Pechoracaris, Dzik & Ivantsov & Deulin, 2004

Dzik, Jerzy, Ivantsov, Andrey Yu. & Deulin, Yuriy V., 2004, Oldest shrimp and associated phyllocarid from the Lower Devonian of northern Russia, Zoological Journal of the Linnean Society 142 (1), pp. 83-90 : 84-85

PECHORACARIS GEN. NOV.

Type species: A. aculicauda sp. nov.

Diagnosis: Hoplostracan with very long spine-like telson without furca, elongated carapace reaching fifth pleomere; pleopods transformed into spines.

Etymology: Derived from the Pechora River region, where the fossils were found, and Latin caris (shrimp).

Affinities: The most striking aspect of this arthropod is its single caudal spine ( Figs 3 , 5 ). In this respect it somewhat resembles the enigmatic ‘trilobitomorph’ Burgessia bella Walcott, 1912 from the famous Middle Cambrian Burgess Shale of British Columbia ( Hughes, 1975). Such affinity is unlikely, however, as the new Russian arthropod shows strongly sclerotized mandibles, which indicates its advanced crustacean affinities.

Probably the closest relative of Pechoracaris aculicauda is ‘ Elymocaris ’ urvantsevi Dunlop, 2002 from roughly coeval strata of the Severnaya Zemla archipelago. Although the presence of a medial dorsal plate and rostral plate is claimed in the original description ( Dunlop, 2002), the evidence for this seems rather weak. ‘ E. ’ urvantsevi shows a similar shape of the carapace to the new archaeostracan, covering all but the last three segments of the abdomen. Its spinose furca is normally developed, but is significantly shorter than the telson spine.

Among the archaeostracans, an elongated caudal spine and reduced furca are known in the Early Devonian Heroldina and Aristozoe, and in the Early Carboniferous Sairocaris . The giant Heroldina rhenana (Broili, 1928) from the Hunsrück Slate of Germany, reaching up to 60 cm in length, is different from the Russian crustacean in the presence of a large rostral plate and dorsal hinge of the carapace ( Bergström et al., 1989; Bartels et al., 1998). In its strongly elongated last abdominal segment, Heroldina resembles Aristozoe regina Barrande, 1972 from the Konĕprusy Limestone of Bohemia (Chlupac˘, 1963) and A. virga Chlupac ˘, 1970 from the earliest Devonian Lochkov Limestone. Another Bohemian aristozoid, Pygocaris schuberti Perner 1916 from the Lochkov Limestone, had a thin cuticle (Chlupac˘, 1963) but still does not show even a remote similarity to the Russian form. Archaeostracans with somewhat reduced furca, elongated medial spine and possibly lacking separate rostral plate are known from as far back in the geological past as the Middle Ordovician ( Hannibal & Feldmann, 1997).

The hoplostracan Sairocaris elongata (Peach, 1882) , that notably co-occurs with Anthracophausia in the Early Carboniferous Glencartholm Volcanic Beds of Scotland, has a very short carapace, exposing posterior thoracic segments ( Schram, 1979). If the Russian form is truly related to Sairocaris , a carapace reduction took place in the evolution of the lineage.

Figures

Figure 3. Archangeliphausia spinosa sp. nov. A, specimen with oblique dorsoventrally compressed carapace and well preserved scaphocerite of 2nd antenna (PIN 4983/2). B, specimens with partially preserved telson and uropods (PIN 4983/ 32a, b). C, specimens with partially preserved telson and uropods (PIN 4983/12a, b). D, posterior part of abdomen with well preserved 6th pleosomite (PIN 4983/4a). E, posterior part of abdomen with well preserved pleura (PIN 4983/39). F, dorsoventrally compressed 6th pleosomite and partial uropods (PIN 4983/36). G, specimen with well preserved spinose pleura (PIN 4983/31).

Figure 5. Restoration of (A) Pechoracaris aculicauda sp. nov. and (B) Archangeliphausia spinosa sp. nov.