Zootaxa 3765 (5): 401–417
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Copyright © 2014 Magnolia Press
Article
ISSN 1175-5326 (print edition)
ZOOTAXA
ISSN 1175-5334 (online edition)
http://dx.doi.org/10.11646/zootaxa.3765.5.1
http://zoobank.org/urn:lsid:zoobank.org:pub:EC778AFE-04E2-4211-A464-CE7050A992F9
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
KRZYSZTOF JAŻDŻEWSKI1, MICHAŁ GRABOWSKI1 & JANUSZ KUPRYJANOWICZ2
1
Department of Invertebrate Zoology & Hydrobiology, University of Lodz, Banacha 12/16, 90-237 Lodz, Poland.
E-mail: [email protected], [email protected]
2
University Museum of Nature, University of Bialystok, Swierkowa 20B, 15-950 Bialystok, Poland. E-mail: [email protected]
Abstract
Two pieces of Baltic amber with amphipod inclusions were studied. One of them contained approximately twenty individuals identified as belonging to the extinct genus Palaeogammarus and described as P. debroyeri sp. nov. Interestingly,
among the individuals there are two pairs preserved in an evident prae-copula position. This is the first finding of such
mating behaviour in fossil amphipods. Based on this behavioural trait and on the observed morphological features, we
conclude that the genus Palaeogammarus should be placed in Gammaridae and not in Crangonyctidae. The second amber
piece contains two individuals identified as belonging to the still extant genus Synurella and described as S. aliciae sp.
nov.
Key words: Crustacea, Amphipoda, Gammaridae, Crangonyctidae, Palaeogammarus, Synurella, fossil, amber, new species
Introduction
Organisms living in aquatic habitats are not supposed to be found frequently in fossil tree resin, such as amber, as it
is a highly hydrophobic medium. However, the fossil record provides numerous amber-preserved limnetic
arthropods including water beetles, water striders, and crustaceans (Schmidt and Dilcher 2007). Among the latter,
findings of amphipod inclusions in the pieces of Baltic Eocene amber are the most common (Zaddach 1864, Lucks
1928, Just 1974, Jażdżewski and Kulicka 2000 a, b, 2002, Coleman and Myers 2001, Coleman and Ruffo 2002,
Weitschat et al. 2003, Coleman 2004, 2006, Jażdżewski and Kupryanowicz 2010). So far, some 10 fossil amphipod
taxa were described, all of them being attributed either to the family Niphargidae Bousfield, 1977 (Niphargus
Schiödte, 1849) or Crangonyctidae Bousfield, 1973 (Palaeogammarus Zaddach, 1864, Synurella Wrzesniowski,
1877 and one unidentified crangonyctid). Interestingly, now both the above mentioned families are represented in
Europe mostly by subterranean or groundwater taxa. On the other hand, members of the family Gammaridae
Latreille, 1802 clearly predominate in the extant epigean fauna (Meijering et al. 1995, Väinöla et al. 2008).
In the present paper we provide a description of two new amphipod species of the genera Palaeogammarus
and Synurella found in Baltic Eocene amber. A short discussion upon the systematic position of Palaeogammarus
and first evidence of prae-copulatory behaviour among fossil amphipods, are provided.
Material and methods
The two studied amber pieces (Figs 1–3) with amphipod inclusions came from the private collection of Mr Jürgen
Velten (Idstein, Germany) and were purchased by the Museum of Earth, Warsaw, Poland). Their catalogue numbers
are no. 22611 (amber piece 1) and no. 22612 (amber piece 2). Most probably both pieces were found on the Sambia
Peninsula (south-eastern shore of the Baltic Sea) and originate from the Eocene period (ca. 45–50 Mya).
Observations of morphological details and respective drawings were done under a stereoscopic microscope
Accepted by J. Lowry: 23 Dec. 2013; published: 20 Feb. 2014
401
Nikon SMZ 1500 supplemented with camera lucida. The drawing procedure followed Coleman’s (2003) method.
Photos were done using a stereoscopic microscope Nikon SMZ 800 with Nikon DS-Fi1 camera attached.
Results
Amber piece 1 (cat. no. 22611)
The amber piece 1 is of flat trigonal shape, with a size of some 40x20x10 mm (Figs 1 and 2). It contains 21
specimens of Amphipoda preserved in various states. Most of them lie deep in the piece and/or are covered with air
bubbles or various dust particles, making them indiscernible. Fortunately at least 8 specimens are located near the
polished surface. The study of all sufficiently visible details of their structures allowed to decide that all specimens
fossilized in this amber piece belong to the same species. The photos of three best situated specimens are presented
in Figs 4 and 5, whereas two of them are drawn in the whole (Fig. 6). Additionally, some better visible fragments of
other specimens are also drawn in Figs 7 and 8. As a holotype we have chosen the specimen A, but the description
is based on the discernible morphology of several individuals.
