Prochydorus rotundus, SMIRNOV, 1992

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 : 254-257

PROCHYDORUS ROTUNDUS SMIRNOV, 1992 (FIGS 1A,

B, 2A–H, 3A–C)

Smirnov, 1992a: 112, figs 4E, 8C–E, 9E, F, 10, 12E, 13; Dumont & Negrea, 2002: 226–227.

Type locality: Khotont. The northern part of Mount Ukra , 6 km west of Khotont Somon in the valley of the Orkhon, north-eastern Hangay, Arhangay Aimag, Mongolia. More than 1500 impressions of different insects were collected from this locality ( Rasnitsyn & Quicke, 2002) .

Age: The absolute age of this locality was not estimated, but its palaeofauna consists of both Jurassic and Cretaceous taxa, and the age is estimated as near the Jurassic–Cretaceous boundary ( Rasnitsyn & Quicke, 2002).

Holotype: PIN, 4307/2024 and 2040 (impression and counter-impression).

Paratypes: PIN 4307 /2019, 2020, 2025, 2038, 2039 (counter-impression of 19) .

The current condition of Smirnov’s (1992a) paratype 4307/2020 prevents accurate determination. Impression 4307/2017, also listed by the author as a paratype of P. rotundus , may belong to Archeoxus mirabilis .

Other material studied: PIN 4307/2002, 2007, 2013, 2016, 2032 (counter-impression of 13), 2041.

Diagnosis: As for the genus.

Description: Body very high (height/length = 0.83– 0.92), rounded in lateral view, dorsum regularly convex, postero-dorsal angle well defined, posteroventral portion broadly rounded. Head about 0.4¥ body

› 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.

view, postabdominal claws, antenna II and apical segment of its exopod. E, holotype PIN 4307 /2040 (counter-impression) .

F, G, paratype PIN 4307 /2019, general view and mandibles. H , paratype PIN 4307 /2020. Scale bars: A, F, H, 1 mm; B–E ,

G, 0.1 mm.

·

length, massive, head capsule of the ‘daphniid-type’, with head shield supplied with a distinct fornix, rostrum pointed. Valve ventral margin without denticles or setae. Postabdomen with claw supplied with minute denticles. Antenna II short. Exopod length 0.21–0.25¥ body length, its proximal segment large, longer than 2 + 3 segments. Endopod somewhat shorter than exopod, its proximal segment only somewhat longer than second segment. Antennal formula (5)–(1)–(3)/(1)–(1)–(3). A short spine on apical segment of exopod. Mandible small, mandibular joint ( Figs 2G, 3B: maj) located on valve margin ( Figs 2G, 3B: vam), at a distance from point where margins of head and valve come together ( Figs 2G, 3B: phv), instead of being attached to head shield ( Figs 2G, 3B: hsm). Length 2.0– 2.5 mm.

Comments: The animal may have been subglobular in shape, as are the extant chydorids Chydorus Rak and Indialona ( Smirnov & Timms, 1983; Smirnov, 1996; Kotov, 2000).