Palaeogammarus debroyeri, Jażdżewski, Krzysztof, Grabowski, Michał & Kupryjanowicz, Janusz, 2014

Jażdżewski, Krzysztof, Grabowski, Michał & Kupryjanowicz, Janusz, 2014, Further records of Amphipoda from Baltic Eocene amber with first evidence of prae-copulatory behaviour in a fossil amphipod and remarks on the taxonomic position of Palaeogammarus Zaddach, 1864, Zootaxa 3765 (5), pp. 401-417 : 405

Palaeogammarus debroyeri sp. nov.

Description. Head without rostrum, distinctly (by 1/3) longer than first pereon segment. Eyes not visible. Cephalic lobe rounded. Coxal plates 1–4 long with successive plates covering hind margins of former ones. Coxal plates 1– 3 with single, rather long setae on lower margins. Fourth coxal plate large, with prominent posterodistal lobe ( Fig. 7 a). Antenna I reaches more or less to half of body length, antenna II distinctly shorter—reaching only to 1/4 or to 1/3 of body length. Tips of antennal flagella broken or hidden, estimation of number of articles of A I flagellum slightly over 15 and of A II flagellum—as less than 10. Accessory flagellum ( Fig. 6 a), visible only in one case and possesses 2 articles. First article of A I peduncle somewhat longer (by 1/3) than second one, second longer by 1/3 than third article. Peduncle articles 4 and 5 of A II subequal in length. In one specimen a subacute triangular gland cone, reaching middle of the 3rd article of A II peduncle visible ( Fig. 6 b). Mouthparts generally invisible except one mandibular palp ( Fig. 6 b), where the second article bears several long setae on its lower margin and is twice as long as third article. Third article of mandibular palp almond-shaped, with 3–4 long apical setae ( Fig. 6 b).

Pereonite II shortest. Pereon and pleon segments, as well as urosomite I smooth, without any structures on surface or on posterior margins. Posterior margins of urosomites II and III armed with several robust setae. This feature is visible only in two specimens, but in our opinion it is simply a result of their better preservation and visibility if compared to other individuals ( Figs. 6 a, 7c).

Anterior pereopods mostly invisible, except two cases when gnathopod apical articles were distinguishable. In one case ( Fig. 8 a), regularly oval shape of propodus seems to indicate the distal part of G I, probably a female. Anterodistal part of propodus and posterodistal part of carpus of this appendage armed with several long and short robust setae. In the second case ( Fig. 7 a), the oblique and somewhat concave shape of palm margin suggests the distal part of male G II. Posterior pereopods (V–VII) quite well visible in several specimens. Their basal articles widely oval, some 50% longer than wide, with large rounded postero-distal lobes ( Figs 6 a, b). Serration or any armature of posterior margins of the basal articles not visible in all but one case, where a delicate serration may be noted ( Fig. 7 b). The state of preservation of most specimens allows the supposition that such serration of these basal articles is not visible due to the poor optical properties of the medium.

Of the posterior pereopods, P VI longest and P VII shortest. Merus, carpus and propodus articles of these appendages subequal in length ( Figs 6 a,b, 8b). Their margins armed with 1–3 groups of robust setae, longer in distal part of each article; longest setae surpass width of article. Dactyls of posterior pereopods are nearly straight, acute, long and slender. Their length ca. 1/3 of propodus length ( Fig. 8 b). Dactyls of pereopods III and IV less slender. Longest distal setae of pereopod propoduses long but somewhat shorter than dactyl length ( Figs 6 a, b, 8b).

Postero-distal parts of epimeral plates 2 and 3 acutely produced ( Figs 6 a, b). Hind margins not serrated.

Uropod I strong, much longer than U II, reaching to nearly middle of U III. Uropods I and II similar in structure, with peduncle somewhat longer than rami. Latter more or less equal in length and armed with 1–3 groups of robust setae on lateral margins and at the tips. Among the tip with robust setae, one bigger than the others ( Fig. 7 c). Uropod III with long lanceolate exopodite, whose distal end could be recognized as a small second article with 1–3 accompanying robust setae. Outer lateral margin of exopodite with 2–3 groups of robust setae. Endopodite small ( Figs. 6 a, b, 8d) and rather difficult to discern in most specimens. Length ca. 1/4 of exopodite length; tip with a setule ( Fig. 8 d). Protopodite of U III short, rectangular.

Telson not well visible in most cases; it seems to be deeply cleft, usually turned upwards to urosome, reaching up to 2/3 of the length of last urosomite. Both telson lobes have 2 apical setae, one bigger and second smaller, both turned upwards ( Figs 8 c,d). Telson length less than half of U III length.

Etymology. The species name is given in honour of Dr. Claude De Broyer (Royal Belgian Institute of Natural Sciences, Belgium), an illustrious specialist in the knowledge of morphology, taxonomy and biogeography of Amphipoda , with many thanks for his incessant help and friendship.

Figures

FIGURE 6. Habitus of Palaeogammarus debroyeri sp. nov.: a) holotype (specimen A), b) paratype (specimen B).

FIGURE 7. Palaeogammarus debroyeri sp. nov., details of morphology: a) anterior part of specimen A 1, b) basis P VII of specimen A 1, c) posterior part of specimen K.

FIGURE 8. Palaeogammarus debroyeri sp. nov., details of morphology: a) distal part of G I in specimen D 1, b) detached distal part of hind pereiopod, c) telson lobes and exopodites of U III in specimen F 1, d) inner side view of U III and side view of telson in specimen H 1.