Palaeorak, 2009

Kotov, Alexey A., 2009, A revision of the extinct Mesozoic family Prochydoridae Smirnov, 1992 (Crustacea: Cladocera) with a discussion of its phylogenetic position, Zoological Journal of the Linnean Society 155 (2), pp. 253-265 : 259

PALAEORAK SCHERBAKOVI SP. NOV.

( FIGS 1G–J , 5A–H)

Type locality: Khasurty. Outcrops at Khasurty River, 10 km south of Tsakir, Zakamensk District, Buryat Autonomous Republic, Asian Russia. Based on palaeofauna analysis, the Mesozoic water body represented by these sediments was reconstructed as a small mountain lake surrounded by a forest composed of different plants (D. E. Scherbakov, pers. comm.).

Age: Glushkov Formation, probably, earliest Cretaceous. This recently discovered locality is poorly studied. Among fossil insects, characteristic Cretaceous taxa are more prevalent than Jurassic taxa (D. E. Scherbakov, pers. comm.).

Etymology: The species is dedicated to Dr D. E. Scherbakov ( PIN), who found micro-crustacean impressions on rock fragments from Khasurty.

Holotype: An impression of a specimen in lateral position, 2.5 mm in length, in the collection of the Palaeontological Institute of Russian Academy of Sciences, PIN 5026 /178.

Paratypes: 97 relatively well-preserved specimens on 22 rock fragments, PIN 5026 /89-177, 179-186 (each number is the access number for each specimen, rather than each rock fragment) .

Description: Body subovoid in lateral view, moderately high (height/length = 0.71–0.79), highest point of dorsum greatly prominent under head level, postero-dorsal angle well defined, postero-ventral angle widely rounded. Head about one-third body length, covered with a ‘flat’ head shield, which ends proximally in a blunt rostrum. In some specimens there is a shallow depression at the posterior border of head, which may be a result of specimen deformation. The valve ventral margin is supplied with small denticles and short marginal setae (preserved only in some specimens). The postabdomen is supplied with a pair of elongated, slightly curved terminal claws, their incurved (dorsal) side armed with small denticles, but lacking a basal spine. Antenna I was not found. Antenna relatively long, about half body length or longer, its basal segment long, exopod 0.33–0.38 body length. Both exopod and endopods with proximal segments very long, longer than two following segments. Antennal formula (5–7)–(1)–(3)/(1)–(1)–(3). No spines were found on any segments. Mandibles relatively small, attached at body where head and valve join (instead of attached to the head shield in chydorids). Limbs are not distinct. Size 1.8–3.0 mm.

Comments: In contrast to Khotont, only one fossilized cladoceran, P. scherbakovi sp. nov., was found in Khasurty.

Figures

Figure 1. Drawings of Upper Mesozoic Prochydoridae based on SEM photographs. A, Prochydorus rotundus from Khotont, reconstruction based on a series of specimens. B, antenna II of holotype, PIN 4307/2040. C, specimen PIN 4307/2031 from Khotont, antenna II. D, Archeoxus mirabilis from Khotont, reconstruction based on a series of specimens. E, antenna II of specimen PIN 4307/2018. F, ‘Archeoxus’ vetrosus from Khotont, antenna II of holotype PIN 4307/2027. G, Palaeorak scherbakovi gen. nov., sp. nov. from Khasurty, holotype 5026/178. H, reconstruction of general view based on series of specimens. I, postabdominal claw, 5026/177. J, antenna II, 5026/179. Scale bars: 1 mm.