Archelatona zherikhini, Kotov & Korovchinsky, 2006

Kotov, Alexey A. & Korovchinsky, Nikolai M., 2006, First record of fossil Mesozoic Ctenopoda (Crustacea, Cladocera), Zoological Journal of the Linnean Society 146 (2), pp. 269-274 : 270

ARCHELATONA ZHERIKHINI SP. NOV.

Etymology. This species is dedicated to the outstanding palaeontologist, Professor V. V. Zherikhin, whose many contributions include organizing PIN expeditions, collecting fossils and determining taxonomies of fossils. His untimely death in December 2001 was a great loss to Russian science.

Type locality. Khutel Khara, mudstones 75 km southeast of Sain Shand, East Gobi Aymag, Mongolia. The geological structure of this locality was reported by Verzilin & Kalmykova (1993). The sediments were deposited in a small (but permanent), oligotrophic mountain lake that formed when a volcanic basalt field dammed a valley. More than 3300 impressions of insects from 14 orders and 49 families have been recovered from these sediments (A. G. Ponomarenko, pers. comm.). Ctenopod remains are very common; they appear to have dominated among the group of microscopic animals. Mono-egged daphnid ephippia are also relatively common, but no single adult specimen has been found.

Age . Lower Tsagan Tsab Formation, Lower Cretaceous, 129 ± 9 Mya ( Verzilin & Kalmykova, 1993).

Holotype. PIN 3965 /3332 (stub sputtered with gold). AII: (7)–(13)/(0)–(1)–(4). There are two antennae II (belonging to different specimens) on the fragment; the holotype is the better preserved one, marked by an arrow. Label of the holotype: ‘ Archelatona zherikhini gen. nov., sp. nov., Khutel-Khara, Mongolia, 3965/ 3332, Holotype’.

Paratypes. (1) PIN 3965 /3293. AII: (8)–(> 11)/(0)–(1?)– (4). AII: (8)–(13)/(0)–(1?)–(4). (2) PIN 3965 /3295. AII: (9)–(13)/(0)–(?)–(?). AII: (7)–(≥ 10)/?. AII: (7)–(> 8)/?. (3) PIN 3965 /3298. AII: incomplete. AII: incomplete. (4) PIN 3965 /3299. Unclear body contour, mandibles and AII: (?)–(≥ 10)/?. AII: (7)–(?)/?. (5) PIN 3965 /3301 AII: (7)–(≥ 8)/(0)–(1)–(4). (6) PIN 3965 /3303. Unclear body contour, mandibles and AII: (8)–(13)/(0)–(?)–(4). (7) PIN 3965 /3304. AII: (8)–(15)/(0)–(?)–(?). (8) PIN 3965 / 3308. AII: incomplete. (9) PIN 3965 /3317. Body contour, mandibles and AII: unclear. (10) PIN 3965 /3320. AII: (7)–(?)/(0)–(?)–(?). (11) PIN 3965 /3321. AII: (8)–(?)/ (0)–(2?)–(4). (12) PIN 3965 /3323. AII: unclear. (13) PIN 3965 /3324. AII: (11)–(8)/(0)–(1)–(4). AII: unclear. (14) PIN 3965 /3325. AII: (≥ 7)–(12)/?. AII: unclear. (15) PIN 3965 /3326. AII: incomplete. (16) PIN 3965 /3332. AII: (?)–(≥ 10)/(0)–(1)–(4) – on the same fragment as the holotype. (17) PIN 3965 /3333. AII: (≥ 6)–(≥ 12)/(0)–(1)– (4). Body contour, mandibles, two AII (unclear), limbs. (18) PIN 3965 /3334. AII: (6?)–(≥ 12)/0-(1)–(4). (19) PIN 3965 /3335. AII incomplete. (20) PIN 3965 /3337. Many mandibles. (21) PIN 3965 /3338. Many mandibles .

Diagnosis. As for the genus.

Description. Mostly rami of swimming antennae are preserved, sometimes accompanied by body contours, mandibles and filtering setae of thoracic limbs ( Fig. 1A–C ). Accordingly, the morphological features of these cladocerans may be reconstructed as follows. Body is probably widely ovoid. Antenna II long and massive, with two-segmented exopod and threesegmented endopod ( Figs 1D–F , 2A–D ). In exopod, the proximal segment relatively short and wide, with a small distolateral spine ( Fig. 2B ), whereas distal segment is relatively long, also having a small distolateral spine ( Fig. 2C,D ). Number of setae of the inner margin of the exopod somewhat variable, ranging from seven to nine on the proximal segment and from 11 to 15 on the distal segment. Endopod seems to be always armed with five setae. Antennal formula (7–9)–(11–15)/(0)– (1)–(4). Each seta of exopod sits on a conspicuous basal prominence. Length of exopod 0.63–0.95 mm. Endopod composed of a short basal segment, a long middle segment with an apical seta, and a short, rounded distal segment bearing a single lateral seta and three apical seta. In some specimens (i.e. 3965/3321), it seems that there is a second seta on the middle segment, but this is probably an artefact because this seta is absent on clear impressions ( Figs 1D–F , 2A–C ). All setae of endopod uniform, unspecialized. Mandibles massive, asymmetrical ( Fig. 1A,B ). Several thoracic limbs similar, with numerous filtering setae ( Fig. 1C ). The proportion between body length and antennal exopod length in specimen 3965/3306 is about 3: 1, similar to proportions of some Recent ctenopods. Thus, we may conclude that the approximate body size of these cladocerans was about 1.8–3.0 mm.

Figures

Figure 1. Archelatona zherikhini gen. nov., sp. nov., paratypes, Khutel Khara, Mongolia, Lower Cretaceous. (A) Whole body, impression PIN 3965/3333. (B) The same fragment, mandibles and thoracic limbs. (C) Filtering setae on thoracic limbs. (D) Rami of swimming antenna, and other impressions on fragment PIN 3965/3333. (E) Rami of swimming antenna, PIN 3965/3334. (F) Rami of swimming antenna, PIN 3965/3332. Scale bars: A, 1000 µm; B–F, 100 µm.

Figure 2. Archelatona zherikhini gen. nov., sp. nov., holotype, Khutel Khara, Mongolia, Lower Cretaceous, PIN 3965/ 3332 (A–D), and Sididae indet. from Khotont, Jurassic/Cretaceous boundary, PIN 4307/2005 (E–F). (A) Rami of swimming antenna. (B) Articulation of segments of antennal exopod. (C) Reconstruction of antennal rami. (D) Distal segment of exopod. (E,F) Rami of swimming antenna, and their reconstruction. Scale bars: 100 µm.