Apocyclops californicus, Hołyńska & Leggitt & Kotov, 2016

Hołyńska, Maria, Leggitt, Leroy & Kotov, Alexey A., 2016, Miocene cyclopid copepod from a saline paleolake in Mojave, California, Acta Palaeontologica Polonica 61 (2), pp. 345-361 : 347-354

Apocyclops californicus sp. nov.

Figs. 2–9 .

Etymology: In reference to its origin from the state California in USA. Holotype: UMNH IP 4824, adult female, ventral aspect showing nine body segments and caudal rami, caudal setae II–VI (VI intact, other setae are partly injured), antennule except for three terminal segments, antennal endopodite (allobasis cannot be verified), maxilla, P1–P4 (medial spine on P1 basipodite cannot be verified), spinule ornamentation on free segment of P5 and spinule ornamentation of pediger 5 anteriorly to P5.

Type locality: Mud Hills , 17 km north of Barstow, Southern California, USA .

Type horizon: Latest Burdigalian or earliest Langhian in the middle member of the Barstow Formation, Miocene.

Material.— Thirty eight specimens including adults (both sexes) and copepodid III− V instars from Rainbow Canyon , Mud Hills ( USA); see SOM 3 .

Description.— Female: Body length excluding caudal setae 730–945 μm (holotype 775 μm; mean 862 μm; n = 13). Body nine-segmented ( Fig. 2A 1 , SOM 4: A). Prosome length/urosome length 1.1–1.4 (n = 10); cephalothorax length/width 1.05 UMNHIP 4834) to 1.20 (UMNH IP 4852); width of cephalothorax/width of genital double-somite 2.1 (UMNH IP 4852) to 2.3 (UMNHIP 4834). Body surface smooth, without rows of pits. Posterolateral corner of pediger 4 weakly produced. Genital double-somite as long as wide, with two low medial humps dorsally in anterior half of segment (SOM 4: B). In one female ( Fig. 2B ) two spermatophores ~43 μm long, forming nearly right angle, attached next to copulatory pore on ventral surface of genital double-somite, and an extra pair attached near anterior margin of double-somite. Anal operculum with straight posterior margin. Two medial hair-sensilla present (insertion sites verified in UMNH IP 4846) anterior to anal operculum. Posterior margin of anal somite bearing tiny spinules on ventral surface ( Fig. 2A 2 ), spinules absent (or very tiny) on dorsal surface (SOM 4: C). Caudal rami ( Fig. 2A 2 , SOM 4: C) 4.2–4.8 times as long as wide (n = 11) (holotype: 4.4; mean: 4.48), with lateral crest more distinct near insertion of posterolateral (III) seta. Tiny spinules present on dorsal surface of rami, no hairs on medial margin. Caudal ramus bearing six setae. Anterolateral (II) seta inserted at distance of 0.43–0.46 (n = 9) ramus length measured from posterior end ( Fig. 2A 2 , SOM 4: C). No spinules present at insertion of antero- (II) and posterolateral (III) setae. Relatively large gap between insertion sites of posterolateral and outer terminal (IV) setae ( Fig. 2A 2 , D). Dorsal (VII) and posterolateral setae inserted nearly the same distance from posterior end of ramus ( Fig. 2D ). Seta III 1.9 (UMNH IP 4852; Fig. 2D ) to 2.1 (UMNH IP 4839) times as long as terminal accessory seta (VI). Outer (IV) and inner terminal (V) setae thick at their base. Seta V (SOM 4C) ~1.5 and 4.0 times as long as seta IV and III, respectively, and about half as long as urosome length (UMNH IP 4852).

Antennule ( Fig. 2C 1 , see Fig. 9C ) reaching posterior fourth of cephalothorax to middle of pediger 2, and composed of 11 segments: I–V, VI–VII, VIII–XI, XII–XIII, XIV, XV–XVI, XVII–XX, XXI–XXIII, XXIV, XXV, XXVI– XXVIII. Setation formula (Arabic numerals mean the number of setae on each segment): 8, 4, 6, 2, 2, 2, 3, 2+ae, 2, 2 (+ae?), 6 (?)+ae (UMNH IP 4824 [holotype], 4829, 4834, 4836, 4844 and OUMNH NT.233). Setae Id, IIp− d, IVd, Vd, VIp− d, VIIp− d, VIIId, Xd, XIp− d, XIId, XIIId, XIVp− d, XVd, XVIId, XVIIId, XXd, XXId, XXIIId, XXIVp− d, XXVp− d, XXVIp, XXVIIp− d, XXVIII-2–4, and aesthetascs AeXXI and AeXXVIII present ( Figs. 2C 1 , 3A 1 , see Fig. 1 ); presence of setae IIIp− d, Vp, IXp− d, and XVId was inferred from the positions of integumental holes indicating insertion sites of these setae. Seta XXVIII-1 and aesthetasc AeXXV, present in extant Apocyclops , could not be verified in fossils. Setae

VIIIp, Xp, XIIp, and XIIIp, missing in extant Apocyclops , were not found in the fossils either.

