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Cretodineutus rotundus gen. et sp. nov., the oldest adult whirligig beetle from the Upper Cretaceous of Myanmar (Coleoptera, Gyrinidae, Gyrininae)
Cretaceous Research 106 (2019) 104251
Contents lists available at ScienceDirect
Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes
Short communication
Cretodineutus rotundus gen. et sp. nov., the oldest adult whirligig beetle from the Upper Cretaceous of Myanmar (Coleoptera, Gyrinidae, Gyrininae) Zulong Liang a, Zhihao Qi b, Jiahui Chen a, Fenglong Jia a, * a b
Institute of Entomology, Life Science School, Sun Yatesen University, Guangzhou, 510275, Guangdong, China College of Tea and Food Science, Wuyi University, Wuyishan City, 354300, Fujian, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2019 Received in revised form 21 August 2019 Accepted in revised form 13 September 2019 Available online 18 September 2019
A fossil whirligig beetle Cretodineutus rotundus gen. et sp. nov. is described from Upper Cretaceous Burmese amber. Cretodineutus resembles the extant genus Dineutus and is assigned to the subfamily Gyrininae, tribe Dineutini. Cretodineutus is the oldest known adult fossil whirligig beetle of the subfamily Gyrininae. The new species is described in detail, and photographs and illustrations are provided. © 2019 Published by Elsevier Ltd.
Keywords: Gyrinidae Dineutini Burmese amber Cretaceous New genus New species
1. Introduction Gyrinidae are the second largest family of aquatic Adephaga, with appropriately 900 extant species worldwide (Short, 2018). According to the latest studies (Miller and Bergsten, 2012; Gustafson and Miller, 2017; Gustafson et al., 2017), extant Gyrinidae includes three subfamilies, Spanglerogyrinae, Heterogyrinae and Gyrininae. The subfamily Gyrininae is the major group of Gyrinidae and consists of eleven extant genera assigned to three tribes and contains the majority of species in the family. In contrast, Spanglerogyrinae and Heterogyrinae are very small subfamilies, each with only one extant species, endemic to southeastern USA and Madagascar respectively. Gyrinidae are an ancient group of beetles with a long evolutionary history. The origin of gyrinids was dated to the Late Permian or early Triassic at 236e271 Ma in a tip-dated phylogenetic analysis (Gustafson et al., 2017). The oldest known gyrinid fossil is from the Lower Jurassic at 189.6e182.7 Ma (Gustafson et al., 2017). Yan et al.
* Corresponding author. E-mail addresses: [email protected], lssjfl@mail.sysu.edu.cn (F. Jia). https://doi.org/10.1016/j.cretres.2019.104251 0195-6671/© 2019 Published by Elsevier Ltd.
(2018) described a new fossil species Tunguskagyrus planus Yan et al., 2018 from Late Permian and assigned it to the stem group of Gyrinidae. However, this assignment was subsequently rebutted by Kirejtshuk and Prokin (2018). There is a total of about 30 fossil gyrinid species described so far (Hatch, 1927; Nel, 1989; Gustafson et al., 2017; Yan et al., 2018), most of which are fossil impressions. The only formally described amber inclusion of whirligig beetles was an Oligocene fossil from Baltic Amber (Motschulsky, 1856). However, most recently a gyrinid larva was described from Burmese amber (Zhao et al., 2019). Interestingly, although nearly half of those species were described in Mesozoic, the earliest record of subfamily Gyrininae was not earlier than Paleocene (Gustafson et al., 2017). Heterogyrinae instead were the dominant group of Gyrinidae back in the Mesozoic according to the fossil record (Gustafson et al., 2017). Gyrininae radiated dramatically and have taken the dominant position following the endeCretaceous extinction event as shown in fossil record (Gustafson et al., 2017). The origin of Gyrininae was estimated to be Late Triassic or Early Jurassic at 187e226 Ma (Gustafson et al., 2017), but without strong support from existing fossil evidence.
