Were Arenga Palms (Arecaceae) Present in the Eocene? A Review of the Genus Succinometrioxena Legalov, 2012

It has been suggested that palms of the genus Arenga (Arecales: Arecaceae) or forms close to it were distributed in the Eocene of North America and Europe. Records of Metrioxenini (Belidae), which are monophages on these palms, confirm this assumption. A new species, Succinometrioxena andrushchenkoi Legalov, sp. n. from Baltic amber is described. The new species differs from S. poinari Legalov, 2012 in the smaller body sizes, elytral punctation larger than the distances between them, and a rostrum weakly curved in females. It is distinguished from S. bachofeni Legalov, 2013 and S. attenuata Legalov et Poinar, 2020 by the forehead lacking horn-like tubercles on either side of the eyes. A description of male of S. poinari was herein compiled for the first time. A list and key to fossil Metrioxenini were given. The modern and fossil distribution of the tribe Metrioxenini and Arenga palms was shown.


Introduction
In the study of fossil fauna and flora, the situation occurs when either plants or insects are poorly represented in the locality. A striking example is late Eocene amber of Europe, where a huge number of inclusions of various insects were found [1], while the plant remains were much rarer and poorly studied [2,3].
Coleoptera are an important component of both modern and fossil ecosystems. Many of these insects have a well-defined food specialization. Herbivorous insects are divided into poly-, oligo-, and monophages [4]. In paleontological reconstructions, it is optimal to use representatives of the last two groups associated in their development with several genera of plants within a family or with several closely related species within a genus.
Obtaining results from Quaternary studies is the simplest study since these are modern species whose trophic relationships are known. The presence of these species in sediments allows us to confirm the presence of its host plants in that locality. For example, finds of Trichapion simile (Kirby, 1811) indicate the presence of Betula, that of Phytobius leucogaster (Marsham, 1802) the presence of Myriophyllum, that of Aizobius sedi (Germar, 1818) the presence of Sedum, and finally that of Hylobius excavatus (Laicharting, 1781) and of Pissodes insignatus (Boheman, 1843) the presence of Larix [5][6][7].
Modern species of beetles, as a rule, are absent in the Paleogene and Neogene. Therefore, the reconstruction of vegetation has to be carried out based on the trophic relationships of weevil genera or tribes, which can also be associated with certain plants. For example, the records of some species of Ceutorhynchus Germar, 1823 in Baltic and Rovno amber show the presence of Brassicaceae [8][9][10][11], and Oxycraspedus Kuschel, 1955 indicates the presence of Araucaria [10][11][12] in the amber-bearing forests.
In the presented study, the possible presence of Arenga Labill. ex DC. palms in the Eocene are discussed based on weevils of the tribe Metrioxenini Voss, 1953 which Life 2023, 13, 1121 3 of 13 and rounded; temples quite short; antennae inserted near rostrum base ventrally; antennal club two-segmented, fused, two or more times as long as eighth antennomere; pronotum bell-shaped, weakly flattened, with two longitudinal carinae and distinct pronotal constriction; lateral carinae consist of six obtuse convexities; scutellum small; elytra elongate, with two long and two short carinae; scutellar striole indistinct; striae usually irregular; interstriae weakly convex or flat; epipleuron distinct; apex of elytra separately rounded; precoxal portion of prosternum elongate; procoxal cavities separate; postcoxal portion of prosternum short; mesocoxal cavities widely separate; metepisternum narrow; abdomen convex; first and second ventrites weakly elongate; legs long; femora weakly clavate; tibiae almost straight; second and third tarsomeres bilobed; claws large, without teeth.
