A new fossil piddock (Bivalvia: Pholadidae) may indicate estuarine to freshwater environments near Cretaceous amber-producing forests in Myanmar
Altogether nine polished pieces of the lower Cenomanian Kachin amber from northern Myanmar (Figs. 1A–D, 2A–E) were examined in this study (depository: Russian Museum of Biodiversity Hotspots, N. Laverov Federal Center for Integrated Arctic Research of the Ural Branch of the Russian Academy of Sciences, Arkhangelsk, Russia). A brief description of each amber piece is given below.Figure 1Lower Cenomanian Kachin amber samples with specimens and borings of †Palaeolignopholas kachinensis gen. & sp. nov. from northern Myanmar used in this study. (A) RMBH biv1115 (frontal view with the holotype). (B) RMBH biv1101 (lateral view with two paratypes and a shell fragment). (C) RMBH biv1116 (frontal view with the fossilized paratype). (D) RMBH biv1100 (frontal view with borings). The red frames indicate position of the type specimens (holotype and some paratypes). The red arrows indicate bivalve borings. Scale bars = 5 mm. (Photos: Ilya V. Vikhrev).Full size imageFigure 2Lower Cenomanian Kachin amber samples with borings of †Palaeolignopholas kachinensis gen. & sp. nov. from northern Myanmar used in this study. (A) RMBH biv1102 (frontal view). (B) RMBH biv1103 (frontal view). (C) RMBH biv1114 (frontal view). (D) RMBH biv1118 (frontal view). (E) RMBH biv1117 (frontal view). The red arrows indicate bivalve borings. Scale bars = 5 mm. (Photos: Ilya V. Vikhrev).Full size imageRMBH biv1115: Size 8.5 × 5.8 × 8.1 mm (Fig. 1A). Inclusions: articulated shell of †Palaeolignopholas kachinensis gen. & sp. nov., “floating” in the resin (the holotype).RMBH biv1101: Size 15.6 × 6.4 × 11.5 mm (Fig. 1B). Inclusions: two complete articulated shells (paratypes) and a shell fragment of †Palaeolignopholas kachinensis gen. & sp. nov., “floating” in the resin.RMBH biv1116: Size 22.5 × 8.3 × 16.5 mm (Fig. 1C). Inclusions: fossilized shell of †Palaeolignopholas kachinensis gen. & sp. nov. (paratype), borings of this species (filled with fine gray sand), unidentified fly specimens (Insecta: Diptera), and unidentified organic fragments (probably, plant debris).RMBH biv1100: Size 17.5 × 4.9 × 12.0 mm (Fig. 1D). Inclusions: borings of †Palaeolignopholas kachinensis gen. & sp. nov. (filled with fine gray sand), and an unidentified caddisfly specimen (Insecta: Trichoptera).RMBH biv1102: Size 15.6 × 5.1 × 12.7 mm (Fig. 2A). Inclusions: borings of †Palaeolignopholas kachinensis gen. & sp. nov. (filled with fine gray sand), and unidentified organic fragments (probably, plant debris).RMBH biv1103: Size 19.6 × 4.7 × 14.3 mm (Fig. 2B). Inclusions: borings of †Palaeolignopholas kachinensis gen. & sp. nov. (filled with fine gray sand), an unidentified beetle specimen (Insecta: Coleoptera), and unidentified organic fragments (probably, plant debris).RMBH biv1114: Size 33.1 × 7.8 × 21.7 mm (Fig. 2C). Inclusions: multiple borings of †Palaeolignopholas kachinensis gen. & sp. nov. (filled with fine gray sand), and unidentified plant remains.RMBH biv1118: Size 25.1 × 8.4 × 14.3 mm (Fig. 2D). Inclusions: separate borings of †Palaeolignopholas kachinensis gen. & sp. nov. (filled with fine gray sand), a plant fragment with a cluster of borings around, and an unidentified insect specimen.RMBH biv1117: Size 15.5 × 3.9 × 10.7 mm (Fig. 2E). Inclusions: borings of †Palaeolignopholas kachinensis gen. & sp. nov. (filled with fine gray sand), and an unidentified insect specimen.Additionally, six amber samples containing adult and sub-adult specimens of †Palaeolignopholas kachinensis gen. & sp. nov. were examined using photographs in published works as