Leptodorosida zherikhini, Kotov, 2007

Kotov, Alexey A., 2007, Jurassic Cladocera (Crustacea, Branchiopoda) with a description of an extinct Mesozoic order, Journal of Natural History 41 (1 - 4), pp. 13-37 : 17-20

Leptodorosida zherikhini sp. nov.

( Figures 1 , 2 A–C)

Etymology

This species is dedicated to the outstanding palaeontologist, Professor V. V. Zherikhin, who published images of Ust’-Baley cladocerans as ‘‘crustaceans of unclear taxonomic status’’ (Zherikhin 1985, Plate 7), and participated greatly in the reconstruction of Jurassic palaeocenoses (Zherikhin 1985; Zherikhin and Kalugina 1985; Sinitshenkova and Zherikhin 1996). His untimely death in December 2001 was a great loss to Russian palaeontology.

Type locality

Ust’-Baley.

Material studied

Holotype: complete body in lateral position, with distinct head, valves and AII, PIN 1873 / 106 ( Figure 1A ) . Paratypes: AII, PIN 1873 /27; complete body with AII (0)-(7)-(?)-(?)/(0)- (6)-(4?)-(?), PIN 1873 /31; complete impression with AII (0)-(?)-(?)-(?)/(0)-(8)-(4)-(?), PIN 1873 /32; unclear impression with AII (0)-(7)-(4)-(?)/(0)-(8)-(?)-(?), PIN 1873 /50; AII (0)-(.6)-(4)-(8)/(?)-(6)-(5)-(5), PIN 1873 /57; AII (0)-(7)-(?)-(?)/(0)-(> 7)-(?)-(?), PIN 1873 /65; AII (0)-?/(0)-(.5)-(4)-(?), PIN 1873 /67; AII (0)-(.7)-(5)-(6)/(0)-(7)-(4)-(5), PIN 1873 /69; AII, PIN 1873 /70; AII (0)-(8)-(4)-(7)/(0)-(7)-(?)-(?), PIN 1873 /76; AII (0)- (7)-(5)-(6)/(0)-(7)-(?)-(?), PIN 1873 /78; complete body with AII (0)-(.5)-(5)-(?)/(0)-(8)- (?)-(?), PIN 1873 /79; AII (0)-(6)-(5)-(?)/(0)-(7)-(4)-(.3), PIN 1873 /80; complete body with distinct head, mandibles and unclear AII, PIN 1873 /81; complete body with distinct head, mandibles and unclear AII, PIN 1873 /82; AII (0)-(.5)-(4)-(6)/(0)-(.6)-(4)-(?), PIN 1873 /83; unclear body contour with mandibles, AII (0)-(?)-(?)-(?)/(0)-(.6)-(?)-(?), PIN 1873 /85; head, unclear valves, AII (0)-(6)-(.5)-(?)/(0)-(.4)-(4)-(.3), PIN 1873 /86; AII (0)-(7)-(6)-(.6)/(0)-(6)-(4)-(.3), PIN 1873 /87; AII (0)-(7)-(6)-(.8)/(0)-(8)-(4)-(5), PIN 1873 /88; complete body with distinct head and valves, PIN 1873 /089; complete body with unclear head and valves, PIN 1873 /90; complete body with distinct head, mandibles, AII (0)-(.5)-(6)-(?)/(0)-(> 6)-(?)-(?), PIN 1873 /91; complete body, 1873/94; complete body, PIN 1873 /95; AII, PIN 1873 /96; complete body, AII (0)-(.5)-(5)-(.4)/(0)-(?)-(4)- (?), AII (0)-(> 7)-(> 6)-(?)/(0)-(6)-(?)-(?), PIN 1873 /101; complete body, PIN 1873 /102; complete body with distinct head and traces of limbs, PIN 1873 /107; AII, PIN 1873 /110; AII, PIN 1873 /113; AII (0)-(> 8)-(> 4)-(.7)/(0)-(7)-(4)-(> 4), PIN 1873 /115; AII, mandibles and limbs, PIN 1873 /116; AII, PIN 1873 /117; AII (0)-(> 7)-(4)-(7)/(0)-(8)- (4)-(4), PIN 1873 /123.

