Aquatic organisms are rarely found in amber, but when they occur they provide invaluable evidence for the better understanding of amber taphonomy and past ecosystems. We report an ammonite and several marine gastropods alongside a mixed assemblage of intertidal and terrestrial forest floor organisms in mid-Cretaceous Burmese amber. Our discovery indicates that the Burmese amber forest was living near a dynamic and shifting coastal environment. The ammonite also provides supporting evidence for the age of the amber, which is still debated, and represents a rare example of dating using fossils present inside the amber.

Amber is fossilized tree resin, and inclusions usually comprise terrestrial and, rarely, aquatic organisms. Marine fossils are extremely rare in Cretaceous and Cenozoic ambers. Here, we report a record of an ammonite with marine gastropods, intertidal isopods, and diverse terrestrial arthropods as syninclusions in mid-Cretaceous Burmese amber. We used X-ray–microcomputed tomography (CT) to obtain high-resolution 3D images of the ammonite, including its sutures, which are diagnostically important for ammonites. The ammonite is a juvenile Puzosia (Bhimaites) and provides supporting evidence for a Late Albian–Early Cenomanian age of the amber. There is a diverse assemblage (at least 40 individuals) of arthropods in this amber sample from both terrestrial and marine habitats, including Isopoda, Acari (mites), Araneae (spiders), Diplopoda (millipedes), and representatives of the insect orders Blattodea (cockroaches), Coleoptera (beetles), Diptera (true flies), and Hymenoptera (wasps). The incomplete preservation and lack of soft body of the ammonite and marine gastropods suggest that they were dead and underwent abrasion on the seashore before entombment. It is most likely that the resin fell to the beach from coastal trees, picking up terrestrial arthropods and beach shells and, exceptionally, surviving the high-energy beach environment to be preserved as amber. Our findings not only represent a record of an ammonite in amber but also provide insights into the taphonomy of amber and the paleoecology of Cretaceous amber forests.

Amber provides a unique mode of preservation for organisms, and when inclusions are present they are usually 3D fossils of terrestrial plants, microorganisms, arthropods, and even vertebrate remains (1⇓–3). Amber deposits are therefore considered to be exceptional Lagerstätten, providing unique windows into past ecosystems (4⇓–6). Given that amber is formed by the fossilization of terrestrial plant resins, the capture of marine inclusions may be considered extremely rare. However, some recent findings of marine and freshwater fossils, particularly, microfossils such as diatoms, radiolarians, ostracods, and copepods, have provided fresh insights into amber taphonomy (7⇓⇓⇓⇓–12).

Burmese amber (from northern Myanmar) contains the most diverse biota of all known Cretaceous ambers (13, 14). Over the last 100 years, and particularly in the past two decades, Burmese amber has received worldwide scientific interest; more than 500 families of invertebrates, vertebrates, protists, plants, and fungi have been reported (15). Here, we provide an account of an exceptional piece of amber that preserves a unique assemblage of marine macrofossils, alongside intertidal, fully terrestrial, and possibly freshwater aquatic arthropods.

Results

The ammonite-bearing piece of amber (BA18100) was obtained from an amber mine located near Noije Bum Village, Tanaing Town (ref. 16 and Fig. 1). It is 33 mm long, 9.5 mm wide, and 29 mm high, and its weight is 6.08 g. There is a diverse assemblage (at least 40 individuals) of arthropods in this amber sample that live today in both terrestrial and marine habitats. Of the terrestrial fauna, Acari (mites) are the most abundant, with 23 specimens; also present are Araneae (spiders), Diplopoda (millipedes), and representatives of the insect orders Blattodea (cockroaches), Coleoptera (beetles), Diptera (true flies), and Hymenoptera (wasps). The arthropod assemblage consists mostly of forest floor-dwelling taxa, and living representatives are generally associated with leaf litter or the top layers of soil. There are several isopods preserved which are consistent with littoral or supralittoral taxa. In addition to the ammonite itself, four definitively marine gastropod shells and one putatively marine isopod are present.