FIGURE 1. Amber piece 1, upper side, individuals of Palaeogammarus indicated with letters (scale bar in mm).
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FIGURE 2. Amber piece 1, lower side, individuals of Palaeogammarus indicated with letters (scale bar in mm).
FIGURE 3. Amber piece 2, individuals of Synurella indicated with letters (scale bar in mm).
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FIGURE 4. Palaeogammarus, specimen A.
FIGURE 5. Palaeogammarus, specimens B and C.
What is interesting enough, among these quite numerous amphipod specimens there are two pairs fossilized in
“prae-copula” position. In the first pair, the prae-copulatory position is evident and clearly recognizable (Fig 9).
The bigger individual (male) is attached to the back of an evidently smaller individual (female) and embraces its
body with pereopods. Unfortunately, the individuals in pairs are covered with mould, so details of their
morphology (including gnathopods), that could help in reconstruction of the attachment mechanism, are difficult to
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study. In the second pair the larger individual adheres to the back of the smaller individual in a position that also
may be recognised as prae-copulatory, yet its pereopods are displaced so no embracement is visible.
The length of amphipod individuals in this amber piece oscillates between 4 and 7 mm. Beside the numerous
amphipods there are two insect inclusions in this amber piece—an adult of brachyceran Diptera and a trichopteran
larva of the family Philopotamidae.
Gammaridae Latreille, 1802
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.
7a). 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. 6a), 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. 6b). Mouthparts generally invisible except one
mandibular palp (Fig. 6b), 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. 6b).
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. 6a, 7c).
Anterior pereopods mostly invisible, except two cases when gnathopod apical articles were distinguishable. In
one case (Fig. 8a), 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. 7a), 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 6a, 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. 7b). 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 6a,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. 8b). Dactyls of pereopods III and IV less
slender. Longest distal setae of pereopod propoduses long but somewhat shorter than dactyl length (Figs 6a, b, 8b).
Postero-distal parts of epimeral plates 2 and 3 acutely produced (Figs 6a, 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.
7c). 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. 6a, b, 8d) and rather difficult to discern in most specimens. Length ca. 1/4 of exopodite length; tip with
a setule (Fig. 8d). 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 8c,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.
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FIGURE 6. Habitus of Palaeogammarus debroyeri sp. nov.: a) holotype (specimen A), b) paratype (specimen B).
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FIGURE 7. Palaeogammarus debroyeri sp. nov., details of morphology: a) anterior part of specimen A1, b) basis P VII of
specimen A 1, c) posterior part of specimen K.
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FIGURE 8. Palaeogammarus debroyeri sp. nov., details of morphology: a) distal part of G I in specimen D1, b) detached
distal part of hind pereiopod, c) telson lobes and exopodites of U III in specimen F1, d) inner side view of U III and side view
of telson in specimen H1.
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Amber piece 2 (cat. no. 22612)
Amber piece 2 is flat and rectangular in shape, with the size of 20x11x3 mm (Fig. 3). Two amphipod specimens (A
and B) are visible in this piece (Fig. 10). One is preserved much better than the other. The anterior part of the latter
one is covered with dust particles. However, some details are seen better in the first, other in the second specimen
(Figs 11 and 12). The discernible details of the morphology of both specimens indicate that they are conspecific;
therefore the description is based on both individuals. The length of individuals in this piece is ca. 5 mm. Specimen
A is chosen as a holotype.
FIGURE 9. Palaeogammarus debroyeri sp. nov., sketch of the pair in prae-copula.
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FIGURE 10. Synurella, specimens A and B.
Crangonyctidae Bousfield, 1973
Synurella aliciae sp. nov.
Description. Head without visible rostrum, slightly longer than first pereon segment; shape of cephalic lobe
difficult to define. No eyes visible. Antenna I long, nearly of the body length (Fig. 10a). Number of articles in A I
main flagellum around 20 (tips of flagella broken), accessory flagellum not visible due to some dust particles
covering this part of A I. Antenna II length reaches half of A I. Flagellum of A II with less than 10 articles. Margins
of pereonites and abdominal segments well seen only in dorsal parts. Margins devoid of armature; only 1 short
setule set on dorsal surface of third abdominal tergite near hind margin (Fig. 11b). Epimeral plates II and III,
discernible only in specimen B, postero-ventrally acutely produced (Fig. 11b). Urosome definitely unsegmented.
