Crayfish are rare in the fossil record and therefore it is important to investigate each occurrence in detail. The only known fossil crayfish from France, Astacus edwardsi Van Straelen, 1928, is known from a replica made by pouring plaster of Paris inside the holotype (subsequently destroyed), an external mould extracted from a travertine cavity from the Thanetian of Sézanne. An evaluation of the taxonomic name, A. edwardsi, is provided; A. edwardsi is considered valid in accordance with ICZN rulings. It possesses atypical features for all other astacid genera, thus Emplastron gen. nov. is erected. Emplastron edwardsi gen. et comb. nov. inhabited a warm climate with calm waters, abundant food sources, and an ample supply of calcium carbonate: so much so that it is surprising that it is the only recovered specimen. Despite apparent North American faunal and floral affinities in the vicinity, E. edwardsi is more closely related to European crayfishes than it is to American ones.
Introduction
Astacoidea Latreille, 1802 and Parastacoidea Huxley, 1879 (crayfishes) are diverse superfamilies of decapod crustaceans that have played a central role in biology for over 130 years since being proposed as a model organism (Huxley, 1880). Crayfishes likely diverged from marine lobsters (Nephropoidea Dana, 1852) during the Permian or Triassic, resulting in their radiation and dispersal before the breakup of Pangaea (Schram and Dixon, 2003; Porter et al., 2005; Crandall and Buhay, 2008). Since they are almost entirely restricted to freshwater environments and devoid of a planktic larval stage, their dispersal potential is poor compared to that of marine lobsters. Therefore, they are good palaeogeographical indicators (Pârvulescu, 2019).
Of the two superfamilies, Astacoidea inhabits the Northern Hemisphere and Parastacoidea inhabits the Southern Hemisphere. Astacoidea contains four families (Crandall and De Grave, 2017): the extant Astacidae Latreille, 1802, Cambaridae Hobbs, 1942, Cambaroididae Villalobos, 1955 and the extinct Cricoidoscelosidae Taylor et al., 1999. The biogeography of extant astacoid families is puzzling (see Dǔriš and Petrusek, 2015): Pacifastacus Bott, 1950 from western North America and Astacidae from Europe are united by their mutual lack of female spermatheca (annulus ventralis) and male coxal hooks, but they are separated geographically by the Cambaridae of eastern North America. Their relationships are also unclear: according to Breinholt et al. (2009), astacids and cambarids are more closely related to each other than they are to Pacifastacus; according to Bracken et al. (2009), conversely, astacids are more closely related to Pacifastacus than they are to cambarids.
Fossil data would help resolve these relationships, but fossil crayfishes are rare (Bell et al., 2020). Herein we revise the only known fossil of Astacus edwardsi Van Straelen, 1928 from the upper Paleocene freshwater travertine deposits of Sézanne, France. Even at the time of its original description, its generic assignment was in doubt. It had been reported that contemporary continental European and North American faunas bore strong affinities (Dollo, 1923), which Van Straelen (1928) corroborated. We reinvestigate A. edwardsi, in light of recent progress in freshwater crayfish systematics (notably the understanding that there are three astacid genera in Europe), and investigate whether this species might be related to North American genera, as suggested by Van Straelen (1928).
We demonstrate that Emplastron edwardsi gen. et comb. nov. is sufficiently dissimilar to all astacid genera to warrant the erection of Emplastron gen. nov., which seems to be more closely related to Astacus Fabricius, 1775, Austropotamobius Skorikov, 1907 and Pontastacus Bott, 1950 than it is to any other genus, placing it confidently within European astacids.
Geological setting
The holotype of Emplastron edwardsi gen. et comb. nov. was recovered by Ernest Munier-Chalmas in 1872 (see Vélain, 1889) from travertine beds situated in Sézanne (Marne department, France; see Van Straelen, 1928). These travertines were deposited in a river flowing in carbonate landscapes (Thanetian age: Van Straelen, 1928; Gingerich, 2000). This period was warmer than the current climate with evergreen broad-leaf forests present in the area and temperate conditions extending up to high latitudes (Mai, 1991; Scotese, 2000; Collomb et al., 2008). The simoedosaurid choristoderan Simoedosaurus lemoinei Gervais 1877, representatives of other crustaceans, i.e., isopods (e.g., Milne-Edwards, 1866), insects (see Nel and Blot, 1990), molluscs, and plants indicative of a hot, humid, and probably tropical climate (Saporta, 1868; Langeron, 1899; Lapparent, 1964; Pentecost, 2005), coexisted.
Travertines are chemical deposits of calcium carbonate as calcite or aragonite that can occur in various bodies of waters (seepage, streams, rivers, and springs) (Pentecost, 2005). In the case of Sézanne, as reported by Ernest Munier Chalmas (in Velain, 1889), both springs and the river deposited the travertines (Velain, 1889). The abundance of calcium carbonate in the water of the river and springs may well have come from dissolution of the surrounding chalk. The river bend was probably a meandering one, as banks do not display similar deposits: the northern bank is characterized by a bottom of pebbles, a sign of a fast flowing water; the southern bank is characterized by the deposition of travertines (Velain, 1889), a sign of slower flow. Ernest Munier-Chalmas apparently recognized these deposits extending for over 2 km (see also the map in Lapparent, 1964) and supposed the river to end in the Rilly lake, as the deposits of this lake contain a similar fauna (Velain, 1889). Note that the eponymous locality (Rilly-la-Montagne) of the Rilly lake is situated about 50 km north of Sézanne, and that Velain (1889) indicated that further away the lake transitioned to the ‘sea of sands’, an epicontinental sea.
