A male representative of the extinct species
Records of Galerucinae sensu stricto (excluding Alticini) from fossil resins are
scarce. Until now only four species have been described:
Baltic amber originally occurs within the “blaue Erde” (“Blue Earth”)
horizon, which can be found throughout the Baltic Sea coastal area in Europe
(Kaliningrad region, Russia; Poland; Denmark; Sweden; Germany; and
Lithuania) (Engel, 2001). Although most estimates of the age of Baltic amber
have suggested that it is derived from the early part of the middle Eocene
(Lutetian, 47.8–41.2 Ma) based largely on K–Ar dating (Ritzkowski, 1997),
palynological biostratigraphy of the specific region where the sample
originated suggests a younger, late Eocene (Priabonian, 37.8–33.9 Ma) age
(Aleksandrova and Zaporozhets, 2008). An intermediate, middle Eocene (mostly
Bartonian, 41.2–37.8 Ma) age was recently proposed for the extinct central
European resin-producing forests according to the stratigraphy of the
Sambian amber deposits (Bukejs et al., 2019). Following classical views
(e.g. Andrée, 1951; Poinar, 1992; Turkin, 1997), Baltic amber is
thought to have been produced by
The material examined is deposited in the Palaeontology Collection of the Royal Saskatchewan Museum (Regina, Saskatchewan, Canada) (RSKM specimen number prefix).
Observations of the studied beetle specimen were made using a Nikon SMZ745T stereomicroscope. Photographs were taken using a Visionary Digital imaging system, consisting of a Canon EOS 5D camera with a Canon MP-E 65 mm macrophotography lens, attached to an automated camera lift with studio flash lighting. Extended depth of field at high magnifications was achieved by combining multiple images from a range of focal planes using Helicon Focus 6.8.0 software, and the resulting images were edited to create figures using Adobe Photoshop CS5.
The X-ray micro-CT observations of specimen RSKM_P3300.139
were conducted at Daugavpils University, Daugavpils, Latvia (DU), using a
Zeiss Xradia 510 Versa system. Scans were performed with a polychromatic
X-ray beam at an energy of 40 kV and power of 3 W. Sample-to-detector
distance was set to 17.5 mm, and source-to-sample distance was 47.7 mm. Tomographic
slices were generated from 3001 rotational steps through a 360
Family Subfamily Tribe Subtribe Genus Figs. 1–4
Collection number RSKM_P3300.139, male. A complete beetle is
included in an elongate, transparent, yellow amber piece with dimensions of
41 mm
Baltic amber from Eocene amber-bearing Blue Earth layers (a primarily Bartonian age is interpreted for the extinct central European resin-producing forests, according to Bukejs et al., 2019).
Yantarny settlement (formerly Palmnicken), Sambian (Samland) Peninsula, Kaliningrad region, Russia.
Specimen RSKM_P3300.139 is morphologically similar to the
female holotype of
Measurements: total body length 3.5 mm; pronotum length 0.75 mm, pronotum maximum width 1.0 mm; elytra length 2.7 mm, elytra maximum width 1.9 mm.
Body elongate, slender, slightly convex dorsally and ventrally; reddish-brown as preserved, shiny, with weak metallic lustre; glabrous dorsally, with very fine, short, recumbent setation ventrally (distinctly visible on pygidium and abdomen).
Head hypognathous, relatively small, without distinct punctures; head
together with eyes slightly wider than anterior margin of pronotum; vertex
slightly convex. Compound eyes directed laterad, oval in outline, moderately
large, prominent, entire, with distinct facets; vertical diameter
Legs slender, moderately long; covered with short, semierect setae.
Procoxae widely oval and slightly transverse,
Abdomen with five visible ventrites, densely covered with fine punctures. Ventrite 5 with deep, oval fovea medially; posterior margin trilobed (median lobe moderately narrow with rounded apical margin; incisions short). Relative length ratios of ventrites 1–5 equal to 18 : 10 : 5 : 6 : 12 (measured medially). Pygidium evenly covered with dense, fine punctures.
Aedeagus (Fig. 4) lanceolate, with sharp triangular apex; in lateral view, aedeagus moderately curved, with apical one-third of length thin, and middle part wide; in ventral view, aedeagus with wide, V-shaped, longitudinal furrow in middle part of length, and with longitudinal, medial carina in apical one-fifth of length. Tegmen Y-shaped.
Sexual dimorphism can be an important feature useful in morphological, taxonomic, and phylogenetic studies. Sexual dimorphism consists of both primary and secondary sexual characteristics. The former include the features of the reproductive system (e.g. chromosomes, gonads, and genitalia), while the latter are very diverse and include different external (morphological) and internal (e.g. hormonal) adaptations. Secondary characteristics develop within each sex to facilitate ecological (e.g. feeding), behavioural (e.g. copulation, migration, offspring care) and other differences between specimens belonging to each sex. Features such as wing development, antennal length ratios, the form and size of sense organs or mandibles, tarsal setation and dilatation, horns, teeth, and different structural modifications of the exoskeleton (like furrows, foveae, projections on pronotum and elytra etc.), total body size, and colour variability are among the many well-known sexually dimorphic characters in Recent Coleoptera (Arrow, 1951; Crowson, 1981; Klausnitzer, 2002; Gullan and Cranston, 2014).
