Ghost Shrimps (Callianassidae) are Decapod Crustaceans noted for the large, complex burrows they build. They excavate their burrow systems in intertidal or shallow subtidal waters in the tropics and subtropics, which tend to be high energy environments. Because of this intact Ghost Shrimps are rare in the fossil record, where typically only the tough claws of the animals are preserved, making it difficult for palaeontologists to use the classification system used for Ghost Shrimps used by marine biologists; instead two parallel classification systems have sprung up, one based on soft tissue and used by biologists and one based on claws and used by palaeontologists. The method used by palaeontologists is known to be a poor reflection of actual biology, with many species placed in a catch-all genus Callianassa.
In a paper published in the journal Zootaxa on 19 June 2014, Matúš Hyžný of the Geological-Paleontological Department at the Natural History Museum in Vienna and the Department of Geology and Palaeontology at Comenius University, and Rok Gašparič of Ljubljanska in Slovenia examine a series of specimens of Ghost Shrimp claws from the Miocene and Oligocene of Central Europe and reconsider their taxonomic assignment.
Hyžný and Gašparič studied material from the Oligocene and Miocene of Italy, Germany, Austria, Slovenia, Slovakia and Hungary assigned to the species Callianassa michelotti, and conclude that this can be placed in the extant genus Calliax. They therefore re-assign all the specimens to Calliax michelotti, a new combination.
The first specimen of Callianassa michelotti as described by Alphonse Milne Edwards in1860. (A–B) refigured from Milne Edwards (1860); (C–D) photo of the holotype. (A) and (C) inner surface; (B) and (D) outer surface. Arrows indicate ridges. Photo by Lilian Cazes. Hyžný & Gašparič (2014).
Calliax michelottii from the Alpine-Carpathian orogenic system. (A–B) Isolated propodi from Jarenina (Slovenia); (C) right propodus articulated with dactylus from Pucking (Austria); (D–H) fragmentary right propodus from Rohožník (Slovakia), note large setal pores on the outer lateral surface (E), arrows (in F) indicate ridges; (I) counterpart of the major left cheliped from Neuhofen bei Tettenweis (Germany), arrows indicate ridges; (J–K) from Szob (Hungary); (L) left major propodus from Želiezovce (Slovakia). Scale bar equals 5 mm. Hyžný & Gašparič (2014).
Hyžný and Gašparič also examined material assigned to the species Callianassa szobensis from the Miocene of Hungary, and conclude that this is the same species as Calliax michelotti.
Hyžný and Gašparič also note that while most Ghost Shrimps are shallow water inhabitants, members of the genus Calliax have been found living around cold seeps in the Straight of Sicily at depths of about 800 m, in a community of chemosymbiotic organisms (organisms who survive through symbiotic relationships with bacteria that obtain energy through processing chemicals from the cold seeps, or at least feed on other animals which do this, rather than being dependent on the consumption of photosynthetic organisms). They further note that the deposits at Rohožník in Slovakia, which have yielded specimens of Calliax michelotti, along with other Crustaeans, Fish and Molluscs, appear to be from similar cold seeps, and that some other Calliax michelotti producing deposits are also thought to be from deeper waters. From this they conclude that it would be unsafe to use Calliax michelotti as an indicator of a shallow-marine environment when studying fossil-producing sites, though they cannot rule out it having lived in shallow waters too, and do not go as far as recommending the species be treated as an indicator of deeper environments.
The oldest specimens currently assigned to the genus Calliax originate from the Palaeocene of Pakistan, suggesting that the genus has a Cretaceous origin within the (now vanished) Tethys Ocean. It is currently known in the Mediterranean and Atlantic only, though fossil specimens are known from the Miocene of Chile. The presence of the genus in central Europe in the Oligocene and Miocene would tend to support the Tethys origin of this species, suggesting a westward distribution through the Tethys into the (then narrower) Atlantic, then through the Caribbean Sea and into the eastern Pacific.
Biogeographical evolution of Calliax. Hyžný & Gašparič (2014).
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