Material and methods
The holotype of Astacus edwardsi Van Straelen, 1928 (SU.Pal.2017.2.60) was an external mould, which Munier-Chalmas infilled with plaster of Paris (axiotype sensu Lucas and Harris, 2020) before he dissolved the surrounding limestone with hydrochloric acid to make a replica of the original morphology of the crayfish (Vélain, 1889). This axiotype is housed at Sorbonne University (SU).
The specimen was imaged with a digital single-lens reflex camera equipped with a 105 mm macro lens. In capturing photographic stills of the specimen, crosspolarised light was employed to avoid reflection of light on the surface of the specimen; images were combined using image stacking software to obtain a satisfactory depth of field (Bengtson, 2000; Haug et al., 2011; Kerp and Bomfleur, 2011). Some of the images were variously coloured prior to the stacking process to produce micro-topographical maps (see Sabroux et al., 2019). The three-dimensional model was a product of 75 photographs that had been captured in natural, non-polarised light and combined with Agisoft LLC Agisoft PhotoScan. The model was then processed in MeshLab (Cignoni et al., 2008), the open source system for processing and editing three-dimensional triangular meshes. Measurements were made on digital photographs using the image processing software ImageJ (Schneider et al., 2012).
The terminology developed by Van Straelen (1925) and Tshudy and Sorhannus (2003) is followed wherever possible. The term cephalothoracic shield, and not carapace, is applied herein.
Systematic palaeontology
Order Decapoda Latreille, 1802
Infraorder Astacida Scholtz and Richter, 1995
Superfamily Astacoidea Latreille, 1802
Family Astacidae Latreille, 1802
Genus Emplastron gen. nov.
Type species.—Astacus edwardsi Van Straelen, 1928.
Diagnosis.—Cephalothoracic shield with well-developed anterior and posterior postorbital carinae; epistome without spine or ridge posterior to urinary orifice; wide rostrum flanked by well-marked lateral carinae without spines; subdorsal carinae extending on the shield (as far as it is possible to observe); postrostral carina raised above the cephalic area; postorbital carinae subdivided into anterior and posterior postorbital carinae; tergopleurae of pleonites 2-5 rounded.
Occurrence.—Travertine from Sézanne (Marne department, circa 100 km east of Paris, France).
Etymology.—(Gr. Emplastron), n. plaster-of-Paris.
Remarks.—Emplastron edwardsi gen. et comb. nov. was described based upon a single specimen, which had been recovered 56 years prior to the work of Van Straelen (1928) (see Munier-Chalmas, 1872). The study of this specimen is complicated due to the incomplete preservation of the axiotype. For instance, some structures that are invaluable for systematic placement (Hobbs, 1974) are not visible or preserved: the first maxilliped, the carpal hook on the pereiopods, the first and second pleopods, and spermatheca. Some of these structures may have been lost to sparmicritisation, destructive activities of microorganisms etching sparry carbonate rocks, e.g. in travertine, by cyanobacteria (see Bathurst, 1976; Kahle, 1977; Pentecost, 1978, 1992; Chafetz et al., 1994). Furthermore, it is probable that some of these missing structures may be due to the casting process: the plaster of Paris might not have reached delicate structures, e.g. appendages. Further details may also have been obscured due over the years to abrasion of the plaster. Thus, Van Straelen (1928), considering the anatomical preservation, or lack thereof, was reticent to assign it to a genus and alluded to no characters in its generic assignment.
Nevertheless, Emplastron gen. nov. is ascribed to Astacoidea because it has a telson divided by a transverse suture (see Hobbs, 1974, p. 5). However, it would be a weakly substantiated assignment to rely on one distinguishing character, hence our decision to compare the specimen to all genera of Astacoidea and Parastacoidea.
Van Straelen (1928) considered the axiotype to have too few diagnostic characters preserved for a precise generic assignment. Therefore, he considered it to be an astacoid a priori based on its palaeogeographic considerations (a supposition, however logical, but without empirical evidence). Furthermore, of the three genera restricted to Eurasia and North America that had been described prior to his 1928 publication (Astacus, Cambarus Erichson, 1846 and Cambaroides Faxon, 1884), he considered his new species to be Astacus, again a priori and further to his original assumption, the modern-day distribution of which overlaps the locality where Emplastron gen. nov. had been recovered. Conversely, Van Straelen (1928) noted in addition that the continental fauna of the area at the time had North American affinities and that it might, consequently, belong to a North American genus. Freshwater crayfish taxonomy has, since then, been subject to splitting of both Astacus and Cambarus, which significantly complicates the assignment of Emplastron edwardsi gen. et comb. nov.
Since its first description, Emplastron edwardsi gen. et comb. nov. has never been studied in detail. Our revision of the axiotype of Emplastron edwardsi gen. et comb. nov. and comparison with all genera led to a reconsideration of its generic assignment. Most characters listed by Van Straelen (