A New Species of Eocene Whitefly—Gregorites michalskii sp. nov. (Hemiptera, Sternorrhyncha, Aleyrodidae) from Baltic Amber

Abstract

This paper describes a new species of extinct whitefly in the genus Gregorites Drohojowska & Szwedo, 2024, from the family Aleyrodidae. This species has been identified based on an inclusion in Eocene Baltic amber. This represents the eighth documented species of the genus and contributes new data to our knowledge of the disparity and taxonomic diversity of whiteflies in Eocene Baltic amber. The discovery of a further Gregorites species, previously known only from succinite on the western coast of Jutland, raises questions about the amberiferous deposits, their geology and history, and on palaeodistributions of the whiteflies. These are briefly reviewed and discussed.

1. Introduction

The Hemiptera, an order of hemimetabolous insects, is distinguished by its piercing–sucking mouthparts, which are employed for feeding on plants, fungi, and other animals. This order is classified as one of the ‘Big Five’ insect orders, distinguished by its unique characteristics within the group. Notably, it is the sole insect order that exhibits incomplete metamorphosis. The other insect orders within this group are characterized by their richness in species. It is evident that the orders Coleoptera, Hymenoptera, Diptera and Lepidoptera collectively represent holometabolous insects [1,2]. The classification system under consideration is divided into six suborders. One of these suborders is the Sternorrhyncha, which comprises the following lineages: modern aphids, phyloxerans, adelgids, scale insects, psyllids and whiteflies. It should be noted that this suborder also includes some extinct lineages [3,4]. Whiteflies (Aleyrodidae) have been identified in the fossil record since the Middle Jurassic [5], yet their presence in the Eocene appears to be particularly abundant. The infraorder comprises a single family, Aleyrodidae, Westwood, 1840 [6], which is currently subdivided into four subfamilies: the extinct Bernaeinae Shcherbakov, 2000 [7], the recent Aleyrodinae Westwood, 1840 [6], Aleurodicinae Quaintance et Baker, 1913 [8], and Udamoselinae Enderlein, 1909 [9]. The latter of these is of questionable taxonomic status. The oldest insectiferous Eocene resin in Europe is Oise amber, which yielded several Aleurodicinae taxa (four genera and four species) [10]. The most abundant resins, collectively designated as Baltic amber (succinite), have produced Eocene specimens of the subfamilies Aleyrodinae (comprising two genera and three species) and the Aleurodicinae (consisting of five genera and 11 species). The broad concept of “Baltic amber” with respect to inclusions should be abandoned in favour of a more restrictive and precise definition of the area of origin of the resin [11,12]. The majority of these deposits are allochthonous, whilst a smaller number may be parautochthonous, and they appear to have been alimented from independent geographical sources [11,12].

In this study, we present a new species of the genus Gregorites, which was discovered as an inclusion in succinite, from the secondary deposits of the Gulf of Gdańsk.

2. Materials and Methods

2.1. Geological Setting

Baltic amber, a collective name for various fossil resins and more commonly referred to as succinite [12,13,14], is the earliest known and still the best studied fossil resin [15,16,17]. Geological exploration for Baltic amber deposits has been carried out historically and in contemporary times in many places across Poland, but the Vistula Delta and the Vistula Spit are invariably the sources of greatest interest [18,19]. The amber in Poland occurs in the Paleogene formations (in situ accumulations), connected with the northern marginal zone of the Eocene Sea, and the Quaternary formations (secondary accumulations). The in situ amber accumulations in the northern zone, the so-called Chłapowo-Sambian delta, are the most commonly explored—the richest amber-bearing sediments (the layers of so-called ‘blue earth’) occur at a depth between 60 m and 130 m [20]. The Quaternary amber-bearing accumulations can be spotted in sediments relocated by the glacier and post-glacier rivers from the degraded Paleogene deposits, mainly originating from the Chłapowo-Sambian delta sediments. The amber concentrations are also encountered on the Baltic beaches from Kołobrzeg to the eastern border of Poland on the Vistula Spit, where amber occurs in beach fossil sediments. Another amber occurrence in the Paleogene sediments is the Możdżanowo region near Ustka—the amber-bearing sediments are represented by sands and gravel of the Upper Eocene. They form the glacial ice float within the Quaternary sediments. The amber-bearing series is characterized by an average thickness of 1.98 m and is located at a depth of about 11 m [19]. The age of the amber is controversial, though it is generally interpreted as having been produced during the Eocene epoch (56–34 million years ago). Different authors [12,13,20,21] have given estimates of 40–47 million years and 35–43 million years as the age of the amber. Precisely dating the amber is difficult due to its redeposited context [12,13,15,16,21].