Fig. 1. Geological and paleogeographic maps of Burmese amber. (A) Geological map showing the position of Burmese amber in Hukawng Valley, northern Myanmar. (B) Paleogeographic map showing the position (red triangle) of Burmese amber site during Late Albian (14, 17).

Ammonite. The ammonite is a juvenile (the adapertural septa are not crowded), has a maximum preserved diameter of 12 mm, and appears to retain the original aragonitic shell, on the basis of its appearance in reflected light (Fig. 2A). It is composed in part by the body chamber, but the apertural part is damaged, as revealed by the survival of a 60° sector of the umbilical wall extending beyond the fragment of the inner flanks of the shell (Fig. 2). Coiling is moderately involute, with ∼64% of the previous whorl covered. The small, shallow umbilicus comprises 18% of the diameter, the low umbilical wall is very weakly convex, and the umbilical shoulder is broadly rounded. The whorl section is compressed, with a whorl breadth-to-height ratio of around 0.7 (the specimen has undergone some postmortem deformation). The inner flanks are very weakly convex, the outer flanks flattened and weakly convergent, the ventrolateral shoulders broadly rounded, and the venter very weakly convex. Ornamentation consists of low falcoid folds, lirae, and riblets that are prorsiradiate and very weakly concave on the inner flank, flexing back and weakly convex at midflank before flexing forward and weakly concave on the outer flank, sweeping forward over the ventrolateral shoulders and crossing the venter in a broad convexity. The suture of the penultimate whorl, revealed in X-ray–microcomputed tomography, is only partially decipherable. E/A is broad, bifid, and moderately incised; A is narrower and possibly trifid; and A/U2 is narrow, little incised, and bifid (Fig. 2B; see Movie S1 for detailed account). Fig. 2. Ammonite Puzosia (Bhimaites) Matsumoto. (A) Lateral view under light microscopy. (B) Flattened sutures reconstructed by microtomography. (C) Microtomographic reconstruction, apparent view. (D) Microtomographic reconstruction, surface rendering; (E) Microtomographic reconstruction, virtual section. (Scale bars, 2 mm.) Given the age of the amber (discussed below), a compressed, involute, weakly ornamented ammonite could belong to one of three principal groups, the superfamilies Phylloceratoidea, Lytoceratoidea, or Desmoceratoidea. The visible structure of the sutures and the lack of distinctive even lirae eliminate the first two superfamilies from consideration. Within Desmoceratoidea, compressed weakly ornamented taxa comparable to the present specimen occur in two subfamilies of the family Desmoceratidae, the Beudanticeratinae and Puzosiinae. Among the Beudanticeratinae, a possible assignation is to Beudanticeras Hitzel (18), a genus that ranges from the Lower to Upper Albian and is known from Europe, North Africa, the Middle East, KwaZulu-Natal (South Africa), Mozambique, Madagascar, northern Pakistan, Australia, Patagonia, and Antarctica. There are similarities with juveniles of the Lower Albian Beudanticeras caseyi Collignon (ref. 19, p. 72, pl. 267, fig. 1165; holotype refigured by ref. 20, text-fig. 3j, k), known from Madagascar and northern KwaZulu-Natal (South Africa), and comparably sized juveniles of the Tunisian upper Lower Albian Beudanticeras dupinianum var. africana (ref. 21, p. 133, pl. 5, figs. 16, 17; text-fig. 49), as figured by Latil (ref. 22, pl. 3, figs. 1–19). These species, however, do not develop the low folds and undulations of the present specimen, and Beudanticeras is restricted to the Albian. Among the Puzosiinae, there are similarities with Puzosia (Bhimaites) Matsumoto (23), which ranges from the Upper Albian to the Upper Cenomanian and is known from western Europe, North Africa, Angola, KwaZulu-Natal (South Africa), Madagascar, South India, Japan, and Venezuela. The falcoid course of the ornament, which matches our specimen, is seen in several representatives of the genus, for example, the Upper Albian Puzosia (Bhimaites) pinguis (24) illustrated by Kennedy and Klinger (ref. 25, text-fig. 12a–g). A feature of Puzosia (Bhimaites) is the development of constrictions on the internal mold; their position is marked on the shell surface by much weaker depressions and associated collar ribs. These are very weakly expressed or absent in specimens comparable in size to the present specimen [see, for example, the somewhat larger (30 mm diameter) individual of Bhimaites stoliczkai (26) figured by Renz (ref. 27, pl. 8, fig. 2)]; this species ranges from the Upper Albian to Lower Cenomanian. To conclude, features of the ammonite preserved most strongly suggest a juvenile Puzosia (Bhimaites), a subgenus that first appeared in the Upper Albian and ranged through the Cenomanian.