Antenna ( Fig. 2C 1 , C 2 ) with allobasis and two free endopodal segments (UMNH IP 4824 [holotype], OUMNH NT.233 and UMNH IP 4844). Presumptive coxobasis bearing exopodal seta and two medial setae, presumptive enp1 with one medial seta; enp2 (second free segment) with six setae on medial margin and four setae at mediodistal angle ( Fig. 3A 1 ).

Mandibular palp reduced to two long and one short setae verified in copepodid V (UMNH IP 4832).

Maxillule ( Fig. 2C 2 ) praecoxal arthrite bearing three mediodistal claws, one seta on ventral (or posterior) surface and one robust seta on medial margin next to mediodistal claws (OUMNH NT.233 and UMNH IP 4845). Close to base of arthrite one long (feathered?) and one short seta as proximalmost element; other setae present in extant species could not be verified. Palp bearing one proximal seta, and at least two setae inserted on lateral lobe (three setae present in extant species). Robust spinous seta inserted on apical margin of the maxillulary palp, other elements (two more slender setae present in extant species) could not be verified.

Maxilla ( Figs. 2C 2 , 3A 1 ) composed of praecoxopodite, coxopodite, basipodite, and two-segmented endopodite (UMNH IP 4824 [holotype] and OUMNH NT.233). Arthrodial membrane between praecoxopodite and coxopodite failed to form on frontal surface; praecoxopodite bearing one endite with two setae, coxopodite with proximal and distal endite bearing one and two setae, respectively. Claw-like medial attenuation of basipodite with many fine spinules on concave margin. One robust seta inserted proximal to medial attenuation, and one shorter seta inserted on caudal surface near base of medial attenuation. Proximal endopodal segment bearing two setae, and distal endopodal segment bearing one large and two distinctly smaller setae Fig. 3A 1 ). Arthrodial membrane between distal endopodal segment and large terminal seta failed to form on frontal surface, but discernible on caudal surface.

Maxilliped is covered by other structures therefore the armature could not be verified.

Leg 1–4 rami ( Fig. 3B , SOM 4: D) two-segmented. Setation (UMNH IP 4824 [holotype], 4826, 4829, 4844, 4845, 4860 and OUMNH NT.233) is shown in Table 1. Transverse row (s) of small spinules present on caudal surface of P1 ( Fig. 2C 2 ) and P4 intercoxal sclerites. Distal margin of P1 intercoxal sclerite ( Fig. 2C 2 ) with large and obtuse protuberances bearing small spinules; protuberances on P4 intercoxal sclerite reaching well beyond distal margin of sclerite and likely spinulose (inferred from the presence of spinules in male). P4 coxopodite ( Fig. 3B ) bearing many small, equal-sized spinules arranged in one row along distal margin, transverse medial row of small spinules near proximal margin, and tiny spinules arranged in group laterally to medial row on caudal surface (UMNH IP 4845). P1, P3, and P4 basipodite with rounded medial expansion (P2 not verified).

Lateral spine of exp1 distinctly oblique to exp2 spines in P1–P4 ( Fig. 2C 2 , SOM 4: E). On terminal exopodal segments of P2–P4, spines conspicuously increasing in length from proximal to distal: length proportion of distalmost and proximalmost spines ~ 2.8 in P2 (UMNH IP 4826), 3.4–3.8 in P3 (UMNH IP 4826, 4860; see SOM 4: E) and 3.5 in P4 (UMNH IP 4826).

P4 enp2 (45 μm) apically bearing one lateral spine (25 μm) and one medial seta (70 μm) (measured on UMNH IP 4826).

