A structurally preserved seed was found within a calcareous siltstone nodule collected at Pombetsu, Mikasa City, Hokkaido, Japan (Figure 1). The Hikagenosawa Formation of the Yezo Group outcrops in the area and contains ammonoids indicating a date in the early Late Albian 3031. Palaeomagnetic studies show that these sediments were deposited in palaeolatitudes of 35 to 40°N and that the sedimentary basin was located on the eastern side of Laurasia 3032. Because the formation mainly consists of offshore sediments, there are few palaeobotanical records within it 333435, but a subtropical climate with dry seasons seems to have prevailed in the area, as suggested by the occurrence of a cheirolepidiaceous conifer 34 and a variety of ephedroid palynomorphs 33.

Although seed structures have not been examined in all Trimenia species, available information on four species (T. moorei, T. neocaledonica, T. papuana, and T. weinmanniifolia) suggests that Trimeniaceae is uniform family in regard of ovule and seed structures 14567. The seed of extant Trimenia is characterized by the testa, consisting of a lignified multilayered exotesta with an alveolate surface and a nonlignified multilayered mesotesta (Figures 2b,d,i,j) 147. Although many families possess seed coats with a hardened exotesta, none other than Trimeniaceae is known to possess a multilayered exotesta 14736. The seeds of some taxa are similar in appearance to the Trimeniaceae seed in that the testa consists of outer sclerotic and inner non-sclerotic cell layers. These taxa include the genus Nuphar (Nymphaeaceae) 3738, the families Buxaceae 36, Simmondsiaceae 39, Melianthaceae 40, Myrtaceae 3641, and Theaceae 36, and the order Sapindales 3642. However, in these taxa, the stony outer layer is composed of a single-layered columnar exotesta and the outer part of the isodiametric mesotestal cells, whereas the exotesta of Trimenia is composed uniformly of sclerotic isodiametric cells, which are derivatives of the outer epidermis of the outer integument 7. Thus, to our knowledge, no other seeds can be compared to the fossil seed.

The fossil seed is not distinguishable from that of extant Trimenia in terms of shape, size, anatropy, bitegmy, endo and exostomic micropyle, crushed inner integument with an operculum, lignified multilayered exotesta composed of isodiametric cells, alveolate surface of the exotesta, or nonlignified multilayered mesotesta (Figure 2). These remarkable similarities unequivocally indicate the close affinity of the fossil seed to the Trimeniaceae. The fossil seed differs from Trimenia seed in the absence of antiraphal vascular bundle (Figure 2e) and in the size of membranous structures surrounding the nucellus (Figures 2c,g; see below for details). Thus, a new genus of Trimeniaceae should be assigned to the fossil seed. The seed also differs from Trimenia seed in having a stalklike structure at the nucellar base (Figure 2a). This stalklike structure might represent a diagnostic feature of the seed, but the structure could be artificially formed by shrinkage.

Molecular phylogeny indicates that the Trimeniaceae diverged after the Nymphaeales and before the Illiciaceae 891011. Thus, diversification of the Trimeniaceae in the Early to earliest Late Cretaceous (from 125 to 90 million years ago) is indicated 22 by the Late Barremian flower of the Nymphaeales from Portugal 18 and the Cenomanian to Turonian seeds of the Illiciaceae from Kazakhstan 16. Although Trimeniaceae-like triporate or polyforate pollen was collected in the Albian to Cenomanian of Brazil 4344, the Barremian of Portugal 1517 and the Campanian to Maastrichitian (83–65 million years ago) of Australia and Antarctica 4445, the possibility could not be ruled out that these are assigned to other families 20294446. Longstrethia varidentata, a foliar species reported from the Cenomanian of Nebraska, is similar to leaves of Trimeniaceae in presence of an intermarginal vein, but L. varidentata would represent a stem-group taxon of Trimeniaceae-Illiciaceae clade because some characters are also shared with Illiciaceae 47. Therefore, no unequivocal record of Trimeniaceae in the Cretaceous existed. The fossil seed provides the first unequivocal evidence of Trimeniaceae 100 million years ago that fills the gap between molecular data and paleobotanical records.

The fossil seed coat structures are strikingly similar to those of Trimenia (Figure 2). Although these characters were acquired 100 million or more years ago and are conserved in Trimeniaceae, the mode of nutrient storage of the fossil seed would have differed considerably from that of the extant Trimenia. The Trimenia embryo is enclosed by the endosperm, which is further surrounded by the perisperm, an additional storage tissue (Figures 2d,i) 467. However, the fossil seed has two membranous structures inside the nucellus, and the area encircled by the inner structure corresponds in size to the dormant mature embryo of extant Trimeniaceae (Figures 2g,i). If this comparison is correct, the outer structure may be comparable to the endosperm membrane. In albuminous seeds without a perisperm, the epidermis of the nucellus is either completely disintegrated by the enlarged endosperm or vestigially retained as a crushed layer, whereas in perispermous seeds, it is retained as the epidermis of the perisperm 6736. The fossil has a relatively well preserved nucellar epidermis, which may imply the presence of a perisperm. The putative endosperm in the fossil occupies a large area inside the nucellus, and thus the endosperm would have played a major storage role. Among the earliest families to diverge, only Nymphaeales 38, Hydatellaceae 48, and Trimeniaceae 467 have a diploid maternal perisperm, which occupies a larger area than the diploid or polyploid fertilized endosperm. This character variation implies that the storage function was largely taken over by the perisperm secondarily 4950 and that the fossil seed may have been on the way toward perispermy.

The fossil seed vasculature is different from that of extant Trimenia. In Trimenia, the vascular bundle supplied from the fruit wall enters the raphe of the seed and extends to the antiraphal side beyond the chalaza 4. Lack of antiraphal vascular bundles in the fossil seed suggests that the antiraphal bundle is a derived character in the Trimeniaceae. This is consistent with the hypothesis, based on character distribution in extant basalmost angiosperms, that the antiraphal bundle is a derived character in the angiosperm seed 651.

So far, we know only the seed of this trimeniaceous fossil, but the areoles sculptured in the seed surface may imply that the fossil seed was contained in a berry because the pressure of berry endocarp cells forms areoles on seeds of extant Trimenia 1234567.

Extant Trimeniaceae species are distributed in eastern Australia and an island chain stretching from Celebes to the Moluccas, New Guinea, New Caledonia, Fiji, Samoa, and the Marquesas 124. Along with the Trimeniaceae, other basalmost families (Amborellaceae, Hydatellaceae, Austrobaileyaceae) and many eumagnoliid families (e.g., Degeneriaceae, Monimiaceae, Winteraceae) now grow only in derivative fragments of Gondwanaland 1452. Contrary to their extant austral distribution, palynological records suggest that early angiosperms originated in the low latitudes 2526 and migrated both northward and southward 262728. Thus, the paleobotanical data indicate that the current austral distributions do not reflect the cradle of angiosperms but the area of conservation 262728. At the same time, the poleward migration model predicts the past occurrence of these basalmost angiosperm families in the Northern Hemisphere. Some pollen grains, tentatively assigned to the Amborellaceae 53 or Trimeniaceae 151720, are reported from the Northern Hemisphere, but familial assignations of pollen are difficult 202946. The fossil seed reveals that Trimeniaceae occurred in the eastern margin of Laurasia in the Northern Hemisphere during the Albian period and indicates a reduction to the relict area subsequent to the hypothesized bipolar migration.