The amber occurrences within the beach zone of the Baltic Sea (especially the eastern part) are the subject of collectible and ‘professional’ gathering (so-called amber divers), and the specimen studied comes from such a source.

2.2. Morphology and Documentation

The specimen studied is a piece of Baltic amber (succinite), probably from the Gulf of Gdańsk deposit, and comes from the collection of Mr. Artur Michalski (Wrocław, Poland). Observations were made at the Institute of Biology, Biotechnology and Environmental Protection, University of Silesia, using Nikon SMZ25, Nikon SMZ1500, Nikon SMZ1270, and Leica M205C stereoscopic microscopes and a Nikon Eclipse E600 microscope; observations and documentation were made with direct and transmitted light (Nikon, Tokyo, Japan; Leica, Wetzlar, Germany). Additional observations and photographs were made with a Nikon Microphot-FX microscope and a Nikon DS-Fi2 digital camera with a DS-U3 controller, under the control of NIS Elements D 5.20.01 software, and with a Nikon Eclipse E600 microscope with a Nikon DSFi2 digital camera, under the control of NIS Elements D 4.20.03 software; and Nikon SMZ1500 with a Nikon DS Fi3 camera, under the control of NIS Elements D 5.01.00 software, the photographs were edited using Adobe Photoshop Elements 6.0. Photographs were also taken in the Laboratory of Evolutionary Entomology and Museum of Amber Inclusions, Department of Invertebrate Zoology and Parasitology, University of Gdańsk, using an Olympus SZX10 equipped with an EP50 digital camera and EPview Version 3.7.2 (Olympus Soft Imaging Solutions GmbH freeware, Münster, Germany). The Fourier transform infrared spectrum (Figure 1) was obtained in the Amber Experts company laboratory, Gdańsk, using a Thermo Scientific Nicolet 380 FT-IR Spectrometer (Waltham, MA, USA), with ATR and baseline correction, for the reasons and according to the procedure proposed by Szwedo & Stroiński [22]. The photographs were readjusted using CorelDRAW X7 (Corel Corporation, Ottawa, ON, Canada). Drawings were made with the aid of a camera lucida attached to an Olympus SZX10 stereomicroscope. The morphological terminology is given by Drohojowska & Szwedo [23]. The publication is registered in ZooBank under the following LSID identifier: LSID: urn:lsid:zoobank.org:pub:EDB7F3BB-80A1-4B72-93D2-EA7863F7B7D5.

3. Results

Systematic Palaeontology

• Order: Hemiptera Linnaeus, 1758 [24]
• Suborder: Sternorrhyncha Amyot et Audinet-Serville, 1843 [25]
• Infraorder: Aleyrodomorpha Chou, 1963 [26]
• Family: Aleyrodidae Westwood, 1840 [6]
• Subfamily: Aleurodicinae Quaintance et Baker, 1913 [8]