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2. Methods and material The specimen was preserved in a fairly transparent piece of Burmese amber, which is from Hukawng Valley of northern Myanmar (Dong et al., 2015: fig 1). Burmese amber is a productive Cretaceous amber deposit with an age of ca. 99 Ma (Shi et al., 2012). After polishing with sandpaper and polishing powder of different grain sizes, the specimen was examined
with a Nikon SMZ1270 Stereomicroscope. The photographs and measurements were completed with a Zeiss SteREO Discovery V20 Microscope. Illustrations were drawn with Adobe Illustrator CS6 based on photographs and the examination of the specimen. The amber is a collection of Zhihao Qi and deposited in the Entomological Collection of Sun Yatesen University, Guangzhou, China (SYSU).
Fig. 1. habitus of Cretodineutus rotundus gen. et sp. nov. A. dorsal view, B. illustration of dorsal view, C. ventral view, D. illustration of ventral view. Scale bar ¼ 1 mm.
Z. Liang et al. / Cretaceous Research 106 (2019) 104251
The taxonomic system and terminology used in this study largely follow Gustafson and Miller (2015, 2017), Miller and Bergsten (2012) and Holmen (1987). 3. Systematic palaeontology Order: Coleoptera Family: Gyrinidae Subfamily: Gyrininae Tribe: Dineutini Genus: Cretodineutus gen. nov. Type species: Cretodineutus rotundus sp. nov. Diagnosis. Body elongate oval (Figs. 1AeB). Labrum protruding, semicircular; Clypeus short and wide, frontoclypeal suture distinct, the status of frontolateral bead unclear (Fig. 2A), pedicel of antenna large, subtriangular; flagellum compact, apex pointed; dorsal and ventral compound eyes widely separated; Gular suture complete, gula with a posteromedial suture (Fig. 2C). Surface of
3
elytra smooth, without pubescence and regular longitudinal striae, sutural border absent; reticulation distinct, formed by small rounded meshes, posterior half of elytra scattered with small punctation and short impressed longitudinal lines; lateral margin very broad, strongly irregularly plicate (Fig. 2B). Mesoventrite large, with a narrow median groove in posterior 2/5. Metaventrite medially triangular in shape, without transverse suture; metaventral wings sub-equilateral triangular. Metanepisternum lobiform. Metacoxae transverse; metacoxal process broad, but not expanded distolaterally (Figs. 1CeD). Fore legs robust, protibiae narrow basally, apically strongly expanded at inner side; male protarsi Deshaped, about as wide as protibiae, discus on protarsomere I absent (Fig. 2D). Abdomen with 6 visible ventrites, abdominal sternite VIII entire, apically rounded, without a longitudinal row of long hairs along the middle (Figs. 1CeD). Etymology. The generic name consists of the prefix Creto-referring to Cretaceous, the geological period of this genus and the suffix -dineutus referring to the most similar extant genus.
Fig. 2. Cretodineutus rotundus gen. et sp. nov. A. labrum and clypeus, B. lateral margin of elytron, C. ventral view of head, D. fore leg, E. hind leg. Scale bar ¼ 0.2 mm in AeB, ¼ 0.5 mm in CeE.
