Palaeorak, Kotov, 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 GEN. NOV.

Type (sole) species: Palaeorak scherbakovi sp. nov.

Etymology: The name of the new genus comes from Greek Palaeo (ancient), and Rak , the generic name of a chydorid with blunt rostrum and well-developed setae on the ventral margin of the valve, described by Smirnov & Timms (1983). The name Rak is derived from the Russian word (rak) for a crustacean.

Diagnosis: Body moderately high, head capsule of ‘chydorid-type’ (compressed dorso-ventrally), covered with ‘flat’ head shield with rostrum truncated, valves with denticles and short setae along ventral margin of valves, mandible attached to point of joint of head shield and valve.

Comments: The genus is probably a few million years younger than Prochydorus and Archeoxus .

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.