Succinometrioxena andrushchenkoi Legalov, sp. n. (Figure 1). LSIDurn:lsid:zoobank.org:act: ventrite is about 1.7 times as long as the fourth ventrite. The legs are long. The m is transverse. The femora are weakly clavate, rugose-punctate. The trochanter is t lar. The tibiae are almost straight, weakly flattened, and weakly oblique at the ape apical dark setose fringes. The tarsi are long, with thick and light setae dorsally. T tarsomere is triangular. The second tarsomere is widely bilobed. The third tar elongate-bilobed. The fifth tarsomere is elongated. The claws are large, contiguo without teeth. Material examined. Holotype, female, KRAM, no. BX 12-23, Baltic amber, cene.
Derivation of name. The species is named in honor of K.V. Andrushchenk ningrad, Russia), who provided the type specimen for description.
Diagnosis. The new species differs from S. poinari in the smaller body si mm), the elytral punctation is larger than the distances between them, and the ros weakly curved in the females. It is distinguished from S. bachofeni and S. attenuat forehead without horn-like tubercles on either side of the eyes.
Remarks. I saw a specimen (female) of the new species on eBay.  Description. Size. The body length (without rostrum), is 3.6 mm, rostrum length, and 1.1 mm. The body is black, with short, sparse, adpressed setae, appearing silveryshiny from the presence of a cavity between the specimen and the internal surface of its impression. Rostrum is subcylindrical, quite long, about 1.1 times as long as the pronotum, 7.7 times as long as wide at the apex and in middle, 5.9 times as long as wide at the base, weakly curved, and finely punctate. Head about 0.2 times as long as the rostrum. Forehead wide, about 1.1 times as wide as rostrum base width, depressed, coarsely punctate, with horn-like tubercles on either side. Eyes quite small, rounded, distinctly convex, finely faceted. Vertex is weakly flattened, and densely punctate. Temples about 1.6 times as long as the eye, punctate. Antennae inserted near base of rostrum ventrally, under small convexities. Antennae long, almost reaching the base of the pronotum. The scape and second antennomere are almost teardrop-shaped. Scape 1.6 times as long as wide at apex. The second antennomere is 1.7 times as long as it is wide at the apex, slightly shorter than and about 0.9 times as narrow as the scape. The third-ninth antennomeres are subconical. The third-seventh antennomeres are subequal in width. Third antennomere is about 1.8 times as long as it is wide at the apex, slightly shorter, and about 0.8 times as narrow as the secooonnnd antennomere. The fourth antennomere is about 2.2 times as long as it is wide at the apex, slightly longer than the third antennomere. The fifth antennomere is about 1.6 times as long as it is wide at the apex, about 0.8 times as long as the fourth antennomere. The sixth and seventh antennomeres are subequal to the fifth antennomere. The eighth antennomere is about 1.1 times as long as it is wide at the apex, slightly shorter, and about 1.3 times as wide as the seventh antennomere. The ninth antennomere (first club article) is similar to the eighth antennomere (last article of the flagellum), about 1.6 times as long as it is wide at the apex, about 1.4 times as long, as subequal in width to eight antennomere. The tenth and eleventh antennomeres (second and third club articles) are fused, about 2.0 times as long as the ninth antennomeres. The tenth antennomere is about 1.1 times as long as wide at the apex, about 1.1 times as long, as and about 1.6 times as wide as the ninth antennomere. The eleventh antennomere is slightly shorter than its length, about 0.7 times as long, as and slightly narrower than the tenth antennomere, and weakly acuminate. The pronotum is almost bell-shaped, with a wide apical constriction, 1.4 times as long as it is wide at the apex and the base, about 1.3 times as long as it is wide in the middle, and narrower than the elytral base. The disk is densely and coarsely punctate, with two longitudinal carinae. The distance between the punctures are smaller than their diameter. The sides of pronotum with carinae consisting of obtuse convexities. The scutellum is small and triangular. The elytra are elongated and distinctly convex, about 2.3 times as long as the pronotum, about 2.4 times as long as wide at the base, and at the apical fourth about 1.8 times as long as wide in middle, with two long carinae. The humeri are weakly