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Z. Liang et al. / Cretaceous Research 106 (2019) 104251
Differential diagnosis. The widely separated dorsal and ventral eyes and broadened mide and hind legs indicate that this genus clearly does not belong to Spanglerogyrinae. The entire abdominal sternite VIII, and absence of the dorsal pubescence and sutural border of this genus suggest that it is not a member of Heterogyrinae either. All these diagnostic characters mentioned above suggest that this genus should be assigned to subfamily Gyrininae. And it is assigned to tribe Dineutini for its sub-equilateral triangular metaventral wings, lobiform metanepisternum and transverse metacoxae. Furthermore, the median metaventrite of this genus is triangular in shape instead of pentagonal, which distinguishes it gimbart. It can be further from Enhydrus Laporte and Macrogyrus Re distinguished from Macrogyrus in the unbordered metacoxal process and the absence of discus on male protarsomere I. It clearly differs from Porrorhynchus Laporte in the semicircularly shaped labrum (elongate triangular in Porrorhynchus), the absence of setose patch on male protrochanter and the protibia being unexpanded distolaterally. This genus resembles Dineutus Macleay and the fossil genus Mesodineutes Ponomarenko especially in the shape of medial metaventrite and metacoxal process. But it could be distinguished from Dineutus in the broaden protibiae and protarsi, and from Mesodineutes in the truncate elytral apex and the absence of elytral striae. Cretodineutus rotundus sp. nov. Type material. Holotype, SYSeENAM0010, male, Late Cretaceous (ca. 99 Ma), from an amber mine in northern Myanmar, specifically located on Noije Bum, Hukawng Valley of Kachin State (ca. 26 150 N, 96 340 E) (Shi et al., 2012). Etymology. The species name comes from the Latin adjective rotundus (¼ rounded), referring to the rounded labrum of this species. Description. Body elongate oval, moderately convex in shape, length 7.0 mm, width 3.4 mm uniformly black dorsally. Ventral surface black, with mid and hind legs reddish brown (Figs. 1AeD). Head. Labrum protruding, semicircular in shape, about twice as long as wide, rounded anteriorly, with long bristles along anterior margin; dorsal surface with green and purple sheen, and scattered strong punctation. Clypeus short and wide, anteriorly slightly concave, frontoclypeal suture distinct, status of frontolateral bead unclear (Fig. 2A). Dorsal and ventral eyes widely separated; dorsal eyes slightly smaller than ventral ones, posteriorly nearly in contact with posterior margin of head, ventral eyes not reaching lateral margin of head. Antennae with pedicel large, subtriangular; flagellum compact, apex pointed. Gular suture distinct and complete, gula narrow, with a deep posteromedian suture (Fig. 2C). Pronotum. Pronotum short and wide, ca. 0.4 as long as wide, regularly attenuated from base to apex; both anterior and posterior margin slightly sinuated. Lateral margin very broad, strongly irregularly plicate. Status of scutellum unclear. Elytra. Microreticulation distinct, formed by small rounded meshes, posterior half of elytra with small punctations and scattered short narrow longitudinal impressed lines. Sutural border absent. Truncature oblique, apex strongly convex. Epipleural angle obtuse, almost obliterated; sutural angle almost 90 , narrowly rounded. Dorsal pubescence and longitudinal striae absent. Lateral margin similar to that of pronotum (Fig. 2B). Ventral surface. Mesoventrite large, with a narrow posteromedian groove in posterior 2/5, but not deepened into a pit at the anterior end. Mesocoxae separated by metaventrite, not contiguous. Metaventrite medially triangular in shape, without transverse suture; metaventral wings sub-equilateral triangular, about as long as median part. Metanepisternum lobiform. Metacoxal process broad, but not expanded distolaterally; truncature of metacoxal process oblique, straight at the end (Figs. 1CeD).