Gnathopods in specimen A poorly visible—only anterior dorsal side of comparatively large propoduses could be
observed. In the specimen B, only palmar margin of propodus and dactylus of gnathopod (I or II) visible (Fig. 12a).
Palmar margin delicately concave, armed with 2 rows of several short robust setae, whereas palmar antero-distal
angle widely rounded and armed with 2 groups of rather long setae. Short palmar setae seem to be not bifid. Dactyl
ventral margin set with 3 setae. Pereopods III and IV invisible (broken or hidden), pereopods V–VII of similar
length, although P V seems to be a little shorter than the next appendages. Bases of these posterior pereopods of
widely rounded oval shape; their hind margins delicately serrated (Fig. 11b). In specimen A serrations of P V basis
are set with short setules, whereas in specimen B serration only visible on hind margins of P VI and P VII basal
articles. Merus, carpus and propodus of P V–VII of similar length. Distal parts of these articles armed with 2–4
robust setae of length equal or shorter than article width (Fig. 11b). Single seta also on the anterior and/or posterior
margins of these articles, more numerously on P V propodus in specimen A. The length of P V–VII dactyls equal to
half of propodus length. Apical setae of propodus not longer than half of dactyl length. Peduncles of uropods I and
II strong. Their margins set with row of robust setae (Fig. 12c). Uropod rami poorly discernible in details but it
seems that they are similar in length or somewhat shorter than peduncles. Only the armature of inner margin of U II
endopodite in specimen B is better visible. This article apically armed with 3 robust setae and 1 such seta on its
lateral margin. Uropod III not visible (possibly broken off) in both specimens. Telson visible in both specimens and
turned upwards along urosome (Figs 11b and 12c). In specimen A it was possible to observe ventral surface of
telson (Fig. 12d) deeply concave to more than half of its length and the tips of each lobe armed with 4 robust setae.
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Etymology. This species is named in honour of Dr. Alicja Konopacka (Department of Invertebrate Zoology &
Hydrobiology, University of Lodz, Poland), colleague and good friend of the first two authors, in recognition of
long-lasting friendship and her contribution to the studies of Amphipoda over the years.
FIGURE 11. Synurella aliciae sp. nov.: a) habitus of specimen A, b) posterior part of specimen B.
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FIGURE 12. Synurella aliciae sp. nov., details of morphology: a) distal part of G I or G II in specimen B, b) coxal plates I–IV,
P V and P VI in specimen A, c) side view of urosome with protopodites of U I and U II and telson in specimen A, d) telson of
specimen A.
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FIGURE 13. Palaeogammarus polonicus Jażdżewski & Kulicka, 2002, protopodite and exopodite of U III wrongly identified
as telson in the original description.
Discussion
The amphipod specimens from the amber piece 1, although many of them are only partly visible, all seem to belong
to a single species. Such an unusual “mass” occurrence, over 20 specimens in a piece of resin of the volume of ca.
4 cm3, can be attributed to aggregational behaviour observed in many freshwater amphipods, eg. in Gammarus
Fabricius, 1775 during feeding (MacNeil et al. 1997) or spring upstream migrations (own observations).
Affinity of Palaeogammarus debroyeri sp. nov. to other fossil taxa. The morphology of this newly described
species evidently locates it in the extinct genus Palaeogammarus. This is evidenced by the length of antennae,
short accessory flagellum with 2 articles, shape of coxal plates and basal segments of posterior pereopods, armature
of hind abdominal segments, deeply cleft telson, comparatively long uropod III with well developed outer ramus
and short inner ramus (when discernible). Robust setae ornamenting hind margins of urosomites are lacking only in
P. danicus Just, 1974, whereas only in P. balticus Lucks, 1928, these hind urosomite margins are somewhat
produced and notched (“...seitlich und hinten starke Ausrandungen.”, Lucks, 1928).
