A new fossil piddock (Bivalvia: Pholadidae) may indicate estuarine to freshwater environments near Cretaceous amber-producing forests in Myanmar

The lower Cenomanian Kachin amber from Myanmar contains a species-rich assemblage with numerous plant and animal fossils. Terrestrial and, to a lesser degree, freshwater species predominate in this assemblage, while a few taxa with marine affinities were also discovered, e.g. isopods, ammonites, and piddocks. Here, we describe the Kachin amber piddock †Palaeolignopholas kachinensis gen. & sp. nov. It appears to be an ancestral stem lineage of the recent Lignopholas piddocks, which are estuarine to freshwater bivalves, boring into wood and mudstone rocks. Frequent occurrences and high abundance of †Palaeolignopholas borings and preserved shells in the Kachin amber could indicate that the resin-producing forest was partly situated near a downstream (estuarine to freshwater) section of a river. Multiple records of freshwater invertebrates (caddisflies, mayflies, stoneflies, odonates, and chironomids) in this amber could also manifest in favor of our paleo-environmental reconstruction, although a variety of local freshwater environments is known to occur in coastal settings.

, 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. 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. www.nature.com/scientificreports/ 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 surface 21 .
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,     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.

Discussion
Taxonomic placement of the Kachin amber piddock. Previously, †Palaeolignopholas kachinensis gen. & sp. nov. was considered a member of the recent genus Martesia 15 . However, the latter genus does not share a divided mesoplax and periostracal lamellae 11 , hence, the new species cannot be placed in Martesia (nor Diplothyra). In its turn, these characters are diagnostic for the genus Lignopholas [11][12][13] . Conversely, Lignopholas shares a posterior slope with concentric sculpture, a smooth valve, and a specific shape of periostracal lamellae (being large, flat, and fringed). Therefore, we prefer to place the fossil species from Cretaceous Kachin amber within the new genus †Palaeolignopholas that appears to be on the stem lineage of the recent Lignopholas clade. The †Palaeolignopholas differs from Lignopholas by having a posterior slope without concentric sculpture, a sculptured valve with Martesia-like "rasping teeth", and fine, hair-like lamellae.
Taphonomy of the new fossil taxon. All available specimens and trace fossils of †Palaeolignopholas kachinensis gen. & sp. nov. are characterized by very small size [15][16][17]19,20 . The shell length of the largest specimen ever known is 9.3 mm 20 , while other samples are much smaller 15,19 . For example, the maximum length was 4.2 mm in a comprehensive sample of shells and borings (N = 131) examined by Mayoral et al. 15 . At first glance, such a small size might be a diagnostic feature of this taxon, as species from conchologically similar genera such as Lignopholas, Martesia, and Diplothyra share much larger size, usually up to 20-30 mm in length 11,12 . However, Lignopholas chengi Turner & Santhakumaran, 1989 was described based on a series of small (possible immature) specimens, the largest of which was 11.5 mm long 11 that agrees with our dataset on †Palaeolignopholas.
It should be noted that fossil specimens of †Palaeolignopholas kachinensis gen. & sp. nov. commonly occur "floating" in the amber (see Figs. 1A,B, 3A, 4A-C) indicating that they often bored into freshly produced (liquid or soft) resin 15,19 . Smith & Ross 19 assumed that these "floating" clams died within 1-2 weeks since their settlement on the resin, while larger examples bored into hardened resin. Mayoral et al. 's taphonomic model 15 predicts that at first stage the bivalves bored into a living resin-secreting tree, which was periodically immersed in seawater. Later, when the resin hardens, wood can partially decay, being replaced by episodic flows of resin, while the bivalves could repeatedly bore the resin after its hardening 15 . Among recent pholadid taxa, Martesia nairi Turner & Santhakumaran, 1989 bores into living mangroves, and damages the trees 11 . However, it was shown that the Kachin amber was produced by Taxodiaceae and Araucariaceae and that several conifer families were the primary sources of Cretaceous amber globally 19,24 . There is not much evidence that conifers such as taxodioids and araucarians could survive in seawater/brackish tidal environments, while the presence of Taxodiaceae pollen in the Noije Bum's amber horizon 25 could indicate riparian and freshwater wetland communities 26 . Pyrite inclusions in the Kachin amber pieces 19 consistent with low oxygenated environments of freshwater and brackish sediments 27 . There are several possible ways of the pyrite origin in freshwater settings, e.g. diagenetic formation from slow oxidation of acid volatile sulphide by organic matter 27 . Hence, we could assume that †Palaeolignopholas clams were boring the resin dropped into the water from trunks and branches of conifer trees growing along the downstream river valley, probably having a tidal influence. Our novel records of a bunch of borings situated around a small plant (wood?) inclusion in the amber (see Fig. 7A) and a boring of adult piddock with a specific bioglyph revealing the shell rotation in hardening resin (Fig. 7C) also support this hypothesis.