follows: BMNH 20205 (Department of Palaeontology, Natural History Museum, London, UK)15, NIGP 169623 and NIGP 169624 (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China)20, RS.P1450 (Ru D. A. Smith collection, Kuala Lumpur, Malaysia)19, and AMNH (Division of Invertebrates, American Museum of Natural History, New York, NY, United States of America)16.Based on morphological analyses of the fossil piddock shells, it was found to be a genus and species new to science, which is described here.Systematic paleontologyPhylum Mollusca Linnaeus, 1758Class Bivalvia Linnaeus, 1758Family Pholadidae Lamarck, 1809Subfamily Martesiinae Grant & Gale, 1931†Palaeolignopholas gen. novLSID: http://zoobank.org/urn:lsid:zoobank.org:act:1D686DCE-A5E9-41DA-9504-2EC58C93D988Type species: †Palaeolignopholas kachinensis gen. & sp. nov.Etymology. This name is derived from the prefix ‘Palaeo-’ (ancient), and ‘-lignopholas’, the name of a recent genus of estuarine and freshwater piddocks boring into wood, mudstone rocks, brickwork, laterites, etc.11,13. Masculine in gender.Diagnosis. The new monotypic genus is conchologically similar to several other piddock genera such as Lignopholas, Martesia, and Diplothyra Tryon 1862 but can be distinguished from these taxa by the following combination of characters: mesoplax relatively small, triangular, divided longitudinally, posterior slope without concentric sculpture, sculptured valve with concave parallel ridges (Martesia-like “rasping teeth”) curved anteriorly, periostracal lamellae dense, fine, hair-like. The fossil genus †Opertochasma Stephenson, 1952 shares a divided mesoplax but it clearly differs from both †Palaeolignopholas gen. nov. and Lignopholas by having two radial grooves on the shell surface21.Distribution. Kachin State, northern Myanmar; Upper Cretaceous (lower Cenomanian)15,19,22.Comments. Both †Palaeolignopholas gen. nov. and Lignopholas appear to be closely related to each other because they share a longitudinally divided mesoplax and periostracal lamellae, which are considered diagnostic features distinguishing this clade from Martesia + Diplothyra. Based on available conchological characters, we assume that †Palaeolignopholas gen. nov. might be placed on the ancestral stem lineage of the Lignopholas clade, although a possibility of homeomorphy could not entirely be excluded.†Palaeolignopholas kachinensis gen. & sp. nov = Plant Antheridia or Fungal Sporangia indet. sensu Grimaldi et al. (2002): 9, fig. 2a,b (bivalve specimens), fig. 3 (borings), fig. 5 (shell reconstruction of an immature specimen), figs. 6 and 7 (SEMs of borings surface showing rasped ornament at different magnifications)16. = Palaeoclavaria burmitis Poinar & Brown (2003): 765, figs. 1–4 (borings) [this fungal taxon was introduced using a trace fossil (boring) as the holotype]17; Poinar (2016): 2, figs. 10, 15, 16 (borings)18. = Martesiinae indet. sensu Smith & Ross (2018): 4, figs. 1a–c, 2a,b, 3a–d (borings), 4a,b, 5a–e (bivalve specimens)19. = Pholadidae indet. sensu Mao et al. (2018): 99, figs. 8a–f (borings), 8g,h (bivalve specimens)20. = Martesia sp. 2 sensu Mayoral et al. (2020): 10, figs. 4a (borings), 7b, 8a–l (bivalve specimens)15. = Pholadidae indet. sensu Balashov (2020): 623.Figures 1, 2, 3, 4, 5, 6 and 7.Figure 3Holotype and a paratype of †Palaeolignopholas kachinensis gen. & sp. nov. from lower Cenomanian Kachin amber, northern Myanmar. (A) Holotype: ventro-lateral view of articulated shell. (B) Paratype: anterio-lateral view of fossilized shell. VN ventral margin; DR dorsal margin; AN anterior margin; PS posterior margin; d disc; rs rasping surface of the valve; uvs umbonal ventral sulcus; pg pedal gape; pl