that are rarely preserved in fossils. Clearly, Mesozoic cirolanoid* taxonomy needs a complete revision using phylogeny as the guide, but will have to wait until I finish with phreatoicideans and some important asellotan groups. Afterwards, the abundance of monotypic genera created in modern isopod groups needs careful inspection from a phylogenetic viewpoint.


Tae-Yoon Park spent time with me on an Australian postdoctoral fellowship to describe a species of Archaeoniscus (Park et al, 2011). We will continue to work on the higher relationships of this odd but often abundant genus.


My exposure to fossil isopods prompted other scientists to send me much new material, which was surprizing because I once thought the fossil record for isopods was extremly poor. People must have begun to realize that the strange crustaceans in their collections were actually isopods. Nicholas Morel from Le Mans France sent me 5 fossil species that have been in his collection from the Cenomanian stratotype since the 1800s (Wilson & Morel, 2016 conference poster). I'm still working these descriptions.

In 2016, Paul Selden sent me a strange isopod that turned out to be the first published fossil of the suborder Asellota from the Upper Triassic, now published (Selden &Wilson, et al. (2016). The Morel collection has an Asellotan (Stenetriidae) and, earlier, Italian scientist Paolo Schirolli sent me images of a strange little asellotan fossil - I still hope to work on that one directly.


Now, I have a backlog of fossils to study, even though my work is primarily neontology, anatomy and my interest in isopod phylogeny. (See my recent paper on Eophreatoicus.)


*End note: I use the term "cirolanoid" here and elsewhere because the evidence suggests that cirolanoid taxa are not closely related to the Cymothoida. More on this later!

Bibliography


Fu, W.-L., Wilson, G.D.F., Jiang, D.-Y., Sun, Y.-L., Hao, W.-C. & Sun, Z.-Y. (2010) A New Species of Protamphisopus Nicholls (Crustacea, Isopoda, Phreatoicidea) from Middle Triassic Luoping Fauna of Yunnan Province, China. Journal of Paleontology, 84, 1003-1013. https://doi.org/10.1666/10-042.1


Guinot, D., Wilson, G.D.F. & Schram, F.R. (2005) Jurassic isopod (Malacostraca: Peracarida) from Ranville, Normandy, France. Journal of Paleontology, 79, 954-960. https://doi.org/10.1666/0022-3360(2005)079[0954:JIMPFR]2.0.CO;2


Hegna, T.A., Ahyong, S.T. & Wilson, G.D.F. (2016) Squilloides Tchindonova and Tchesunov, 1993 is a spelaeogriphacean, not an isopod. Journal of Crustacean Biology, 36, 860-862. https://doi.org/10.1163/1937240X-00002484

Park, T.-Y., Wilson, G.D.F., Lee, D.-C. & Choi, D.K. (2012) Occurrence of the Isopod Archaeoniscus coreaensis New Species from the Lower Cretaceous Jinju Formation, Korea. Journal of Paleontology, 86, 626-640. https://doi.org/10.1666/11-131R.1

Selden, Paul A., Wilson, George D.F., Simonetto, L. & Dalla Vecchia, Fabio M. (2016) First fossil asellote (Isopoda: Asellota), from the Upper Triassic (Norian) of the Carnic Prealps (Friuli, northeastern Italy). Journal of Crustacean Biology, 36, 68-86. https://doi.org/10.1163/1937240X-00002387

Wilson, G.D.F. & Edgecombe, G.D. (2003) The Triassic isopod Protamphisopus wianamattensis (Chilton) and comparison with extant taxa (Crustacea, Phreatoicidea). Journal of Paleontology, 77, 454-470. https://doi.org/10.1666/0022-3360(2003)077<0454:TTIPWC>2.0.CO;2

Wilson, G.D.F. (2012) Fossils In the phylogeny of Isopod crustaceans (Arthropoda), p. 478. In: The Origins of Arthropod Diversity: Phylogenetic Insights from the Living and the Dead. Geological Society of America Annual Meeting. Vol. 44, no. 7; paper no. 200-10. Hegna, T.A. & Wolfe, J.A. (eds.). Geological Society of America, Charleston, N.C., USA.

Wilson, G.D.F. & Morel, N. (2016) Isopod crustacean fossils from the Cenomanian stratotype: five new species in suborders Cymothoida, Asellota and Valvifera. In: Colloque Cénomanien. Morel, N., Desmares, D. & Neraudeau, D. (eds.). Musée Vert, Muséum d'Histoire Naturelle du Mans, France, Le Mans, France.

Wilson, G.D.F., Paterson, J.R. & Kear, B.P. (2011) Fossil isopods associated with a fish skeleton from the lower Cretaceous of Queensland, Australia - direct evidence of a scavenging lifestyle in Mesozoic Cymothoida. Palaeontology, 54, 1053-1068. https://doi.org/10.1111/j.1475-4983.2011.01095.x

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