P5 with one wide (width/length 1.6–2.1) free segment ( Table 2, Fig. 3A 2 , C), whole surface covered with small spinules (UMNH IP 4824 [holotype], 4826, 4829, 4846, 4852). Free segment bearing medial spine shorter than segment and one lateral seta inserted far from medial spine, near laterodistal corner of segment. Lateral seta of presumptive first segment of cyclopine P5 ( Fig. 3C ) inserted on laterodorsal surface of pediger 5. Pediger 5 ornamented with many small spinules arranged in field rather than row on dorsal surface next to insertion of lateral seta ( Fig. 3C ) and on ventral surface anterior to free segment ( Fig. 3A 2 ). Presence of spinules could not be verified on ventral surface of pediger 5, medially to P5.

Male: Body length 590–765 μm (mean 709 μm; n = 9). Body 10-segmented ( Fig. 3D, E ). Prosome length/urosome length 1.07–1.17 ( Fig. 3D, E ). Cephalothorax length/width 1.06—the cephalothorax dorsoventrally is slightly compressed therefore true proportion may be higher (UMNH IP 4856; Fig. 3E ); cephalothorax width/genital segment width 2.5—true proportion may be lower as the cephalothorax is dorsoventrally compressed. Somites devoid of surface ornamentation formed by cuticular pits. Medial hair-sensilla(?), inferred from insertion sites, present on dorsal surface of pediger 5 (UMNH IP 4856). Anal operculum with straight posterior margin. Caudal rami 3.9–4.8 (mean 4.3; n = 8) times as long as wide, no hairs on medial margin. Anterolateral (II) seta inserted at distance of 0.45–0.46 ramus length measured from posterior end (UMNH IP 4828, 4856). Large gap between insertion sites of posterolateral (III) and outer terminal (IV) setae. Dorsal (VII) and posterolateral (III) setae inserted nearly the same distance from posterior end of ramus. Seta III ~1.5 times as long as terminal accessory (VI) seta (OUMNH NT.234). Inner (V) and outer (IV) terminal caudal setae thick at their base.

Antennule ( Figs. 4 , 5 ) 16-segmented: I–V, VI–VII, VIII, IX, X, XI, XII, XIII, XIV, XV, XVI, XVII, XVIII, XIX– XX, XXI–XXIII, XXIV–XXVIII. In one male (UMNH IP 4858) the antennule is 17 (or 18)-segmented ( Fig. 3D , SOM 5: A), segments XXIV and XXV (segments 9 and 10 in female) remained separated instead of being fused, and segment XXV at least partly, also separated from segment XXVI–XXVIII. Fused condition of segments XXIV and XXV in the male is widely distributed in Cyclopoida ( Huys and Boxshall 1991) , and is also common, likely representing the plesiomorphic state in male Cyclopidae ( Fig. 9B, D ), therefore the separated state of these segments is considered here as a developmental aberration (an atavism?). The following setal elements and aesthetascs were identified (UMNH IP 4830, 4850, Figs. 4 , 5 , see also Figs. 1 , 9 , SOM 2): Id, IIp− d, IIIp− d, IVd, Vp− d, VIp− d, VIId, VIIId, IXd, AeIX, Xd, XIp− d, XIVp (?), XIVd, XVp, XVId, XVIIp− d, XVIIIp, AeXVIII (?), XIXp− d, XXd, XXId, AeXXI, XXIId, XXIIId, XXIVp− d, XXVp, XXVIp, XXVIIp− d. Presence of setae Id, and IVd was inferred from the position of integumental holes indicating insertion sites of these setae. Setae XIXd, XXId, and XXIId transformed to striated plate-like structures on segment 14 (XIX–XX) and segment 15 (XXI–XXIII). One large oval pore (SOM 5: A) with longer diameter ~2 μm on proximal plate (modified seta XXId) of segment 15 (UMNH IP 4858).

Antenna ( Figs. 4A , 5A ) composed of allobasis and two free endopodal segments. Exopodal seta present on presumptive coxobasis. Second endopodal segment (second free segment of antenna) bearing 8 setae (4 on medial margin and 4 at mediodistal angle). The antennal setation was verified in one specimen only ( UMNH IP 4850 ). The nine-setae state of the second endopodal segment presumably is also present in the adult male, because one male copepodid V ( UMNH IP 4825 ) had nine setae on this segment .

Endopodite of maxilla: arthrodial membrane formed on caudal surface between distal endopodal segment and large terminal seta.