Legs. Profemora with 5 setigerous punctures uniformly along anterior margin; protibiae slightly shorter than profemora, base very narrow, gradually broadened toward apex, outer apical angle sub-rectangular, narrowly rounded; inner margin with a row of short setae; protarsi strongly dilated, large and broad, ca. 0.70 as long as protibia, and almost the same width as it, discus on protarsomere I absent, a row of adhesive setae visible along outer margin; claws developed and equal, about one third as long as protarsi (Fig. 2D). Mid and hind legs short and broadly flattened, strongly modified into paddleeshaped for swimming. Tibial apices and tarsi with a row of long swimming hairs (Fig. 2E). Abdomen. Abdomen with 6 visible ventrites. abdominal sternite VIII entire, semicircular, apically rounded, without a longitudinal row of long hairs along the middle (Figs. 1CeD)
4. Discussion The specimen is quite well preserved in the amber so that the whole body is nearly intact. But unfortunately, some important characteristics, such as the scutellum and frontolateral bead, cannot be distinguished due to the heavy opaque resin on the surface of the specimen. Nevertheless, the available characteristics are sufficient for comparing this specimen with other genera of Gyrinidae. Cretodineutus gen. nov. is assigned to tribe Dineutini according to our analysis above. Dineutini were classically considered as a gimbart, 1882; Brink, 1955) but sometimes monophyletic group (Re were considered paraphyletic (Beutel, 1990). The recent comprehensive phylogenetic analysis has proved the monophyly of this tribe (Miller and Bergsten, 2012; Gustafson and Miller, 2017). Cretodineutus confirms a Late Cretaceous origin of Dineutini with a history of at least 99 Ma, which has been indicated by tip-dating analysis (Gustafson and Miller, 2017) but was not well supported by existing fossil records. Fossils of Gyrininae were primarily only found in the Cenozoic before the discovery of Cretodineutus, the first adult Gyrininae found in Mesozoic, which in addition to Cretogyrus (Zhao et al., 2019) adds more than 30 million years to the minimum age of this subfamily. 5. Conclusions A new fossil genus and species of Gyrinidae, Cretodineutus rotundus gen. et sp. nov. is described from Burmese amber based on comparison with other whirligig beetles. Cretodineutus is assigned to tribe Dineutini and resembles the extant genus Dineutus. Cretodineutus confirms a Late Cretaceous origin of the Dineutini and is among the oldest known fossil of Gyrininae, adding more than 30 million years to the minimum age of this subfamily. Acknowledgements We are grateful to Dr. Qiang Yang for his valuable opinions on the manuscript submission and his assistance in polishing the amber. We really appreciate the anonymous reviewers for their professional opinions on manuscript revision. We are indebted to Dr. Robert B. Angus (The Natural History Museum, London, UK), who improved the English of the manuscript. We are also indebted to Zuqi Mai for providing the information about this amber. References Beutel, R.G., 1990. Phylogenetic analysis of the family Gyrinidae (Coleoptera) based on mesothoracic and metathoracic characters. Quaestiones Entomologicae 26, 163e192. Brink, P., 1955. A revision of the Gyrinidae (Coleoptera) of the Ethiopian region. I. Acta Universitatis Lundensis. Nova Series 51, 1e140.
Z. Liang et al. / Cretaceous Research 106 (2019) 104251 Dong, F., Shih, C., Dong, R., 2015. A new genus of Tanyderidae (Insecta: Diptera) from Myanmar amber, Upper Cretaceous. Cretaceous Research 54, 260e265. Gustafson, G.T., Miller, K.B., 2015. The New World whirligig beetles of the genus Dineutus Macleay, 1825 (Coleoptera, Gyrinidae, Gyrininae, Dineutini). ZooKeys 476, 1e135. Gustafson, G.T., Miller, K.B., 2017. Systematics and evolution of the whirligig beetle tribe Dineutini (Coleoptera: Gyrinidae: Gyrininae). Zoological Journal of the Linnean Society 181, 118e150. Gustafson, G.T., Prokin, A.A., Bukontaite, R., Bergsten, J., Miller, K.B., 2017. Tip-dated phylogeny of whirligig beetles reveals ancient lineage surviving on Madagascar. Scientific Reports 7, 8619. Hatch, M.H., 1927. A revision of fossil Gyrinidae. Bulletin of the Brooklyn Entomological Society 22, 89e96. Holmen, M., 1987. The aquatic Adephaga (Colcoptera) of Fennoscandia and Denmark. I. Gyrinidae, Haliplidae, Hygrobiidae and Noteridae. Fauna Entomologica Scandinavica 20, 1e168. Kirejtshuk, A.G., Prokin, A.A., 2018. The Position of the Palaeozoic Genus Tunguskagyrus Yan, Beutel et Lawrence in the Family Triaplidae sensu n. (Coleoptera, Archostemata: Schizophoroidea). Entomological Review 98, 872e882.