periostracal lamellae. Scale bars = 500 µm. (Photos: Ilya V. Vikhrev).Full size imageFigure 4Paratypes of †Palaeolignopholas kachinensis gen. & sp. nov. from lower Cenomanian Kachin amber, northern Myanmar. (A) Paratype: dorsal view of articulated shell. Scale bar = 500 µm. (B) Paratype: dorsal view of articulated shell. The detached and deflected umbonal paired fragment of the valves is framed by red square. The blue contour indicates the lifetime position of this fragment. The blue arrows show the shell breakages. Scale bar = 200 µm. (C) Umbonal paired fragment of the holotype valves (inner view). The blue arrows show the shell breakage. Scale bar = 200 µm. VN ventral margin; DR dorsal margin; AN anterior margin; PS posterior margin; ms longitudinally divided mesoplax (inner view); pr prora; d disc; rs rasping surface of the valve; uvs umbonal ventral sulcus; pg pedal gape; pl periostracal lamellae; sb shell breakage. (Photos: Ilya V. Vikhrev).Full size imageFigure 5Rasping surface of †Palaeolignopholas kachinensis gen. & sp. nov. shell. (A) Holotype shell. The red frame marks position of the enlarged area. (B) Undulated micro-sculpture of the rasping surface. Scale bar = 100 µm. (Photos: Ilya V. Vikhrev).Full size imageFigure 6Schematic reconstruction of †Palaeolignopholas kachinensis gen. & sp. nov. from lower Cenomanian Kachin amber, northern Myanmar based on the type series and other fossil material15,16,19,20. (A) Lateral view of adult specimen. (B) Dorsal view of adult specimen. (C) Ventral view of adult specimen (based on a paratype BMNH 2020515). (D) Anterio-ventral view of immature specimen. (E) Dorsal view of immature specimen. (F) Mesoplax of adult specimen. (G) Mesoplax of immature specimen. d disc; mt metaplax; ms mesoplax; hp hypoplax; ca callum; uvs umbonal ventral sulcus; pg pedal gape; pl periostracal lamellae. Scale bars = 1 mm (A–C). (Line graphics: Yulia E. Chapurina).Full size imageFigure 7Clavate borings of †Palaeolignopholas kachinensis gen. & sp. nov. from lower Cenomanian Kachin amber, northern Myanmar. (A) Cluster of borings. It marks drilling of immature piddocks into soft resin from the unidentified plant (wood?) fragment. (B–D) Clavate borings of adult piddocks. Scale bars = 1 mm. Abbreviation: bg a characteristic bioglyph indicating the shell rotation inside hardening resin. (Photos: Ilya V. Vikhrev).Full size imageLSID: http://zoobank.org/urn:lsid:zoobank.org:act:F6659EBF-B0A4-4B21-A99B-2C56BDB7EC9B.Common name. Kachin Amber Piddock.Holotype. RMBH biv1115, the adult shell with length 3.07 mm and width 1.13 mm “floating” in the resin (Figs. 1A, 3A, 5A,B), local collector leg., Russian Museum of Biodiversity Hotspots, N. Laverov Federal Center for Integrated Arctic Research of the Ural Branch of the Russian Academy of Sciences, Arkhangelsk, Russia.Paratypes. RMBH biv1116, the fossilized adult shell with length 4.05 mm and width 1.83 mm (Figs. 1C, 3B); RMBH biv1101, the immature specimen with articulated shell (width 1.86 mm) sharing a detached and deflected umbonal paired fragment of the valves due to the shell breakage (Figs. 1B, 4B,C); RMBH biv1101, the other immature specimen with shell length 2.68 mm and shell width 2.52 mm in this amber piece (Figs. 1B, 4A); BMNH 20205, adult specimen [illustrated in Mayoral et al. (2020): fig. 7B15], Department of Palaeontology, Natural History Museum, London, UK; NIGP 169623, adult specimen [illustrated in Mao et al. (2018): 100, fig. 8G20], and NIGP 169624, two adult specimens [illustrated in Mao et al. (2018): 100, fig. 8H20], Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China; RS.P1450, two sub-adult specimens [illustrated in Smith & Ross (2018): 5, fig. 