P1–P4 rami 2-segmented, setation pattern as in Table 3 (UMNH IP 4828, 4840, 4850, 4856, 4858). P3 and P4 intercoxal sclerites ( Fig. 6A 1 , A 2 ) with transverse rows of spinules in middle line and near distal margin on caudal surface; large obtuse protuberances on distal margin, those of P4 bearing spinules. P4 coxopodite ( Fig. 6A 2 ) bearing many small equal-sized spinules arranged in single row along distal margin, small spinules present at proximolateral and distalolateral angle. Medial expansion of P4 basipodite Fig. 6A 1 , A 2 ) rounded and bearing spinules. Spines on P2 exp1 and P3 exp1 distinctly oblique ( Fig. 3D ), and to less extent the proximalmost spine of P2 exp2 also oblique to other spines of P2 exp2; P2 exp1 spine with fine setules (UMNH IP 4828, 4858). On terminal exopodal segments of P2−P4, spines conspicuously increasing in length from proximal to distal: length proportions of distalmost and proximalmost spines 3.2, 3.8, and 2.9 in P2, P3, and P4, respectively (P2−

P3, UMNH IP 4840; P4, UMNH IP 4850). P4 enp2 (length 42 μm) ~2 times as long as wide (UMNH IP 4850).

P5 free segment 1.2–1.6 times as wide as long ( Fig. 6A 1 , B, SOM 4: F, SOM 5: B), likely covered with spinules (UMNH IP 4850, 4856, 4858). Free segment with medial spine 1.4–2.7 times longer than segment (UMNH IP 4850, 4856) and lateral seta inserted at laterodistal corner, far from medial spine. Lateral seta of presumptive first segment of cyclopine P5 inserted on laterodorsal surface of pediger 5 ( Fig. 6B ). Anteriorly to P5 free segment, pediger 5 ornamented with small spinules arranged in field rather than single row ( Fig. 6A 1 , A 3 ); medially to P5 spinules arranged in triangular pattern on ventral surface of somite (UMNH IP 4850).

P6 bearing three elements ( Fig. 6C ), medial (13 μm) and middle (12 μm) setae subequal in length, and long lateral seta (40 μm) (UMNH IP 4828; body length: 765 μm). P6 flap with transverse rows of spinules (UMNH IP 4850).

Copepodid V (female and male): Body length 620 μm ( UMNH IP 4854 ) to 875 μm ( UMNH IP 4841 ) ; mean 748 μm; n = 10. In one female ( UMNH IP 4857 ; Fig. 8A , SOM 5: C) and one male ( UMNH IP 4825 ; Figs. 7A , 8B ), in which the sex could be identified by the laterodorsal and ventral position of P6, respectively, the body size significantly differed: female ~780 μm, and male 640 μm. Body composed of nine somites, anal somite longer than wide. Prosome length/urosome length 1.20–1.35 ( UMNH IP 4825 , 4832 , 4835 , 4841 , 4851 ) . In one CV larva ( UMNH IP 4841 ) two spermatophores attached ventrally, next to posterior margin of genital somite ( SOM 5 : D); length of spermatophore 43 μm. Caudal rami 3.4–3.7 times as long as wide (mean: 3.5; n = 6), bearing six setae ( SOM 5 : E). Anterolateral ( II) seta inserted at distance of 0.42 ramus length measured from posterior end ( UMNH IP 4851 ) . Large gap between insertion sites of posterolateral (III) and outer terminal (IV) setae. Seta III ~1.6 times as long as terminal accessory (VI) seta ( UMNH IP 4851 , body length 770 μm). Inner ( V) and outer (IV) terminal setae thick at their base. Dorsal (VII) seta 1.3 times as long as seta III ( UMNH IP 4851 ) .

Antennule ( Fig. 7B , SOM 6: A) as in adult female 11-segmented, and reaching to or slightly beyond posterior margin of cephalothorax (UMNH IP 4825, 4832, 4848, 4857). The following setae and aesthetascs were identified: Id, IId, IIIp− d, IVd, Vp− d, VIp (?) in UMNH IP 4848, VId (?) in UMNH IP 4848, VIIp− d, VIIId, IXd, Xd, XIp− d, XIId, XIVp− d, XVd, XVIId, XVIIId, XXd, XXId, AeXXI, XXIVp− d, XXVp− d, AeXXV, XXVIp (?) in UMNH IP 4857, XXVIIp− d, XXVIII-2–4, AeXXVIII (UMNH IP 4825, 4848, 4851, 4857). Presence of seta XXIVd and aesthetasc AeXXI were inferred from the insertion sites of these elements. Setae XIIp and XIIIp absent in adult female, were not found in CV female either (UMNH IP 4857; Fig. 7B ); presence of setae XIIp and XIIIp could not be verified in CV male (UMNH IP 4825; SOM 6: A). Setae VIIIp and Xp (missing in both adult female and male), which might be inserted on ventral surface of the compound segment 3 (VIII–XI) could not be verified, as the copepodids (UMNH IP 4825, 4848, 4851, 4857) showed the dorsal surface of the antennule. At least one of the CV larvae is a male (UMNH IP 4825), yet none of the additional armature elements, typical of the male, could be verified on the antennule (SOM 6: A).