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Miller, K.B., Bergsten, J., 2012. Phylogeny and classification of whirligig beetles (Coleoptera: Gyrinidae): relaxed-clock model outperforms parsimony and time-free Bayesian analyses. Systematic Entomology 37, 706e746. M. Me ne trie s. Etudes Motschulsky, V., 1856. Lettres de M. de Motschulsky a Entomologique 5, 3e38. te Nel, A., 1989. Les Gyrinidae fossiles de France (Coleoptera). Annales de la Socie Entomologique de France 25, 321e330. gimbart, M., 1882. Essai monographique de la famille des Gyrinidae:1re partie. Re te Entomologique de France 6, 379e458. Annales de la Socie Shi, G., Grimaldi, D.A., Harlow, G.E., Wang, J., Wang, J., Yang, M., Lei, W., Li, Q., Li, X., 2012. Age constraint on Burmese amber based on UePb dating of zircons. Cretaceous Research 37, 155e163. Short, A.E.Z., 2018. Systematics of aquatic beetles (Coleoptera): current state and future directions. Systematic Entomology 43, 1e18. Yan, E.V., Beutel, R.G., Lawrence, J.F., 2018. Whirling in the late Permian: ancestral Gyrinidae show early radiation of beetles before PermianeTriassic mass extinction. BMC Evolutionary Biology 18, 33. Zhao, X., Zhao, X., Jarzembowski, E.A., Wang, B., 2019. The first whirligig beetle larva from mid-Cretaceous Burmese amber (Coleoptera: Adephaga: Gyrinidae). Cretaceous Research 99, 41e45.
Contents lists available at ScienceDirect
Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes
Short communication
Cretodineutus rotundus gen. et sp. nov., the oldest adult whirligig beetle from the Upper Cretaceous of Myanmar (Coleoptera, Gyrinidae, Gyrininae) Zulong Liang a, Zhihao Qi b, Jiahui Chen a, Fenglong Jia a, * a b
Institute of Entomology, Life Science School, Sun Yatesen University, Guangzhou, 510275, Guangdong, China College of Tea and Food Science, Wuyi University, Wuyishan City, 354300, Fujian, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2019 Received in revised form 21 August 2019 Accepted in revised form 13 September 2019 Available online 18 September 2019
A fossil whirligig beetle Cretodineutus rotundus gen. et sp. nov. is described from Upper Cretaceous Burmese amber. Cretodineutus resembles the extant genus Dineutus and is assigned to the subfamily Gyrininae, tribe Dineutini. Cretodineutus is the oldest known adult fossil whirligig beetle of the subfamily Gyrininae. The new species is described in detail, and photographs and illustrations are provided. © 2019 Published by Elsevier Ltd.
Keywords: Gyrinidae Dineutini Burmese amber Cretaceous New genus New species
1. Introduction Gyrinidae are the second largest family of aquatic Adephaga, with appropriately 900 extant species worldwide (Short, 2018). According to the latest studies (Miller and Bergsten, 2012; Gustafson and Miller, 2017; Gustafson et al., 2017), extant Gyrinidae includes three subfamilies, Spanglerogyrinae, Heterogyrinae and Gyrininae. The subfamily Gyrininae is the major group of Gyrinidae and consists of eleven extant genera assigned to three tribes and contains the majority of species in the family. In contrast, Spanglerogyrinae and Heterogyrinae are very small subfamilies, each with only one extant species, endemic to southeastern USA and Madagascar respectively. Gyrinidae are an ancient group of beetles with a long evolutionary history. The origin of gyrinids was dated to the Late Permian or early Triassic at 236e271 Ma in a tip-dated phylogenetic analysis (Gustafson et al., 2017). The oldest known gyrinid fossil is from the Lower Jurassic at 189.6e182.7 Ma (Gustafson et al., 2017). Yan et al.
