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

publication ID	https://doi.org/ 10.1111/j.1096-3642.2008.00412.x
DOI	https://doi.org/10.5281/zenodo.5815734
persistent identifier	https://treatment.plazi.org/id/0391904F-FFFB-FFA2-F693-FABF3E33F9C5
treatment provided by	Carolina
scientific name	Prochydorus
status

Treatment

PROCHYDORUS ROTUNDUS SMIRNOV, 1992

( FIGS 1A, View Figure 1 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 View Materials /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 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 View Figure 2 , 3B View Figure 3 : maj) located on valve margin ( Figs 2G View Figure 2 , 3B View Figure 3 : vam), at a distance from point where margins of head and valve come together ( Figs 2G View Figure 2 , 3B View Figure 3 : phv), instead of being attached to head shield ( Figs 2G View Figure 2 , 3B View Figure 3 : 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).

SPECIMEN 4307/2031 ( Figs 1C View Figure 1 , 3D–F)

A specimen ( PIN 4307/2031) from Khotont possessed unusual features. Unfortunately, its postero-dorsal half is not preserved. The animal, about 2.1 mm in length, has a high body and head of Prochydorus - type, with type of mandibular joint unknown. Each left and right antenna II has formula (?)–(1)–(1)–(1)–(1)–(1)– (?)–(?)/(1)–(1)–(3). Most probably, it is an atypical specimen of Prochydorus , with the large proximal segment of antennal exopod subdivided into six small ‘segments’, each with a single seta. The structure of antenna II may be an atypical recapitulation to an ancestral ‘conchostracan-like’ state. If the specimen is not atypical, it belongs to a new, undescribed order of the branchiopods instead of Prochydorus .