Antenna ( Fig. 7C ) with allobasis, setation of allobasis as in adults (UMNH IP 4825, 4854). Enp2 with five setae on medial margin and four setae at mediodistal corner.

Mandibular palp ( Fig. 7D 1 ) bearing two long and one short setae (UMNH IP 4832).

P1–P4 rami ( Fig. 7D 2 ) two-segmented (P3 endopodite was not verified), armature formula shown in Table 4 (UMNH IP 4825, 4832, 4854, 4857). Transverse row of spinules present near distal margin on caudal surface of P4 intercoxal sclerite ( Fig. 8B ), and a second row likely also present at the middle of sclerite. Two obtuse protuberances on distal margin of intercoxal sclerite with relatively large spinules (UMNH IP 4825). P4 basipodite ( Fig. 8B ) with rounded medial expansion (UMNH IP 4825, 4854). On terminal exopodal segments of P2–P4 distalmost spines significantly longer than proximalmost spines (exact measurements were not possible).

P5 (UMNH IP 4825, 4832, 4841, 4857) appearing in female copepodid as a lateroventral lobe (UMNH IP 4857; Fig. 8A ) rather than separated segment, with slight cuticular invagination between presumptive free segment and pediger 5, while separation of P5 segment rather distinct in the male UMNH IP 4825; Fig. 8B ). P5 segment (or lobe) bearing two setae (UMNH IP 4857, medial/lateral seta = 21/17 μm, body length 780 μm; UMNH IP 4832, medial/lateral seta = 24/23 μm, body length 840 μm; UMNH IP 4841, medial/ lateral seta = 24/23 μm, body length 875 μm), with wide gap between setae. Lateral seta (UMNH IP 4857, 19 μm) of presumptive first segment of cyclopine P5 similar in length to lobe setae, and inserted on laterodorsal surface of pediger 5 ( Fig. 8A ). Presence of spinules on P5 lobe could not be confirmed unambiguously, but few spinule-like structures were observed in UMNH IP 4825, 4841, and 4857. Medially to P5, row of longer spinules present on ventral surface of pediger 5 (UMNH IP 4841; SOM 5: D).

P6 sexually dimorphic: in female ( Fig. 8A ) low hump likely bearing three elements on laterodorsal surface near posterior margin of genital segment − posteriormost element unambiguously confirmed, two more anterior setae injured UMNH IP 4857); in male one thick (medial spine) and one slender element (supposed to be one of the two lateral setae, see Fig. 8B ) were confirmed on ventral surface next to posterior margin of genital segment (UMNH IP 4825).

Copepodid IV: Body length 575 μm (UMNH IP 4835); prosome length/urosome length 1.2. Body ( Fig. 8C 1 ) composed of eight segments, anal somite longer than wide. No spinules present on posterior margin of anal somite (lateral view). Caudal rami measured across its widest part 3.2 times as long as wide, and bearing six setae (SOM 6: B). Anterolateral (II) seta inserted at distance of 0.44 ramus length measured from posterior end. Large gap between insertions of posterolateral (III) and outer terminal (IV) setae. Spinules absent at insertion of seta II and III. Seta III ~1.8 times as long as terminal accessory seta (VI). Outer (IV) and inner (V) terminal setae thick at their base. Dorsal (VII) seta (42 μm) 1.6 times as long as seta III.

Antennule not reaching posterior margin of cephalothorax. More proximal segments are covered with sediments, only five terminal articles (XVII–XX, XXI–XXIII, XXIV, XXV and XXVI–XXVIII) could be confirmed.