From the formerly described Palaeogammarus species, P. debroyeri differs in the following features:
1) From P. polonicus Jażdżewski & Kulicka, 2002— by the lack of a row of setules along the posterior margin
of basal articles of hind pereopods. The longest setae of propoduses of these pereopods are shorter than the dactyls
in P. debroyeri and longer in P. polonicus. Outer margin of U III exopodite in P. debroyeri is armed with, regularly
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set, three groups of robust setae (Figs 6b, 7c), whereas in P. polonicus there are three long, slightly curved setae
situated in the distal half of this margin (Fig. 13). On this occasion it is important to note, that in the paper by
Jażdżewski and Kulicka (2002) the U III of Palaeogammarus polonicus was wrongly identified and described to be
“telson”. Having at our disposal the holotype of this Baltic Eocene amber species and using now a much better
microscope and illumination, we have discovered that this “telson” (Jażdżewski and Kulicka 2002: Fig. 2), is in
fact, the exopodite of left U III (Fig. 13). This appendage is still inadequately visible because of the perpendicular
crack of the amber piece, mentioned by Jażdżewski and Kulicka (2002). The true telson is definitely invisible in the
specimen. But it is now obvious that the U3 exopodite of P. polonicus, declared by these authors as “broken or nondiscernible” is comparatively wide and laterally armed (see above). The presence or absence of the 2nd exopod
article cannot be unequivocally recognized; maybe it was crashed. Also, presence of endopodite could not be
verified due to the above mentioned crack in the amber piece.
2) From P. sambiensis Zaddach, 1864—by a more delicate and smooth (not acute) serration of the posterior
margins of basal articles of hind pereopods, and by the lack of numerous small setae located along the anterior
margins of these basal articles. In P. debroyeri coxal plates I–III are armed with single setae, that are absent in P.
sambiensis.
3) From P. sambiensis and P. balticus, the newly described species differs by having the comparatively slender
first article of A I peduncle (length/width ratio ca. 3), which is distinctly longer than the next article. In the first two
species the first A I article is thick, somewhat egg-shaped (length/width ratio ca. 2) and of the same length as the
second one.
4) From P. danicus, the new species differs by the presence of robust setae on hind margins of urosomites and
by long setae on lower margins of coxal plates I–III (Fig. 7a). In P. debroyeri the third article of mandibular palp is
almond-shaped and armed with 3–4 long apical setae, whereas in P. danicus this article is semifalcate with mediodistal stiff setae. In P. danicus U3 exopodite is unisegmented with few apical setae, its outer margin is devoid of
setae, whereas in P. debroyeri this exopodite has a small second article with 1–3 accompanying setae and the outer
margin of U III exopodite is armed with 3 groups of robust setae.
5) Palaeogammarus sp. described by Coleman & Myers (2001) has strongly expanded meruses of pereopods
V–VII, which is not the case in P. debroyeri.
Affinity of Synurella aliciae sp. nov. to other fossil taxa. The amphipods from the amber piece 2 can be
classified as Synurella for their fused urosome segments and the lack of robust setae on the hind margin of the
urosome. Also the shape of the telson resembles that of some extant Synurella species (e.g. Karaman 1974),
although in comparison to the latter it is deeply concave in our specimen. On the other hand the row of the telson
robust setae is very similar to such an armature of many Synurella species. The only feature differing from the
extant congeneric species is the shape of gnathopod palmar setae. These setae are bifid in all extant species of
Synurella described until now (Karaman 1974). We could not observe such a shape in our material, yet this may be
an artefact resulting from the minute size of the setae, which is not easy to discern in details when embedded in
amber. However, all the other observed features do not leave any doubts about the generic classification of the
specimens. So far, two findings of Synurella preserved in amber are known (Coleman 2004, 2006). However, due
to the rather poor preservation state of the individuals and weak visibility of important diagnostic features, they
were not given any specific names. For the same reason it is impossible to discuss their morphological affinity to
the presently described S. aliciae.
Remarks on the taxonomic position of Palaeogammarus Zaddach, 1864
Just (1974), describing Palaeogammarus danicus, was the first who formally placed it in the family
Crangonyctidae Bousfield, 1973 (Bousfield 1977, emended), mentioning at the same time that it was Bousfield
(pers. comm. in Hessler, 1969), who suggested the kinship of Palaeogammarus and Crangonyx Bate, 1859.
Supporting his point of view, Just (1974) mentioned a few features of the so far described Palaeogammarus species
common with several crangonyctid genera (particularly Crangonyx): the shape of the latero-frontal margin of the
cephalon, accessory flagellum of antenna I very short and composed of two segments, pereopod VI longer than V
and VII, exopodite of uropod III well developed, longer than the peduncle and uni-segmented. Simultaneously, Just
(1974) admits that other even more important characters, such as mouthparts, endopodite of uropod III and telson
could not be compared directly.