flattened. Scutellar striole indistinct. Punctate striae regular and distinct near elytral suture, irregular and indistinct in other elytral parts. The punctures are rounded and dense. The distance between the punctures are smaller than their diameter. The interstriae are almost flat. The epipleuron is distinct. The apex of the elytra is separately rounded, obtuse, and without elongated teeth. The margin of the elytra is sharp and carinate. The prosternum is coarsely punctate. The precoxal portion of the prosternum is elongated, 2.7 times as long as the length of the procoxa. The procoxal cavity is round and narrowly separated. The postcoxal portion of the prosternum is short. The metaventrite is about 2.2 times as long as the mesocoxal length, weakly convex, and densely punctate. The abdomen is flattened. The first ventrite is weakly elongated, about 1.4 times as long as the mesocoxal length. The second ventrite is about 0.7 times as long as the first ventrite. The third ventrite is equal to the second ventrite. The fourth ventrite is slightly shorter than the third ventrite. The fifth ventrite is about 1.7 times as long as the fourth ventrite. The legs are long. The metacoxa is transverse. The femora are weakly clavate, rugose-punctate. The trochanter is triangular. The tibiae are almost straight, weakly flattened, and weakly oblique at the apex, with apical dark setose fringes. The tarsi are long, with thick and light setae dorsally. The first tarsomere is triangular. The second tarsomere is widely bilobed. The third tarsomere elongate-bilobed. The fifth tarsomere is elongated. The claws are large, contiguous, and without teeth.
Material examined. Holotype, female, KRAM, no. BX 12-23, Baltic amber, late Eocene. Derivation of name. The species is named in honor of K.V. Andrushchenko (Kaliningrad, Russia), who provided the type specimen for description.
Diagnosis. The new species differs from S. poinari in the smaller body sizes (3.6 mm), the elytral punctation is larger than the distances between them, and the rostrum is weakly curved in the females. It is distinguished from S. bachofeni and S. attenuata in the forehead without horn-like tubercles on either side of the eyes.

Description.
Male. Size. The body length (without rostrum) is 3.9-4.1 mm, rostrum length 1.1-1.2 mm. The rostrum is weakly curved, 1.3 times as long as the pronotum, about 6.7 times as long as wide at the apex and in the middle, densely punctate, with small granulations dorsally. The forehead is wide, flattened, coarsely punctate, and with horn-like tubercles on either side. The eyes are small, rounded, and convex. The vertex is weakly flattened and coarsely and densely punctate. The temples are short, slightly shorter than the eyes, and punctate. The antennae are inserted near the base of the rostrum ventrally, under small convexities, quite long, and almost reaching the base of the pronotum. The first antennomere is almost teardrop-shaped. The second antennomere is oval. The third-eighth antennomeres are subconical. The eighth antennomere is about 1.3 times as long as it is wide at the apex. The ninth antennomere is about 1.1 times as long as it is wide at the apex, about 1.1 times as long as and about 1.3 times as wide as the eighth antennomere. The tenth and eleventh antennomeres are fused. The tenth antennomere is about 1.4 times as long as it is wide at the apex, about 1.6 times as long as, and about 1.3 times as wide as the ninth antennomere. The eleventh antennomere is about 0.7 times as long as wide at the base, about 0.4 times as long as, and about 0.8 times as narrow as the tenth antennomere. The pronotum is bell-shaped. The disk narrowed at the apex and at the base, densely punctate, and with two longitudinal carinae. The elytra are elongated and weakly flattened, 2.6 times as long as the pronotum, and with two long and two short carinae. The carinae and seam have semierect, short, brown setae. The humeri are weakly flattened. The punctate striae are irregular and indistinct. The punctures are rounded, small, and dense. The distance between punctures subequal or smaller than their diameter. The interstriae between the punctures are weakly convex. The prosternum is coarsely punctate. The precoxal portion of the prosternum and elongated, about 1.9 times as long as the procoxal length. The procoxal cavity is round and narrowly separated by the prosternal process. The postcoxal part of the prosternum is short, about 0.6 times as long as the procoxal