* Corresponding author. E-mail addresses: [email protected], lssjfl@mail.sysu.edu.cn (F. Jia). https://doi.org/10.1016/j.cretres.2019.104251 0195-6671/© 2019 Published by Elsevier Ltd.
(2018) described a new fossil species Tunguskagyrus planus Yan et al., 2018 from Late Permian and assigned it to the stem group of Gyrinidae. However, this assignment was subsequently rebutted by Kirejtshuk and Prokin (2018). There is a total of about 30 fossil gyrinid species described so far (Hatch, 1927; Nel, 1989; Gustafson et al., 2017; Yan et al., 2018), most of which are fossil impressions. The only formally described amber inclusion of whirligig beetles was an Oligocene fossil from Baltic Amber (Motschulsky, 1856). However, most recently a gyrinid larva was described from Burmese amber (Zhao et al., 2019). Interestingly, although nearly half of those species were described in Mesozoic, the earliest record of subfamily Gyrininae was not earlier than Paleocene (Gustafson et al., 2017). Heterogyrinae instead were the dominant group of Gyrinidae back in the Mesozoic according to the fossil record (Gustafson et al., 2017). Gyrininae radiated dramatically and have taken the dominant position following the endeCretaceous extinction event as shown in fossil record (Gustafson et al., 2017). The origin of Gyrininae was estimated to be Late Triassic or Early Jurassic at 187e226 Ma (Gustafson et al., 2017), but without strong support from existing fossil evidence.
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Z. Liang et al. / Cretaceous Research 106 (2019) 104251
2. Methods and material The specimen was preserved in a fairly transparent piece of Burmese amber, which is from Hukawng Valley of northern Myanmar (Dong et al., 2015: fig 1). Burmese amber is a productive Cretaceous amber deposit with an age of ca. 99 Ma (Shi et al., 2012). After polishing with sandpaper and polishing powder of different grain sizes, the specimen was examined
with a Nikon SMZ1270 Stereomicroscope. The photographs and measurements were completed with a Zeiss SteREO Discovery V20 Microscope. Illustrations were drawn with Adobe Illustrator CS6 based on photographs and the examination of the specimen. The amber is a collection of Zhihao Qi and deposited in the Entomological Collection of Sun Yatesen University, Guangzhou, China (SYSU).
Fig. 1. habitus of Cretodineutus rotundus gen. et sp. nov. A. dorsal view, B. illustration of dorsal view, C. ventral view, D. illustration of ventral view. Scale bar ¼ 1 mm.
Z. Liang et al. / Cretaceous Research 106 (2019) 104251
The taxonomic system and terminology used in this study largely follow Gustafson and Miller (2015, 2017), Miller and Bergsten (2012) and Holmen (1987). 3. Systematic palaeontology Order: Coleoptera Family: Gyrinidae Subfamily: Gyrininae Tribe: Dineutini Genus: Cretodineutus gen. nov. Type species: Cretodineutus rotundus sp. nov. Diagnosis. Body elongate oval (Figs. 1AeB). Labrum protruding, semicircular; Clypeus short and wide, frontoclypeal suture distinct, the status of frontolateral bead unclear (Fig. 2A), pedicel of antenna large, subtriangular; flagellum compact, apex pointed; dorsal and ventral compound eyes widely separated; Gular suture complete, gula with a posteromedial suture (Fig. 2C). Surface of
3
elytra smooth, without pubescence and regular longitudinal striae, sutural border absent; reticulation distinct, formed by small rounded meshes, posterior half of elytra scattered with small punctation and short impressed longitudinal lines; lateral margin very broad, strongly irregularly plicate (Fig. 2B). Mesoventrite large, with a narrow median groove in posterior 2/5. Metaventrite medially triangular in shape, without transverse suture; metaventral wings sub-equilateral triangular. Metanepisternum lobiform. Metacoxae transverse; metacoxal process broad, but not expanded distolaterally (Figs. 1CeD). Fore legs robust, protibiae narrow basally, apically strongly expanded at inner side; male protarsi Deshaped, about as wide as protibiae, discus on protarsomere I absent (Fig. 2D). Abdomen with 6 visible ventrites, abdominal sternite VIII entire, apically rounded, without a longitudinal row of long hairs along the middle (Figs. 1CeD). Etymology. The generic name consists of the prefix Creto-referring to Cretaceous, the geological period of this genus and the suffix -dineutus referring to the most similar extant genus.