• Genus: Gregorites Drohojowska et Szwedo, 2024 [12]
• Type species: Gregorites aegiri Drohojowska et Szwedo, 2024; by original designation
• Composition: Gregorites aegiri Drohojowska et Szwedo, 2024; G. bergelmiri Drohojowska et Szwedo, 2024; G. skadii Drohojowska et Szwedo, 2024; G. bestlae Drohojowska et Szwedo, 2024; G. thrymi Drohojowska et Szwedo, 2024; G. halogii Drohojowska et Szwedo, 2024; G. ymiri Drohojowska et Szwedo, 2024 [12]; and G. michalskii sp. nov.
• Gregorites michalskii sp. nov., Drohojowska et Szwedo
• LSID: urn:lsid:zoobank.org:act:CB63DC80-5ACC-4990-B65F-212456458D6D
• (Figure 2A–D and Figure 3A–E)
• Etymology. The specific epithet was given in honour to Mr. Artur Michalski (Wrocław, Poland), an amber collector and an excellent photographer of amber inclusions, who kindly donated the specimen for research.
• Holotype. Male. It was part of the Artur Michalski collection and was deposited in the collection of the Museum of Amber Inclusions of the University of Gdańsk, MAIG 7228. It is a small piece of amber measuring 21 mm × 14 mm × 3 mm, and is approximately oval in shape. The specimen is preserved and in good condition, and the dorsal and ventral sides of the inclusion are generally well preserved; the anteroventral side is partly covered by a milky veil (milky coating emulsion, ‘Verlumung’). Gas bubbles are partly obstructing the hind wing and tip of the antenna. Biosyninclusions: Acari (larva), stellate hairs; other inclusions: gas bubbles and mineral intrusions between layers of resin.
• Locality and stratum. Southern coast of the Gulf of Gdańsk; secondary deposit (Quaternary) or xenolith of the middle to upper Eocene age (Pomeranian Formation) [18,19,20,27].
• Diagnosis. The fore wing vein CuP (claval vein) reaches half of the fore wing length (longer than half of the fore wing in G. aegiri, shorter in G. ymiri, G. hallogi and G. skadii). The metatibia lacks a row of setae forming a comb (the metatibial comb is present in other species of the genus, except G. bestlae). There are long claspers that are longer than the pygofer, distinctly hooked at the apex and crossed apically. The total length of the body is the longest among the known species of the genus.
• Description. The total length is ca. 1.48 mm, including the claspers. The head includes compound eyes and is narrower than the pronotum, with a vertex trapezoid, convex disc, a slightly arcuate anterior margin, an acutely concave posterior margin and lateral margins that diverge posteriad. The compound eyes are easily visible in the dorsal view, with ommatidia of similar size that are arranged in rows. The lateral ocelli are easily visible in the dorsal view, and the anteriad is half of the vertex length and is placed above the compound eyes. The vertex smoothly shifts to the frons, and the frons has a median concavity but lacks a median ocellus. The postclypeus is concave, with an enlarged and protruding anteclypeus and a triangular and small clypellus. Narrow loral plates and narrow genae are present. The compound eyes are not divided, and the ommatidia are of a similar size and are arranged in regular rows. The antennal fovea margins are slightly elevated. Antenna with seven antennomeres, a cylindrical scapus (first antennomere) that is about twice as long as it is wide, an elongated and barrel-shaped pedicel (second antennomere) and a slightly widening apicad are present. The third antennomere is the longest, and the fourth and seventh antennomeres are cylindrical and of a similar length. The apical one is a tapering apicad (Figure 3A,B). The base of the rostrum is thick and easily visible, with two segments of a visible labium that is relatively short and an apex that reaches the tips of the procoxae.

Figure 2. Gregorites michalskii sp. nov., holotype, MAIG 7228: (A,B) Body in dorsal view. (C,D) Body in ventral view; scale bars = 0.25 mm.

Figure 2. Gregorites michalskii sp. nov., holotype, MAIG 7228: (A,B) Body in dorsal view. (C,D) Body in ventral view; scale bars = 0.25 mm.

A description of the pronotum is as follows: short, wider than the head with compound eyes, a widening laterad, arcuate, the anterior margin is subparallel to the posterior margin and rounded lateral margins. A description of the mesopraescutum is as follows: fan-shaped, arcuate anterior margin, straight posterolateral margins, converging posteriad and a rounded apex. A description of the mesoscutum is as follows: wide, M-shaped, narrower than the pronotum, distinctly narrow in the median portion, a deeply incised anterior margin, a posterior margin that is deeply and arcuately incised in the middle, concave lateral margins, elevated and rounded anterior angles, straight anterolateral margins, diverging and acute lateral angles and straight posterolateral margins. A description of the mesoscutellum is as follows: in the form of a narrow band, arcuate anterior margins, the posterior margin is shallowly arcuate, acute lateral sections and it diverges posteriad. A description of the mesopostnotum is as follows: triangular, anterior margin with a slightly arcuate median portion, the lateral portions more arcuate and the posterolateral margins are slightly convex and converge to the acute tip. The parapteron is relatively large and ellipsoid. A description of the metascutum is as follows: large, distinctly narrow, incised in the median section, the anterior margins are almost straight, the anterolateral angles are acute, the posterolateral margins are slightly arcuate, the posterolateral angles are wide and angulate and the posterior margin is shallowly concave. A description of the metascutellum is as follows: transversely widened, the anterior margin is slightly arcuate in the median portion, the posterolateral sections are narrow and slightly arcuate and it is directed slightly posteriad.