length. The mesocoxal cavities are rounded, and widely separated. The metaventrite is about 1.7 times as long as the mesocoxal cavity length, weakly convex, and densely punctate. Mesocoxal cavities transverse, narrowly separated. The abdomen is weakly convex, and impressed in the middle. The first ventrite is about 0.8 times as long as the metacoxal length. The second-fourth ventrites are subequal in length. The second ventrite is about 0.8 times as long as the first ventrite. The fifth ventrite is about 1.1 times as long as the fourth ventrite. The legs are long and the femora are weakly clavate, and rugose-punctate. The profemur is thicker than the meso-and metafemora. The tibiae are almost straight, weakly flattened, and weakly oblique at the apex, with an apical dark Remarks. This species is fairly common in Baltic amber. I have seen at least 10 specimens on eBay and online stores.   Remarks. This species is fairly common in Baltic amber. I have se specimens on eBay and online stores.   The forehead has small horn-like tubercles on either side of the eyes. The pronotum is densely punctated, almost matte ( Figure 5).........................................A. electrica -The forehead is simple. The pronotum is sparsely punctated and lustrous (Figure 4)  ....................................................................................................................................A. zherikhini 3. The sides of the pronotum have obtuse teeth. The forehead usually has horn-like tubercles on either side (Figures 1-3 4. The forehead has horn-like tubercles on either side of the eyes (Figures 1 and 2)....5 -The forehead is without horn-like tubercles on either side of the eyes ( Figure 3A)...6 5. The elytral punctation is larger than the distances between them. The rostrum is weakly curved in the females (Figure 1 4. The forehead has horn-like tubercles on either side of the eyes (Figures 1 and 2)....5 -The forehead is without horn-like tubercles on either side of the eyes ( Figure 3A)...6 5. The elytral punctation is larger than the distances between them. The rostrum is weakly curved in the females (Figure 1 (Figure 6) "P." bisculcatus Scudder, 1893-Roan Mountain [24] Remarks. Scudder [24] described a smaller species (3.7 mm) conditionally placed in the genus Paltorhynchus. The shape of the body and the carinae on the elytra confirm the Remarks. Scudder [24] described a smaller species (3.7 mm) conditionally placed in the genus Paltorhynchus. The shape of the body and the carinae on the elytra confirm the correct placement in the tribe Metrioxenini. However, the structure of the abdomen is not known, which does not allow it to be reliably placed in any subtribe. The body length of Metrioxenina is 2.1-3.8 mm, but usually does not exceed 3 mm; Zherichinixenina species range from 3.5 to 8.3 mm. Since it is habitually similar to P. narwhal, I keep it in the genus Paltorhynchus, but to make a conclusion about the systematic position, it is necessary to study the types or new materials from the locality.

List of fossil Metrioxenini
Life 2023, 13, x FOR PEER REVIEW 9 of 13 correct placement in the tribe Metrioxenini. However, the structure of the abdomen is not known, which does not allow it to be reliably placed in any subtribe. The body length of Metrioxenina is 2.1-3.8 mm, but usually does not exceed 3 mm; Zherichinixenina species range from 3.5 to 8.3 mm. Since it is habitually similar to P. narwhal, I keep it in the genus Paltorhynchus, but to make a conclusion about the systematic position, it is necessary to study the types or new materials from the locality.
The weevils of the tribe Metrioxenini are confined to the palms of the genus Arenga [13]. Their larvae develop in the stems of these plants [13], but it is possible that in some genera Life 2023, 13, 1121 10 of 13 they develop in fruits. They have not been found on other genera of the tribe Caryoteae, but they may be covered with them.
Palms of the genus Arenga include more than 20 species distributed (Figure 7) in southern China and South and Southeast Asia (including the Philippines, Sunda, New Guinea, and Ryukyu), and northern Australia [58]. The tribe Caryoteae Scheff. also includes the genera Caryota L. and Wallichia Roxb., distributed in the Oriental region. [24,27]. Pleurambus strongylus Poinar et Legalov, 2014 from the modern tribe Allocorynini Sharp, 1890 was described from early Miocene Dominican amber [57]. There are no other fossil finds of Belidae.