Sexual dimorphism in extant members of the tribe Luperini (Coleoptera:
Galerucinae) is characterized as follows: males always have a trilobed
abdominal ventrite 5, often with the middle lobe impressed; meanwhile,
abdominal ventrite 5 is always simple and entire in females; often the males
have slightly wider first tarsomeres and longer antennae (e.g. Wagner,
2003; Bezděk, 2015). Rarely the males of some luperin genera have also
modified head, elytra or legs (Mohamedsaid and Furth, 2011; Prado, 2013).
However, the sexually dimorphic characters present on the abdomens of fossil
representatives of Luperini have never been described. The description of
In general, amber specimens differ from compression fossils, where the sex
of specimens is rarely determinable, and reports of sexually dimorphic
characters are the exception, rather than the rule. Consequently,
three-dimensionally preserved inclusions in fossil resins have the capacity
to provide unique data on sexual dimorphism in extinct lineages. Sexual
dimorphism in fossil beetles has usually been documented as an incidental
observation in taxonomic papers (e.g. Lyubarsky and Perkovsky, 2015; Jiang
et al., 2019). It has only received more extensive coverage in a few papers
focused on groups such as fossil silvanid beetles like
There are several characteristics that can be used for sex determination in
fossil species within different beetle groups known from Baltic amber, and
these appear to have been stable since at least the Eocene. Dimorphic
characteristics include:
modification of abdominal ventrites: shape of the last abdominal segments
in Cantharidae (e.g. Kuśka and Kupryjanowicz, 2005; Fanti and Sontag,
2019 among others), and in Staphylinidae (Bogri et al., 2018); ventrite 1
with median, oval plaque in Monotomidae (Bukejs and Alekseev, 2015a); modification of antennomeres in Ripiphorinae (Batelka et al., 2020); pronotal shape in Cerambycidae (Vitali, 2006); elytral shape in Staphylinidae (Shavrin and Yamamoto, 2019); relative lengths of antennae in Cerambycidae (Vitali, 2018) and in
Cantharidae (Parisi and Fanti, 2019); lateral pronotal lobes in Cantharidae (e.g. Parisi and Fanti, 2019); shape of protarsi and ventral surface of protarsomeres in Carabidae
(Gamboa and Ortuño, 2015, 2018; Schmidt, 2015), Chrysomelidae (Bukejs
and Konstantinov, 2013), Dytiscidae (Balke et al., 2010), and
Melandryidae (Bukejs and Alekseev, 2015b); exposed distal parts of male and female genitalia in Aderidae (Alekseev
and Grzymala, 2015), Anthicidae (Telnov and Bukejs, 2019), Cantharidae
(Kazantsev, 2013), Chrysomelidae (Bukejs and Chamorro, 2015; Bukejs et al.,
2015, 2016), Cupedidae (Kirejtshuk, 2005), Ischaliidae (Alekseev and Bukejs,
2017), Lycidae (Kazantsev, 2019), Melandryidae (Alekseev, 2014),
Pyrochroidae (Bukejs et al., 2019), Scirtidae (Heuss, 2008; Klausnitzer,
2012), Staphylinidae (Jałoszyński et al., 2018; Brunke et al., 2019;
Shavrin and Yamamoto, 2019), Tenebrionidae (Nabozhenko et al., 2019a, b;
Telnov et al., 2019; Alekseev et al., 2020a), Zopheridae (Alekseev and
Bukejs, 2016) and other groups.
Recent technological advances have allowed researchers to identify the sex
of specimens with greater confidence and expand their study of features
with significance to sex determination in amber inclusions. X-ray
microtomography data obtained from lab-based machines or synchrotron
beamlines have allowed sex determination in fossils without visibly exposed
genitalia. The optically hidden, internal genitalia have been successfully
studied in fossil species of Carabidae (Schmidt et al., 2016, 2017, 2019;
Schmidt and Michalik, 2017), Latridiidae (Reike et al., 2017), Leiodidae
(Perreau and Tafforeau, 2011; Perreau, 2012; Perreau and Perkovsky, 2014),
Merophysiidae (Reike et al., 2020), Mycetophagidae (Alekseev et al., 2020b), Tenebrionidae (Nabozhenko et al., 2020), and other coleopteran families. To date,
the sexual dimorphism that has been discovered in Eocene Baltic amber
beetles can be characterized as “evolutionarily stable” and limited to
characters known in present-day relatives: no new sexually dimorphic
characters have been discovered in Eocene fossils. The continued search and
accurate study of the Baltic amber beetle assemblage may possibly yield
future discoveries in this respect, enabling us to provide further details
on the morphology of fossil beetles and the range of variation present
within individual taxa.
Volume renderings of X-ray microtomography of the habitus, pterothorax and
aedeagus of male
X-ray micro-CT volume renderings of the habitus, pterothorax, and aedeagus of the male
AB designed the study. AB and JB identified the specimen and performed
systematic placement. AB prepared specimen description and plates. KK
performed
The authors declare that they have no conflict of interest.
The authors are grateful to José Miguel Vela (Instituto Andaluz de Investigación y Formación Agraria y Pesquera (IFAPA), Málaga, Spain) and Shuhei Yamamoto (Field Museum of Natural History, Integrative Research Center, Chicago, USA) for their helpful comments and corrections to an earlier version of this manuscript. The study of Vitalii I. Alekseev was done with the support of the state assignment of IO RAS (theme no. 0149–2019–0013).
This research has been supported by the IO RAS (grant no. 0149-2019-0013).
This paper was edited by Torsten Scheyer and reviewed by José Miguel Vela and Shuhei Yamamoto.