Fig. 2. Cretodineutus rotundus gen. et sp. nov. A. labrum and clypeus, B. lateral margin of elytron, C. ventral view of head, D. fore leg, E. hind leg. Scale bar ¼ 0.2 mm in AeB, ¼ 0.5 mm in CeE.
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Z. Liang et al. / Cretaceous Research 106 (2019) 104251
Differential diagnosis. The widely separated dorsal and ventral eyes and broadened mide and hind legs indicate that this genus clearly does not belong to Spanglerogyrinae. The entire abdominal sternite VIII, and absence of the dorsal pubescence and sutural border of this genus suggest that it is not a member of Heterogyrinae either. All these diagnostic characters mentioned above suggest that this genus should be assigned to subfamily Gyrininae. And it is assigned to tribe Dineutini for its sub-equilateral triangular metaventral wings, lobiform metanepisternum and transverse metacoxae. Furthermore, the median metaventrite of this genus is triangular in shape instead of pentagonal, which distinguishes it gimbart. It can be further from Enhydrus Laporte and Macrogyrus Re distinguished from Macrogyrus in the unbordered metacoxal process and the absence of discus on male protarsomere I. It clearly differs from Porrorhynchus Laporte in the semicircularly shaped labrum (elongate triangular in Porrorhynchus), the absence of setose patch on male protrochanter and the protibia being unexpanded distolaterally. This genus resembles Dineutus Macleay and the fossil genus Mesodineutes Ponomarenko especially in the shape of medial metaventrite and metacoxal process. But it could be distinguished from Dineutus in the broaden protibiae and protarsi, and from Mesodineutes in the truncate elytral apex and the absence of elytral striae. Cretodineutus rotundus sp. nov. Type material. Holotype, SYSeENAM0010, male, Late Cretaceous (ca. 99 Ma), from an amber mine in northern Myanmar, specifically located on Noije Bum, Hukawng Valley of Kachin State (ca. 26 150 N, 96 340 E) (Shi et al., 2012). Etymology. The species name comes from the Latin adjective rotundus (¼ rounded), referring to the rounded labrum of this species. Description. Body elongate oval, moderately convex in shape, length 7.0 mm, width 3.4 mm uniformly black dorsally. Ventral surface black, with mid and hind legs reddish brown (Figs. 1AeD). Head. Labrum protruding, semicircular in shape, about twice as long as wide, rounded anteriorly, with long bristles along anterior margin; dorsal surface with green and purple sheen, and scattered strong punctation. Clypeus short and wide, anteriorly slightly concave, frontoclypeal suture distinct, status of frontolateral bead unclear (Fig. 2A). Dorsal and ventral eyes widely separated; dorsal eyes slightly smaller than ventral ones, posteriorly nearly in contact with posterior margin of head, ventral eyes not reaching lateral margin of head. Antennae with pedicel large, subtriangular; flagellum compact, apex pointed. Gular suture distinct and complete, gula narrow, with a deep posteromedian suture (Fig. 2C). Pronotum. Pronotum short and wide, ca. 0.4 as long as wide, regularly attenuated from base to apex; both anterior and posterior margin slightly sinuated. Lateral margin very broad, strongly irregularly plicate. Status of scutellum unclear. Elytra. Microreticulation distinct, formed by small rounded meshes, posterior half of elytra with small punctations and scattered short narrow longitudinal impressed lines. Sutural border absent. Truncature oblique, apex strongly convex. Epipleural angle obtuse, almost obliterated; sutural angle almost 90 , narrowly rounded. Dorsal pubescence and longitudinal striae absent. Lateral margin similar to that of pronotum (Fig. 2B). Ventral surface. Mesoventrite large, with a narrow posteromedian groove in posterior 2/5, but not deepened into a pit at the anterior end. Mesocoxae separated by metaventrite, not contiguous. Metaventrite medially triangular in shape, without transverse suture; metaventral wings sub-equilateral triangular, about as long as median part. Metanepisternum lobiform. Metacoxal process broad, but not expanded distolaterally; truncature of metacoxal process oblique, straight at the end (Figs. 1CeD).