Regional paleo-environmental reconstruction. Rare discoveries of supposed marine taxa in the Kachin amber 20,28,29 were used as a basis to establish the hypothesis on a close proximity of the amber-producing forest to the coastal marine 20 or brackish water habitats 19 . Records of a juvenile ammonite shell 29 and several isopods of possible saltwater (littoral or supralittoral) affinities 28,29 might reveal that the amber was produced from coastal trees located close to a marine bay or other kind of shoreline environments 29 . Numerous records of piddocks and their traces in the amber were also used as a robust evidence supporting the hypothesis on coastal origin of the resin 15,19 . It should be noted that alleged marine gastropods from the Burmese amber 30 were found to be terrestrial snails from the operculate family Pupinidae (Cyclophoroidea) 31 . A few occurrences of marine oysters, corals, and crinoids adhering to the amber pieces 20 are not relevant to the paleoenvironmental discussion since they post-date reworking of the amber into a marine environment.
Conversely, the Kachin amber houses a diverse assemblage of freshwater taxa such as caddisflies [32][33][34][35] , mayflies [36][37][38][39] , stoneflies 40 , chironomids 41 , water measurers 42,43 , odonates [44][45][46][47][48] and their stem lineage 49 . It is known that insects rarely occur in marine and brackish environments 50 , although recent saltwater-tolerant or even marine species exist among several orders, e.g. Hemiptera 51-53 , Odonata [54][55][56] , and Trichoptera 57 . Hence, a species-rich and abundant fauna of freshwater insects occurring in the amber could indicate rather a proximity www.nature.com/scientificreports/ of resin-producing trees to freshwater habitats than to coastal marine environments. Frogs and amphibian egg masses discovered in the Kachin amber 58,59 are also hardly correspond to saltwater environments. Based on our new results and the published data, outlined above, we could assume that the Kachin amberproducing forest was partly situated near a downstream (estuarine to freshwater) section of a tropical river. Furthermore, we could hypothesize that these forests as a whole extended across a variety of freshwater and estuarine environments such as coastal rivers, river deltas, lakes, lagoons, and coastal bays. It is known that environmental conditions in downstream sections of rivers draining into the sea are changing periodically from freshwater to slightly brackish or brackish due to the tidal influence 60,61 . There is a possibility of seasonal changes driven by rainfall, when the salinity in lower tidal parts of coastal rivers significantly decreases during the rainy season 62 . Although the Hukawng Valley is currently a mainland site, which is far from the sea, the Burma Terrain (i.e. a microplate covering a large part of Myanmar with amber deposits west of the Sagaing fault) was a tropical island during the Albian-Cenomanian 63,64 . In general, the hypothesis that the resin-producing forest in northern Myanmar was partially situated near a largely freshwater body such as a downstream riverine section could explain the high diversity of freshwater taxa discovered in the Kachin amber. The scarce presence of saltwater (chiefly estuarine) fauna can be explained through occasional marine ingressions into the system. At first glance, habitats of the recent freshwater piddocks Lignopholas fluminalis in the middle reaches of the Kaladan River in Myanmar 13,14 (Fig. 8A-C) can be considered a remote modern analogue of the Mesozoic ecosystem inhabited by †Palaeolignopholas. It is known that †Palaeolignopholas bored into the amber and wood 15 , while Lignopholas fluminalis bores into the bottom rocks 13,14 . The fossil species probably occurred in a downstream (estuarine to freshwater) riverine section, while the recent taxon is known to occur in the middle reaches of a river but may present itself in the estuary as well 13 . Besides, we do not know if they are really related, as the age difference between them is 100 Myr and there is no direct evidence on the missing link available. Theoretically, it could be a homeomorphy. There are other Mesozoic taxa with a portioned mesoplax 21 , providing candidates for an ancestry.
Finally, our novel results not only establish a taxonomic concept for a fossil amber-preserved piddock but also show that the bivalves on their own do not necessarily indicate the presence of brackish to normal marine salinity water. Morphological study. Measurements of amber pieces were performed using a digital caliper (Digimatic Coolant Proof, Mitutoyo, Japan). Images of amber pieces were taken using a Canon EOS 6D digital camera (Canon Inc., Japan) with Canon EF 100 mm f/2.8L IS USM macro lens (Canon Inc., Japan). Morphological study of fossil bivalve specimens was based on an approach of Turner & Santhakumaran 11 with a special focus to the shell shape, size, valve shape and sculpture, the shape of mesoplax, metaplax, hypoplax, and umbonal reflection, and the character of periostracal lamellae. Images and measurements of specimens and trace fossils were taken with a Leica M165C microscope (Leica, Germany).

Methods
Nomenclatural acts. The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature (ICZN), and hence the new name and synonymy contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank (http:// zooba nk. org), the online registration system for the ICZN. The LSID for this publication is: urn:lsid:zoobank.org:pub:C14A4E2F-9280-48A6-9C95-8298BD1F964F. The electronic edition of this paper was published in a journal with an ISSN and has been archived and is available from PubMed Central.