Figure 3. Gregorites michalskii sp. nov., holotype, MAIG 7228: (A) Head and thorax in dorsal view. (B) Head in ventral view. (C) Wings. (D) Wax plates and genitalia. (E) Legs. Scale bars: 0.2 mm.

Figure 3. Gregorites michalskii sp. nov., holotype, MAIG 7228: (A) Head and thorax in dorsal view. (B) Head in ventral view. (C) Wings. (D) Wax plates and genitalia. (E) Legs. Scale bars: 0.2 mm.

A description of the fore wing is as follows: 1.26 mm long, 0.68 mm wide at its widest point, narrower at the base, a widening apicad and membranous with no traces of coloration. The margins of the fore wing are distinctly crenulate, including the claval margin, the ‘merlons’ and ‘embrasurses’ are equidistantly dispersed, the costal margin is thickened (slightly more thickened at the base), the costal margin is more arcuate at the base, there is a mildly arcuate to a widely arcuate anteroapical angle, the apex is widely arcuate, the posteroapical angle is truncate, the tornus is merely arcuate, the tornal angle is very widely open, the claval margin is slightly arcuate and the anteroclaval angle is widely rounded. The base of the veins Sc + R + CuA is short, and branch Sc + R is slightly shorter than branch Rs and is forked at about ½ of the fore wing length. Branch R1 diverges at an angle of about 45° from the main axis and is directed towards the anteroapical angle but does not reach the margin. Branch Rs diverges at an angle of about 20° from the main axis, is almost straight, is directed toward the apex of fore wing but does not reach the margin. The CuA stem is slightly sinuate, diverges at an angle of ca. 30° from the main axis and is directed towards the posteroapical angle but does not reach the margin. The claval fold (vein CuP) is visible, the apex of the clavus is at the level of ½ of the fore wing length. A very faint claval vein is visible.

The hind wing is membranous and shorter than the fore wing (ca. 0.88× of the fore wing length), is 1.19 mm long and 0.51 mm wide. The margins of the hind wing are crenulate and less distinct in the apical margins. The costal margin is almost straight, the anteroapical angle is widely arcuate, the apex is rounded, the posteroapical angle is widely rounded and the posterior margin is distinctly arcuate. There is no setae at the base, and no distinct setae of the coupling apparatus are visible. The stem Sc + R is slightly arcuate. Branch R1 diverges at an angle of about 45° from the main axis, and is slightly arcuate and more bent at the base. Branch Rs slightly diverges posteriad from the main axis, is slightly arcuate and does not reach the hind wing margin.

The prolegs appear to be the shortest among the legs, the procoxae are conical and partly visible, the protrochanter is scaphoid, the profemur is laterally compressed, the protibia is longer than the profemur and is 0.31 mm long, the subquadrate is in a cross section, the protarsus is two-segmented, the basiprotarsomere is 0.13 mm long and is longer than the apical protarsomere (0.11 mm), the tarsal claws are distinct and the paronychium is distinctly shorter than the claws.

The mesoleg is longer than the prolegs, the mesocoxa is subconical, the mesotrochanter is scaphoid, the combined length of the mesotrochanter and mesofemur is shorter than the length of the mesotibia (0.36 mm), the mesofemur is laterally compressed, the mesotibia is subquadrate in a cross section, there are no distinct combs or rows of setae, the mesotarsus is about 0.55 times the length of the mesotibia, the basimesotarsomere is longer (0.12 mm) than the apical tarsomere (0.08 mm), the tarsal claws are distinct and the paronychium is distinctly shorter than the claws.

The metalegs are the longest, the metacoxae are conical and slightly larger than the mesocoxae, the metatrochanter is well developed and scaphoid, the metafemur is shorter than the metatibia and is laterally compressed, the metatibia is slender and circular in a cross section, there are no combs of rows of setae, the apex is slightly wide with three apical setae, the basimetatarsomere is longer (0.17 mm) than the apical metatarsomere (0.1 mm), its combined length is 0.6 times that of the metatibia, the tarsal claws are distinct and the paronychium is distinctly shorter than the tarsal claws.