Legs. Profemora with 5 setigerous punctures uniformly along anterior margin; protibiae slightly shorter than profemora, base very narrow, gradually broadened toward apex, outer apical angle sub-rectangular, narrowly rounded; inner margin with a row of short setae; protarsi strongly dilated, large and broad, ca. 0.70 as long as protibia, and almost the same width as it, discus on protarsomere I absent, a row of adhesive setae visible along outer margin; claws developed and equal, about one third as long as protarsi (Fig. 2D). Mid and hind legs short and broadly flattened, strongly modified into paddleeshaped for swimming. Tibial apices and tarsi with a row of long swimming hairs (Fig. 2E). Abdomen. Abdomen with 6 visible ventrites. abdominal sternite VIII entire, semicircular, apically rounded, without a longitudinal row of long hairs along the middle (Figs. 1CeD)
4. Discussion The specimen is quite well preserved in the amber so that the whole body is nearly intact. But unfortunately, some important characteristics, such as the scutellum and frontolateral bead, cannot be distinguished due to the heavy opaque resin on the surface of the specimen. Nevertheless, the available characteristics are sufficient for comparing this specimen with other genera of Gyrinidae. Cretodineutus gen. nov. is assigned to tribe Dineutini according to our analysis above. Dineutini were classically considered as a gimbart, 1882; Brink, 1955) but sometimes monophyletic group (Re were considered paraphyletic (Beutel, 1990). The recent comprehensive phylogenetic analysis has proved the monophyly of this tribe (Miller and Bergsten, 2012; Gustafson and Miller, 2017). Cretodineutus confirms a Late Cretaceous origin of Dineutini with a history of at least 99 Ma, which has been indicated by tip-dating analysis (Gustafson and Miller, 2017) but was not well supported by existing fossil records. Fossils of Gyrininae were primarily only found in the Cenozoic before the discovery of Cretodineutus, the first adult Gyrininae found in Mesozoic, which in addition to Cretogyrus (Zhao et al., 2019) adds more than 30 million years to the minimum age of this subfamily. 5. Conclusions A new fossil genus and species of Gyrinidae, Cretodineutus rotundus gen. et sp. nov. is described from Burmese amber based on comparison with other whirligig beetles. Cretodineutus is assigned to tribe Dineutini and resembles the extant genus Dineutus. Cretodineutus confirms a Late Cretaceous origin of the Dineutini and is among the oldest known fossil of Gyrininae, adding more than 30 million years to the minimum age of this subfamily. Acknowledgements We are grateful to Dr. Qiang Yang for his valuable opinions on the manuscript submission and his assistance in polishing the amber. We really appreciate the anonymous reviewers for their professional opinions on manuscript revision. We are indebted to Dr. Robert B. Angus (The Natural History Museum, London, UK), who improved the English of the manuscript. We are also indebted to Zuqi Mai for providing the information about this amber. References Beutel, R.G., 1990. Phylogenetic analysis of the family Gyrinidae (Coleoptera) based on mesothoracic and metathoracic characters. Quaestiones Entomologicae 26, 163e192. Brink, P., 1955. A revision of the Gyrinidae (Coleoptera) of the Ethiopian region. I. Acta Universitatis Lundensis. Nova Series 51, 1e140.
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