P1–P4 exopodites two-segmented (endopodites not verified). Presence of lateral basipodite setae was confirmed in P1 and P3 ( Fig. 8C ). Proximal exopodal segment with one spine in P1–P4, distal exopodal segments of P1–P4 with 3, 4, 4, 3 spines. Distalmost spines significantly longer than proximalmost ones on terminal exopodal segments of P2– P4. P5 ( Fig. 8C 2 ) lobe-like, not separated from pediger 5 with arthrodial membrane, and bearing medial (15 μm) and lateral (14 μm) setae. Lateral seta (14 μm) of presumptive first segment of cyclopine P5 inserted on laterodorsal surface of pediger 5. P6 ( Fig. 8C 2 ) composed of two setae (medial ~12 μm, lateral 10 μm) inserted ventrally on posterior margin of genital segment (urosomite 2).

Copepodid III: Body length 560 μm (UMNH IP 4849), prosome length/urosome length: 1.6. Body ( Fig. 8D 1 , SOM 6: C) composed of 7 segments, anal somite 1.4 times as long as wide. Anal operculum with straight posterior margin. Caudal rami (SOM 6: C) 2.8 times as long as wide (true proportion may be higher, as the specimen is dorsoventrally compressed), bearing six setae. Anterolateral (II) seta (13 μm) inserted at distance of 0.44 ramus length measured from posterior end. Wide gap between insertions of posterolateral (III) and outer terminal (IV) setae. Outer (IV) and inner (V) terminal setae thick at their base. Length of terminal accessory seta (VI), and setae V, IV, and III, 18 μm, 148 μm, 123 μm, and 28 μm, respectively. Dorsal (VII) seta 1.4 times as long as seta III. Conspicuous lateral keel present on caudal rami posteriorly to insertion of seta II.

Antennule ( Fig. 8D 2 ) reaching about posterior third of cephalothorax, 9-segmented: I–V, VI–XI, XII–XIV, XV– XVI, XVII–XX, XXI–XXIII, XXIV, XXV, XXVI–XXVIII. The following setal elements were confirmed (dorsal view): IIId (?), VIId, XId, XIVp, XVd, XVIId, XVIIId, XXId, XXIIId, XXIVp− d, XXVp− d, XXVIp, and at least four apical elements on terminal segment (XXVI–XXVIII).

P5 covered with sediment, yet two elements, a robust seta (or spine) and more dorsally a slender seta were confirmed. Insertion site of slender seta (on P5 lobe, or laterodorsally on pediger 5) could not be verified.

Figures

Fig. 2. Adult female of cyclopid copepod Apocyclops californicus sp. nov. from Mud Hills, Southern California; Burdigalian–Langhian, Miocene. A. UMNH IP 4824 holotype, habitus (A1) and anal somite and caudal rami (A2) in ventral view. B. UMNH IP 4833, pediger 5 and genital double-somite with two pairs of spermatophores (arrowed) in ventral view.C. OUMNH NT.233, 11-segmented antennule and antenna with allobasis in ventral view (C1), cephalothoracic appendages (arrow shows oblique spine on the first exopodal segment of leg 1) in ventral view (C2). D. UMNH IP 4852, caudal setae with setules on setae IV and V in dorsal view. A1, antennule; A2, antenna; Md, mandible; Mxl, maxillule; Mx,: maxilla; P1, leg 1.

Fig. 3. Cyclopid copepod Apocyclops californicus sp. nov, from Mud Hills, Southern California; Burdigalian–Langhian, Miocene. A−C. Adult female. A. UMNH IP 4824 holotype, median section of the antennule showing some setae coded on segments 2−5 (VII−XIV) and antennal endopodite in anterior view (A1), spinulose surface ornamentation of pediger 5 and free segment of leg 5 in ventral view (A2); p or d means anteroproximal- or anterodistal seta, arrows point to ten setae on the second (penultimate) endopodal segment of the antenna, and two short setae on the distal endopodal segment of the maxilla (Mx). B. UMNH IP 4845, prosome with four pairs of the swimming legs in lateroventral view. C. UMNH IP 4852, spinulose surface ornamentation of pediger 5 and free segment of leg 5 in dorsal view. D, E. Adult male habitus. D. UMNH IP 4858, in lateroventral view. E. UMNH IP 4856, in dorsal view.

Fig. 4. Cyclopid copepod Apocyclops californicus sp. nov. (UMNH IP 4850) from Mud Hills, Southern California; Burdigalian–Langhian, Miocene, adult male antennule in ventral view. A. Segments 1−8 (I−XIII) and antenna with allobasis. B. Segments 1−11 (I−XVI). C. Segments 10−14 (XV−XX). D. Segments 15−16 (XXI−XXVIII).