The abdomen with forceps is 0.9 mm long and 0.41 mm wide at its widest point, the first segment is distinctly constricted, segments III to V have distinct wax plates, segments VI to VIII are steeply narrow and the pygofer is cylindrical with a median, longitudinal, shallow concavity. The claspers are longer than the pygofer, are 0.21 mm long, are placed laterally and have a narrow base with external protuberances. The external margin is straight and curved mediad in the apical ⅓, the tips are hooked, acute and crossed, the internal margin is shallowly concave to ca. ⅓ its length. Then, it is convex with a protuberance in the median section and is slightly sigmoid towards the base; only the basal portion of the aedeagus is visible. It appears short, subconical, and curved at the base and is directed dorsad.

4. Discussion

Fossil resins have been demonstrated to be a highly valuable source of data, particularly with regard to bioinclusions, that is, ancient organisms and their remains embedded within the fossilized tree resins. These bioinclusions are regarded as being pivotal in facilitating our understanding of past events and the prevailing climatological and ecological conditions. The term “Baltic amber” is used to collectively denote a group of fossil resins (of the succinite type) that have been preserved for scientific studies as a whole, and studies of their inclusions (see [12,13,14,23]). These inclusions have been of scientific interest since the time of Linnaeus. Initially, the amber collected on the Baltic Sea coast, specifically from the Sambia Peninsula mines, constituted the sole source of inclusions. However, this situation underwent a significant transformation with the subsequent discovery of succinite-type fossil resins abundant in inclusions in other regions, including the environs of Bitterfeld in Germany [28], the Rovno and Zhitomir regions of Ukraine [29] and the Lublin area of Poland [30]. Furthermore, other sources of succinite, such as those found in the far north (e.g., Ellesmere Island, Axel Heiberg Island, the Canadian Arctic and western Spitsbergen in Norway [31]), give rise to further questions concerning the origin, depositional history and age of these fossil resins.

The aforementioned fossils provide a pertinent illustration of the recent challenges encountered in the interpretation of inclusions within the context of their geological history. The inclusion, designated as a newly described species, Gregorites michalskii sp. nov., is sourced from a piece identified as “Baltic amber”, which was recovered from the southern coast of the Baltic Sea, in the vicinity of Gdańsk. It is not possible to determine the layers from which the material was collected with any degree of certainty, given that it was collected on the beach. However, an analysis using FT-IR spectroscopy confirmed the fossil resin to be succinite, thereby indicating that the resin was formed during the Eocene period. The area’s complex depositional and geological history has resulted in several periods of amber redepositions [19,27]. Consequently, it is only possible to make general assumptions regarding the precise origin and age of this particular piece of succinite.

The genus Gregorites was described from the amber that had been found along the western coast of the Jutland region of Denmark [12]. Therefore, the discovery of another species assigned to this genus is of interest not only from a taxonomic perspective, but also as an opportunity to enhance our understanding of the taxonomic diversity and morphological disparity of the genus and extinct Eocene whiteflies in general. A further noteworthy observation pertains to the presence of representatives of this genus in deposits that extend significantly further towards the eastern regions. This prompts the following question: does the distribution of the Gregorites whiteflies represent a more extensive distribution that once occurred in ‘Baltic amber’ forests, or is it a consequence of amber transportation and redepositions that have occurred during the depositional history of the amber? It is not possible to ascertain the presence of any transportation marks on the piece, as it was cleaned and polished prior to examination. The new species Gregorites michalskii sp. nov. displays a unique combination of morphological characteristics, thus providing unequivocal evidence for its classification within the subfamily Aleurodicinae and the genus Gregorites [12,32]. The following generic characters were observed: the absence of a median ocellus; seven-segmented antennae, with separated flagellomeres and an acute apical flagellomere; fore wing widening towards the apex, with the widest portion occurring at approximately two-thirds of its length; fore wing with a long and easily visible CuA; and a visible claval fold [12]. The species described above shares features with other species of the Gregorites genus, but in unique combinations. The most notable of these characteristics is the absence of metatibial combs, a feature that distinguishes it from G. bestlae. Furthermore, it possesses a triangular mesopostnotum, a feature it shares with G. ymiri, G. hallogi and G. skadii [12].

5. Conclusions

The present finding of a new species of the genus Gregorites in Baltic amber from the Gulf of Gdańsk underscores the significance of succinite in terms of its role in supporting a wide array of insect diversity. Evidence from studies on fossils demonstrates phenomena such as the evolution of whiteflies and their taxonomic diversity, morphological disparity and palaeobiogeographic distribution. The rate of novel discoveries pertaining to understudied groups, such as whiteflies, continues to accelerate.