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When analysing the successive characteristics of the families Crangonyctidae and Gammaridae in the papers
by Bousfield (1977, 1982), Holsinger (1977, 1986) and Zhang and Holsinger (2003), one may conclude that the
basic diagnostic characters for these groups are: a) in Crangonyctidae—stick-like calceoli, presence of sternal gills,
row of bifid setae on gnathopod palms, lack of abdominal tergite armature, U III often reduced with vestigial or
even absent endopodite, telson entire or concave, if cleft—then not more than half of its length; b) in
Gammaridae—mushroom-like calceoli, setae of gnathopod palms not bifid, no sternal gills, abdominal tergites
usually with setae, U III well developed, endopodite, if reduced, at least distinctly scale-like, telson deeply cleft,
usually to its base. According to the recent classification by Lowry and Myers (2013), members of the parvorder
Crangonyctidira Bousfield, 1973 (i.e. all crangonyctoid families) are characterized by maxilla I ischial endite
setose apically, stalked coxal gills and minute or absent endopodite of uropod III. The same authors define
members of Gammaridira Latreille, 1802 (i.e. all gammaroid families) by having unstalked coxal gills, absent
sternal gills and endopodite of uropod III well developed, shorter than exopodite.
Taking into account the above diagnostic features, in our opinion, placement of the genus Palaeogammarus
within the family Crangonyctidae seems to be at least disputable if simply not wrong.
Obviously, many important diagnostic features are hardly preserved or invisible in fossil taxa. For example,
calceoli were never observed in Palaeogammarus. These delicate structures, if present, were surely the first to
break off or disappear. Also, any structures situated in the sternal cavity cannot be visible in amphipods embedded
in amber. The structure of gnathopod palms was usually not visible or poorly distinguishable. However, in the
presently described P. debroyeri as well as in P. balticus the gnathopod palms were relatively well visible, but it
was impossible to recognize the detailed structure of palmar spines. In the remaining Palaeogammarus species the
gnathopod palms were invisible. On the other hand, contrary to crangonyctid taxa, the armature of posterior
margins of abdominal segments is well illustrated (Zaddach 1864, Jażdzewski & Kulicka 2002) or described
(Lucks 1928) for all Palaeogammarus, including P. debroyeri, with the only exception of P. danicus. Also, in all
Palaeogammarus species, the uropod III, or at least its outer ramus, is rather strong (except in P. sambiensis).
Besides, in the newly described species, the U III exopodite is clearly bi-segmented. In contrast to crangonyctoid
taxa, the presence of small but rather well developed U III endopodite was observed in the presently described
species. It was also illustrated by Coleman (2006: Fig. 2c) and seems to be present already in the drawing by Lucks
(1928: Fig. 4). Finally, the telson in Palaeogammarus, if visible, is comparatively long and narrow, deeply cleft and
definitely more resembling that body part in gammaroids than in crangonyctoids.
However, an even more convincing argument for placing Palaeogammarus in Gammaridae instead of
Crangonyctidae is the mate guarding we discovered in P. debroyeri (Fig. 9)—the very first insight in praecopulatory behaviour of fossil amphipods. Among modern amphipod taxa, mate guarding is known in Hadzioidea
S. Karaman, 1943 (Bousfield 1983), Corophioidea Leach, 1814, Caprellidea Leach, 1814, Talitroidea Rafinesque,
1815 and Gammaroidea Latreille, 1802 (Bousfield, 1977), while Crangonyctoidea Bousfield, 1973 do not guard
females during mating (Conlan 1991). It may not be excluded that extinct crangonyctoids also displayed the
“gammarid-style” of female guarding in praecopula and lost it in course of evolution, yet such scenario would be
difficult to justify, particularly that their lifestyle seems not changed that much since Eocene (Just 1974, Zhang &
Holsinger 2003).
Acknowledgments
The authors wish to thank Mr. Jürgen Velten for offering the studied amber pieces for purchase. Thanks are also
due to Dr. Ryszard Szczęsny, the director of the Museum of Earth, Polish Academy of Sciences in Warsaw for their
purchase and a loan to us. The authors wish to acknowledge Dr. Piotr Jóźwiak for preparing all drawings. We are
grateful to Prof. Dr. John R. Holsinger, as well as to Prof. Dr. Edward L. Bousfield for fruitful discussions. The
review from Dr. Charles O. Coleman helped us to improve the quality of our work. Last but not least, we cordially
thank Dr. Jim Lowry for all his efforts in editing the manuscript.
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