Fig. 5. Cyclopid copepod Apocyclops californicus sp. nov. (UMNH IP 4850) from Mud Hills, Southern California; Burdigalian–Langhian, Miocene, adult male antennule in ventral view. The line drawings based on the scanning electron micrographs (see Fig. 4) show some setal elements or their insertion sites (coding as in Fig. 1). A. Segments 1−2 (I−VII). B. Segments 2−11 (VI−XVI). C. Segments 11−14 (XVI−XX). D. Segments 15−16 (XXI−XXVIII).

Fig. 6.Adult male of cyclopid copepod Apocyclops californicus sp. nov. from Mud Hills, Southern California; Burdigalian–Langhian, Miocene. A. UMNH IP 4850. Protopodite and proximal segments of the exo- and endopodite of leg 4 (caudal view), and pediger 5 in ventral view (A1). Drawings of protopodite and proximal segments of the exo- and endopodite of leg 4, and intercoxal sclerite of leg 3 in caudal view (A2) and pediger 5 with leg 5 in ventral view A3). B. UMNH IP 4856, pediger 5 with leg 5 in dorsal view; lateral seta of presumptive proximal segment of leg 5 is inserted on pediger 5. C. UMNH IP 4828, leg 6 in lateral view; arrowheads show the three setal elements, whole length of the lateralmost seta is indicated by bracket.

Fig.7. Copepodid V of cyclopid copepod Apocyclops californicus sp. nov.from Mud Hills, Southern California; Burdigalian–Langhian, Miocene. A. UMNH IP 4825, male, habitus in ventral view. B. UMNH IP 4857, female, 11-segmented antennule. C. UMNH IP 4854, sex cannot be determined, antenna with allobasis in ventral view; arrows show the insertion sites of five setae on the medial margin of the penultimate segment of the antennal endopodite. D. UMNH IP 4832, sex cannot be determined.Antennule (segments 1−8), coxopodite of the mandible (Md) with palp bearing two long and one short setae, maxillulary arthrite and maxilla (D1). Swimming legs 1 to 4 in lateral view (D2), arrow points to the medial spine of leg 1 basipodite.

Fig. 8. Cyclopid copepod Apocyclops californicus sp. nov. from Mud Hills, Southern California; Burdigalian–Langhian, Miocene. A, B. Copepodid V. A. UMNH IP 4857, female, swimming legs 3−4, leg 5 and leg 6, in lateral view; arrowheads point to lobe setae and lateral seta of leg 5, and posteriormost seta of leg 6. B. UMNH IP 4825, male, leg 4 protopodite in caudal view, urosomites 1−3 in ventral view; arrowheads point to setae of leg 5 and leg 6. C. Copepodid IV, UMNH IP 4835, habitus in lateral view (C1), coxopodite, basipodite, and first exopodal segment of leg 3 and 4, leg 5, and leg 6 (C2). D. Copepodid III, UMNH IP 4849, habitus (D1) and nine-segmented antennule (D2) in dorsal views.

Fig. 9. Schematic representation of the antennule segmentation and setation in Cyclopidae. A, B. Euryte robusta. C, D. Apocyclops panamensis. Female (A, C), male (B, D). Symbols: short line, anteroproximal seta; long line, anterodistal seta; ellipse, aesthetasc; filled black triangle, spinous seta on segment XIV; trapezoids, modified setae. Structures indicated with thick lines are present in the male but not expressed in the conspecific female. Roman numerals denote the ancestral segment homologies in the male of A. panamensis.

Fig. 1. Coding of the antennulary setae, shown on the female of cyclopid copepod Apocyclops cf. ramkhamhaengi (MIZ 2/2015/9) from Townsville (Australia), extant species. A. Segments 1−5 (I−XIV). B. Segments 6−8 (XV−XXIII). C. Segments 9−11 (XXIV−XXVIII). Armature elements denoted by black and grey codes are present in the male of Euryte robusta; codes in black denote setae or aesthetascs present in female in the Apocyclops panamensis group; Roman numeral refers to the ancestral segment on which the seta is inserted; p or d means anteroproximal- or anterodistal seta; Ae with Roman numeral in subscript means an aesthetasc inserted on the ancestral segment given in the subscript.