Isopods. There are four isopod specimens in the amber (Fig. 3 A–C) and a further three specimens, which cannot be determined but may also be isopods. The first isopod (Fig. 3A) is consistent with terrestrial isopods in body shape: the eyes appear to be reduced, although this is not entirely clear, and there are six to seven pereonite segments with all pereopods ambulatory. The form of the uropods, if present, is not entirely clear, which is unfortunate, as this is a key character for distinguishing marine and terrestrial taxa. It is similar to Armadillidae, which is recorded from Burmese amber (28), but also exhibits characters of marine taxa, such as having a larger posterior part, but many important characters are obscured, so it is difficult to identify with certainty. Although Armadillidae is generally considered to be terrestrial, Poinar (28) considered that the features present in his fossil excluded it from those Oniscoidea mostly adapted to terrestrial habitats, such as the strictly terrestrial Myanmariscus, also recorded from Burmese amber (29). Fig. 3. Isopods of uncertain taxonomic affinity, but generally consistent with littoral or supralittoral taxa. (A) Isopod 1. (B) Isopod 2. (C) Isopod 3. (D) Circular structure attached to the dorsal surface of isopod 2. (Scale bars, 1 mm in A and C. Scale bar, 0.5 mm in B and D.) The second isopod (Fig. 3B) exhibits an elongated exopod uropod and is similar to extant Sphaeromatidae in general habitus, indicating that it is possibly a marine or intertidal isopod. Sphaeromatids are typically marine, but many have been known to occur in estuaries (30) and intertidal zones and even to extend into freshwater habitats, including karstic streams and caves (31). The pale coloration and reduced eyes could indicate a stygobiont, but the color could also be taphonomic. These characters are also not restricted to cave dwellers, as some open-marine isopods also exhibit them (32). There is a peculiar circular structure seemingly attached to this specimen (Fig. 3D), although the association could be taphonomic. The other two isopods (e.g., Fig. 3C) exhibit characters of terrestrial or supralittoral isopods and are possibly associated with the extant Oniscidea: Tylidae. The uropods appear to be reduced, as is typical of the more terrestrial taxa, and the visible antennae are thick at the base with strong basal segments tapering gradually toward the apex with a two- or three-jointed flagellum. The first pair of antennae appear to be strongly reduced, which is also an indication of the Oniscidea, with Tylidae only retaining the proximal article (33). There are a few other specimens that are probably isopods, including one that is badly damaged with most of its ventral side obscured by a gastropod. The coloration and coxal plates are similar to those of the specimen shown in Fig. 3A, but there is a partial eye preserved, and it is larger. It is closely associated with a gastropod, but this is probably taphonomic. There are two other very badly damaged specimens, which may be isopods and two others, which may be isopod larvae, but they are poorly preserved. Although taxonomic assignment is difficult based on specimens in which key characters cannot be observed, the specimens present in the piece of amber seem to be consistent with littoral or supralittoral isopods, with one possible fully marine species.

Gastropods. Four marine gastropod shells are also preserved with the ammonite (Fig. 4), of which two are well-preserved and can be attributed to the genus Mathilda Semper (Mathildidae) by the small, conical shell with heterostrophic protoconchs, whorl sides rounded and basally subcarinate, base broadly arched, and ornament of strong spiral cords and fine axial threads (34). Mathilda was mainly distributed in the western Tethys sea during the Cretaceous, and our fossils are a Cretaceous record of this genus from the eastern Tethys sea. Fig. 4. Gastropods. (A) Mathilda sp. (B) Mathilda sp. (C) Undetermined specimen. (D) Undetermined specimen. (Scale bars, 1 mm.)