First inclusion of a trichiine beetle (Coleoptera: Scarabaeidae) from Baltic amber

Abstract

Baltic amber forms the largest known Konservat-Lagerstätte of fossil plant resin and the richest repository of fossil insects of any age on Earth. Despite a long history of palaeobiological research of Baltic amber (over 200 years) and really intensive taxonomic study in the last decade, many interesting and new bioinclusions await scientific attention. In the current paper, the brief report of an unique large flower chafer inclusion from the Early Tertiary Baltic amber is provided. The author decided not to formally create a new genus and not to subjectively assign the Eocene beetle to an extant genus. However, the specimen seems to be remarkable and surprising in different aspects. The reported Eocene beetle is: 1) one of the oldest members of the subfamily Cetoniinae and also the earliest fossil record of the tribe Trichiini; 2) the first known representative of the subfamily in Baltic amber; 3) the largest known beetle fossilized in amber; and 4) an unique example of mineralized fossil included in amber.

Full text

Palaeoentomology 002 (5): 425–429
https://www.mapress.com/j/pe/
Copyright © 2019 Magnolia Press
Short PALAEOENTOMOLOGY
ISSN 2624-2826 (print edition)
ISSN 2624-2834 (online edition)
Accepted by D.-Y. Huang: 10 Oct. 2019; published: 31 Oct. 2019 425
PE
Communication
Baltic amber forms the largest known Konservat-
Lagerstätte of fossil plant resin and the richest repository
of fossil insects of any age on Earth. Despite a long
history of palaeobiological research of Baltic amber (over
200 years) and really intensive taxonomic study in the
last decade, many interesting and new bioinclusions await
scientific attention. In the current paper, the brief report
of an unique large flower chafer inclusion from the Early
Tertiary Baltic amber is provided. The author decided not
to formally create a new genus and not to subjectively
assign the Eocene beetle to an extant genus. However,
the specimen seems to be remarkable and surprising in
different aspects. The reported Eocene beetle is: 1) one of
the oldest members of the subfamily Cetoniinae and also
the earliest fossil record of the tribe Trichiini; 2) the first
known representative of the subfamily in Baltic amber;
3) the largest known beetle fossilized in amber; and 4) an
unique example of mineralized fossil included in amber.
Material and methods
The amber piece “6651” containing the studied specimen
is deposited in the collection of Kaliningrad Amber
Museum, Russia [KAM]. The specimen is included in
yellow semitransparent amber piece with approximate
dimensions of 31×16×14 mm. Syninclusions consist
of numerous detrital particles. The amber piece is not
subjected to any supplemental fixation.
Systematic palaeontology
Family Scarabaeidae Latreille, 1802
Subfamily Cetoniinae Leach, 1815
Tribe Trichiini Fleming, 1821
Genus indet.
Figs 1, 2A.
Material. No. 6651 [KAM], adult, sex unknown.
Preservation. Strongly damaged and partially
air-exposed remains of beetle in the split amber piece,
abdomen and metathorax covered with dense “milky”
amber opacity. Head and prothorax completely absent,
elytral dorsal surface, metasternum and scutellar shield
damaged, ultimate segments of metatarsi cut off. The air-
exposed body parts are mineralized (probably replaced by
calcium phosphate due to the anaerobic conditions of the
glauconitic lagoon sediments).
Horizon and locality. Baltic amber (mid-Eocene,
45.0–38.0 Ma according to Bukejs et al. 2019). Yantarny
settlement (formerly Palmnicken), Sambian (Samland)
peninsula, the Kaliningrad region, Russia.
Description. Thorax. Elytra subparallel-sided.
Length of elytron 12 mm, humeral width of elytron 6.6
mm. Metepisternum 1.3 mm wide, with rounded outer
posterior margin. Scutellar shield wide, triangular,
rounded apically.
Legs. Metacoxae approximate. Metafemora simple,
6 mm long. Femoral maximal width 2.5 mm. Length of
metatibia 6.5 mm, maximal metatibial width 1.5 mm.
Metatibiae with 2 apical spurs of a little unequal length
and one rounded medial denticle on outer margin.
Tibial apices weakly lobate. Relative length ratios of
metatarsomeres 1–3 equal to 3.5-3-3.
Remarks. Large body size and the specific body form
suggest the interpretation of the beetle as a member of the
family Scarabaeidae. Despite absence of many body parts
(e.g., head with antennae, pronotum, procoxae, protibia,
tarsal claws) bearing important diagnostic characters and
insufficient visibility of obscured characters (e.g., the
number and proportions of ventrites), several remaining
characters led us to conclusion about possible placement
of the fossil. The specimen under consideration was
assigned to the tribe Trichiini within the subfamily
Cetoniinae based on a combination of the following
characters: 1) elytra shortened, wide (about 0.9× as long
as their combined width) and separately rounded at apices;
2) pygidium and apical portion of propygidium exposed;
https://doi.org/10.11646/palaeoentomology.2.5.4
http://zoobank.org/urn:lsid:zoobank.org:pub:81A9F646-9120-457A-BB82-9FBA1863712C
First inclusion of a trichiine beetle (Coleoptera: Scarabaeidae) from Baltic amber
VITALII I. ALEKSEEV1, 2
1 Shirshov Institute of Oceanology, Russian Academy of Sciences, Nahimovskiy prospekt 36, 117997 Moscow, Russia.
2 Kaliningrad Regional Amber Museum, Marshal Vasilevskii square 1, Kaliningrad, 236016, Russia.
E-mail: alekseew0802@yahoo.com

ALEKSEEV
426 • Palaeoentomology 002 (5) © 2019 Magnolia Press
FIGURE 1. Amber piece No. 6651 [KAM] with trichiine beetle inclusion. A, Pygidium and elytral apex, dorso-caudal view. B,
Metathorax and hind leg, ventro-frontal view. C, Metathorax, abdomen, and hind leg, lateral view. Scale bar = 1.0 mm.