4A,B19], Ru D. A. Smith collection, Kuala Lumpur, Malaysia.Type locality and strata. The Noije Bum Hill mines, Hukawng Valley, near Tanai (26.3593°N, 96.7200°E), Kachin State, northern Myanmar; Upper Cretaceous (lower Cenomanian; absolute age of youngest zircons in enclosing marine sediment: 98.79 ± 0.62 Ma)19,22.Etymology. The name of this species reflects its type locality, which is situated in the Kachin State of Myanmar.Diagnosis. As for the genus.Description. Shell small (up to 9.3 mm in length15,19,20), conical, with a rounded anterior margin, tapering posteriorly (Figs. 3A,B, 4A–C, 6A–E); its shape is similar to those in the recent Lignopholas, Martesia, and Diplothyra. Valve sculptured, with concave parallel ridges (Martesia-like “rasping teeth”) curved anteriorly (Fig. 5A,B). The ridges share a characteristic wave-like micro-sculpture (Fig. 5B). Sulcus deep (Figs. 3A, 4C, 6A–C). Mesoplax longitudinally divided, relatively small, triangular, tapering or lobed anteriorly (Fig. 3A, 6B,F), in immature specimens sometimes with lateral lobes (Figs. 4C, 6E,G). Metaplax and hypoplax long, narrow, not longitudinally divided but sometimes slightly bifurcated posteriorly (Fig. 6A–C). Periostracum densely covered by fine, hair-like lamellae (Figs. 4B,C and 6D). Umbonal reflection with large flattened ridge. Pedal gape presents in immature (Figs. 4A,B, 6D) and some adult specimens (Fig. 3A) but it is covered by callum in older specimens (Figs. 3B, 6C). Morphological details of the new species were also presented in a series of micro-CT images published Mayoral et al. (see Fig. 8 in that paper15) and in the reconstruction of Grimaldy et al. (see Fig. 5 in that work16).Figure 8Recent freshwater piddock Lignopholas fluminalis (Blanford, 1867) in the middle reaches of the Kaladan River, Rakhine State, Myanmar13. (A) Habitat of the freshwater piddock: river pool with siltstone rocks at the bottom, a possible modern analogue of the Mesozoic riverine ecosystem with †Palaeolignopholas. (B) Siltstone rock fragment with living freshwater piddocks inside their clavate borings. (C) Ethanol-preserved piddock (dorsal view). (D) Living piddock with fully developed callum (ventral view). (E) Living piddock with pedal gape (ventral view). Abbreviations: d disc; mt metaplax; ms mesoplax; ca callum; uvs umbonal ventral sulcus; pg pedal gape; pl periostracal lamellae. Scale bar = 2 mm. (Photos: Olga V. Aksenova).Full size imageBorings and corresponding ichnotaxon. The borings produced by †Palaeolignopholas kachinensis gen. & sp. nov. represent club-shaped (clavate) structures (Figs. 1C,D, 2A–E, 7A–D), sometimes with a characteristic bioglyph revealing the shell rotation in hardening resin (Fig. 7C). These borings were illustrated in detail15,16,17,19,20, and were considered belonging to Teredolites clavatus Leymerie, 184215. Initially, the trace fossils produced by the Kachin amber piddock were described as sporocarps of Palaeoclavaria burmitis Poinar & Brown, 2003, a non-gilled hymenomycete taxon17. The holotype of this taxon represents a club-shaped piddock crypt labelled as follows: “Amber from the Hukawng Valley in Burma; specimen (in piece B with accession number B-P-1) deposited in the Poinar amber collection maintained at Oregon State University (holotype)17”. Hence, Palaeoclavaria Poinar & Brown, 2003 and P. burmitis Poinar & Brown, 2003 must be considered ichnogenus and ichnospecies, respectively. New ichnotaxonomic synonymies are formally proposed here as follows: Teredolites Leymerie, 1842 (= Palaeoclavaria Poinar & Brown, 2003 syn. nov.), and Teredolites clavatus Leymerie, 1842 (= Palaeoclavaria burmitis Poinar & Brown, 2003 syn. nov.). More