Diagnosis

As for the genus.

Description

Body elongated (height/length 5 about 0.3–0.4), subovoid ( Figures 1A, B , 2A ), with slightly convex dorsal margin and distinct postero-dorsal angle, postero-ventral angle absent. Head massive, 0.3–0.4 body length, strongly chitinized, forming a head shield, with its distalmost extremity protruding anteriorly. Posterior portion of head slightly inflated (most probably, powerful mandibular muscles were attached here), posterior head margin as a shallow incision. A distinct fold (fornix) on lateral surface of head ( Figure 2A , arrow), covering coxal portion of AII. Valve firm, probably thick-walled, oval-elongated, without marginal setae.

Antenna I not found. AII strong and long, length from base to tip of exopod more than half of body length. Basal segment strong, thick. Both antennal branches long, thin, foursegmented ( Figures 1 C–F, 2B, C). Exopod longer than endopod. First segment of exopod of moderate size, lacking setae, second to fourth segments elongated, each with a series of setae, length of second segment less than that of third plus fourth segments. Endopod with long first segment, lacking setae, second segment large, with a series of setae, third and fourth segments small, each with a series of setae. Antennal formula (0)-(5–8)-(4–.6)-(6–.8)/(0)-(6–8)-(4–5)-(4–5). No spines were found on any segments and on the basal segment of AII. Mandibles asymmetrical, large, strongly chitinized, with molar surfaces of primitive, triturating type, lacking any large projections. Thoracic limbs were not distinct in the majority of specimens, probably hidden in most under strong valves, but two impressions of Leptodorosida were associated with uniform limbs of filtering type ( Figure 1G, H ), although their number and details remain unclear. Eggs were not found .

Size 1.8–2.2 mm. Apparently, specimens had strong cuticle and were weakly deformed.

Specimens with postabdominal claws were absent. Thin paired claws 0.5–0.8 mm long, lacking denticles, were found on many fragments without clear association with body parts of other specimens, but they are too large to belong to Leptodorosida .

Comments

In the Jurassic lake Ust’-Baley, L. zherikhini sp. nov. co-occurred with Smirnoυidaphnia smirnoυi sp. nov. (see below). But on rock fragments the former can be easily differentiated from the latter in having a strongly chitinized head and AII with numerous setae on the endopod.

Figures

Figure 1. SEM (A–E, G, H) and optical (F) micrographs of Leptodorosida zherikhini gen. nov., sp. nov., Ust’- Baley, Asian Russia. (A) Holotype, PIN 1873/106; (B) paratype, general view, PIN 1873/102; (C) antenna II, PIN 1873/101; (D, E) antenna II, PIN 1873/115; (F) antenna II, PIN 1873/123; (G) head and thoracic limbs, PIN 1873/107; (H) thoracic limbs, PIN 1873/116. Scale bars: 1 mm (A, B); 0.1 mm (C–H).

Figure 2. Mesozoic Cryptopoda ord. nov. (A) Leptodorosida zherikhini gen. nov., sp. nov. from Ust’-Baley, reconstruction based on several impressions; (B, C) antenna II, PIN 1873/115 and 1873/121; (D) Leposida ponomarenkoi gen. nov., sp. nov. from Chalunikha, general view of holotype, PIN 4042/60, see also Figure 3A; (E) reconstruction based on several impressions; (F) postabdomen, PIN 4042/65; (G–I) antenna II, PIN 4042/60 (holotype), 4042/61, and 4042/62; (J, K) Leposida ponomarenkoi gen. nov., sp. nov. from Unda, antenna II, PIN 3015/2415 and 3015/2421. Scale bars: 1 mm.