FIRST INCLUSION OF A TRICHIINE BEETLE Palaeoentomology 002 (5) © 2019 Magnolia Press • 427
FIGURE 2. Eocene trichiine inclusion in Baltic amber and representatives of the tribe in the natural conditions of the Recent
Kaliningrad region. A, Amber piece No. 6651 [KAM] with trichiine beetle inclusion, blue arrow indicates the shortened elytra with
separately rounded apices and exposed pygidium. B, Gnorimus variabilis (Linnaeus, 1758). C, G. nobilis nobilis (Linnaeus, 1758).
D, Trichius fasciatus (Linnaeus, 1758). Not reproduced to the same scale.

ALEKSEEV
428 • Palaeoentomology 002 (5) © 2019 Magnolia Press
3) apex of metatibia with two spurs; 4) metatibia with one
external elevation; 5) posthumeral elytral emargination
lacking and mesepimeron not visible from above; and
6) metacoxae not widely separated. The beetle size and
above mentioned characters may distinguish this fossil
specimen from the representatives of the other extant
Palaearctic tribes of Cetoniinae (Krikken, 1984; Bezděk,
2016), such as Cetoniini, Cremastocheilini, Goliathini,
Gymnetini, Osmodermatini, Phaedimini, Schizorhinini,
Taenioderini and Valgini. A full morphologic comparison
with these cetoniine tribes is impossible using the
incomplete specimen at our disposal. Therefore, the
proposed placement of the fossil within tribe Trichiini
should be considered provisional.
The fossil specimen seemingly resembles the recent
Old World genus Gnorimus Le Peletier & Audinet-
Serville, 1828 in size, general form and proportions
of body fragments, but the fossil unfortunately lacks
systematically important characters for indisputable and
objective attribution to any extant or extinct genus.
Discussion
The largest known fossil in the family Scarabaeidae is
the Middle Eocene Oryctoantiquus borealis belonging
to the subfamily Dynastinae [known from external mold,
reconstructed body length 50.0 mm] (Ratcliffe et al.,
2005). Amber inclusions tend to be dominated by small
organisms: there are few records of large insects and small
vertebrates in any amber (Martínez-Delclòs et al., 2003).
The maximum size of inclusion in fossil resin is unknown,
but probably is a few centimeters in length. The beetles
larger than 10 mm in length are very rare in museum
collections and scientific records. The possibly largest
described beetles from Baltic amber are the representatives
of the family Lucanidae: Succiniplatycerus berendti
(Zang, 1905) [total body length 12 mm (Zang, 1905)] and
Dorcasoides bilobus Motschulsky, 1856 [“5.5 lign.” i.e.,
12.4 mm long (Motschulsky, 1856)]. The largest beetle
known from Dominican amber is the trichiine beetle
Paleotrichius dominicanus Poinar, 2011 [15.7 mm long].
The herein reported beetle inclusion from Baltic amber
is remarkable in body size, the total length of the beetle
remains comprises 14 mm. The comparison of the body
fragments with the analogous ones of similarly looking
recent European Gnorimus—species and extrapolation
from the specimens of recent Gnorimus variabilis
(Linnaeus, 1758) and Gnorimus nobilis (Linnaeus, 1758)
allows the estimation of the fossil beetle total body length,
which should be approximately 20 mm long. Hence, the
reported specimen is unique in respect to body size and is
one of the largest beetles known from any fossil resin.
The representatives of the subfamily Cetoniinae
(Coleoptera: Scarabaeidae) are extremely rare in amber
and are poorly represented in the fossil record (Krell,
2000, 2006, 2007). The only species described from amber
is Paleotrichius dominicanus Poinar, 2011 from Miocene
Dominican amber, approximately 17 Ma (Poinar, 2011).
The oldest known fossil reported as Cetoniinae is an
unnamed taxon from Eckfelder Maar [Eocene oil shales,
approximately 44 Ma] (Wappler, 2003). The herein
reported specimen represents the first known cetoniine
beetle in Baltic amber, the earliest fossil record of the tribe
Trichiini, as well as one of the oldest subfamily records
(approximately contemporaneous to a previous record
from mid-Eocene of the westernmost Germany).
The studied specimen No. 6651 [KAM] is remarkable
also in the taphonomic respect. The air-exposed beetle
body parts are seemingly mineralized (probably replaced
by calcium phosphate) during the process of diagenesis.
Such naturally phosphatized inclusion in Baltic amber
is unique and was probably formed in the anaerobic
conditions of the glauconitic lagoon sediments, where
several cracks in the copal-like resin were formed. The
defragmentation of the final amber piece and the partial
loss of the phosphatized inclusion took place during
the present-day extraction of the amber piece from the
succinite-bearing layers.
Larvae of the extant representatives of the tribe
Trichiini occur in rotten wood of deciduous trees, but
adults feed on flowers and effluent tree sap (Figs. 2B–D).
In all likelihood, the fossil was related to such a kind of
food and inhabited the broad-leaved trees of the Eocene
“amber forest”.
Acknowledgments
I thank Mrs. Anna V. Smirnova and Dr. Andranik R.
Manukyan (Kaliningrad Regional Amber Museum,
Russia) for the opportunity to study this intriguing fossil.
I am very thankful to Mrs. Christel Hoffeins (Hamburg,
Germany) and two anonymous reviewers who provided
valuable feedback to help me improve the manuscript.
The study of the author was done with support of the
state assignment of IO RAS (Theme No. 0149–2019–
0013).
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