The weevils of the tribe Metrioxenini are confined to the palms of the genus Arenga [13]. Their larvae develop in the stems of these plants [13], but it is possible that in some genera they develop in fruits. They have not been found on other genera of the tribe Caryoteae, but they may be covered with them.
Palms of the genus Arenga include more than 20 species distributed (Figure 7) in southern China and South and Southeast Asia (including the Philippines, Sunda, New Guinea, and Ryukyu), and northern Australia [58]. The tribe Caryoteae Scheff. also includes the genera Caryota L. and Wallichia Roxb., distributed in the Oriental region. The current distribution of the tribe Metrioxenini covers Southeast Asia, from China (Yunnan), Indochina (Vietnam, Thailand, Laos, Malaysia), the Sunda Islands (Java, Sumatra, Kalimantan, Sulawesi, Timor, etc.), and the Philippines (Figure 7). Thus, the area of the tribe Metrioxenini completely fits the area of the genus Arenga. It can be assumed that the Metrioxenini species will be found in most of the range of the Arenga palm.
The earliest finds of the subfamily Coryphoideae Burnett are from the Aptian of North Africa [59]. Reliable macrofossils of representatives of the genus Arenga are not known [60]. Seed from the middle Oligocene of Puerto Rico was considered to resemble closely those of the genera Arenga or Iriartea Ruiz & Pav. [61]. The earliest fossil pollen of Arenga is from the early Eocene of India [62] and the early Miocene of Kalimantan [63]. Seeds probably corresponding to the genus Caryota were described from the early Eocene London Clay [64]. Pollen similar to Caryota was recorded from the Oligocene of the Isle of Wight [65]. Three palm species (Phoenix eichleri Conwentz, 1886, Palmophyllum kuenowi (Casp. 1872), Bembergia pentatrias Caspary 1881) are known from Baltic amber [2]. Phoenix eichleri belongs to the subfamily Coryphoideae of the tribe Phoeniceae. Palmophyllum kuenowi and Bembergia pentatrias have an unclear taxonomic position. Palms are known The current distribution of the tribe Metrioxenini covers Southeast Asia, from China (Yunnan), Indochina (Vietnam, Thailand, Laos, Malaysia), the Sunda Islands (Java, Sumatra, Kalimantan, Sulawesi, Timor, etc.), and the Philippines (Figure 7). Thus, the area of the tribe Metrioxenini completely fits the area of the genus Arenga. It can be assumed that the Metrioxenini species will be found in most of the range of the Arenga palm.
The earliest finds of the subfamily Coryphoideae Burnett are from the Aptian of North Africa [59]. Reliable macrofossils of representatives of the genus Arenga are not known [60]. Seed from the middle Oligocene of Puerto Rico was considered to resemble closely those of the genera Arenga or Iriartea Ruiz & Pav. [61]. The earliest fossil pollen of Arenga is from the early Eocene of India [62] and the early Miocene of Kalimantan [63]. Seeds probably corresponding to the genus Caryota were described from the early Eocene London Clay [64]. Pollen similar to Caryota was recorded from the Oligocene of the Isle of Wight [65]. Three palm species (Phoenix eichleri Conwentz, 1886, Palmophyllum kuenowi (Casp. 1872), Bembergia pentatrias Caspary 1881) are known from Baltic amber [2]. Phoenix eichleri belongs to the subfamily Coryphoideae of the tribe Phoeniceae. Palmophyllum kuenowi and Bembergia pentatrias have an unclear taxonomic position. Palms are known from the Green River, including the modern genus Phoenix L. [66]. Indeterminate retirements of palm trees are found in Florissant [67].
Fossil records of Metrioxenini are from the early Eocene of Roan Mountain, Colorado (USA), the late Eocene of Europe (Baltic amber), and the terminal Eocene of Florissant,